2.1. Patient Characteristics, Simulation, and Planning

Institutional Review Board approval was obtained for this study. One-hundred and sixty-five iCBCT (every 3 fractions) from 14 patients who underwent salvage PPRT were analyzed. The most common dose/fractionation was 66 Gy in 33 fractions, delivered in 10 patients (71.4%). Other dose/fractionation schemes, including 52.5 Gy/20 fractions and 70.2 Gy/39 fractions, were delivered to 2 patients each (17.4%).

All patients were instructed to empty the bladder and drink 16 oz of water 45 min before simulation and daily treatment. Based on physician preference, an endorectal balloon was used for 8/14 (57%) patients.

Prostate bed CTV was contoured per the Faculty of Radiation Oncology Genito-Urinary Group (FROGG) consensus guideline. An MRI was utilized to contour the apex of the prostate to the plane where the puborectalis is at the level of the urethra. The ipsilateral seminal vesicle bed was included if seminal vesical invasion was present. The retropubic space was included for the initial inferior half of the pubic bone. OARs, including bladder, rectum, femoral heads, and small and large bowel, were contoured for treatment planning and optimization. The rectal wall was defined as the outermost 3 mm of the rectum. Volumetric modulated arc therapy (VMAT) was planned for all patients. For making consistent plans with multiple PTV margins, an inverse planning optimizer, RapidPlan™ (Varian Medical Systems, Palo Alto, CA, USA), was used with minimal human intervention, adapting to PTV margins using the same prostate-bed model: bladder V65% < 60%, bladder D0.03cc < 108%, rectal wall D0.03 < 108%, CTV D95% > 95% of the prescription dose.