To present the radiotherapy quality assurance (RTQA) results from a prospective multicenter phase II randomized trial of short vs. protracted urethra-sparing stereotactic body radiotherapy (SBRT) for localized prostate cancer (PCa).
Between 2012 and 2015, 165 PCa patients from nine centers were randomized and treated with SBRT delivered either every-other-day (arm A, n=82) or once-a-week (arm B, n=83). 36.25 Gy in 5 fractions were prescribed to the prostate with (n=92) or without (n=73) inclusion of the seminal vesicles (SV), while the urethra planning-risk volume (uPRV) received 32.5 Gy. Patients were treated either with VMAT (n=112) or with IMRT (n=53). Deviations from protocol dose constraints, PTV homogeneity index (HI), PTV Dice similarity coefficient (DSC), and number of monitor units (MU) for each treatment plan were retrospectively analyzed. Dosimetric results of VMAT vs. IMRT and treatment plans with vs. without inclusion of SV were compared.
At least one major protocol deviation occurred in 51 patients (31%), while none was observed in 41. Protocol violations were more frequent in the IMRT group ( p<0.001). Furthermore, the use of VMAT yielded better dosimetric results than IMRT for uPRV D 98% (31.1 vs. 30.8 Gy, p<0.0001), PTV D 2% (37.9 vs. 38.7 Gy p<0.0001), HI (0.09 vs. 0.10, p<0.0001), DSC (0.83 vs. 0.80, p<0.0001) and bladder wall V 50% (24.5% vs. 33.5%, p=0.0001). To achieve its goals VMAT required less MU than IMRT (2275 vs. 3378, p<0.0001). The inclusion of SV in the PTV impacted negatively on the rectal wall V 90% (9.1% vs. 10.4%, p=0.0003) and V 80% (13.2% vs. 15.7%, p=0.0003).
Protocol deviations with potential impact on tumor control and/or toxicity occurred in 31% of patients in this prospective clinical trial. Protocol deviations were more frequent with IMRT. Prospective RTQA protocols should be strongly recommended for SBRT trials in order to minimize potential protocol deviations.