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 (Dp ) 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 (Drec ). 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 D95%  ≥ 95%Dp to PTV, and D1cc  ≤ Dp 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 Drec . A cumulative dose (Dsum ) was calculated from the five daily Drec deformed onto the simulation MRi.

RESULTS: For each patient, CTV coverage resulted in D95%  > 95%Dp when estimated as ifa by Drec . No significant difference for D95% and D99% metrics to CTV resulted between groups I and II. D1cc was < Dp for rectum, urethral-PRV, and penile bulb, whereas < 103.5%Dp for the bladder. Significant correlations resulted between metrics computed by Dsum and as ifa by Drec , 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 Drec and Dsum .