ESTRO 2023 - Abstract Book

S225

Saturday 13 May

ESTRO 2023

DailyPlan consisted of daily online replanning with 1mm setup robustness setting (SRS) and 3% range robustness setting (RRS). ClinAdapt consisted of an initial treatment plan generated for the planning CT (pCT) using 3 mm/3% SRS/RRS, and ad-hoc replanning based on clinical evaluation. The investigated ART alternatives were: 1) wOffR: weekly offline replanning with 2mm/3% SRS/RRS; 2) PL1: using daily in- room imaging for plan selection from a pre-treatment generated PL (created using the pCT with 0, 1, 2, 3, or 5 mm SRS and 3% RRS for each patient); 3) PL2: a progressive library following the concept of PL1, but with the PL weekly extended with offline replans using rCTs. For PL1&2, daily online plan selection was based on target coverage recomputed on rCTs. For evaluation, 25 fractionated treatment courses were simulated for each patient for all 5 strategies, using known setup and range error distributions. All strategies used propagated contours that were manually adjusted. Fraction doses were accumulated. Near-minimum target coverage and NTCPs (using Wilcoxon-signed-rank) in the simulated treatments were compared to DailyPlan. Results In DailyPlan, 15/15 patients had a near minimum CTV V95% above the requested 98% for both CTVs. For PL1&2 this was 14/15, compared to 13/15 for ClinAdapt and 12/15 for wOffR (Fig.1). NTCP differences from DailyPlan are presented in Fig.2. Average differences between DailyPlan and ClinAdapt were 4.1±1.9%-point for xerostomia (XS) and 3.9±2.2%-point for dysphagia (DP), in favor of DailyPlan (p<0.001 for both). NTCPs of PL2 approached those of DailyPlan with differences of only 1.4.±1.7%-point for XS and 1.6±2.4%-point for DP (p<0.001 for both) in favor of DailyPlan. With PL2, 74% (XS) and 73% (DP) of the NTCP reduction could be achieved that can be realized with DailyPlan on average. For wOffR, these percentages were 40% and 43%. Conclusion With acceptable target coverage, NTCP reductions with the progressive plan library approach (PL2) approached those obtained with daily in-room re-planning (DailyPlan). Weekly offline re-planning (wOffR) also showed NTCP reductions compared to ClinAdapt, but to a lesser extent. Implementation of PL2 would avoid problems of DailyPlan with computation speed and online plan QA.

Proffered Papers: Physics

OC-0290 Investigation of the feasibility of selenium-75 as a viable brachytherapy source J. Reid 1,2 , J. Kalinowski 3,4 , J. Munro III 5 , A. Armstrong 6 , S. Enger 3,4

1 Mcgill University, Oncology, Montreal, Canada; 2 Jewish General Hosptial, Lady Davis Institute for Medical Research, Montreal, Canada; 3 McGill University, Oncology, Montreal, Canada; 4 Jewish General Hospital, Lady Davis Institute for Medical Research, Montreal, Canada; 5 Montrose Technology, Inc., N/A, Worcester, USA; 6 McMaster University, Nuclear Operations & Facilities, Hamilton, Canada Purpose or Objective Selenium-75 (75Se, t1/2 = 118 days, E γ ,avg = 210 keV) is a radioisotope that is widely used in industrial gamma radiography. Its lower photon energy and longer half life compared to Iridium-192 (192Ir, t1/2 = 74 days, E γ ,avg = 380 keV) make it a viable candidate for use as a brachytherapy source. The goal of this study was to investigate the feasibility of using a 75Se