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Large breast volume has long been considered as a risk factor of radiation-induced skin toxicities in breast cancer. Few studies have assessed its impact in hypo-fractionated radiotherapy regimens. Several definitions of large breast volume exist, the most common is based on a bra size  (>=D). However, asking about the bra size in daily practice remains embarrassing for both the doctor and the patient. Hence, the need for an easy and practical tool to estimate the bra size  and subsequently the breast volume has become eminent in radiation oncology field. The aim of the study is to assess the radiation induced skin toxicities during hypo-fractionated radiotherapy regimens in patients with breast cancer and to look for an eventual correlation of these toxicities with breast volume.

This is a retrospective single-center descriptive analytical study carried out on all patients with breast cancer treated with curative radiotherapy according to a hypo-fractionated regimen during the year 2021. We used a bra size estimator which can estimate it from the distance measured between the upper and lower limit of the breast delineated on the simulation CT scan carried out in radiotherapy.

One hundred and thirty-five patients were included. The average age was 51[26-77 y.o]. The patients were postmenopausal in 50.4% of cases (68/135) and had treatment on the linear accelerator in 60.7% of cases (82/135) and on cobalt therapy in the rest of patients. Tumor stage (II) was the most represented in 54.8% (74/135) and surgery was radical in 60.7% (82/135). The majority of cancers were “Luminal B Her negative” in 50% of cases (66/135). The radiotherapy regimens were delivered as 40 Gy in 15 fractions in 59.3% of cases (80/135) and 42.5 Gy in 16 fractions in the rest of cases. The radiotherapy boost was delivered in 44.4% of cases (60/135).

According to the bra size estimator, the sizes of the bra were distributed as follows; "AA" in 2 patients (1.5%), "A" in 30 patients (22.2%), "B" in 63 patients (46.7%), "C" in 32 patients (23.7%), "D" in 6 patients (4 .4%) and "E" in 2 patients (1.5%). Thus medium breast volume was the most represented in 70.4% (95/135) then small breasts (23.7%) and large breasts (5.9%).

Radio-induced skin toxicities were observed in 72.6% of cases (98/135). According to the RTOG classification, they were "grade I" in 81.6% of cases (80/98), "grade II" in 15.3% of cases (15/98) and "grade III" in 3.1% of cases (3/98).

The correlation between radio-induced skin toxicities and the different breast volumes was not significant.

The bra size estimator seems to be a useful and practical tool in the current practice of the radiotherapist in order to estimate the breast volume of patients with breast cancer undergoing radiotherapy. However, in the contrary to what has been demonstrated in the literature about breast volume in normo-fractionated regimens, the hypo-fractionated regimen seems to be well tolerated regardless of breast volume.