How are medical physicists involved in radiation treatment?

Radiation treatment involves many steps and the use of advanced technologies. Here are just a few of the actions required to treat a patient:

  • Calibrating radiation machines to deliver dose accurately
  • Creating computer models of machines’ radiation output
  • Imaging the patient in their treatment position
  • Creating an individualized treatment plan for each patient
  • Positioning the patient accurately and precisely on the treatment couch
  • Delivering the planned radiation treatment

Medical physicists are responsible for overseeing the technical side of these steps. They carry out machine calibrations, verify the correctness of machine performance, supervise treatment planning, and generally make sure all systems work properly to deliver safe and effective treatments. Most patients do not know there is a medical physicist working behind the scenes to ensure their safety during treatment. Medical physicists do much of their work at the end of the day, after patients and staff have gone home and the radiation therapy equipment is available for testing and research.

The Association of American Physicists in Medicine (AAPM) is a professional organization of medical physicists and other affiliated professionals who volunteer their time and expertise to develop and publish recommendations on topics such as safety and quality assurance (QA) procedures. These publications are accepted as the national standards in the USA and often internationally.

Image courtesy of Dr. Lawrie Skinner.
Medical physicists are responsible for the technical aspects of radiation therapy, which includes everything involved in the production, measurement, and calculation of radiation.
Radiation beams are calibrated and radiation dose data are collected using a special water tank as shown here.

What is machine quality assurance?

Radiation is not detectable by the human senses and therefore special equipment is required to measure the radiation delivered by treatment machines.

Physicists are responsible for making sure that measurement equipment and treatment machines work properly. They ensure measurement instruments are properly calibrated, and they use those instruments to check machine performance on a daily, monthly, and annual basis. If necessary, physicists adjust the machines to maintain their radiation output within tight tolerances. If a machine is repaired or modified by a service engineer, the physicist is responsible for verifying the machine operates correctly afterwards. This process of routinely checking and adjusting machine performance is known as machine quality assurance, or “machine QA.”

Medical physicists are also responsible for using the data from quality assurance measurements to create an accurate software model of the treatment machines. This model is used in turn to create treatment plans for each patient.

What is patient-specific quality assurance?

Each patient’s personalized radiation plan is reviewed by a medical physicist prior to treatment. For certain complex plans the radiation delivery is also measured before patient treatment. A physicist may use ion chambers, film, or detector arrays to measure the radiation emitted by a machine executing a particular treatment plan, and the physicist verifies the measured dose agrees with the planned dose. This procedure is known as patient-specific quality assurance, or “patient QA.”

Medical physicists use specialized equipment to measure the radiation dose delivered by complex treatment plans before the plans are approved for treating patients.
This ensures that the distribution of radiation delivered by each plan accurately matches what is intended. 
Image courtesy of Dr. Taoran Li.

How are new clinical procedures developed?

New technologies and clinical procedures are brought into clinical use through the work of medical physicists. As new software, radiation-delivery systems, and support equipment become commercially available, the medical physicist performs rigorous testing, analyzes data, and assists in developing clinical workflows so that those technologies can be safely used.

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