Simulation of an HDR "Boost" with Stereotactic Proton versus Photon Therapy in Prostate Cancer: A Dosimetric Feasibility Study

Purpose/Objectives: To compare the dose escalation potential of stereotactic body proton therapy (SBPT) versus stereotactic body photon therapy (SBXT) using high-dose rate prostate brachytherapy (HDR-B) dose-prescription metrics. Patients and Methods: Twenty-five patients previously treated with radiation for prostate cancer were identified and stratified by prostate size (<= 50cc; n = 13, > 50cc; n = 12). Initial CT simulation scans were re-planned using SBXT and SBPT modalities using a prescription dose of 19Gy in 2 fractions. Target coverage goals were designed to mimic the dose distributions of HDR-B and maximized to the upper limit constraint for the rectum and urethra. Dosimetric parameters between SBPT and SBXT were compared using the signed-rank test and again after stratification for prostate size (<= 50cm(3) and >50cm(3)) using the Wilcoxon rank test. Results: Prostate volume receiving 100% of the dose (V100) was significantly greater for SBXT (99%) versus SBPT (96%) (P <= 0.01), whereas the median V125 (82% vs. 73%, P < 0.01) and V200 (12% vs. 2%, P < 0.01) was significantly greater for SBPT compared to SBXT. Median V150 was 49% for both cohorts (P = 0.92). V125 and V200 were significantly correlated with prostate size. For prostates > 50cm(3), V200 was significantly greater with SBPT compared to SBXT (14.5% vs. 1%, P = 0.005), but not for prostates 50cm(3) (9% vs 4%, P = 0.11). Median dose to 2cm(3) of the bladder neck was significantly lower with SBPT versus SBXT (9.6 Gy vs. 14 Gy, P < 0.01). Conclusion: SBPT and SBXT can be used to simulate an HDR-B boost for locally advanced prostate cancer. SBPT demonstrated greater dose escalation potential than SBXT. These results are relevant for future trial design, particularly in patients with high risk prostate cancer who are not amenable to brachytherapy.