Toxicity, quality of life outcomes, and dosimetric impact in phase III trial of hypofractionated or standard fractionated proton therapy for low-risk prostate cancer patients: PCG GU 002.

Abstract

48 Background: The purpose of this study is to identify differences in toxicity, quality of life (QOL), and dosimetric endpoints among patients with low-risk prostate adenocarcinoma treated with either standard fractionation or hypofractionation proton-beam therapy. Methods: We analyzed the results of the first 75 patients treated in our phase III trial comparing 38Gy relative biological effectiveness (RBE) in 5 fractions (fx) (n = 46) vs. 79.2Gy (RBE) in 44 fx. (n = 29). All patients had low-risk prostate cancer and were treated with proton radiation using fiducial markers and daily image guidance. We evaluated American Urological Association (AUA) Symptom Index, adverse events (AEs), and Expanded Prostate Index Composite (EPIC) domains at pretreatment and 3, 6, 12, 18, 24, 36 and 48 months. Doses to the prostate were selected based on similar long term effects to normal tissue based on an α/β = 3.5 for an EQD2 of 76Gy. Results: The median follow-up was 36 months for both groups; 23 (30%) patients reached a follow-up of 48 months. We observed a small difference in AUA score (3.2 vs. 8.6, p = 0.002) at 1-year favoring the 79.2Gy arm. AUA scores were not statistically different at 18 months and after. Similarly, EPIC Urinary symptoms favored the 79.2Gy arm at 1 year (92.3 vs. 84.5, p = 0.009) and 18 months (92.3 vs. 85.3, p = 0.03); bother scores were not significantly different at any other time points. G2 or higher GU toxicity was similar between arms (p = 0.8). No differences in EPIC Bowel symptoms, EPIC Sexual symptoms, or bowel G2 or higher toxicities were seen. One death was observed in the 38Gy arm due to an unrelated case of multiple myeloma. The bladder V80 (79.2Gy arm, p = 0.04) and V39 (38Gy arm, p = 0.05) were predictive for G2 or higher GU AE. There was no association observed between rectal dose and G2 or higher rectal toxicity. Conclusions: Patients treated in the hypofractionated and standard arms tolerated treatment well. A small and temporary difference in the GU domain was seen favoring the standard arm. As predicted long term effects on the GU domain and bowel were similar for both arms, suggesting that the α/β is similar to 3.5 for long term effects. Clinical trial information: 01230866.