Effectiveness and toxicity of five-fraction prone accelerated partial breast irradiation

Purpose Our institution was an early adopter of 5-fraction accelerated partial breast irradiation (ABPI) to treat women with early-stage breast cancer. This study reports long-term oncologic and cosmetic outcomes. Methods We included patients receiving APBI 600 cGy × 5 fx delivered every other day or every day between 2010 and 2022. Logistic regression models were used to identify factors associated with development of late toxicities, clinician, and patient-rated cosmesis. Kaplan–Meier methodology was used to calculate overall survival (OS), disease-free survival (DFS), and locoregional recurrence-free survival (LR-RFS). Results 442 patients received APBI either daily (56%) or every other day (44%) in the prone position (92%). At a median follow-up of 48 months (range: 5.96–155 months), 12 (2.7%) patients developed a local recurrence (LR). Out of 258 patients with > 3-month toxicity data available, the most common late grade ≥ 2 adverse event was breast fibrosis (6.2%). On multivariate analysis, daily APBI treatment (vs every other day) did not correlate with an increased risk of any late grade ≥ 2 toxicity though it did correlate with a lower risk of any late grade ≥ 2 fibrosis. Overall, at a median follow-up of 80 months, the rates of good–excellent physician and patient-rated cosmesis were 95% and 85%, respectively, with no difference between patients treated on consecutive vs. every other day. On multivariate analysis, patients who did not receive any adjuvant therapy were at increased risk of developing a LR. Five-year OS, LRFS, and DFS were 97.2%, 97.7%, and 89.5%, respectively. Conclusions Five-fraction APBI delivered primarily in the prone position either daily or every other day was effective with low rates of local recurrence, minimal toxicity, and excellent cosmesis at long-term follow-up.