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Hypofractionated radiotherapy (HRT) is becoming a valid option in the treatment of breast cancer. However, data are missing concerning the normal tissues dosimetric constraints. The purpose of this study is to evaluate the cumulative doses in the brachial plexus during hypofractionated locoregional irradiation for breast cancer with a clinical toxicities correlation in order to propose dosimetric constraints adapted to this type of fractionation.   

This is a retrospective study analyzing the dosimetric plans of 41 patients treated with adjuvant locoregional radiotherapy for localized breast cancer between January and December 2020. The treatment plan was carried out according to a three dimensional (3D) conformational technique. The dose prescribed was 42.5 Gy in 16 fractions with a boost of 10 Gy in 4 fractions in the tumor bed in case of a conservative treatment (35%) and 40 Gy in 15 fractions in case of a radical treatment (65%) at a rate of 5 fractions per week. The delineation of the brachial plexus was performed according to the RTOG guidelines and approved by two radiotherapy physicians. By analyzing the dose-volume histogram, we retrospectively recorded the mean dose (Dmean), the maximum dose (Dmax), the volume receiving more than: 32 Gy (V32) and  41 Gy (V41).These dosimetric constraint levels were defined by calculating the biological equivalent dose. Clinical assessment of radiation-induced brachial plexitis was performed at 12 months after the end of radiotherapy using a questionnaire based on the modified LENTA-SOMA clinical scales. Data analysis was performed by SPSS version 20.

The median of the Dmean, Dmax, V32 and V41 were  respectively of 31Gy [14-41], 41 Gy [4.5-41], 73% [0-91] and 0.08% [0-80]. The 3^rd quartile (Q3) of Dmean, Dmax, V32 and V41 were respectively of 34 Gy, 43 Gy, 80% and 23%.

The following table summarizes the data of the cumulative doses in the brachial plexus: 

At twelve months after the end of the radiotherapy, fourteen patients (34.14%) reported a radiation-induced brachial plexitis (grade 2 in 13 cases and grade 3 in only one case). For all these patients, the Dmax was > 40 Gy and the Dmean > 30 Gy.

The radiation-induced brachial plexitis is one of the most underestimated complications in the locoregional treatment of localized breast cancers. In the absence of radiation-induced plexitis prevention guidelines, we propose a systematic delineation of the brachial plexus in case of locoregional breast cancer irradiation in order to minimize as much as possible the cumulative doses at its level. The results of this study show that these constraints could be reasonable to respect during dosimetric optimization: Dmoy £30 Gy, Dmax £40 Gy and V41 £23%. However, these results should to be confirmed by larger series.