IMRT vs Protons: The Information You Need to Make the Best Decision
Many men with prostate cancer want to understand the difference between IMRT, conventional radiation offered by their local radiation oncologist, and specialized radiation with Proton Therapy. IMRT stands for Intensity Modulated Radiation Therapy and has been the standard radiation treatment for prostate cancer for the past decade.
Historically I was not convinced there was a significant benefit to proton therapy for prostate cancer and felt that I was able to provide an unbiased opinion due to my training. However based on the most recent scientific data, as well as newer proton treatment techniques (pencil beam scanning), I now encourage men to consider proton therapy in addition to IMRT for their prostate cancer.
Check out this study
This study from University of Florida (UF) is a great example of more recent data coming out of proton centers to really try and examine if there is a clinical benefit to proton treatment. Remember from my prior blogs – a proton plan is typically always going to look better than a photon plan, but we still don’t know if all that low dose radiation matters clinically.
In 2016, the University of Florida (UF) Proton Center published a study that reporting the 5-year outcomes of more than 1,300 prostate cancer patients treated with either IMRT or Proton therapy from 2006 to 2010. The study shows that proton therapy is a highly effective treatment for low-risk, intermediate-risk and high-risk (click here if you do not understand the difference in risk categories for prostate cancer because it’s important) prostate cancer. It also reports a low rate of serious side effects.
Below is a table from the UF publication showing the treatment delivered and the 5-year BCR (biochemical control rate) as well as side effects.
Note: the reason prostate cancer studies look at “biochemical control rate” or “freedom from progression” is because most men die WITH prostate cancer and not FROM prostate cancer. If we just looked at how many men survived after a prostate cancer treatment we wouldn’t be able to tell if the treatment was any good because most men die of cardiovascular disease, diabetes, and other causes.
So BCR basically means the percentage of patients whose PSA stayed low and there was no concern for recurrent prostate cancer. You can see that both IMRT and Proton therapy provided excellent control of the PSA. Especially for Low Risk (LR) prostate cancer patients where BCR rates are as high as 99%. This means that for 99% of men treated, the PSA remained low and stayed low. As expected, the BCR drops for patients having intermediate and high risk disease, because these men had more advanced cancer prior to even starting treatment. Overall the BCR rates are excellent for low risk and intermediate risk disease.
Bryant C, Smith TL, Henderson, RH, et al. Five-year biochemical results, toxicity, and patient-reported quality of life after delivery of dose-escalated image guided proton therapy for prostate cancer. Int J Radiat Oncol Biol Phys 2016;95:422-434
Do Either Treatments Have Side Effects?
Since both IMRT and Proton therapy provide good control of the PSA, what about side effects? Look at the circled data above looking at grade 3 toxicity. GI toxicity refers to rectal side effects such as diarrhea, bloody stools, or other rectal bleeding. GU toxicity refers to irritation of the bladder from the radiation, such as difficulty urinating, pain with urination or blood in the urine.
Bladder and rectal side effects from both IMRT and Proton therapy were low, below 3%. When compared to the rates of urinary leakage and erectile dysfunction caused by surgery, non-invasive management with either type of radiation appears to have fewer side effects.
So, why am I now offering protons therapy to my patients with prostate cancer? A couple of reasons:
- Urologists have become much more aggressive in telling men that they need to have their prostate removed (separate discussion on why this is happening – coming soon).
- Because of #1, I am seeing more men post – operatively who still need full course radiation because even though the prostate is out, there is microscopic disease left behind causing the PSA to go up (which, begs the question, why these men were offered surgery in the first place … but I digress)
- The men that I see post op are often wearing pads or diapers due to urinary leakage, and sadly, are no longer potent. Often times they depend on a penile pump or injections to maintain a sex life. Consequently, I am very conscious of not causing further damage with radiation.
For men who have had their prostate removed and have a rising PSA, my challenge is to deliver radiation to the pelvic lymph nodes and prostate bed, to eradicate microscopic prostate cancer cells.
I need to deliver my radiation using the most precise technique possible and that appears to be proton therapy using pencil beam scanning (PBS). While the low dose scatter from IMRT appears to be fairly well-tolerated for men with intact prostates, we are now seeing significant reductions in that low dose scatter with PBS. Several studies from Massachusetts General as well as University of Florida, have shown significantly lower doses delivered to the bladder and the rectum with proton therapy when compared to IMRT. I expect that to translate into an even further reduction with PBS. ,
Men also are told they should avoid radiation due to the risk of a second cancer, such as a rectal or bladder cancer. While the risk of this is very low for a man in his 70’s, it can be a concern for younger men in their 50’s. With IMRT, the radiation delivered to the bladder and rectum has caused another cancer 20 years down the road. This is yet another reason, I have been impressed with protons and specifically PBS. There are some studies demonstrated as much as a 40% reduction in risks of secondary malignancies related to radiation exposure when PBT is used.
In summary, I recommend men with prostate cancer should request a consultation with both a local radiation oncologist as well as at a Proton Center. I am seeing younger patients in their 50’s and 60 are who want to avoid surgery due to the risk of leakage and losing their erectile function. Proton radiation offers a non-invasive method of delivering very precise and accurate treatment which has been shown to reduce low dose radiation to the bladder and rectum. Newer proton technology using pencil beam scanning (PBS) provides another level of precision which allows for avoidance of normal tissue and can achieve a higher dose to the tumor. PBS seems particularly helpful when treating men who have had a radical prostatectomy and the PSA continues to rise.
I am happy to discuss all radiation options with men with prostate cancer to ensure they have a plan that is personalized to them, whether it be IMRT, Proton therapy, SBRT or brachytherapy (seed implants).
1 Trofimov A, Nguyen P, Coen J, et al. Radiotherapy treatment of earlystage prostate cancer with IMRT and protons: a treatment planning comparison. Int J Radiat Oncol Biol Phys. 2007;69(2):444–453.
2 Vargas C, Fryer A, Mahajan C, et al. Dose-volume comparison of proton therapy and intensity-modulated radiotherapy for prostate cancer. Int J Radiat Oncol Biol Phys. 2008;70(3):744–751
3 Rechner L, Howell R, Zhang R, Etzel C, Lee A, Newhauser W. Risk of radiogenic second cancers following volumetric modulated arc therapy and proton arc therapy for prostate cancer. Phys Med Biol. 2012;57(21): 7117–7132.
4 Yoon M, Ahn S, Kim J, et al. Radiation-induced cancers from modern radiotherapy techniques: intensity-modulated radiotherapy versus proton therapy. Int J Radiat Oncol Biol Phys. 2010;77(5): 1477–1485.
5 Fontenot J, Lee A, Newhauser W. Risk of secondary malignant neoplasms from proton therapy and intensity-modulated X-ray therapy for early-stage prostate cancer. Int J Radiat Oncol Biol Phys. 2009;74(2): 616–622.