2673 - Factors Influencing the Utilization of the Integrated Magnetic Resonance Linac for Accelerated Partial Breast Irradiation

J. Dean¹, F. Foroudi^2,3, M. Tacey¹, L. McDermott², and M. Chao^1,3; ¹Olivia Newton John Cancer Wellness and Research Centre, Austin Hospital, Melbourne, VIC, Australia, ²Olivia Newton-John Cancer Wellness & Research Centre, Melbourne, VIC, Australia, ³Monash University, Melbourne, VIC, Australia

Purpose/Objective(s): Accelerated Partial Breast Irradiation (APBI) is an accepted short course treatment option for patients with early breast cancer. The objective of this retrospective audit was to consider the factors that influence the decision to treat APBI patients on the Magnetic Resonance Linac (MRL) or the conventional C-arm linac. Secondary objectives were to compare the dose distributions and side effect profile post-treatment. Materials/

Methods: Electronic medical records and planning data were accessed for patients that were treated on either the conventional or MR Linac with APBI in 2023 under a human research ethics committee approved protocol. Data was collected for oncological history, suitability for treatment on the MRL, planned dose (for targets and Organs at Risk (OAR)) and 2 week, 3 and 6 month follow-up, Assessment was based on Common Terminology Criteria for Adverse Events (CTCAE) v5 classifications. Statistical analysis compared patient profiles, dose and toxicity rates between the two treatment groups.
Results: Over the 12-month study period, 29 patients were eligible for treatment with APBI. Ten patients were unsuitable for treatment on the MRL, with reasons including claustrophobia/anxiety (4), bilateral breast lesions (3), tumour position in relation to OAR and body habitus (2) and unable to tolerate treatment time (1). The average age was similar between groups (Conventional: 64.1 ± 7.9, MRL: 65.8 ± 12.2, p=0.70), with no noted differences in laterality, tumour size and histological features. Dose analysis demonstrated that, on average, target coverage and OAR doses were not significantly different overall. Small variations were noted for the median ipsilateral lung V10Gy (7.6% conventional vs 2.95% for MRL) and Heart V3Gy (6.9% vs 2.4%) constraints. Dose to 95% of the PTV was on average lower for MRL compared to conventional; 28.56Gy and 29.03Gy, yet D2% was slightly higher 31.39Gy (MRL) and 31.05Gy (conventional). Audited plan data confirmed that all plans in both groups met ideal constraint criteria. At 3-months follow-up, there were four recorded grade 1 toxicities in the conventional treatment group compared to two grade 1 toxicities in the MRL treatment group. There were no new toxicities reported at 3 or 6-month follow-up across either treatment group when compared to baseline (2-week follow-up).
Conclusion: Unfortunately not every patient eligible for APBI is suitable for treatment on the MRL. In this small retrospective analysis, bilateral lesions, anatomical and psychological factors meant that only 2/3 proceeded to treatment on the MRL. However, if they are eligible, the MRL was found to provide similar dose and early toxicity outcomes when compared to conventional APBI. Follow up data collection is ongoing.