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) 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). 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%D to PTV, and D ≤ D 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. A cumulative dose (D) was calculated from the five daily D deformed onto the simulation MRi.

RESULTS

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