ESTRO 2021 Abstract Book

S1300

ESTRO 2021

Maximum bladder dose was reduced by 0.6Gy with AP (p<0.05) but increased by 0.4Gy for Bone marrow. Mean V95% was not significantly different between MP and AP; D2% had a mean increase of 0.3Gy (p<0.05). Fig.2 shows typical hotspots from an AP plan. These had not been seen on any of the plans used for template development.

All MP were clinically acceptable. The AP was deemed unacceptable in 3/10 cases; 2 due to doses >107% outside of PTV and 1 due to poor coverage. In 8/10 cases the MP was preferred.

Conclusion AP generation with little or no user interaction produced clinically acceptable plans for the majority of cases. MP was typically preferred although PTV coverage, bowel and bladder doses were comparable, and rectal doses lower with AP. Further optimization of the AP template (e.g. to reduce priority of rectal dose reduction and/or introduce additional structures to limit hotspots adjacent to PTV) may further improve AP quality. In the context of the introduction of oART, the potential to significantly reduce treated volume may outweigh any residual loss of plan quality. Further work is required to determine the significance of initial plan statistics, in particular the propagation of hotspots through a treatment course with daily plan generation. Despite some drawbacks of AP the time savings are significant and should enable the benefits of oART to be realised for the majority of cervical radiotherapy cases. PO-1575 Evaluation of accumulated dose with residual uncertainties: an example with 4D-MRI guided liver SBRT T. van de Lindt 1 , T. Janssen 1 , M. Witte 1 , V. van Pelt 1 , A. Betgen 1 , M. Nowee 1 , J. Sonke 1 1 NKI-AvL, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective PTV margins account for geometric uncertainties to ensure adequate target coverage for a patient population. Dose accumulation (DA) is a method to quantify the impact of geometric uncertainties for individual patients and typically accounts for only a subset of these uncertainties. The purpose of this study was therefore to develop a method to evaluate the accumulated dose (D DA ) in the presence of residual uncertainties.

Materials and Methods Ten patients with liver metastases, treated with 4D-MRI guided liver SBRT on the Unity MR-linac (Elekta AB,