ESTRO 2022 - Abstract Book

S703

Abstract book

ESTRO 2022

Conclusion In the presence of large anatomical variation, full adaptation for prostate SBRT is beneficial for rectal sparing; particularly in the high dose region when compared to 3DOF. Dose to bladder D5cc correlating to the trigone of the bladder, on the other hand, was less sensitive to the full adaptation. Further investigation correlating the dosimetric advantage of full adaptation with toxicity and patient reported outcomes will help evaluate the role of this treatment workflow.

OC-0784 Assessment of intraprostatic tumour motion in prostate radiotherapy on the MR-Linac

V. Kong 1 , J. Padayachee 1 , J. Winter 1 , J. Dang 1 , W. Li 1 , I. Navarro 1 , R. Glicksman 1 , V. Malkov 1 , J. Helou 1 , A. Berlin 1 , P. Chung 1

1 Princess Margaret Cancer Centre, Radiation Medicine Program, Toronto, Canada

Purpose or Objective There is increasing interest in dose escalation to the GTV in patients receiving radiotherapy for localised prostate cancer. The MR-Linac (MRL) presents a unique opportunity to facilitate this approach through improved target definition and adaptive radiotherapy delivery. We aimed to assess intra-fraction GTV motion during MRL treatment delivery and to evaluate the efficacy of a PTV derived based on internal target volume (ITV). Materials and Methods Five patients with an identifiable GTV who received whole gland prostate radiotherapy on the MRL were selected for this retrospective planning study. During each treatment session, three T2 MR images were obtained – localization for adaptation (MR Loc ), verification pre-treatment (MR Ver ), and during beam-on (MR BeamOn ). Average session time was 38 minutes. MR Loc and MR BeamOn from 5 treatment sessions for each patient were retrieved. To assess for intra-fraction motion, displacement of the GTV on MR BeamOn was calculated using coordinates of the centre of mass. An ITV was generated summing GTV Loc and GTV BeamOn . Two PTVs were defined to compare dosimetry: GTV Loc + 2 mm (GTV-PTV), and ITV + 2 mm (ITV-PTV). A 9-beam IMRT plan was generated to deliver 15 Gy in a single fraction to each PTV on the MR Loc scans. Rectum, urethra and bladder were included in OAR assessment. Results A total of 50 GTVs and 25 ITVs were analysed. Median GTV was 1.05 cm 3 (range: 0.46 – 2.77). Intra-fraction GTV motion was greatest in the transverse direction (mean = 2.2 mm), followed by cranio-caudal (mean = 1.4 mm), and lateral (mean = 0.7