Accelerated Partial Breast Irradiation (APBI) For Ductal Carcinoma In Situ.

[PURPOSE/OBJECTIVES] Suitability criteria for partial breast irradiation (PBI) are narrowly constrained for those with ductal carcinoma in situ (DCIS). In comparison to invasive disease, guidance is limited regarding the optimal application of PBI to treat DCIS. Here, we report disease outcomes for a heterogeneous cohort of patients with DCIS who received PBI.

[MATERIALS/METHODS] Using a prospectively maintained institutional database, we identified patients with DCIS who underwent lumpectomy and adjuvant PBI from 2008 to 2022. Based on clinicopathologic characteristics patients were classified as suitable, cautionary, or unsuitable for PBI by American Society for Radiation Oncology (ASTRO) criteria. The primary endpoint was local recurrence (LR).

[RESULTS] The cohort comprised 176 patients with DCIS who received PBI, median age 60 years (interquartile range (IQR) 52, 66). Median DCIS size was 9 mm (IQR 4, 15 mm), and approximately 20% had multifocal disease. Most patients had negative (≥ 2 mm) final surgical margins (n = 160, 91%), 10 had < 2 mm margins and 6 had ≤ 1 mm margins for DCIS. 18 (10%) patients had nuclear grade 1 disease, 111 (63%) had grade 2 disease, and 33 (19%) had grade 3 disease, with a small subset classified as grade 1-2 or 2-3. Most patients (n = 161, 91%) had estrogen receptor (ER) positive DCIS, and 72 (41%) received endocrine therapy. By ASTRO criteria, 118 (67%) patients were suitable for PBI, 57 (32%) were considered cautionary, and 1 (0.6%) was unsuitable. At a median 24 months of follow up (range 2-127 months) we observed a single LR, yielding a 2-year LR rate of 0.6%. The LR occurred in a patient classified as cautionary, yielding a 2-year LR rate of 1.8% for the cautionary subset. No breast cancer mortality events were observed.

[CONCLUSIONS] These early-term results exhibit excellent local control for patients with DCIS who received lumpectomy and PBI, 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 PBI for patients with DCIS may warrant further investigation.