SAFE–IOERT prospective study: clinical outcomes of exclusive intraoperative radiotherapy in early breast cancer according to molecular subtype and tumor-infiltrating lymphocytes

Intraoperative electron radiotherapy (IOERT) can be applied during breast conserving surgery to treat invasive breast cancer. Tumor-infiltrating lymphocytes (TILs) are fundamental elements of the specific immunological response against tumor cells and have prognostic importance in many types of cancer. The aim of the study was to analyze the local recurrence rate, adverse effects, surrogate molecular subtype, and stromal percentage of TILs in patients with breast cancer treated with exclusive IOERT.

Eighty-one patients with early-stage breast carcinoma were included in a prospective study and were treated performing a conservative surgery, followed by IOERT with a dose of 20 Gy as accelerated partial breast irradiation (APBI). TILs were evaluated in hematoxylin–eosin sections of surgical specimens before IOERT.

The median follow-up of the patients was 58 months (range 15–128). Fifty-eight patients (71.6%) were classified as Luminal A-like, 21 patients (25.9%) Luminal B-like and 2 patients (2.5%) were classified as HER2 positive-like. The local recurrence rate was 2.5%. Two patients presented ipsilateral local recurrence, and one patient presented distant recurrence. Eight patients (9.9%) suffered chronic toxicity with grade 1 localized fibrosis, fat necrosis, and pain. Eighty-one percent of stromal TILs were low and 14.9% were intermediate. In 3 patients (4.1%) the stromal percentage of TILs could not be assessed.

In this prospective cohort of carefully selected patients with early breast cancer, exclusive IOERT delivered as accelerated partial breast irradiation achieved favourable local control with minimal chronic toxicity. Predominantly low stromal TIL levels in luminal-like tumours may be associated with recurrence risk. Larger prospective studies are needed for validation.