All Questions

Thank you for your question, which we have broken down into a series of smaller questions:

What is the difference between an “x-ray” and a “CT scan”?
The term “x-ray” refers to a radiograph, a 2-dimensional picture created by passing x-rays through an object such as a body part, and capturing a stationary shadow image that shows differences in internal structures. A CT scan or CAT scan is an image obtained by passing x-rays through an object at many different angles around the object, measuring the transmission of those x-rays at every angle, and then using those measurements to create images that look like slices through the object, similar to slices from a loaf of bread.

What is CTDI?
CTDI stands for Computed Tomography Dose Index. It is a measurement of the x-ray output that was used during a CT scan. CTDI (also known as CTDIvol) is measured in milliGray or mGy. It is a dose index, not a direct measure of a patient’s radiation dose. The CTDIvol value mentioned in the dose report (55.75 mGy) is a typical value for a CT scan of the temporal bones in a pediatric patient, and is within the expected range.

What is DLP?
DLP stands for Dose Length Product. It is a calculated quantity obtained by multiplying the CTDI of a CT scan by the length of the scan in cm. The unit of measure of DLP is mGy x cm, often written as mGycm. The DLP mentioned in the dose report (420 mGycm) is typical for a pediatric patient.

What is the difference between a radiation dose reported in mGy vs a radiation dose reported in mSv?
mGy is a unit of measure of radiation dose, which is defined as the energy deposited in a sample of matter divided mass of that sample. Therefore, dose is energy (measured in Joules) per unit mass (measured in kg). 1 mGy is equal to 1 mJ/kg. mSv is the unit of measure of “effective dose”. This is a calculated value that takes into account the part of the body that was exposed to radiation. To convert a radiation dose in mGy to an effective dose in mSv requires knowing which organs were exposed. For example, a DLP of 420 mGycm for a pediatric head CT corresponds to an effective dose of about 1.7 mSv.

Should pediatric patients be imaged with the same radiation dose that would be used for an adult?
Most radiographs and CT scans are performed using automatic exposure control. This means that the amount of radiation used to form the image is automatically tailored to the body part, to use the correct amount of radiation that will create a good quality image with the least amount of radiation necessary. Using too little radiation can produce an image with inferior quality. Using too much radiation is wasteful and, in extreme cases, potentially harmful. Small children can be imaged with less radiation than adults, but larger teenagers may need adult-sized doses of radiation to produce images of adequate quality for diagnosis.

What would be a concerning level of radiation dose for an x-ray or a CT scan?
Medical imaging professionals receive extensive training in how to operate their x-ray machines and CT scanners. This training emphasizes the use of the appropriate amount of radiation to achieve the best results for the patient with the least amount of radiation. In addition, modern imaging technology includes features such as automatic exposure control that makes the job of the imaging professional easier and reduces the likelihood of errors such as incorrect radiation dose to nearly zero. The amount of radiation used in medical imaging is far below the levels that have been associated with injury or disease. Therefore, there is little need for concern about the radiation from an x-ray image or a CT scan, or even a few x-rays or CT scans. The widely accepted effective dose level above which there may be some cause for concern is 100 mSv. In medical imaging, the effective dose may reach as high as 25 mSv in rare cases, but most exams use less than a 10 mSv, and very often less than 1 mSv. The risk of harm from a single medical imaging exam (such as a CT scan) is so small that it cannot be reliably estimated.

The use of oral or intra-venous contrast material does not increase the radiation dose that the patient receives during a CT scan. If a scan protocol includes scans both without and with a contrast agent, then the radiation dose received will be the sum of the radiation from the two scans. Any contrast material present inside the body does not affect the radiation dose received during that scan. (For more information about contrast agents, see https://www.medicalradiationinfo.org/radiationandmedicine/medical-imaging/contrast-agent-needed/).

Publications that refer to the typical radiation dose for a certain diagnostic imaging exam should state clearly whether the estimate refers to a single picture or a complete examination that may include multiple pictures (otherwise known as "views"). If that information is not given, then the reader is left uncertain about the meaning of the dose estimate. X-rays of the hip typically involve 2-4 views of the hip depending on what the doctor requested.

The AAPM's public education website and “Ask the Expert” section is intended to provide basic education about medical physics. The AAPM members who respond to questions are not medical doctors and cannot make medical decisions or recommendations about specific clinical situations. However, we can provide some general information about medical imaging physics, technology, and acceptable practice.

The decision about the most effective test to answer a clinical question is best made by the care provider who is most familiar with the patient's condition. Providers can utilize "clinical decision support" tools such as the American College of Radiology's Appropriateness Criteria (1), which produces evidence-based guidance on the proper imaging exam for a given condition. The decision about whether to have CT vs MRI is not just a matter of the lowest cost or the patient’s preference. The images produced by CT and MRI provide different information, and one test or the other may be more appropriate depending on the clinical question that needs to be answered.

Many experts believe that a patient's previous radiation exposure history should not be considered when a decision is made about a diagnostic test in the future (2). There is no evidence that the effects of radiation are cumulative, so each new exposure is independent of the previous ones. Consequently, there is no lifetime radiation dose upper limit for medical exposures (2). Patients should not be denied appropriate medical care (such as x-rays or CT scans) just because they received medical radiation in their past.

Regarding the risks associated with diagnostic imaging, the risk of harm from any diagnostic imaging exam is statistically insignificant (3). There is an association between the radiation dose received and the risk of future cancer at high doses such as those from atom bombs (eg, Hiroshima and Nagasaki) and nuclear power accidents (eg, Chernobyl), but no such association has been proven at the much lower doses associated with medical imaging. It should be noted that MRI does not use ionizing radiation.

To summarize, there is no CT radiation dose limit, the best exam for you may be an MRI or it may be a CT (or something else), and your medical team is advised to utilize evidence-based guidelines to help decide which test is best for you. Your prior radiation exposure from exams like dental x-rays, chest radiographs or even CT or nuclear scans should not influence whether you can receive additional imaging. To do so would deprive you of the benefits of that imaging. Imaging facilities that follow best practices ensure that CT radiation levels are kept to the necessary minimum to provide adequate image quality for making an accurate diagnosis.

(1) https://www.acr.org/Clinical-Resources/ACR-Appropriateness-Criteria
(2) https://www.aapm.org/org/policies/details.asp?id=2545&type=PS
(3) https://www.aapm.org/org/policies/details.asp?id=3615&type=PS

CT scans, MRI scans and cell phones have one thing in common. They all emit energy known as electromagnetic radiation. However, the energy range and the intensity levels used are quite different, and the ways that the energy interacts with the human body are also different. There is no reason to be concerned that the energy from MRI will affect your body in a way that enhances the effect from the CT scan, or vice versa. There is also no reason to be concerned about cell phone use, because cell phones cannot cause cancer, and neither can MRI scans. CT scanning uses x-rays, which are a type of ionizing radiation. At very high radiation dose levels of ionizing radiation, cancer risk is increased. We know this because of studies of atomic bomb survivors and nuclear workers who received very high doses. For patients receiving lower doses such as from medical x-rays like CT scans, there is no evidence of an increased cancer risk above the baseline risk that we all face by simply living on Earth, unless many such scans are performed in a compressed time period. Natural DNA repair mechanisms can counteract the effects of ionizing radiation at low doses and dose rates. Using diagnostic imaging to detect a potentially serious disease is generally considered to be a good trade-off when compared with the small (or possibly non-existent) theoretical risk of causing a cancer that would not appear until many years into the future, if at all.

Biological effects from radiation are categorized into two types: “deterministic” and “stochastic.” Deterministic effects occur at high levels of exposure wherein the severity of effects increases with dose. (Hair loss and skin irritation are two common examples.) The amount of radiation required to produce deterministic effects is beyond the range used in diagnostic imaging, but they are sometimes encountered at the high doses involved in radiation therapy.
With stochastic effects, the risk of effect increases with radiation dose. That is, an effect may or may not occur in a patient but its probability of occurring is assumed to scale with dose. The most common example of a stochastic effect is cancer induction (ie, carcinogenesis) which is known to be associated with radiation exposure based on evidence from groups such as atomic bomb survivors, residents near sites of severe nuclear accidents, etc, who received moderate to high radiation doses.
Not all tissues and organs are equally sensitive to radiation, and some are more likely to undergo DNA changes that can eventually lead to stochastic effects such as cancer. The quantity that accounts for these differences in tissue response is called “effective dose.” It is a sum of the radiation doses received by individual organs and tissues after applying weighting factors that account for the radiation sensitivity of the exposed organs, with more sensitive organs carrying more weight than less sensitive organs. The unit of effective dose is the sievert (Sv) and 1 Sv corresponds to 1 joule/kilogram (1 J/kg) of photon energy absorbed uniformly in the whole body. It is common to report effective doses in milliSieverts (mSv, or 0.001 Sv).
Now, to finally address your specific questions: Effective dose is a useful quantity for roughly comparing the relative risks associated with partial-body exposures (such as from X-ray imaging) and whole-body exposures (such as from natural background radiation). By its definition, 1 mSv of effective dose correlates to the same risk of overall detriment regardless of which parts of the body were actually irradiated.
A couple final comments: For the relatively low dose of 1 mSv being considered the duration of irradiation is not a factor, so it is not unreasonable to compare the effective dose from a brief medical imaging procedure to that from a year of background radiation. It is also important to emphasize that effective dose was originally designed as a metric for use in conservative radiological protection calculations and it should never be used to estimate the absolute risk of stochastic effects from radiation exposure.

The risks associated with high energy radiation (such as x-rays and gamma rays) have been studied for many years. It is widely recognized that large amounts ("high doses") of such "ionizing" radiation can be harmful. However, there is not widespread agreement about the risks associated with smaller amounts of this radiation, such as the amounts used for medical imaging. The biological effects from low doses of ionizing radiation (x-rays and gamma rays) are actually still something of a mystery. There are theoretical models that predict that low doses (amounts just above the "background" level that we are surrounded by while living on earth) can be the cause of cancers that appear many years in the future. There are other models that support the notion that low levels of ionizing radiation (less than ~100 mSv) are harmless. Both of these ideas have supporters in the scientific community. But due to the fact that cancer is a common disease (42% of individuals are diagnosed with cancer during their lifetime), it is impossible to say that an individual cancer diagnosis was caused by radiation, as opposed to a "natural" cancer or from some other cause. For the same reason, the uncertainty that is inherent in biostatistics suggests that we will never know with confidence whether low levels (less than 100 mSv) of radiation exposure increases one's lifetime cancer risk, or has no effect at all. You can think of this like the risk of getting lung cancer from smoking 1 cigarette, or the risk of dying in a fatal car crash during 1 road trip. In these examples, the risk is very close to zero or may actually be zero.

With respect to your question about 5 x-ray images of the head of a 6-week-old baby, we can say that the total radiation dose from these 5 x-rays is indeed low, only a fraction of 1 mSv. The theoretical risk associated with this exposure is so small that it cannot be calculated. In addition, we know that the body repairs DNA damage caused by ionizing radiation. Small doses of radiation are easily repaired, while high doses can overwhelm the repair system, leading to lasting damage and subsequently more long-term cancer risk. We know this from studies of people who received very high doses of radiation.

The x-rays that your child experienced produced images that undoubtedly aided in your child's care. Such images are invaluable in improving human health. All patients (or their proxies) must weigh the benefits of medical technology against the costs and risks, and one of those risks is the small theoretical risk from ionizing radiation, which is possibly even zero. The Hippocratic oath requires medical professionals to justify tests and procedures by ensuring that the benefits exceed the risks. Through open communication between patients and their doctors, patients can feel confident that their medical tests (such as diagnostic imaging tests with x-rays and gamma rays) are justified and the benefits exceed the very small risks.

The parameters you mentioned are used for the evaluation of radiation therapy treatments, particularly for cervical cancer patients under treatment with brachytherapy (the use of enclosed radioactive material implanted in the body). These parameters (DHI, COIN, CI, CN, OI) are acronyms describing ways we take complex radiation dose and summarize it with numbers we can use to determine if a certain treatment meets the prescription of a radiation oncologist:
DHI: dose homogeneity index
COIN: conformity index
CI: coverage index
CN: conformation number
OI: overdose index
While these metrics may be helpful, they don't tell the whole story and can't be relied on fully to determine the quality of a treatment. Nevertheless, there are published studies that give us some idea of typical numbers we can expect. The following resource is just one example of these published data.

Palled, S. R., Radhakrishna, N. K., Manikantan, S., Khanum, H., Venugopal, B. K., & Vishwanath, L. (2020). Dosimetric comparison of manual forward planning with uniform dwell times versus volume-based inverse planning in interstitial brachytherapy of cervical malignancies. Rep Pract Oncol Radiother. 2020 Nov-Dec; 25(6): 851–855. DOI: https://doi.org/10.1016/j.rpor.2020.08.005

CTDI (CT Dose Index) and DLP (Dose Length Product) are measures of the amount of radiation output from the x-ray tube during a CT scan. Larger patients require more radiation output than small patients to achieve the same level of image quality. Therefore, CTDI and DLP should increase with patient size when the CT scanning is performed properly. CTDI and DLP are not measures of patient dose (how much radiation the patient absorbed). Even though larger patients require more CTDI, the absorbed dose for patients of all sizes when averaged across their body thickness should be about the same when the CT scanning is done correctly.

The medical physics community uses "Diagnostic Reference Levels" (DRLs) to help imaging facilities determine if radiation doses are within a normal range as determined by national surveys. For the CT dose metrics CTDIvol and DLP, the DRLs are a function of patient size. The DRLs for 10 common adult and pediatric CT examinations in the United States have been published:

https://pubmed.ncbi.nlm.nih.gov/28221093/
https://pubmed.ncbi.nlm.nih.gov/34698569/

These values will eventually need to be updated to reflect changes in technology and practice.

It is worth noting that all CT scanners built since ~2005 have the ability to automatically adapt the radiation output to patient size using an approach called "automatic tube current modulation" which estimates the patients' size by analysis of the localizer images (aka scanned projection radiographs). Because of this, it is rarely necessary for CT technologists to choose the CTDI and DLP that will be used for a particular CT exam. It is done automatically to achieve consistent image quality.

Radionuclide therapy or radiopharmaceutical therapy (RPT) is defined as the use of a radionuclide (radioactive material) linked to a targeting molecule that is then injected into a patient to treat various diseases including cancer. The targeting molecule attaches to receptors on certain cancer cells and the radiation disrupts the cell's normal functioning and therefore destroys it.

RPT has been in use since the early 1900s and has become a common treatment, especially for cancers that are difficult to treat with traditional external beam radiation therapy. For example, Iodine-131 has been used safely and effectively for many decades to treat hyperthyroidism (Graves Disease) and thyroid cancer. It is difficult to quantify the number of hospitals and clinics that perform RPT services because that number is constantly increasing and there is no comprehensive database that tracks this. According to the World Nuclear Association, the demand for radioisotopes in general is increasing by up to 5% annually, and much of this use is in medical applications such as nuclear imaging and RPT. There is a great deal of emerging research in this area, and new therapies such as Lutetium-177, Actinium-225 and Radium-223 are changing the landscape of cancer treatment and providing patients with new and improved treatment options. Some RPT agents also emit gamma rays that can be used to produce images that demonstrate the distribution of the therapeutic agent in the body. This is known as "theranostics" because the radionuclide agent is used for both therapy and diagnostic imaging within the same treatment.

DLP stands for Dose x Length Product and is one of the measures used to quantify radiation used during CT scanning. It is measured in units of mGy*cm. You asked about the dose expressed in milliSieverts, so you are interested in knowing the effective dose resulting from this CT scan. Effective dose is the sum of the individual organ doses for all organs that were irradiated, adjusted to account for the sensitivity of each organ to radiation. Because our knowledge of these sensitivities is for the average member of a population, not for any individual, effective dose applies only to populations and not to individuals. Therefore, we can say that a DLP of 582 mGy*cm from a head CT scan corresponds to an effective dose of about 1.2 mSv to the average adult. It may be helpful to compare this value to the average annual "natural background" dose in the US of 3.2 mSv from naturally occurring sources of ionizing radiation such as radon and cosmic rays. Effective doses in this range have not been associated with any negative health consequences, while the diagnostic information from a CT scan can have tremendous health benefits.

Medical physicists are integral members of the healthcare team that provides care for patients - specifically the diagnosing and treating of injuries and diseases. However, because the medical physicist is not the person who is primarily responsible for diagnosing or treating patients, they are not considered “healthcare providers” under the law.

Medical physicists utilize specialized skills and advanced knowledge of medical technology to ensure that medical imaging and many types of therapy are performed with high quality and as safely as possible for the patient and the medical staff. They work alongside radiologists, radiation oncologists, radiologic technologists, medical dosimetrists, radiation therapists, nurses, and many other healthcare professionals with the goal of providing the best possible care for each patient.

Occupational radiation dose limits are in place to ensure that risks to radiation workers remain low and within similar occupational risks of other professions. Currently, there are annual dose limits (January 1st to December 31st) but there are no regulatory or required limits for lifetime occupational radiation exposure.

In 1991, the National Regulatory Commission (NRC) considered a regulatory amendment that would establish a lifetime cumulative occupational radiation dose but ultimately rejected it. This was because the annual limit serves as a de facto lifetime limit but also because an actual lifetime cumulative limit could hinder a person’s employability and their right to choose a job in a chosen profession.

Both the National Commission on Radiation Protection and Measurements (NCRP) and the International Commission on Radiological Protection (ICRP) have proposed lifetime cumulative radiation dose maxima. These scientific expert bodies do not have the authority to impose regulatory limits, only to make recommendations. The NCRP, which focuses within the United States, recommended a lifetime cumulative effective dose maximum value of 10mSv times the worker’s age in years. The ICRP, whose scope is worldwide, recommended a lifetime cumulative effective dose maximum value of 1000mSv, regardless of age.

As always, it is recommended to keep doses as low as reasonably achievable (“ALARA”). While there may be no required lifetime limit on occupational dose, it is always good practice to keep exposures to the minimum amount necessary to properly accomplish the work required.

Yes, it is possible to differentiate between an empty vs. fluid-filled cavity space by comparing the amplitude of the return signal from the bone cavity to a known reference signal.

The reason is that the vibration signal/wave is a mechanical wave. When the vibration wave encounters an empty cavity, it will produce a return signal with low amplitude because air is a poor conductor of mechanical waves and the vibration wave will not interact with the air strongly. In contrast, fluids are good conductors of mechanical waves. When the vibration wave encounters fluids, it will interact strongly with the fluids, producing a strong return signal with a high amplitude. Therefore, by comparing the amplitude of the return signal, it is possible to differentiate between an empty cavity and a cavity filled with fluid.

It is worth noting that the feasibility and accuracy of the above approach depend on many factors, such as the frequency and amplitude of the vibration wave, the shape and size of the cavity, the properties of surrounding materials etc. It may need further investigation.

There are limits on the amount of radiation that workers can receive annually, or that a member of the public can receive when they visit a facility that uses radiation as a part of its business (for example, a medical clinic). However, a patient is allowed to receive any amount of radiation that an authorized medical professional deems necessary to image or treat the patient's condition. Many radiological imaging procedures use "automatic exposure control" which delivers the precise amount of radiation needed to obtain a high-quality image with the appropriate radiation dose. This is similar to the way that photographic cameras work. If automatic exposure control is not available, then the technologist relies on their training and experience to select the correct radiation dose needed to produce a high-quality set of pictures. In modern CT scanners, a Dose Report is produced for every CT examination, which describes the radiation dose that was used to complete that exam.

Let us start with the pros of a magnetic resonance imaging (MRI) examination. MRI is a widely used imaging method that will provide your doctors with high quality images that they will use to see and better understand what is going on inside your body. Also, compared with other commonly used imaging methods such as x-rays, computerized tomography (CT), and nuclear medicine, MRI does not use high-energy radiation, and therefore it does not carry the very small risk of future cancer that is sometimes associated with x-rays and gamma rays (used in nuclear medicine).

As for the cons, in most clinical MRI scanners, patients are placed inside the relatively narrow bore (“donut hole”) of the scanner. This can provoke feelings of claustrophobia and anxiety in some patients. Moreover, you will be lying down on a not very comfortable table and be asked to stay as still as possible because any movement can compromise the quality of the acquired images. MRI exams are noisy, even when wearing hearing protection, and this can increase patient discomfort during the examination.

While MRI is considered to be safer than other imaging methods that use high energy radiation, it has unique safety considerations. The first thing to remember is that the MRI scanner uses a very powerful magnet that is ALWAYS ON and can attract any ferromagnetic (containing iron, nickel or cobalt) object that gets close enough to it. During the acquisition of the images, the MRI scanner uses radio waves (such as the ones used in radios, cellphones and other devices) that could increase your body temperature and any metallic object, ferromagnetic or not, on or inside your body could heat up and cause some burns. Some patients have experienced twitching, tremor, or pain that is associated with the use of electronic components inside the scanner. Contrast dyes commonly used in MRI examinations may cause an allergic reaction in some patients.

In general, MRI examinations are prescribed only if the benefits associated with it outweigh the potential risks (described above). Also, many steps taken before and during the examination are meant to prevent any of those risks from happening. Before your examination you will most likely get many questions from doctors and MRI technologists, all of which should be trained in MRI safety. Let your doctors or the MRI technologist know if you have any metal on or inside your body, any electronic devices like a pump or pacemaker, if you suffer from claustrophobia and/or anxiety, if you have any kidney problems, and/or if you are pregnant. You should be provided with special clothing and hearing protection to wear during your examination. At all times you should have the means to communicate with the MRI technologist to let him/her know about any issue or discomfort.

Most MRI examinations are completed without complications. In such cases, no side effects are known to be associated with the procedure.

For more information see https://www.medicalradiationinfo.org/radiationandmedicine/medical-imaging/mri-scanner-work/

Not all imaging procedures use “ionizing” radiation, which is the type of radiation that has the potential to cause harm when received in high amounts. The ultrasound and MRI procedures you’ve had are examples of non-ionizing procedures, so there is no concern about radiation effects.

Your CT scans, nuclear scans, and radiographs (x-rays) used fairly small amounts of ionizing radiation to produce images. The radiation dose level that you provided for your CT scan of the chest/abdomen/pelvis (300 mGycm) is extremely low compared to national averages for that (very common) imaging exam. The information obtained from medically-necessary imaging like CT scanning, nuclear imaging, and x-rays is often very beneficial to medical diagnosis and patient outcomes. The small amount of ionizing radiation used in medical imaging has not been scientifically proven to cause any harm, and if there is any long-term risk, then it is believed to be extremely small. If radiation exposures are separated by weeks or months, then the body's innate ability to repair DNA damage will go into action, further reducing the small theoretical risk.

There is no limit on the amount of medical imaging a patient can receive. Each imaging exam is ordered based on the patient's situation at that particular time, without regard to their radiation exposure history. This is good, because it means that a patient should not be denied a potentially life-preserving imaging exam just because they may have had prior imaging. With modern imaging equipment, the amount of radiation used to produce images is customized for the patient's body size and for the purpose of the examination.

To answer your question we refer you to this website that describes an "alternative pathway" for experienced international medical physicists who wish to become certified by the American Board of Radiology:
https://www.theabr.org/medical-physics/initial-certification/international-medical-graduates

This letter to the editor of JACMP may be of interest, although it applies specifically to diagnostic imaging physics, not therapy physics:
https://doi.org/10.1002/acm2.13166

There are two types of biological risk from radiation that medical physicists focus attention on. One is a potential skin reaction if a large accumulation of radiation impinges upon a region of skin. The other risk is cancer initiated by an accumulation of radiation delivered to radiosensitive organs.

The treatment that you received used a technique called interventional fluoroscopy to produce x-ray images that guided the placement of the stents and coils in your head. The numbers that you were given (2140 and 2100 mGy) is the cumulative air kerma to a reference point from each procedure. The reference point is defined in industry standards and is intended to approximate the location where the x-ray beam first hits the skin. When the skin dose exceeds about 2000 mGy, there is a potential for “transient erythema” which is a temporary reddening of the skin, like a sunburn(1). Since your reported values are just slightly above the 2000 mGy threshold, it is unlikely that you would have experienced erythema because in most cases the x-ray beam is moved around and the dose to any region of skin is less than the reported cumulative air kerma value to the reference point. Therefore the peak skin dose is very likely less than the threshold dose for early transient erythema. However, if you do notice skin changes in the area of your treatment, you should definitely bring it to the attention of your doctor.

The second risk is an increase in the risk of cancer above the baseline risk that every human faces. This risk is difficult to quantify because our knowledge about radiation and cancer is derived from studies of people who received very high radiation doses to their entire body (for example, atomic bomb survivors, nuclear accident survivors, etc.). Applying this knowledge to medical patients who receive localized, smaller doses requires numerous assumptions. It is generally agreed that a radiation dose from medical imaging increases an individual’s cancer risk very slightly above the natural baseline lifetime risk of 42 diagnosed cancers for every 100 people(2). Higher doses are associated with increased risk, but even at the highest doses used in medicine, the overall increase in cancer risk is still quite small compared to baseline and depends on the age and sex of the patient. Using published values for the typical kerma-area product for cerebral neurovascular embolization for aneurysm (250-300 Gycm2)(3), a published dose conversion coefficient from kerma-area product to effective dose (0.087 mSv/Gycm2)(4), and a published cancer risk coefficient(2), a very rough estimate based on many assumptions is that 1 patient in 400 who undergoes cerebral coil embolization may develop a future cancer as a result of the radiation exposure, while about 168 of these 400 will be diagnosed with cancer from other or natural causes. Because of the high natural incidence of cancer in the population, it is impossible to know if an individual’s cancer resulted from a radiation exposure or if it would have occurred anyway.

You had two procedures performed about six months apart. During the time interval between these procedures, your body’s natural DNA repair mechanisms had the opportunity to fix any DNA base pair mutations that may have been caused by the first radiation exposure. We have no way to quantify the degree of repair, but your body did have time to “heal” from the first exposure. For cancer risk, each radiation exposure is considered independent, and the second exposure carries no greater or less risk than the first.

References:
(1) https://www.iaea.org/resources/rpop/health-professionals/radiology/erythema
(2) NAS/NRC National Academy of Sciences/National Research Council. Health Risks from Exposure to Low Levels of Ionizing Radiation: Phase 2, BEIR VII, Board on Radiation Effects Research (National Academies Press, Washington), 2006.
(3) Miller et al, Radiation Doses in Interventional Radiology Procedures: The RAD-IR Study Part I: Overall Measures of Dose. JVIR 14(6) 711-727, 2003. https://doi.org/10.1097/01.RVI.0000079980.80153.4B
(4) NCRP Report No 160, Ionizing Radiation Exposure of the Population of the Unites States (National Council on Radiation Protection and Measurements, Bethesda, Maryland), 2009.

To answer your question we refer you to this website that describes an "alternative pathway" for experienced international medical physicists who wish to become certified by the American Board of Radiology:
https://www.theabr.org/medical-physics/initial-certification/international-medical-graduates

This letter to the editor of JACMP may be of interest, although it applies specifically to diagnostic imaging physics, not therapy physics:
https://doi.org/10.1002/acm2.13166

Every diagnostic imaging exam is prescribed only if the benefits associated with it outweigh the potential risks. The information obtained from a CT exam can improve the clinical outcome for the patient by providing physicians with insights into what is going on inside the patient. As with every medical procedure, there are risks associated with a diagnostic imaging exam. A typical way to describe the risk associated with radiation from a CT exam is the effective dose. For a standard head exam, the effective dose is generally lower than 2 mSv (milli Sievert). To provide a comparison, the average annual natural background radiation in the US is 3.1 mSv. This means that by simply living on earth, each person in the US is naturally exposed every year to a radiation dose higher than the dose associated with a standard head CT exam. Therefore, we can reasonably say that the risk associated with a single head CT procedure is negligible, and the benefit to the patient from a medically necessary brain CT scan outweighs the radiation risk.

Risk vs benefit is always taken into account for imaging using ionizing radiation. The risk of a screening mammogram is quite low: since a very large number of screening mammograms are done in the US every year, mammographic imaging procedures and radiation dose are closely regulated by the FDA. Screening mammograms are kept at a tightly regulated low dose. Physicists ensure that all of the x-rays produced by the machine fall on the imaging detector, which blocks direct x-rays from going to your uterus. Therefore, a lap apron is not necessary. Due to the positioning of the breast on the imaging plate (pulled away from the body), the amount of low-energy, low-dose scattered radiation that could possibly reach the vicinity of the uterus is incredibly small or negligible. In comparison, if you are at risk for breast cancer, the potential benefit – both for you and for your future baby -- of catching any problems early typically far outweighs the low risk!

The radiation oncology team performs a lot of checks before, during and after treatment to make sure treatments are delivered as precisely and safely as possible. Before treatment, the patient will be set up in a way that will hold them still for every treatment, and their images are used to create a custom-made treatment plan for their body and their disease. During treatment, more imaging is used to make sure they’re positioned precisely where they need to be. The medical physicist(s) on the team will be doing behind-the-scenes checks during the whole process to make sure the treatment plan is correct, the machine is operating as it should, and the radiation dose delivered matches the plan.

For most adults, 10 CT scans in a year does not carry a high risk of cancer development. In general, the risk of radiation causing cancer is a linear relationship with no lower threshold. That means the higher the radiation we received, the higher the risk for us to develop cancer will be. Depending on the type of CT scan, the radiation dose from that scan is approximately equal to the dose that everyone receives in one year from naturally-occurring sources on earth. After our body receives radiation, whether from medical imaging or from nature, the cells in our body will try to repair any cellular damage that may have occurred. In most cases, the cumulative radiation from 10 CT scans spread out over a year will be repaired and will not reach a dangerous level of cancer risk. In almost all cases, CT scans offer more benefit than risk. The information gathered from your CTs helps physicians diagnose and treat your medical ailment(s). This outweighs the small to negligible risk associated with the exposure to x-ray radiation during a CT exam.

In general, no. Radiation treatment delivery is entirely a passive process. The length of treatment depends on the particular case, but usually the patient can expect to remain in the same position for approximately 20 minutes, which may cause some discomfort. Special devices are made for each patient to assist in holding still and minimizing discomfort. The radiation itself cannot be felt - just like with an x-ray, you won’t feel a thing.

For PET/CT, the amounts of 18F-FDG excreted in breastmilk after a PET scan are below the level of concern for the breastfed infant and most international radiation safety organizations state that no interruption of breastfeeding is necessary. However, to follow the principle of keeping exposure "as low as reasonably achievable", some guidelines recommend withholding breastfeeding for 1 to 4 hours. Because of extensive uptake by lactating breasts and the consequent external radiation, nursing mothers should refrain from prolonged close contact with their infants for a period of time. Some authors suggest that the infant be bottle fed with expressed breastmilk by a third person for 1 feeding or 4 to 12 hours, depending on the dose, after a PET scan with 18F-FDG in a nursing mother. Ref: https://toxnet.nlm.nih.gov/newtoxnet/lactmed.htm

For CT scans with an iodine-containing contrast agent, eg. Isovue 370, Intravenous iodinated contrast media is minimally excreted into breastmilk and minimally absorbed orally so it is not likely to reach the bloodstream of the infant or cause any adverse effects in breastfed infants. Guidelines developed by several professional organizations state that breastfeeding need not be disrupted after a nursing mother receives a iodine-containing contrast medium. However, because there is no published experience with iopamidol during breastfeeding, other agents may be preferred, especially while nursing a newborn or preterm infant. Ref: https://toxnet.nlm.nih.gov/newtoxnet/lactmed.htm

No, patients do not need to cease breastfeeding after a chest x-ray exam. The x-ray interactions with the body are complete as soon as the machine finishes taking the image. There is no lingering radiation inside of the body.

Both, protons and photons are used in the treatment of cancer by directing beams towards tumors and killing the cancer cells in the tumor when energy is deposited. The main difference between the two methods lies in the way the energy is deposited in the patient's body.
Photons are highly energetic packets of light that travel at the speed of light. As photons enter the patient's body, they deposit most of their energy at the surface of the patient and the energy deposition weakens as the photons travel through the patient's body and finally exits the patient's body.
In contrast, protons are electrically charged particles that upon entering the body, immediately begin slowing down as they deposit their energy. Protons deposit most of their energy immediately before they have given up all of their energy and come to rest. This leads to a peak in energy right before the protons come to rest, called the Bragg peak. The initial energy of protons may be adjusted in such a way that the protons come to rest in the tumor and minimal energy is deposited beyond the tumor. Therefore the healthy tissues that lie beyond the tumor may be spared better when compared with photons. This can be of great benefit to the patient in most cases.
Accelerating protons to energies suitable for treatment of most cancers is technically difficult and expensive. Therefore there are far fewer proton beam therapy facilities available than photon therapy facilities. In some cases the energy deposition difference described above is not of the highest importance and photon therapy may be more beneficial to the patient. Radiation oncologists use scientific evidence to determine which radiation type is best for any given patient and tumor location.

MRI is a commonly used imaging method that allows our doctors to see and better understand what is going on inside your body. Compared with other imaging methods such as x-ray, CT, and nuclear medicine,, MRI does not use high-energy radiation to create an image, and therefore it does not carry the very small risk of future cancer that is sometimes associated with x-rays and gamma rays. However, MRI has some unique safety considerations because of the strong magnetic fields and radio waves that are used. The magnetic field can attract some metals; therefore, let your doctor or the MRI technologist know if you have any metal on or inside your body. Use only the facility's provided clothing during the exam. MRI exams can be noisy, so you may be offered hearing protection. Some MRI scanner designs (the "narrow bore" type) can provoke feelings of claustrophobia in some patients. Contrast dyes are sometimes used, which may cause an allergic reaction in some patients.

Indeed, some forms of radiation, specifically x-rays or photons, do penetrate through your entire body and will deposit some dose along their entire path of travel. However, the design of common linear accelerators is such that they can rotate around a single point where the radiation from different directions will add up together to collect significant dose. Tissues away from this common point will only see a small fraction of the radiation paths and collect minimal dose. This is similar to how a magnifying glass can focus the sun’s rays down to a single point and cause a quick burn on your hand while putting your hand out into the direct sun will simply feel a little bit warm.

A dose volume histogram, or DVH, is a quick way to graphically show how much radiation dose a structure (like the heart) receives as part of radiation treatment. It shows how much dose is received by a percentage of the structure. It can give several important pieces of information, such as the maximum dose, minimum dose, or how much dose is received by 50% of the structure. However, it is important to know that a DVH does not give information about the location of dose, such as where the maximum dose is located in a structure.

X-ray imaging is like creating a shadow using a light source and any object. When light shines on the object, the amount of light coming through the object will depend on the transparency of that object. With visible light, the shadow of a clear glass is very faint while the shadow of a rock of the same dimension is dark. This difference comes from the difference between the transparency of different objects. The same principle applies to x-ray imaging. The interesting property of x-rays is the ability to penetrate through human body. The penetration ability through an object or human body depends on the energy of x-rays; higher energy x-rays can penetrate through more tissue. For the same energy x-rays, penetration through the bone or through the muscle will be different (similar to light’s transmission through glass or rock). So, if we shine x-rays on the human body, a region with bone structure will allow less penetration and a region of muscle will allow more penetration. When these x-rays are collected using an x-ray film or a digital detector, a difference will be seen in terms of the number of x-rays at two locations and this relative difference allows the structures in your body to be visualized as an x-ray image

Hopefully it won’t disappoint you to find out that radiation only makes people glow in comic books and movies. In reality, the radiation you will be getting is totally colorless and you shouldn’t feel anything at all from your treatment besides the applicator the physician has already placed. If you do feel anything during the treatment, please speak up and say so. We have cameras and an intercom so we can see and hear you at all times during the treatment even though we’ll be out of the room.

Radiation can damage cells in the body. Healthy, normal cells are capable of repairing themselves following exposure to radiation. However, cancerous cells have lost this ability. When radiation is delivered to a part of the body containing a tumor, the cancerous cells are killed but the non-cancerous cells survive.

Generally speaking, such websites should not be used to estimate your personal risk from an imaging exam. They use risk averages derived from the general population, including men and women, adults and children of all ages. Therefore, these values cannot be used to calculate an individual's risk which is unique for each person. If you are concerned about the safety of your exam, please contact the radiology or medical physics department of the facility where you had your exam. They will be able to answer any questions that you may have.

Dental exams, like other x-ray exams, do not require any safety action after the end of the exam. There is no residual radiation after an x-ray. The effect of the x-ray radiation stops as soon as the exam ends and there are no lingering safety issues for the patient’s family members, friends, coworkers, etc.

Normal cells are better at healing themselves than cancer cells. Multiple treatment “fractions” allow normal cells a chance to repair, while the intended radiation damage accumulates in the cancer cells. Also, small imperfections in the treatment setup can exist, such as exact patient positioning, body motion, internal organ motion, and the positions of moving parts in the treatment machine. By spreading the radiation therapy over a number of fractions, those small imperfections average out and their effect is decreased.

Some types of cancer benefit from much higher dose per treatment. In that case, the physician may recommend “stereotactic” therapy in 5 or fewer fractions. In order to minimize technical imperfections, stereotactic treatments may use more devices to prevent patient motion and more imaging for accurate positioning

In some ways, a medical physicist is like a pharmacist. The doctor writes a prescription of radiation for your cancer, and it is the job of the medical physicist to fill the prescription. First, we have to test the quality of the machines that are used to treat cancer. Every day we perform quality testing on the machines to ensure the safety of our patients. Second, with the help of other staff on the radiation therapy team, we develop a unique treatment plans to get the radiation where the doctor wants it and away from the healthy organs. Our quality checks do not stop when the treatment begins - during treatment we make continuous checks to ensure safety throughout the treatment, and verify the delivery of the radiation to the cancer.

Proton therapy is a tool for doctors to treat cancer, and like X-rays, it is a form of radiation therapy. However because of protons' unique stopping characteristics (known as the Bragg peak), proton therapy can stop at an exact depth (cancer location) inside the body, whereas x-rays travel farther. Therefore, proton therapy has the potential of causing less harm to healthy organs that are near the location in the body where the treatment is intended. Depending on the type and location of cancer, proton therapy may be beneficial. For more information please visit The National Association for Proton Therapy’s webpage https://www.proton-therapy.org/.

While you are inside the scanner, the various clicking, buzzing and vibration noises that you hear are produced by magnetic field gradient coils that localize the information inside your body and produces images. It is important to keep still while you hear these noises throughout the exam, or else the images produced will be distorted or blurred. The technologist should advise you when it is ok to move or adjust your position, such as during the quiet period in between imaging sequences. While imaging is taking place, it is generally ok to make small movements using parts of your body not in the imaging field, such as toes or fingers during a head or body exam.

Inside the machine, known as a linear accelerator (or “linac” for short), there is a measurement device between the source of radiation and the patient. This device is called a “Monitor Chamber,” and it is used to monitor the amount of radiation being emitted by the linac. The Monitor Chamber collects electric charges created by radiation as it passes through the device. That electric charge is counted as “Monitor Units.” When a treatment plan is developed, we set a certain number of Monitor Units that we want to be counted with each treatment field. When we “turn on” the beam, the linac begins counting the Monitor Units, and when the number has been reached, the linac knows to turn off the radiation. For safety purposes, we have other safeguards to ensure that the number of monitor units is not exceeded and that the linac turns off the beam at the appropriate time

Prostate seeds refers to a form of low-dose-rate (LDR) brachytherapy, which is a type of radiation therapy in which multiple, small, radiation-emitting devices are implanted directly into the prostate permanently. The procedure is conducted with the patient unconscious, and it is a one-time procedure. The dose to the prostate can be high and will give very little dose to normal organs at a distance from the prostate. The procedure is one of the many options of treatment available for prostate cancer.

No. You are not radioactive and pose no risk to those around you. The radiation you were treated with is a special kind of energy delivered to your disease in the form of small particles (electrons) or waves (gamma or x-ray radiation). When delivered, some of this energy passes through, reflects off, or is absorbed by your body. The portion of that energy that does not pass through or bounce off the body is absorbed and transfers its energy to tissues (which is how it kills cancer cells). None of this energy remains as a radiation source inside of the body.

This concept is distinct from the term “radioactive” which describes something that emits radiation as part of its physical nature. If you were treated via high dose-rate brachytherapy (HDR), you were treated with a form of the metal iridium which is radioactive. This source emitted radiation, which interacted with your body as described above, and then was retracted from your body. Since the radioactive source is no longer in your body, you are not radioactive and are no longer receiving the radiation. If you were treated with a linear accelerator, nothing radioactive ever entered your body.

Medical physicists are clinician scientists who usually specialize in either diagnostic or therapeutic radiological physics. In the diagnostic setting, medical physicists contribute to the effectiveness of radiological imaging procedures by ensuring high image quality obtained safely, and the development of improved imaging techniques and technologies. In the therapeutic setting, medical physicists work in radiation oncology and contribute to the development, optimization, and quality control of therapeutic delivery techniques. They collaborate with radiation oncologists to design treatment plans and monitor equipment and procedures to ensure that patients receive the prescribed dose of radiation to the intended location.

Medical physicists are technical experts in academic, industry, or hospital settings who apply physics knowledge to the diagnosis and treatment of disease. We usually spend our time doing some combination of patient care, research & development, and teaching. As a radiation oncology physicist, I consult with our physicians to make sure our team delivers safe and effective radiation treatments to cancer patients.