For many decades, the treatments most often chosen for localized prostate cancer (PCa) were either surgery (radical prostatectomy or RP) or radiation (radiotherapy or RT). Each of them has evolved and improved, both in effective cancer control and reduced risk of urinary, sexual and bowel side effects.

• RP, relying on the surgeon’s scalpel, has progressed from open surgery to manual laparoscopic RP to robot-assisted RP.
• RT, relying on ionizing photon beams, has been refined from external beam (EBRT) with its scatter effect upon nearby tissues to more targeted radiation, including intensity modulated radiation therapy (IMRT) and stereotactic body radiation therapy (SBRT). Since IMRT and SBRT can be directed to the tumor in a more focused manner, their effect was more lethal while at the same time limiting scatter so side effects were reduced.

Proton beam radiation therapy

In the 1990s, a revolution in radiation therapy was introduced: proton beam therapy (PBT). Unlike photons with difficult to control scatter, protons are focused on a target with negligible scatter. That’s because subatomic protons are more than a thousand times more massive than electrons. This makes it easier to aim as they enter a tumor, with less emission of radiation as they exit. Also, their radiation is short-lived, so it was expected that there would be fewer short- and long-term side effects, and lower odds of secondary cancers than IMRT and SBRT. How well has PBT lived up to those expectations?

That question was put to the test by the randomized PARTIQoL trial, designed to compare PBT vs. IMRT results (side effects and cancer control). Dr. Jason Efstathiou presented the findings at the 2024 annual meeting of the American Society for Radiation Oncology (ASTRO, Sep. 29-Oct 2, Washington DC). A total of 450 PCa patients were enrolled between July 2012-Nov. 2021. All enrolled patients had proven localized PCa (Gleason ≤ 7, PSA < 20 ng/mL, stage T1c-T2c) and no prior RP, RT or hormone therapy. Participants were randomly assigned to receive either PBT (226 patients) or IMRT (224 patients). Follow up averaged 60.3 months. They were tracked for side effects (urinary, bowel, and sexual) based on standardized questionnaires, and they were also tracked for progression-free survival. The investigators found that at 24 months and 60 months, there were no significant differences between the two groups in terms of urinary, sexual and bowel scores. Likewise, according to a urology news report, progression-free survival was roughly equivalent. According to ASTRO press, “Five years after treatment, 93.7% and 93.4% of patients treated with IMRT and protons, respectively, had not experienced tumor progression…”

In short, this clinical trial found no notable difference in cancer control or quality of life between PBT and IMRT, even when correcting for factors like age, use of rectal spacers, dosimetry, etc. It was also pointed out that ongoing innovations and developments in either modality, such as advances in imaging and delivery systems, experimental fractionation of radiation dose or boost, etc. were not included in the trial. Finally, the available data was calculated based on an average of 5 years of tracking, but patients will continue to be tracked.

Dr. Efstathiou commented, “We tested two contemporary, advanced forms of external beam radiation for a very common cancer, and we demonstrated that both are very safe, effective treatments that give patients excellent outcomes in terms of quality of life and cancer control.”^[i]

Therefore, based on this randomized trial, there is no observable advantage to PBT over IMRT. This matters, because there has been reluctance on the part of insurers to cover or reimburse for PBT, which is the most expensive PCa treatment in terms of healthcare dollars. In addition, establishing a PBT center involves great cost, greatly limiting the number and location of such centers, so many patients simply don’t have access to PBT. Given the findings of the PARTIQoL trial that IMRT offers the same results as the more expensive PBT, it is reasonable to expect that resistance to payment will continue.

NOTE: This content is solely for purposes of information and does not substitute for diagnostic or medical advice. Talk to your doctor if you are experiencing pelvic pain, or have any other health concerns or questions of a personal medical nature.

References

[i] “IMRT and proton therapy offer equally high quality of life and tumor control for people with prostate cancer.” ASCO News, Sep. 30, 2024. https://www.astro.org/news-and-publications/news-and-media-center/news releases/2024/astro24efstathiou