Long Term Outcomes Following Adjuvant Accelerated Partial Breast Radiotherapy for Early-Stage Breast Cancer: Are Current Guidelines for Appropriateness Appropriate?

Purpose: Accelerated Partial Breast Irradiation (ABPI) is increasingly being accepted as an alternative to whole breast radiotherapy. However, limited long-term outcome data and variable definitions of ABPI have made widespread adoption of available guidelines challenging. We aim to report long-term follow-up for localized breast cancer treated with surgery followed by adjuvant APBI with high dose rate (HDR) brachytherapy and compare current ABPI appropriateness criteria. We hypothesize that even with long-term follow-up, APBI will result in low recurrence rates irrespective of appropriateness criteria.

Methodology: A retrospective review of patients with breast cancer treated with surgery followed by adjuvant single lumen HDR brachytherapy was conducted between 2002-2007 under an approved IRB. Oncological outcomes including overall survival (OS), breast cancer specific survival (BCSS), ipsilateral breast tumor events (IBTEs), contralateral breast tumor events (CBTEs), regional control (RC), and distant control (DC) were calculated using the Kaplan-Meier method with differences between groups compared using log-rank t-test. We evaluated IBTEs based on appropriateness criteria from existing guidelines including the American Society of Therapeutic Radiation Oncology-2014 & 2017 (ASTRO-14, ASTRO-17), American brachytherapy Society-2022 (ABS-22), Groupe Européen deCuriethérapie-European Society for Therapeutic Radiology and Oncology-2009 (GEC-ESTRO-09), and American Society of Breast Surgeons-2018 (ASBrS-18).

Results: We identified 149 patients with a median follow-up of 15 years (IQR: 8-18). Median age at diagnosis was 62 years (IQR: 55-73), 96% white race, 86% T1, 90% N0 disease, 68% received hormonal therapy alone and 12% received chemotherapy alone. Suitable cases for ABPI per guidelines were 56 (37%) ASTRO-14, 60 (40%) ASTRO-17, 73 (49%) ABS-22, 53 (35%) GEC-ESTRO-09, and 87 (58%) ASBrS-18. A total of 11 IBTEs (7.4%) with 9 invasive and 2 in-situ recurrences, 6 in-field and 5 out-of-field events were observed. The 10-, 15-, 20-years IBTEs were 4% (C.I 1%-8%), 8% (C.I 2%-13%), and 14% (C.I 5%-23%) respectively. The 10-year CBTEs was 2% (C.I 0%-4%), while similar CBTEs at 15-, 20-years were observed 9% (C.I 3%-15%). The 10-, 15-, 20-years OS were 83% (95% C.I 77%-90%), 76% (95% C.I 68%-83%), 70% (95% C.I 62%-79%) respectively. The 10-, 15-years BCSS were 99% (95% C.I 98%-100%), 20-years BCSS was 98% (95% C.I 95%-100%). The 10-, 15-, 20-years RC were similar 98% (95% C.I 95%-100%). The 10-, 15-years DC were similar 98% (95% C.I 95%-100%), while 20-years DC was 94% (95% C.I 86%-100%). No statistically significant difference was observed in IBTEs across different guidelines for ABPI appropriateness criteria per ASTRO-14 (p=0.382), ASTRO-17 (p=0.78), ABS-22 (p=0.13), GEC-ESTRO-09 (p=0.84), and ASBrS-18 (p=0.6).

Conclusions: With similar IBTEs and CBTEs rates at 15-years following APBI, these results confirm low rates of IBTEs following adjuvant ABPI at long-term follow-up which transcend current consensus criteria for appropriateness. These data suggests that more patients may be appropriate candidates for APBI than recommended by current guidelines.

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