ESTRO 2020 Abstract book

S280 ESTRO 2020

study and comparing the dosimetric outcomes to the clinical HDR-BT plan. Material and Methods We randomly selected 18 patients who underwent HDR-BT between February 2018 and July 2019. Delineations from the intraoperative MRI-guided HDR-BT plan were rigidly transferred to a pretreatment PET-CT scan. Planning was IMRT-based with 9 beams, resembling VMAT which is not yet available on the MRL. A PTV margin of 1 mm was used, assuming near-future MRL treatment with on-line treatment plan adaptation. A dose of 19 Gy was prescribed to ≥95% of the PTV and the CTV was allowed to receive >19 Gy, resembling high-dose volumes in HDR-BT. HDR-BT constraints for organs at risk (OAR) were used (table 1). If OAR constraints could not be met, a dose of ≥17 Gy was accepted to ≥90% of the PTV, conform HDR-BT prescription. Treatment plans were optimized for adequate target coverage with minimal OAR dose. The simulated MRL plans were compared to the clinically delivered HDR-BT plans using paired samples T-test. Results For HDR-BT, 10 out of 18 (56%) plans did not reach CTV D95 ≥19 Gy, of which one did not reach D90 ≥17 Gy. For MRL, 8 out of 18 (44%) plans did not reach PTV D95 ≥19 Gy, of which all but one also failed with the HDR-plan. Two of these MRL plans did not reach D90 ≥17 Gy. There were no significant differences between mean D90 and D95 (table 1). In HDR-BT plans, due to the steeper dose gradient, a larger part of the CTV received ≥150% of the prescribed dose while none of the MRL plans reached a dose of ≥200% (table 1 and figure 1). Furthermore, D10% for the urethra and D1cc and D2cc for the rectum were significantly lower in the HDR-BT plans. On the other hand, D1cc and D2cc of the bladder were slightly lower in the MRL plans.

Conclusion Delivering a single 19 Gy dose to a recurrent prostate cancer lesion on the MRL with a D95 comparable to what is achieved with HDR-BT may be feasible. The steeper dose gradient in HDR-BT is reflected by a larger part of the target volume receiving ≥150% of the prescribed dose, however, the clinical implications of this have not yet been determined. Also, urethra and rectal dose are mostly lower with HDR-BT, though none of the MRL plans exceeded the OAR constraints. OC-0468 Delivery of single-fraction lung SABR using MR-guidance T. Finazzi 1 , J. Van Sörnsen de Koste 1 , M. Palacios 1 , F. Spoelstra 1 , B. Slotman 1 , C. Haasbeek 1 , S. Senan 1 1 Amsterdam University Medical Centers VUmc location, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Two prospective randomised clinical trials have established single-fraction stereotactic ablative radiotherapy (SABR) as a standard of care for lung tumors. We evaluated treatment delivery using magnetic resonance (MR)-guidance. Material and Methods An MR-simulation procedure was performed in patients who were eligible for single-fraction peripheral lung SABR. Use of MR-guidance was considered when clinicians were concerned about single-fraction delivery using an internal target volume approach, especially in mobile tumors. Treatment simulation and delivery was performed on an MR Linac (ViewRay Inc., USA), with online treatment plan reoptimization based on a breath-hold 3DMR scan. This scan was repeated mid-treatment due to lengthy This abstract is part of the media programme and will be released on the day of its pres ntatio