ESTRO 2021 Abstract Book

S1284

ESTRO 2021

Conclusion Treating stereotactic prostate on the MR Linac using the fully adaptive ATS workflow results in improved target organ coverage over a non adaptive treatment. Organ at risk constraints were consistently met using the adapt to shape workflow.

PO-1559 PTV margin required for radiotherapy of prostate cancer on an MR linac R.L. Christiansen 1,2 , L. Dysager 3 , O. Hansen 3,2 , C. Brink 1,2 , U. Bernchou 1,2

1 Laboratory of Radiation Physics, Odense University Hospital, Odense, Denmark; 2 Department of Clinical Research, University of Southern Denmark, Odense, Denmark; 3 Department of Oncology, Odense University Hospital, Odense, Denmark Purpose or Objective Inter-fractional setup uncertainties are accounted for in daily MR-guided radiotherapy (RT). Thus, the PTV margin applied to CTV’s need only account for the intra-fractional motion (IFM) and the delineation uncertainty. The aim of this study was to determine the IFM of the prostate and pelvis, respectively, in prostate patients treated with daily MR guidance on a high field MR Linac (MRL) to establish the required PTV margins for high-risk prostate radiotherapy (RT). Materials and Methods In 40 consecutive patients treated for localized prostate cancer with daily MR-guided adapted RT, the IFM during five treatment fractions evenly distributed over the treatment course, was measured. For each fraction, a pre-treatment MR was acquired for plan adaptation as well as a post-treatment MR to assess the patient intra-fraction motion. All patients followed a drinking protocol. The target in high-risk prostate cancer consists of a high-dose CTV (CTV1) including the prostate and proximal part of the seminal vesicles RT and an elective low-dose CTV (CTV2) which includes the pelvic lymph nodes. The pelvic lymph nodes follow the bony structures closely, thus four bony landmarks were used as a surrogate for the IFM of CTV2. The translational motion from pre- to post-treatment MR was measured for each landmark in the right-left (RL), superior-inferior (SI) and anterior-posterior (AP) directions. As CTV1 can move independently of the bony structures of the pelvis its translations were evaluated separately. The systematic error (∑) and random error ( σ ) of the population was calculated for the population. The penumbra ( σ p ) was determined from the distance between 80% and 90% isodose lines of the steepest dose gradients on MRL prostate plans. PTV margins were calculated using van Herk’s margin recipe with the inclusion of delineation uncertainty ( σ d ) as a random error, as new delineations were made for each fraction. Results The required PTV margin in each direction and point of measurement (table 1) show that different PTV margins were required for CTV1 (3 mm RL, 4 mm SI and 5 mm AP) and CTV2 (2 mm in all directions). The MRL penumbra was determined to be σ p = 4.4 mm, and the delineation uncertainty was taken to be σ d = 2 mm.

Conclusion The required PTV margins for the prostate and proximal seminal vesicles are 5 mm AP, 4 mm SI and 3 mm RL. The elective lymph node regions require a uniform PTV margin of 2 mm.

PO-1560 Evaluation of organ motion effect on dose in SBRT treatments for oligorecurrent prostate cancer