Does Breast Surgery Type Alter Incidental Axillary Irradiation? A Dosimetric Analysis of the "Sentinel Envahi et Randomisation du Curage" SERC Trial

Simple Summary Currently, breast surgery type directly affects the management of the axillary fossa in patients with breast cancer. Trials that validated the omission of completion axillary dissections after a positive sentinel lymph node biopsy had excluded patients treated with a total mastectomy. Radiotherapy and its incidental axillary dose seem to be a major element of local axillary control in this setting, but these are only described in the literature after conservative surgeries. Within a sub-study of an SERC trial, which included patients treated with a total mastectomy, we demonstrated that the axillary dose delivered via radiotherapy is of the same level as that already known after conservative surgery. This element could contribute to the data allowing us to dispense with completion axillary lymph node dissections in the case of a positive sentinel lymph node biopsy after a total mastectomy. Background. An incidental axillary dose of adjuvant radiotherapy using tangential beams is usually given after breast-conserving surgery for breast cancer. The goal of this sub-study was to evaluate this incidental dose in the setting of post-mastectomy radiotherapy (PMRT) according to two different radiotherapy techniques. Methods. Patients participating in a randomized SERC trial who received PMRT in a single center were included. We collected the incidental axillary dose delivered to the Berg level 1 using different dosimetric parameters and compared two techniques using Student's t-test: three-dimensional conformal radiotherapy (3D-CRT) and volumetric arc therapy (VMAT). Results. We analyzed radiotherapy plans from 52 patients who received PMRT from 2012 to 2021. The mean dose delivered to the Berg level 1 was 37.2 Gy. It was significantly higher with VMAT than with 3D-CRT-43.6 Gy (SD = 3.1 Gy) versus 34.8 Gy (SD = 8.6 Gy) p < 0.001. Eighty-four percent of the Berg level 1 was covered by 40 Gy isodose in the VMAT group versus 55.5% in the 3D-CRT group p < 0.001. Conclusions. On the Berg level 1, PMRT gives a dose at least equivalent to the one given by post-breast-conserving surgery radiotherapy, making it possible to limit completion axillary lymph node dissections in select pN1a patients treated with a mastectomy. Modern radiotherapy techniques like VMAT tend to increase this incidental dose.