Long-term efficacy and patterns of failure after accelerated partial breast irradiation: a molecular assay-based clonality evaluation

Purpose: To determine the long-term efficacy and cosmetic results of accelerated partial breast irradiation (APBI) by reviewing our institution's experience. Methods and Materials: A total of 199 patients with early-stage breast cancer were treated prospectively with adjuvant APBI after lumpectomy using interstitial brachytherapy. All patients had negative margins, 82% had Stage I disease, median tumor size was 1.1 cm, and 12% had positive lymph nodes. The median follow-up for surviving patients was 8.6 years. Fifty-three patients (27%) have been followed for ≥10 years. Results: Six ipsilateral breast tumor recurrences (IBTRs) were observed, for a 5-year and 10-year actuarial rate of 1.6% and 3.8%, respectively. A total of three regional nodal failures were observed, for a 10-year actuarial rate of 1.6%. Five contralateral breast cancers developed, for a 5- and 10-year actuarial rate of 2.2% and 5.2%, respectively. The type of IBTR (clonally related vs. clonally distinct) was analyzed using a polymerase chain reaction–based loss of heterozygosity assay. Eighty-three percent of IBTRs ( n = 5) were classified as clonally related. Multiple clinical, pathologic, and treatment-related factors were analyzed for an association with the development of an IBTR, regional nodal failure, or contralateral breast cancer. On multivariate analysis, no variable was associated with any of these events. Cosmetic results were rated as excellent/good in 99% of patients. Conclusions: Long-term results with APBI using interstitial brachytherapy continue to demonstrate excellent long-term local and regional control rates and cosmetic results. According to a polymerase chain reaction–based loss of heterozygosity assay, 83% of recurrences were classified as clonally related.