Radiation has been used in medicine for over a hundred years. Radiation is now used safely by carefully controlling its application, and medical physicists play a major role in that control. Medical physicists continually perform checks on equipment and procedures to ensure radiation is delivered correctly. In this way they help keep risks as low as practical while maximizing the benefits of radiation for each patient.

For nearly all patients, medical procedures involving radiation carry low risk. Risks from radiation are related to the amount of radiation dose received and whether it is received over the whole body or just a part. In most cases, patients receive radiation to only a part of their body. Radiation levels are low in diagnostic applications and higher for radiation therapy because the goal of therapy is to destroy diseased cells.

The benefits from medical radiation are great and generally outweigh the risks. When assessing radiation risk, there are two main considerations: the possibility the body’s normal tissues will be noticeably damaged in the short term, and the probability that radiation will cause cancer in the long term.

What are the short-term risks?

There are billions of cells in the human body. If some cells are lost due to radiation damage they are usually replaced by new cells. This is the case for patients receiving radiation from diagnostic procedures, which deliver such low doses of radiation that patients typically don’t experience effects. The average radiation dose a person in the US receives from natural sources in the environment is about 3 milliSieverts (mSv) per year, and simple diagnostic x-ray procedures deliver doses less than 1 mSv for each procedure. (The milliSievert [mSv] is a commonly used unit of radiation dose.) More complex procedures such as x-ray computed tomography (CT) scans may deliver slightly more dose, roughly 5-10 mSv, to the imaged area.

As the amount of radiation delivered to a patient increases, more cells are killed. At relatively high radiation dose levels, the damage and loss of normal cells has an observable effect. For example, during a daily radiation therapy treatment, radiation passes through the patient’s skin and penetrates their body to kill the targeted cancer. Total radiation dose to the skin can be around 3,000 mSv over the entire course of radiation therapy, and the patient may experience a temporary skin reaction like a sunburn.

What are the long-term risks?

Exposure to radiation—whether from the natural environment, a person’s job, or medical procedures—increases the risk of developing cancer in the long term. The question is by how much.

Most healthy cells are able to repair themselves after receiving small doses of radiation. Even if a cell’s repair isn’t perfect, it is unlikely to have any long-term impact. However, there is always a small chance, even at low levels of radiation exposure, that a surviving, damaged cell may eventually reproduce in an uncontrolled way. This can result in cancer.

Based on studies of populations exposed to very high levels of radiation, radiation-induced cancers can appear in a person years after exposure.

For most cancer patients, the increase in the number of radiation-induced late cancers is very small, and the immediate benefits from radiation therapy outweigh the long-term risks. However, data does show that radiation therapy increases the risk for developing a second cancer during the patient’s lifetime. After about seven years, an increased number of bone marrow and blood cancers may appear. After about twenty years, an increased number of solid tumors may appear.

How much risk is there in diagnostic procedures?

To put the risk in perspective, patients undergoing a diagnostic procedure receive the equivalent of about 1 mSv to their whole body. (Even though only part of the body may receive radiation, the dose received is commonly converted to a whole-body equivalent dose for the purpose of performing cancer risk calculations.) The natural cancer death rate in the U.S. is about 20% of the population. This means that out of every 100,000 people, 20,000 will eventually die of cancer. If 100,000 people each received 1 mSv from a diagnostic medical procedure, some models predict an 5 additional cancer deaths some time long after that exposure. That means the risk of developing cancer would increase by 0.025% (20,005 deaths instead of 20,000). The lifetime fatal cancer risk increase from diagnostic radiation procedures is very low for an individual patient, while the medical benefits are immense.

How much risk is there from radiation therapy?

Although radiation treatments deliver much higher doses than diagnostic imaging procedures, those high doses are focused at the sites of disease and the dose delivery is spread out over time to give healthy tissues time to repair themselves. The dose needed to destroy cancer cells is typically about 50,000 mSv, roughly ten thousand times higher than in diagnostic procedures. However, the risk for developing a fatal cancer is not proportionally higher compared to diagnostic imaging.

The purpose of therapeutic radiation is to kill cancer cells which present a near-term threat. In the course of delivering that treatment, the radiation will pass through and deliver some dose to normal tissues. This radiation dose to normal tissues can introduce a risk for developing another cancer later in the patient’s lifetime. This risk is usually low because only a small fraction of the normal tissues receives any radiation, and the dose is generally much less than that received by the cancer target. Depending on the case, normal tissue averaged over the whole body of a patient receives about 1/150th of 50,000 mSv, or about 330 mSv. This will produce a fatal cancer in about 15 in 1,000 people (1.5%) long after treatment. In other words, 98.5% of radiation-therapy patients will not develop a radiation-induced cancer. Though there is nonzero risk associated with radiation therapy, that risk is very low and medical physicists work hard to keep it that way.

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