ESTRO 2021 Abstract Book

S65

ESTRO 2021

body radiotherapy (SBRT) of prostate cancer. Materials and Methods

First ten patients receiving ART-SBRT in our department were included. Prescription dose consisted of 7 fractions of 6.1 Gy according to the HYPO-RT-PC trial. Our ART technique is based on CBCT imaging acquired in a Varian Clinac 2100 CD (5 patients) and a Varian TrueBeam linac (5 patients). Once the patient is setup by using the room lasers, a CBCT scan is acquired and 6D-rigid registered with the simulation CT scan (sim-CT). The target (CTV: prostate), organs-at-risk (OARs) and the reference IMRT plan are mapped from the sim-CT scan to the CBCT scan. The CTV and OARs are adapted to the actual anatomy by a qualified radonc. Then, the reference IMRT plan that was calculated on the sim-CT scan is re-optimized on the CBCT scan to obtain an adapted plan (ART plan), while the patient lies on the linac couch. In order to mimic each treatment session without this adaptive strategy, the reference plan was mapped to the CBCT scan according to the sim-CT/CBCT 6D-rigid fusion (focused on the CTV), and recalculated with the same original monitor units (6D plan). For each patient and treatment session, the ART and 6D plans were compared (70 plan pairs) using the following metrics: Dmin for the CTV; V95 for the PTV; and V65, V75 and V90 for the rectum. Dmin is the minimum dose as % of the prescribed dose, and Vx is the % of volume receiving ≥ X% of the prescribed dose. The HYPO-RT-PC trial establishes these objectives: Dmin ≥ 95%, V95 ≥ 95%, V65 ≤ 45%, V75 ≤ 45%, and V90 ≤ 15%. The required time for the online plan adaptation and total treatment time were accounted. Results • Adapting the plan increased the target coverage significantly compared to non-adapt: 97.8% vs. 90.4% (p < 0.001) and 98.7% vs. 90.7% (p < 0.001), for Dmin of the CTV and V95 of the PTV, respectively. All ART plans met the required objectives, while nineteen and twenty nine 6D plans did not met the requirements for Dmin and V95, respectively (Fig 1). • Not significant differences were found for the three rectum metrics (V65: 20.1% vs. 20.7%, V75: 14.4% vs. 15.3%; V90: 7.8% vs. 8.6 %), between the ART and non-adapted 6D plans. Although all plans met the required objectives, adapting plan did not improve these metrics in about 45% of sessions (Fig 2). • The average time for the online plan adaptation and total treatment time were 23.5 min (range: 15.0 to 43.4 min; 95th percentile: 34.5 min) and 32.5 min (range: 19.5 to 53.3 min; 95th percentile: 45.1 min), respectively.

Conclusion