2818 - Heart and Cardiac Substructure Sparing Intensity-Modulated Proton Therapy for Non-Metastatic Breast Cancer: Real-World Prospective Registry Outcomes from India

Purpose/Objective(s): To report real-world clinical outcomes, toxicity profile, and cardiac dosimetry in non-metastatic breast cancer patients treated with intensity-modulated proton therapy (IMPT).

Materials/Methods: Consecutive patients treated with adjuvant IMPT were prospectively enrolled in an institutional registry, HYPRO-B, at India’s first proton therapy program. Clinical outcomes, cardiac dosimetry (mean heart, LAD, RCA doses and LV-V5), fractionation schedules, and CTCAE-graded acute and early late toxicities were captured prospectively.

Results: The 71-patient cohort represents patients with at least 6 months follow-up and real-world clinical indications. Patients with left-sided disease comprised 75%, right-sided 23%, and bilateral, 3%. 45% of patients received post mastectomy IMPT and the remaining, after breast-conserving surgery (BCS). Nearly 95% of the latter received simultaneous integrated tumour bed boost and 11% of the cohort received an internal mammary node (IMN) boost. Regional nodal irradiation (RNI) was delivered in 73%, the IMNs treated in all. Most patients received moderate hypofractionation (MH), 40Gy/15fractions, 91.5%, 5.6%, conventional fractionation, and 2.8% accelerated partial breast irradiation. Despite the high usage of RNI, the median mean heart dose ( MHD) for patients undergoing MH was 0.74 Gy (IQR,- 0.39-0.93Gy) and the cardiac exposure low, Table 1.

Conclusion: In this prospective real-world registry, IMPT achieved excellent early disease control with sub-Gy mean heart dose, acceptable toxicity and excellent early outcomes in real-world breast cancer practice, even in patients receiving comprehensive nodal irradiation. These findings support IMPT as a cardioprotective treatment approach for diverse breast cancer populations. A longer follow-up is needed to assess long-term cardiac outcomes.