Photons without bolus versus electrons with bolus after upfront mastectomy without immediate reconstruction in breast cancer patients

Abstract Purpose To evaluate locoregional control and outcome after mastectomy in patients treated by post-mastectomy highly conformal electron radiotherapy (PMERT) with bolus compared to patients treated by highly conformal photon radiotherapy (PMPhRT) without bolus in the adjuvant setting of non-metastatic breast cancer (BC). Methods and Materials We studied women undergoing PMRT without immediate reconstruction for BC before 2012 in 2 sites of our hospital using 2 different techniques. All patients received 50 Gy in 25 fractions. Patients previously treated by neoadjuvant chemotherapy were excluded. Results Among the 807 patients, 583 received PMERT and 224 received PMPhRT. The median follow-up was 64 months. Patients in the PMERT group had a median age of 52.7 years (26-91), 6.9% were triple-negative (TN), 16.3% were HER2-positive, and 58.6% had multifocal lesions. Patients in the PMPhRT group had a median age of 56.4 years (28-89), 8.5% were TN, 12.9% were HER2-positive, and 55.8% had multifocal lesions. Lymph node involvement was observed in 66% and 72.8% of cases (p=0.07) treated by PMERT and PMPhRT, respectively. No significant difference in overall survival was observed between the 2 groups: HR = 1.2 [0.67; 2.13] (p=0.54). The risk of locoregional recurrence, estimated using the Fine-Gray method, was significantly higher with PMPhRT than with PMERT: SHR=3.62 [1.07; 12.3] (p=0.04), corresponding to a 5-year LRR rate of 0.53% [0- 1.12%] for PMERT and 2.52% [0.05; 4.6%] for PMPhRT. Conclusions A higher risk of local recurrence was observed in the PMPhRT without bolus group compared to PMERT with bolus group. Prospective randomized trials are needed to confirm these findings.