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Due to their physical characteristics, protons have a great potential to treat patients with prostate cancer. While reduced toxicity, improved outcome and cost-effectiveness have been discussed, there is a lack of robust clinical data so far. Thus, the debate in treating early prostate cancer with proton beam therapy is still active. Therefore, the current study aimed to analyze long-term outcome of a large cohort of prostate cancer patients treated with either photons or photons and protons.

For this retrospective study, we included 549 men with stage T1-T3 N0 M0 treatment-naïve prostate cancer irradiated at our department in the period from 2010 to 2019. Patients received either conventional photon-based intensity-modulated radiotherapy with daily image guidance (IMRT/IGRT) (CT cohort) or combined photon radiotherapy with a proton boost using active raster scanning technique (PT cohort). Acute and late adverse events were evaluated according to CTCAE criteria and quality of life according to QLQ-C30 and -PR25 questionnaire. Further endpoints were overall survival (OS) and biochemical progression-free survival (bPFS). For statistical analyses we used the log-rank and chi-squared test.

The CT cohort consisted of 398 men while 151 patients were included in the PT group. Both cohorts were well-balanced, most patients were classified as intermediate-risk according to d’Amico risk classification (68.3% for CT vs. 70.2% for PT). Median follow-up was 58.9 months for the photon group and 49.0 months for the group with a proton boost. There were no statistically significant differences regarding clinical outcome with a 5- and 10-year overall survival of 92.5% vs. 93.7% and 69.6% vs. 77.5% (CT vs. PT). 5- and 10-year biochemical progression-free survival (bPFS) were 90.0% and 57.1% for the photon group and 84.9% and 50.2% for the proton boost cohort, respectively. During treatment, there was a higher risk for gastrointestinal (GI) toxicity for some items (e.g. grade 2 rectal urgency: 6.1% vs. 0.7%), however, these differences disappeared during long-term follow-up. The global quality of life at the time of the last follow-up demonstrated no statistically significant difference between both groups with a mean score of 70.7 (SD 21.2) for the CT and 70.6 (SD 21.9) for the PT cohort.

Our findings obtained from a large cohort confirmed the long-term efficacy of definitive irradiation of patients with treatment-naïve prostate cancer using a proton boost. However, these results also suggest that there was no considerable decrease of toxicity when compared to conventional photon irradiation. Thus, data from prospective randomized trials like the phase III randomized clinical trial of proton therapy versus IMRT for low or intermediate risk prostate cancer (NCT01617161) are highly expected.