Carl Rossi, MD // Scripps Proton Therapy Center
BASICS OF PROTON THERAPY
Proton therapy for prostate cancer was first performed in 1977, long before the development of intensity modulated radiation (IMRT). Proton therapy first became clinically available in 1990. Proton therapy is a type of external beam radiation. However, unlike the commonly available type of x-ray therapy such as IMRT, SBRT (ex. Cyberknife), proton therapy utilizes subatomic particles (protons). Protons and x-rays have equal anticancer effects. The advantage of protons lies in their ability to reduce radiation exposure to the normal body tissues surrounding the prostate.
Protons interact with human tissue differently than x-rays. X-rays pass straight through the body with a substantial amount of radiation energy exiting out of the body. Everything within the path of the beam receives radiation. In contrast, protons deliver a low “entrance dose” (radiation dose to tissues in front of the target), place their highest dose within the target, and sharply limit exposure beyond the target. These unique physical properties are called the “Bragg Peak.” The Bragg Peak phenomenon is unique to proton therapy.
ENHANCED PROTON TECHNOLOGY
To date, the vast majority of prostate cancer patients who have been treated with proton therapy have been treated with passive-scatter proton therapy (PSPT). With PSPT, the proton beam is shaped by solid lead block which is manufactured and customized for each individual patient. The type of beam it creates completely covers the target volume with a uniform dose of radiation. However, with PSPT it is impossible to vary the radiation dose within the target area. So PSPT, for example, is unable to boost the dose to high-value target area or dose attenuate (minimize radiation) in adjacent normal structures. In addition, PSPT technology, due to limitations inherent to its lead-block methodology, is unable to treat larger target areas, such as the pelvic lymph nodes in the pelvis.
These limitations with PSPT have motivated the development of intensity-modulated proton therapy (IMPT). IMPT steers the proton beam to the target using electromagnetic forces. The proton dose is laid down throughout the target volume in a fashion analogous to a 3D printer manufacturing a complex solid object, with the protons typically being placed in layers that are approximately 1 millimeter thick. This ability to “paint” the proton dose makes
it possible to create differential radiation doses throughout the target volume, so that areas containing the greatest amount of tumor can receive substantially higher doses. IMPT is not constrained by field size limits as was PSPT, making it feasible to treat targets within the pelvis. Commencing in February 2014, the Scripps Proton Therapy Center in San Diego was the first facility in the United States to implement intensity-modulated proton treatment.
OPTIMAL TREATMENT PLANNING AND TARGETING
Multi-modality imaging enables the creation of a three-dimensional map of the target area. At Scripps, all prostate cancer patients undergo a thin-slice pelvic CT and a multi-parametric prostate MRI. The image sets are combined to create a composite, three-dimensional reconstruction of the prostate and pelvis. The addition of multi-parametric MRI has been a significant advance which has enabled us to target intra-prostate disease with a higher radiation dose. The planning session is performed with a rectal balloon, which stabilizes the prostate, and minimizes gland motion. All patients are treated on a six-degree-of-freedom robotic couch that can move in sub-millimeter increments. Patients are typically treated lying on their back.
Prior to entering the treatment room, each patient undergoes a daily bladder ultrasound to verify that a minimum amount of fluid is present within the bladder. The patient’s position is verified by performing a daily CT scan in the treatment room. A planning system then analyzes his position and commands the robotic treatment table to move in such a fashion as to match the original treatment plan developed at the patient’s first visit.
Typically, the “beam on” time is approx- imately 20–30 seconds per radiation field. The average time spent by the patient in the treatment room, including all of the above set-up and positioning, is less than 20 minutes. There are no restrictions placed on physical activity during treatment and most patients tolerate treatment with little if any difficulty.
A SHORTER COURSE OF TREATMENT
Historically, a course of proton therapy was administered over a 9-week period. At Scripps, our most common protocol requires 5 1/2 weeks to complete. During that time, the entire prostate receives a radiation dose equivalent to 80 Gray. The cancer itself is boosted an additional 10 Gray to a radiation dose equivalent of approximately 90 Gray.
CLINICAL RESULTS WITH IMPT
Since the availability of IMPT in the United States is very new, the number of IMPT-specific prostate publications remains limited. The largest study com- paring patients treated with IMPT and patients treated with PSPT concluded that the cure rates were identical (as might have been expected), however, there was a decrease in gastrointestinal toxicity (primarily rectal toxicity) favoring those patients treated with IMPT.
IMPT technology is rapidly evolving with the primary advances being in more sophisticated planning and delivery systems. For example, we expect that within five years that patients will be able to be planned in “real time.” This means that the treatment plan can be adjusted as necessary on a daily basis to reflect any changes in tumor size or patient anatomy. In addition, there are a number of trials taking place which examine the feasibility of shortening the duration of treatment further. Hopefully, these efforts will prove successful which would permit a greater number of prostate patients to take ad- vantage of this technology.
Carl Rossi, MD, is a radiation oncologist specializing in proton beam therapy, specifically for prostate cancer and lymphomas. He is also the current medical director for the Scripps Proton Therapy Center, which will provide treatment to target tumors with high control and precision. Dr. Rossi has a research focus on the quality of life and cure rate in prostate cancer and lymphoma treated with proton beam radiation.