Abstract PS1-07-12: Personalizing Radiotherapy for Left-Sided Breast Cancer: Identifying Candidates for Proton Beam Therapy Based on Cardiac Risk

Abstract Introduction Young patients with left-sided breast cancer face a higher long-term risk of major cardiac events due to incidental cardiac radiation exposure and cardio-toxic systemic therapy agents. Photon-based radiotherapy delivering a mean heart dose (MHD) of <font face=“Symbol”>Ź</font>3 Gy is associated with at least a 2% increase in the risk of major cardiac events (Kirby et al., 2023). This study aimed to stratify patients based on their risk and identify those most likely to benefit from proton beam therapy (PBT), a modality recently introduced at our tertiary care cancer center. Objective To identify patients with left-sided breast cancer who are most likely to benefit from proton beam therapy based on predicted cardiac exposure. Methodology We reviewed records of 737 patients with stage I to III or oligometastatic breast cancer treated with adjuvant radiotherapy at our center in 2023. A dosimetric audit was performed for all left-sided cases. Radiotherapy was delivered using IMRT or field-in-field 3DCRT, with or without DIBH. Fractionation regimens included 40 Gy in 15 fractions, 26 Gy in 5 fractions, and 28.5 Gy in 5 weekly fractions with boost when indicated. Patients were categorized according to the UK PARABLE trial criteria, which stratify cardiac dose thresholds based on age and the presence of risk factors. Statistical analysis was performed using IBM SPSS Version 28.0. Associations between groups were assessed using the Chi-square test, with a significance threshold of p < 0.05. Results The data for 444 patients was analyzed; 69.3% were under the age of 55. Cardiac comorbidities (22.3%) and diabetes (19.8%) were the most prevalent risk factors, while COPD (1.6%) and smoking (5.6%) were less common. Most patients (84.5%) had a BMI below 30.Fourteen patients (3.1%) did not achieve the age- and risk-adjusted desired MHD using photon techniques and could potentially benefit from proton therapy. Among those with risk factors, 6.5% did not meet their MHD thresholds. In contrast, only 1.1% of patients without risk factors failed to achieve the desired MHD. (Table 1) Younger patients (below 4 years) were disproportionately represented among those not achieving target MHD, particularly those with comorbidities. Conclusion Proton therapy may offer significant benefit to a subset of left-sided breast cancer patients at high cardiac risk, particularly younger individuals with comorbidities or unfavorable anatomy who are unable to achieve guideline-recommended mean heart dose thresholds with photon techniques. Risk-adapted selection for PBT is a promising strategy to optimize cardiac sparing in this population.As PBT is a limited and high-cost resource, such data-driven selection is essential for equitable and cost-effective use, ensuring access for those most likely to benefit. Citation Format: R. Sangle, R. Krishnamurthy, K. Bhandarwar, P. Sharma, N. Sharma, S. Nair, G. Yadav, T. Wadasadawala, R. Pathak, S. Kale, P. Sahoo, S. Gadge, R. Sarin. Personalizing Radiotherapy for Left-Sided Breast Cancer: Identifying Candidates for Proton Beam Therapy Based on Cardiac Risk [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS1-07-12.