ESTRO 2022 - Abstract Book

S844

Abstract book

ESTRO 2022

MLD with approximately 3 Gy resulted in an average predicted 2 year overall survival of 49% (range 32–77%), while the predicted 2 year overall survival of the maximum heart sparing plan was very similar: 52% (range 34–80%). Conclusion The mean heart dose can be reduced substantially with only limited increase of mean lung dose. Increasing the mean lung dose with approximately 3 Gy relative to a maximum lung sparing esophageal VMAT plan is recommended to optimally reduce the MHD without compromising plan robustness.

OC-0952 Dosimetry & toxicity comparison between CBCT-Guided and MR-Guided Prostate Ultra-hypofractionated RT

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

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

Purpose or Objective Ultra-hypofractionated ( ³ 6Gy/fraction) radiotherapy for localized prostate cancer has been delivered at our institution with either online CBCT-guided 3 degrees of freedom (3DOF) translational correction or online MR-guided adaptation. Herein, we compared the daily dosimetry and acute toxicity between these two systems. Materials and Methods Fifty patients recruited to a Phase 2 trial evaluating the efficacy of External Beam Radiation Therapy (EBRT) plus High Dose Rate Brachytherapy intraprostatic boost (1500cGy/1) were included in this analysis. Hydrogel spacer was inserted at the discretion of physicians. An EBRT reference plan was generated to deliver 3000cGy/5 to the CTV (Prostate ± Seminal Vesicles) with a margin of 5mm using VMAT for 25 patients treated with CBCT-3DOF. The remainder were planned with IMRT and MR-guided adapt-to-shape (AtS). Daily EBRT delivered dose to CTV, bladder and rectum was computed using the daily CBCT or MR images. Deviation of >10% from reference dose was considered clinically significant. Acute toxicity was prospectively recorded using Common Terminology Criteria for Adverse Events (ver 4.0). Results Delivered dose per fraction to 95% of CTV was acceptable for all 250 fractions, with a median of 660cGy (Range: 649– 680cGy). Hydrogel spacer was present in 17 patients in the CBCT-3DOF and 15 patients in the MRL cohort. VMAT with CBCT- 3DOF was associated with lower daily rectal D50% but higher D1cm 3 compared to IMRT using MR-AtS, especially in the presence of hydrogel spacer (Table 1). The use of MR-AtS resulted in more fractions delivering a significantly lower rectal D20% and D1cm 3 than the reference dose, when compared to CBCT-3DOF (56 vs 26 for D20%, 51 vs 2 for D1cm 3 ; Figure 1). In contrary, difference between reference and delivered dose to 5cm 3 of bladder was significant in <15 fractions by either systems. Median follow up was 26 months for CBCT-3DOF and 25 months for MR-AtS. The cumulative incidence of G2+ urinary/gastrointestinal toxicity by 6 months was 8%/8% for the CBCT-3DOF cohort and 20%/0% for the MR-AtS.