ESTRO 2021 Abstract Book

S1468

ESTRO 2021

Netherlands

Purpose or Objective The PERYTON-trial is a prospective randomized controlled trial, which investigates the potential benefit of a hypofractionated salvage radiotherapy (SRT) schedule of 20x3=60Gy compared to the current standard of 35x2Gy=70Gy. A pre-trial quality assurance (QA) program was undertaken to detect inter-institutional radiotherapy treatment variation to ensure protocol compliance within the PERYTON-trial. Materials and Methods The QA program included 1) A questionnaire on details of the local radiotherapy protocols 2) An audit of delineation of the clinical target volume (CTV) and the organs at risk (OAR) and 3) A treatment planning benchmark exercise. Centers received a CT-scan of an eligible patient for SRT and were asked to delineate the CTV according to the local protocol. The submitted CTV contours were analyzed using the dice similarity coefficient (DSC), and the 50 and 95 percentile Hausdorff distance (HD50 and HD95) for all observer pairs. For the planning exercise, a pre-delineated CT scan was sent and centers were instructed to create PTVs and generate a treatment plan according to the PERYTON-trial protocol constraints for the hypofractionated arm. Reduction up to 4% of PTV V95 coverage was allowed if needed to meet the OAR constraints. Exceeding >5% in rectal volumes constraints was considered a major violation. The treatment plans were reviewed to detect protocol variations. Results Seven centers submitted the questionnaire. All responding centers use CT-scans for delineation (three centers MRI as well). The PTV margin varies from 5-10 mm. Bony anatomy was used for position verification in all centers. Four centers perform online verification using cone beam CT-scans (CBCT), the others perform offline verification using CBCT, 2D kV or MV imaging. In all centers a comfortably full bladder is recommended during radiation. Five centers completed both the delineation and planning exercises. Figure 1 shows all submitted CTV’s together with the delineation guidelines used. The CTV volume varied from 29 to 167 cm3, with the largest variation in the cranial boundary. The large volume differences resulted in a mean pairwise DSC of 0.35, and a mean HD50 and HD95 of 2.4 mm and 22.0 mm, respectively. All centers met the protocol requirements for the PTV coverage (Table 1). Three submitted plans had minor variations concerning the bladder constraints. One center had major violations of the rectal constraints (rectum V55 of 22% of prescription dose (PD), rectal wall V50 of 31.4% of the PD and a rectal wall mean dose of 29.2Gy) and was advised to lower the PTV coverage to meet the rectal constraints.