By Thomas J. Petrone, PhD, DABR, DABMP
Medical physicists, whether practicing in radiation oncology or diagnostic imaging, must be experts in the application of ionizing radiation in medicine. In oncology, the therapeutic physicist works with a team of radiation oncologists, dosimetrists, and therapists to utilize complex systems and techniques for the treatment of cancer. In imaging, the physicist is involved in the use of radiation from a myriad of sophisticated equipment to create diagnostic images of the anatomy and, in the case of nuclear medicine, study the uptake of radiopharmaceuticals by the body.
Calibrating a Response
In both oncology and imaging, the physicist must address calibration and patient/staff safety challenges. Calibration is a process of measuring radiation accurately, utilizing expensive detectors and electronics so that the output of a radiation device is known well before it is allowed to be used on a patient. These detectors come in many forms, such as gas-filled chambers and solid-state devices. The response of the measurement system must be well understood and calibrated at a reference laboratory, traceable to a primary standard such as those set forth by the National Institute of Standards and Technology (NIST).
In oncology, the physicist calibrates linear accelerators, high dose rate after loaders, gamma knives, and individual radioactive sources, which will be used for treatment. The results of these calculations are used to adjust the output of treatment machines and are input into treatment planning systems to determine the exact dose distribution in the patient, well ahead of actual therapy. Large tanks of water are often utilized to obtain a complete profile of the primary and scattered radiation distribution. These profiles are also utilized in the planning computer to simulate the interaction in the patient.
Calibrations are also required of physicists in imaging applications such as radiography, CT, and mammography. Again, calibrated detectors are used to measure the output of those systems accurately. However, the results are not utilized to help treat the patient but rather to help optimize radiation, enabling the best possible diagnostic image while concurrently assuring that only the amount needed is applied to the patient.
In nuclear medicine, medical physicists might use the radiation measured to quantify the amount of pharmaceutical taken up by an organ or abnormality. Here, it is important that the imaging machine is calibrated so the measured uptake is accurate and reliable. Without this accuracy, which rests on precise calibration, the diagnosis could be adversely affected.
Also, nuclear medicine radioactive materials must be measured in a calibrated device known as a dose calibrator. This ensures that the proper amount of material is injected to render an optimized image. Radioactive materials are being used more and more to treat cancers through injection and ingestion. These materials must also be calibrated to ensure the prescribed therapeutic effect is achieved.
Patient and Staff Safety
Medical physicists are also heavily involved in the safety of patients and staff. Soon after the advent of ionizing radiation’s first uses in medicine, it became known that at higher levels, there could be some detrimental effects. Skin burns were observed early on, and the carcinogenic effect was further confirmed through the effects of the bombs used against Japan in World War 2.
Given that understanding, various scientific and regulatory authorities established limits on the exposure of workers. Medical staff in radiation oncology and imaging are monitored to determine whether they are staying within set limits. The medical health physicist monitors the exposure of staff throughout the medical center and intervenes as needed.
Radiation safety is an important consideration when treating patients for cancer. The dose given to a diseased area can be substantial to achieve the desired therapeutic result. The radiation delivered to the rest of the patient must be minimized to avoid unwanted side effects. The physicist is involved in this minimization by working on the treatment plan with the rest of the team to optimize delivery. Some tactics include selecting the right type of radiation (photons vs electrons or other particulate beams); choice of energies from 17 kiloelectronvolts to 18 megaelectronvolts; and utilization of angles, arcs, and collimation to implement what is termed intensity modulation. Shielding is sometimes used in the treatment room to protect healthy tissues, such as the eyes.
Diagnostic imaging physicists also protect patients from unneeded exposure, using tools such as collimation, output modulation, and shielding. CT has contributed significantly to a rise in overall population radiation dose. On the other hand, diagnostic medicine would not be close to where it is today without the widespread use of CT. In mammography, physicists must measure the dose delivered to the breast on a regular basis to ensure not only diagnostic image quality but also that the radiation dose delivered to sensitive breast tissue is kept low.In the past several years, many dose database platforms to record and monitor patient dose expenditure across an entire facility or enterprise, in aggregate and individually, have been implemented. Medical physicists typically take the leading role in utilizing these systems. This allows them to be vigilant and react with corrective measures whenever it appears that the applied dose is trending away from the ideal. Medical physicists spend much of their energy and careers applying their expertise to the numerous aspects of calibration and safety. They safeguard the most effective utilization of the precious tool of ionizing radiation for the health of the population.
— Thomas J. Petrone, PhD, DABR, DABMP, is the CEO and chief medical physicist of Petrone Associates, a full-service medical physics practice focused on regulatory compliance, accreditation quality assurance, treatment planning, commissioning shielding education, and comprehensive consulting.