Adaptive SBRT by 1.5 T MR-linac for prostate cancer: On the accuracy of dose delivery in view of the prolonged session time

Purpose: Adaptive Stereotactic Body Radiotherapy (SBRT) of prostate cancer (PC) by online 1.5 T MRi-guidance prolongs session-time, due to contouring and planning tasks, thus increasing the risk of prostate motion. Hence, the interest to verify the adequacy of the delivered dose. Material and methods: For twenty PC patients treated by 35 Gy (D-p) in five fractions, daily pre- and post- delivery MRi scans were respectively used for adapt-to-shape (ATS) optimization, and re-computation of the delivered irradiation (D-rec). Two expansion recipes, from Clinical (CTV) to Planning target volume (PTV), which slightly differed in the posterior margin were used for groups I and II, of ten patients each. Plans had to assure D-95% >= 95%D-p to PTV, and D-1cc <= D-p to rectum, bladder, penile bulb, and urethral planning-risk-volume (urethral-PRV). The adequacy of the delivered dose was estimated by inter-fraction average (ifa) of dose-volume metrics computed from D-rec. A cumulative dose (D-sum) was calculated from the five daily D-rec deformed onto the simulation MRi. Results: For each patient, CTV coverage resulted in D-95% > 95%D-p when estimated as ifa by D-rec. No significant difference for D-95% and D-99% metrics to CTV resulted between groups I and II. D-1cc was < D-p for rectum, urethral-PRV, and penile bulb, whereas < 103.5%D-p for the bladder. Significant correlations resulted between metrics computed by D-sum and as ifa by D-rec, by both linear-correlation analysis, and Receiver-Operating-Characteristic curve analysis. Conclusions: Our results for PC-SBRT confirm the adequacy of the delivered dose by ATS with 1.5 T MR-linac, and the consistency between dose-volume metrics computed by D-rec and D-sum.