Session Item

To evaluate the ipsilateral breast tumour recurrence (IBTR) rate after two different accelerated partial breast irradiation (APBI) techniques (intraoperative electron radiotherapy, IORT and external-beam APBI, EB-APBI) in patients ≥60 years with early stage breast cancer.

Between 2011 and 2016, women ≥60 years with breast carcinoma or DCIS of ≤30mm and cN0 were included before breast conserving surgery in a two-armed prospective multi-center cohort study. After lumpectomy, IORT (1x23.3Gy) was provided in one hospital and EB-APBI (10x3.85Gy daily) in 2 other hospitals. Primary endpoint was IBTR (all recurrences in the ipsilateral breast irrespective of localization) as a first event at 5 years after lumpectomy. Secondary endpoints were locoregional recurrence(LRR), distant recurrence(DR), disease-specific survival and overall survival. A competing risk model was used to estimate the cumulative incidences of IBTR, LRR and DR which were compared using( Fine and Gray’s test). Kaplan-meier estimates were used for the other endpoints. Univariate Cox-regression models were estimated to identify risk factors for IBTR. Analyses were performed on the intention to treat (ITT) population (IORT n=305; EB-APBI n=295), and sensitivity analyses were done on the per-protocol (PP) population (n=270; n=207 respectively). A p-value of ≥0.05 was deemed significant.

Median follow up was 5.2 years (IORT) and 5.0 years (EB-APBI). The cumulative incidence of IBTR in the ITT-population at 5 years after lumpectomy was 10.6% (95% confidence interval (CI) 7.0-14.2%) after IORT and 3.7% (95%CI 1.2-5.9%) after EB-APBI (p=0.002) (fig 1). LRR was also significantly higher after IORT than EB-APBI (11.6% vs 3.6%,  p=0.001). There were no differences between groups in other endpoints. Sensitivity analysis showed similar results. For both groups, no significant risk factors for IBTR were identified in the ITT population. In the PP population tumour-free surgical margin of ≤2 mm was the only significant risk factor for developing IBTR in both treatment groups. In the PP-population of the IORT group, 40% (12/30) of IBTR were in-field recurrences,  33% (10/30) were new ipsilateral breast tumours and 27% (8/30) were recurrences in the biopsy tract. In the EB-APBI group, all evaluable IBTR’s were new ipsilateral breast tumours.  

Ipsilateral breast tumour recurrences and locoregional recurrence rates were unexpectedly high in patients treated with IORT, and acceptable in patients treated with EB-APBI. Possible explanations for the difference in IBTR are selection bias, a more concise radiotherapy target volume with IORT leading to the occurrence of recurrences in the biopsy tract, and surgical margins.