ESTRO 2023 - Abstract Book

S112

Saturday 13 May

ESTRO 2023

Conclusion Our results identify that the novel NT outlining for STSE are reproducible between the observers and in a multi-professional environment, RTT and clinician scores are consistent. This work has also identified areas of refinement and improvement required to improve the guidelines, particularly for the deep thigh NVB and lateral rotator MCT. These guidelines are currently under revision and a secondary assessment will be conducted to test for improvements. The first author is clinical doctoral research fellow CDR-2018-04-ST2-004, funded by HEE/ NIHR. MO-0150 Accelerated Partial Breast Irradiation dosimetry for a conventional versus an integrated MR-Linac J. Dean 1 , F. Foroudi 1 , M. Chao 1 , F. Height 1 , S. Fisher 1 , D. Smith 1 , S.P. Ng 1 1 Olivia Newton-John Cancer, Wellness and Research Centre, Radiation Oncology, Heidelberg, Australia Purpose or Objective Accelerated Partial Breast Irradiation (APBI) is an accepted short-course treatment option for patients with early breast cancer. This study evaluates our early experience for APBI patients treated on the integrated Magnetic Resonance Linear accelerator (MR-Linac) as part of the departmental Feasibility of Imaging and Radiation Treatment Delivery on the MR Linac (FIRM) Study. Materials and Methods As part of the FIRM study, nine suitable patients were parallel planned for APBI in the supine position (30Gy in 5 fractions, delivered on alternate days) for both conventional and MR-Linac. Protocol constraints and patient selection were based on the Florence APBI Trial. The cohort included left and right sided patients. A 6MV volumetric modulated arc therapy (VMAT) plan was generated for the conventional linac and a 15 field 7MV FFF step-and-shoot intensity modulated radiation therapy (IMRT) plan for the MR-Linac. Beam angles were limited to the ipsilateral side for both techniques. Target and OAR (heart, lung(s), skin, chest wall and contralateral breast) doses were collected and analysed. Results All plans in the study met protocol criteria. Target coverage was comparable (average D95% of 29.6Gy on the MR-Linac vs 29.2Gy for conventional). MR-Linac plans were marginally hotter on average (D2% of 31.4Gy vs 31.1Gy). Ipsilateral and contralateral lung doses were similar. Maximum dose (0.035cc) to the contralateral breast was also comparable. The volume of heart receiving 3Gy was lower for MR-Linac plans, but mean doses were slightly higher. The percentage of 15Gy to the uninvolved ipsilateral breast was almost 10% higher for the MR-Linac plans, but still well under protocol tolerance. Maximum doses to skin evaluation structures were on average 0.5Gy cooler for VMAT plans. Target position provided more challenges for MR-Linac plans due to physical machine limitations as well as proximity to skin and chest wall. Conclusion Our experience indicates that clinically acceptable APBI plans can be generated for patients for the MR-Linac that are comparable to conventional VMAT plans. Additional planning considerations must be applied to ensure robust MR-Linac dosimetry. PD-0151 Effects of radiotherapy interruptions in lung cancer disease control: a multicenter study E. Gomis Selles 1 , Ó. Muñoz Muñoz 1 , B.D. Delgado León 1 , P. Jiménez Labaig 2 , J. Cacicedo Fernández de Bobadilla 2 , P. Romero Pareja 1 , I. Paguey Garrido 1 , S. Pérez Luque 1 , M. Borrego Reina 1 , J.L. López Guerra 1 1 Virgen del Rocio University Hospital, Radiation Oncology, Sevilla, Spain; 2 University Hospital Cruces, Radiation Oncology, Bilbao, Spain Purpose or Objective The evidence about the impact of radiotherapy (RT) interruptions on disease control is scarce and presents biases. The main objective of this study is to assess the impact of RT interruptions on overall survival (OS) and local recurrence-free survival (LRFS) in patients with lung cancer treated with radical intention. Materials and Methods Retrospective cohort study selecting all patients with lung cancer treated with radical intention with RT in three institutions between October 2008 and August 2021. Exclusion criteria included patients with a lack of information about the reason for interruption or the total dose administered. Log-rank test and Cox regression were employed to analyze OS and LRFS. Chi-square test was used to compare distributions. Results A total of 609 patients with lung cancer who received a median of 64Gy were selected. According to the tumor characteristics, most of the patients the most of patients presented non-small cell lung cancer (NSCLC, 85.5%), and stage III (86.0%). Poster Discussion: Lung 1