ESTRO 2021 Abstract Book

S64

ESTRO 2021

MIM algorithm (Accuray Inc, Sunnyvale, USA). Gross Tumour Volume (GTV) and Planning Target Volume (PTV) were rigidly propagated from pCT to FxCTs. Autocontours (AUTO) remained unedited, and were geometrically compared to MAN through the Dice coefficient (DC), mean surface distance (MSD) and Hausdorff distance (HD) metrics. Both MAN and AUTO contours were cut at 3 cm from the PTV to reduce optimization time. Treatment plans on pCTs were optimized using the VOLO TM optimizer in Precision, and the dose was rigidly transferred to FxCTs to evaluate non-adapted (NoAd) doses. Adaptive plans were made on FxCTs using MAN and AUTO. The three resulting doses on FxCTs (NoAd, MANRp and AUTORp) were compared using PTV coverage and OAR constraints (V35<0.5cc). Results Table 1 shows the DC, MSD and HD between MAN and AUTO, and V35Gy and PTV coverage parameters evaluated on NoAd, MANRp and AUTORp doses. NoAd plans showed OAR dose-constraints violations on 70% of FxCTs. Replanning using MAN and AUTO contours resulted in plans satisfying OAR constraints for 93% and 64% FxCTs, respectively. But, MANRp and AUTORp improved V35Gy for 100% and 95% of the fractions compared to non-adapted plans. PTV coverage reduced 2% and 3% after MANRp and AUTORp, with respect to NoAd (Fig.1).

Conclusion Notwithstanding the variation between manual and deformed OARs, replanning in uncorrected deformed contours resulted in plan improvements in all but 5 fractions compared to no plan adaptation. These results indicate that for a large number of fractions manual editing of autocontours could be avoided to reduce the overall time for an ART workflow.

OC-0090 Dosimetric evaluation of an online adaptive strategy for prostate SBRT J. Calvo-Ortega 1 , J. Torices-Caballero 1 , S. Moragues-Femenia 1 , C. Laosa-Bello 1 , J. Casals-Farran 1 1 hospital Quirónsalud Barcelona, Radiation Oncology, Barcelona, Spain

Purpose or Objective To evaluate the dosimetric advantage of performing daily online adaptive radiotherapy (ART) for stereotactic