Accelerated partial breast irradiation (APBI) for ductal carcinoma in situ.

Abstract

e12516 Background: Landmark studies of accelerated partial breast irradiation (APBI) included patients with DCIS, but with limited guidance regarding subgroups of in situ disease. Suitability criteria defined by national guidelines for APBI are narrowly constrained among those with DCIS, and controversy persists regarding optimal patient selection. Here, we report disease outcomes for a heterogeneous cohort of patients with DCIS who received APBI. Methods: Using a prospectively maintained institutional database, we identified patients with DCIS who underwent lumpectomy and adjuvant APBI from 2000 to 2022. Clinicopathologic characteristics were collected, and patients were classified as suitable, cautionary, or unsuitable for APBI by the latest American Society for Radiation Oncology (ASTRO) criteria. Outcomes of interest included local recurrence (LR), overall survival (OS), and follow up time. Cumulative incidence functions were calculated to estimate the incidence of LR over time. All statistical analyses were performed in R version 4.2.2. Results: The cohort comprised 176 patients, median age 60 years (IQR 52, 66). Median DCIS size was 9 mm (IQR 4, 15 mm), and approximately 20% had multifocal disease. Most patients had negative final surgical margins (160, 91%), with 10 patients having ≤2 mm margins and 6 having ≤1 mm margins for DCIS. 18 (10%) patients had nuclear grade I disease, 111 (63%) had grade II disease, and 33 (19%) had grade III disease, with a small subset classified as grade I-II or II-III. Most patients (161, 91%) had ER+ DCIS, and 72 (41%) received endocrine therapy. By ASTRO criteria, 118 (67%) patients were suitable for APBI, 57 (32%) cautionary, and 1 (0.6%) was unsuitable. At a median 24 months of follow up (range 2 to 127 months) we observed only one LR, yielding a 2-year LR rate of 0.6%. The LR occurred in the cautionary group, yielding a 2-year LR rate of 1.8% for the cautionary subset. The patient with LR was 56 years old, with 24 mm of high grade DCIS, ER+, with lumpectomy to negative margins, and did not receive endocrine therapy. No breast cancer mortality events were observed. Conclusions: Our early-term results exhibit excellent local control for patients with DCIS who received lumpectomy and APBI, even among those classified as “cautionary” by national guidelines. Although our findings are limited by short follow up, these results suggest that broadening the application of APBI for patients with DCIS may warrant further investigation.