GUEST CONTRIBUTOR:
Peng Wang, Ph.D. DABR
Chief Proton Physicist
Inova Fairfax Hospital
Adjunct Associate Professor,
Department of Health Professions
University of Wisconsin-La Crosse
The 12th International Day of Medical Physics is observed on November 7, 2024. The discipline of medical physics may be relatively unfamiliar to the general public. A medical physicist is a health professional who applies physics in medicine, particularly in the diagnosis and treatment of cancer. This work involves the development and use of medical devices, technologies, and radiation treatments. Medical physicists’ responsibilities encompass patient care, equipment safety, research and development, education, treatment, and communication.
Therapeutic medical physics is a subcategory of medical physics. A therapeutic medical physicist is responsible for using radiation in the treatment of diseases, primarily solid tumors. The location of the cancer and the dose prescription are provided by the radiation oncologist. The primary responsibilities of physicists include calibrating, modulating, and measuring radiation, as well as calculating the radiation doses delivered to patients. The treatment process, which begins with generating high-energy particles and concludes with interactions of radicals with deoxyribonucleic acid (DNA), involves a series of energy transformations, along with the generation, transportation, and interaction of particles and electromagnetic radiation. It is of paramount importance that the healthcare team has accurate data regarding the doses delivered to different regions of the patient’s body.
One of the most frequently utilized treatment modalities is X-ray therapy, which is generated by a linear accelerator. The electrons are accelerated to a high velocity and directed toward a tungsten target, resulting in the generation of X-rays that can traverse the patient’s body and reach deeply seated malignant lesions. The amount of energy deposition declines as the radiation penetrates deeper into the patient’s body, irrespective of the locations of the targets and healthy organs. To deposit a greater quantity of energy in the tumor region, multiple field angles are employed, targeting the tumor. The X-ray deposits energy along the entire beam path within the patient’s body, resulting in a high dose concentrated in the tumor and a low dose across a substantial portion of the healthy tissues.
In proton therapy, protons are the particles delivered to the patient’s body to deposit energy. The protons are accelerated to approximately two-thirds of the speed of light in the accelerator. Once the proton beam enters the patient’s body, it decelerates due to interactions with surrounding electrons, ultimately reaching zero velocity. Energy is deposited along the beam path, with the greatest energy deposition occurring at the beam’s end—unlike in the X-ray beam. Consequently, the energy of the incoming proton beam can be calibrated to stop precisely at the tumor site, concentrating the majority of the energy within the tumor. The principal advantage of proton therapy is the ability to spare the surrounding healthy tissue beyond the reach of the protons.
Building and maintaining proton systems is a costly endeavor. The first hospital-based proton treatment center began operations at Loma Linda University Medical Center in 1990. It was the only facility of its kind in the United States until 2003. Currently, there are 46 proton centers in operation within the U.S., compared to over 2,000 traditional X-ray centers. Worldwide, less than one percent of radiotherapy patients are treated with protons. Given the limited number of proton treatment centers and the recent emergence of the technology, there is a relatively small cohort of medical physicists specializing in this field. Consequently, proton physicists frequently encounter a dearth of guidelines, consensus, data, and evidence related to their practices. In addition to their regular clinical responsibilities, they often must conduct investigations, experiments, or planning studies to address numerous inquiries. Furthermore, it is essential that proton physicists are capable of independently producing clinical-level patient plans in case of a personnel shortage. Due to the heightened sensitivity of proton plans to field selection and patient anatomy, proton physicists perform a greater number of procedures and quality assurance checks to guarantee the accurate delivery of the prescribed daily dose to the patient. The workload for a typical proton plan is approximately three to four times greater than that of an X-ray external beam plan. Moreover, proton centers frequently experience system downtimes due to the equipment’s complexity. Proton physicists must also be skilled at optimizing the clinic’s workflow to ensure safe and efficient patient care.
Despite the numerous challenges in proton therapy, there have been considerable technical advancements. In 1996, intensity-modulated proton therapy (IMPT) was developed using a proton scanning beam, resulting in a more conformal dose distribution on the proximal side of the target. The use of CT-on-rails and cone beam CT (CBCT) in the treatment room allows clinicians to monitor the patient’s anatomical changes on the day of treatment. Implementing Monte Carlo calculations in clinical settings has enabled accurate estimation of radiation doses in heterogeneous tissues, such as lungs and sinuses. Techniques like tracking, gating, and repainting have facilitated the use of proton pencil beam scanning to treat tumors with motion. These advancements have expanded proton therapy’s applicability to a wider range of diseases while enhancing treatment robustness.
Looking ahead, there is significant potential for further development in both the biological and physical aspects of proton therapy. The implementation of linear energy transfer (LET) and relative biological effectiveness (RBE) optimizations in treatment planning enables clinicians to prescribe higher RBE doses to the tumor, potentially resulting in higher tumor control rates while reducing radiation-induced toxicity in healthy organs at risk (OARs). When the radiation dose rate exceeds 40 Gy/s, the FLASH effect emerges, demonstrating comparable efficacy in cancer cell eradication while sparing normal tissue. In comparison to other radiation forms, FLASH therapy with protons is relatively straightforward from an engineering perspective and is already being explored by researchers in pre-clinical settings and by clinicians in clinical trials. If the anticipated FLASH effect can be demonstrated in human subjects, this modality has the potential to significantly enhance the efficacy of radiation therapy. Another rapidly developing field is proton arc therapy, in which proton beams are delivered from multiple gantry angles with an interval of one degree or a few degrees between adjacent gantry angles. Compared to delivering radiation with two to six gantry angles, proton arc therapy offers greater planning flexibility, potentially enhancing the LET and RBE of the proton beam. Furthermore, proton arc therapy may reduce treatment delivery time, allowing for the treatment of more patients with the same operating machines, staff, and hours. Online adaptive proton therapy is another promising modality that will improve the robustness of proton treatment. New adaptive proton plans can be generated based on the patient’s updated anatomy, ensuring greater consistency between the daily dose received and the radiation oncologist’s prescription.
Beyond the biological and physical aspects, it is crucial to reduce the economic burden of proton therapy for the general patient population to make this treatment more accessible. Currently, only 1% of patients undergoing radiation therapy receive proton therapy. The insurance coverage and appeals process for proton therapy is often challenging for both patients and clinics. This issue could be mitigated if the cost of proton therapy were to decrease significantly. Medical physicists play an essential role in reducing the cost of proton therapy through various approaches. These include reducing the facility’s footprint by developing a compact accelerator, eliminating the rotating gantry by using a patient chair, increasing patient throughput with proton arc therapy, mini-ridge filters, optimizing patient schedules, and employing hypofractionation treatments. It is imperative that all stakeholders in proton therapy, including clinicians and non-clinical organizations like the National Association for Proton Therapy (NAPT), collaborate to expand access to this treatment for a broader patient population.
On the occasion of the International Day of Medical Physics, we are thrilled to acknowledge the remarkable achievements and exciting challenges in this dynamic field.
EXPERTISE UNVEILED: A GUEST CONTRIBUTOR SERIES
We bring together the brightest minds and most experienced professionals in the field of proton therapy. In this distinguished guest contributor series, experts share invaluable insights, breakthrough research, and real-world experiences in advancing proton therapy. Join us as we delve into the world of proton therapy, exploring its intricacies and potential through the eyes of those who know it best. Interested in becoming a contributor? Email info@proton-therapy.org for more information.
