Proton therapy (PT) and intensity-modulated radiation therapy (IMRT) yield comparable prostate cancer treatment outcomes, according to preliminary results of the prospective COMPPARE study presented May 31 at the American Society of Clinical Oncology (ASCO) annual meeting in Chicago.
As a result, both techniques should be considered standard of care, said presenter Nancy Mendenhall, MD, of the University of Florida College of Medicine.
“This large pragmatic clinical study has demonstrated outstanding early outcomes with IMRT and PT,” she said.
The goal of the COMPPARE study was to understand the difference between proton and photon-based radiation treatment in prostate cancer. The researchers also sought to understand early outcomes for the two radiation therapy modalities and to contribute evidence supporting standard-of-care recommendations.
Proton therapy is a promising radiation modality that reduces radiation dose to non-targeted tissues. The technique is broadly accepted in some disease sites, and recent Level 1 evidence demonstrated improved toxicity, quality of life, and survival in oropharyngeal cancer.
Currently, less than 2% of prostate cancer patients who receive radiation therapy undergo proton therapy. However, single institutions have reported that PT has yielded comparable to superior results to conventional IMRT for toxicity, quality of life, and disease control outcomes. Furthermore, modeling and NCDB studies have shown a reduction in second malignancy with proton therapy
However, prospective comparative effectiveness data are lacking. Mendenhall noted that IMRT redistributes integral dose to non-targeted tissue, while PT reduces dose to non-targeted tissue by approximately 50%.
Funded by the Patient-Centered Outcomes Research Institute (PCORI), COMPPARE is a prospective comparison of outcomes of proton and photon radiation in all de novo prostate cancer with the exception of those with very-high risk for metastatic disease.
Fifty-one PT and IMRT centers participated in the trial and 2,524 patients were added to the study from July 2018 to October 2022. Of the eligible patients, 1,500 prostate cancer patients were placed in the PT cohort while 1,000 patients received traditional IMRT.
The patients had an average age of 69 and 79.3% were white and 16.8% were Black. Over 90% of the patients had intermediate or high-risk disease, and 60% were treated with hyperfractionation. In addition, 75% were given a rectal spacer and 50.6% also received androgen deprivation therapy (ADT).
“We hypothesized small differences between proton therapy and IMRT in bowel urgency, bowel frequency, GI toxicity, and freedom from disease progression at two and three years,” she said. “What we actually saw was a significant reduction in the impact on bowel function with both proton therapy and IMRT over what we had hypothesized, and a significant reduction in toxicities over what we had anticipated, and no significant differences between the two groups at this current very early time point of two and three years.”
Prostate cancer treatment outcomes by radiation modality | |||
Outcomes | IMRT | PT | P-value |
Bowel urgency | 6% | 5.7% | 0.28 |
Bowel frequency | 4% | 3.5% | 0.43 |
GI toxicity | 5.6% | 5.2% | 0.80 |
Freedom of progression at 3 years | 97.9% | 98% | 0.90 |
None of the differences were statistically significant.
Mendenhall also noted that temporary rectal spacers significantly reduced radiation-induced rectal and bowel toxicity for both radiation modalities.
“Based on no significant early differences between patient-reported quality of life, outcomes, toxicity, and freedom from PSA progression, both IMRT and PT should be considered standard of care,” she said. “This study applies to all patients with prostate cancer except those with very high risk for metastatic disease.”
She acknowledged that an important limitation of the study is that assessing the most important long-term outcomes of disease control, late toxicity, and secondary cancers will require longer-term follow-up.
