ESTRO 2022 - Abstract Book

S1489

Abstract book

ESTRO 2022

Twenty patients treated for high-risk prostate cancer with 78 Gy to the prostate and proximal seminal vesicles (CTV1) and 56 Gy to elective pelvic lymph nodes (CTV2) were included. MR scans were acquired at simulation (pMR) and at the 10 th , 20 th and 30 th fraction (+/- 2) on a diagnostic scanner to simulate the daily scans on a high field MR-linac (MRL). MRgART workflows were simulated in offline Monaco 5.40 (Elekta AB, Stockholm, Sweden) with an MRL reference plan on the planning MR with nine beams and up to 80 segments. An independent study had previously shown that the appropriate PTV margins for MRgART were 5 mm (AP), 4 mm (SI) and 3 mm (RL) for CTV1 and 2 mm for CTV2. A reference plan created on the pMR was adapted to each of the three session scans, including contour deformations, and the adapted plan doses were warped onto the pMR. Standard plans were generated as single arc VMAT plans with the aid of AutoPlan in Pinnacle 16 (Philips Medical Systems, Madison, WI, USA). Our department standard PTV margins were applied as a uniform 7 mm addition to CTV1 and 7 mm (AP), 12 mm (SI) and 5 mm (RL) to CTV2, to account for CBCT match on fiducials in the prostate. The standard RT plan was rigidly registered to each adaptation MR before it was warped to the planning MR for dose accumulation. Doses to organs at risk were compared by DVH analysis and potential clinical effects were assessed via NTCP models from the literature. Results MRgART yielded statistically significant lower doses for the bladder wall, rectum and peritoneal cavity, compared to the standard RT, as exemplified in Figure 1. Mean population accumulated doses were as good or better for all investigated organs at risk (OAR) when planned for MRgART compared to standard RT. These reductions in dose to the OAR translated into reduced median risks of hematuria ( Δ NTCP 0.5%), urine incontinence ( Δ NTCP 2.8%), urine voiding pain ( Δ NTCP 2.8%) and acute gastrointestinal toxicity ( Δ NTCP 17.4%), as illustrated in Figure 2.

Figure 1. Population mean DVH from MRgART and standard RT for the bladder wall and rectum. The p-value curve is a means to illustrate the dose levels of statistical significant differences.

Figure 2. Boxplots showing the distribution of the patients’ NTCP for investigated clinical endpoints from MRgART (MRL) and standard RT plans (Std). Data points exceeding 1.5 times the interquartile range are considered outliers and marked with a red +.

Conclusion Online adapted radiotherapy may reduce the dose to OAR in high-risk prostate cancer patients, due to reduced PTV margins. This potentially translates to significant and clinically relevant reductions in the risks of acute and late adverse effects.

PO-1690 MR-guided adaptive radiotherapy for gastric lymphomas: a pilot study to select PTV margins

L. Rechner 1 , K. Boye 1 , P.M. Petersen 1

1 Copenhagen University Hospital - Rigshospitalet, Department of Oncology, Center for Cancer and Organ Diseases, Copenhagen, Denmark