Many men with prostate cancer ask if proton therapy is right for them. The answer is: it depends. Proton radiation can achieve a more precise treatment – meaning more high-dose radiation to the prostate and minimize low dose radiation to surrounding tissues.
A proton radiation plan, when compared to a photon radiation plan (also referred to as IMRT) typically shows a significant decrease in the low dose radiation. Dr.’s Royce and Efstathiou published a review in 2018 with the picture below showing the difference in the radiation dose distribution for a proton plan (b) with an IMRT plan (a).
Note: to orient yourself, pretend you are lying on a table and we’re looking at your pelvis from your feet. Bladder is in yellow, hip bones in blue and pink and prostate in red. Rectum is behind the prostate (blue/yellow lines)
T.J. Royce and J.A. Efstathiou / Urologic Oncology: Seminars and Original Investigations 37 (2019) 628−636
As you can see the prostate (red) is receiving 95-100% of the radiation dose in both plans. However, the bladder and the rectum, are receiving more of the lower dose radiation (10-50% of dose) in the traditional IMRT plan.
Therefore, if the prostate is receiving sufficient radiation dose with both plans, the question becomes, does the amount of low dose radiation received by the bladder and rectum matter?
Intrinsically, we assume less radiation is better than more radiation. The effects of low dose radiation are known to impact children significantly more than adults. Most men with prostate cancer are over 60 years old so the risk of long-term damage from lower radiation doses is quite low.
If low dose radiation is not expected to result side effects, then what is the benefit of proton therapy? Especially considering that many men have to travel and get treated away from home. The medical studies are mixed. Many institutions such as Loma Linda, Massachusetts General Hospital, University of Florida, have proton centers and have gone back and reviewed the outcomes of proton therapy for prostate patients. This is called a “retrospective review”. While some of these studies do show a benefit, meaning that prostate patient’s PSA drops low and stays low, the benefit has not been significant enough to show that protons are superior. In addition, there appear to be some tradeoffs including worse side effects in the short term. Several studies showed that rectal bleeding and rectal damage was worse with proton therapy than with IMRT. Of course, many of these used older proton therapy techniques. Newer techniques such as pencil beam scanning may minimize this risk.
What radiation oncologists need is a randomized controlled trial comparing men with prostate cancer treated with proton therapy with men treated with IMRT. This trial exists. The trial is called the PARTiQoL trial and is currently enrolling.
PARTiQoL is a large trial which randomizes men to receive either proton beam therapy (PBT) or IMRT (conventional photon therapy). The trial will assess the “quality of life” of men in each treatment arm with the goal of providing information on the side effects seen with each treatment as well as the overall control of the prostate cancer.
Until we have data from the PARTiQoL trial, here is my general take on recommending protons for prostate cancer. As always, each individual’s case is unique.
1.- The emerging data on proton therapy for prostate cancer is compelling. While that data is not “randomized”, which is the gold standard for medical science, the retrospective studies are generally showing slightly higher control rates (meaning PSA drops low and stays low) with protons.
2.- Radiation Oncologists have known for many years that curing prostate cancer requires a high dose of radiation. Consequently, studies showing better control of prostate cancer with protons makes sense because protons have a higher RBE (relative biologic effectiveness). I’ll spare you the physics and just say that a higher RBE means that same dose of radiation results in more biologic damage with protons.
3.- Proton therapy is still a form of radiation and does cause side effects. In my clinical experience, proton therapy can cause more short-term side effects then IMRT. Meaning that if you are already having trouble urinating due to having an enlarged prostate, proton therapy could require you to take certain medication or even need a catheter short-term.
4.- I recommend consideration for proton therapy if you are under 60 or have access to a proton center near you. Men who develop prostate cancer in their 40-50’s tend to have more aggressive disease. Consequently, protons may provide a higher control rate due to the increased RBE. In addition, there is some data showing slightly less long-term side effects with protons which is an important consideration as these men have a longer expected lifespan than a patient in their 70-80’s.
5.- For men who have to travel long distances to get to proton centers or procure housing and spend time away from family and friends, this burden needs to be weighed against the benefit expected with protons. In addition, the financial burden of the treatment and travel can be significant.
Patient-focused treatment, and an individualized approach to oncology means Dr. Norleena Gullett is not just treating cancer, she's treating the whole person.