Dosimetry Comparison of Incidental Irradiation to the Low Axilla in Whole Breast Irradiation Using IMRT and IMPT Therapy

Abstract

It has been hypothesized that incidental dose to low axilla contributes to regional control in patients receiving whole breast irradiation (WBI), especially in patients with limited metastatic sentinel nodes who were spared from subsequent axillary dissection. We aimed to compare axillary dose between intensity-modulated radiotherapy (IMRT) and intensity-modulated proton radiotherapy (IMPT). Dosimetric comparisons between IMRT and IMPT were conducted in 15 patients treated with WBI without planned regional node irradiation (RNI). The prescribed dose was 40.05 Gy (RBE)/15 Fx. Axillary lymph nodes levels I, II (ALN I, ALN II) and rotter's lymph nodes (RN) were contoured and evaluated. The dose to the axilla was generally low for IMPT, with mean dose to ALN I, ALN II, and RN of 896.31, 194.64, and 2111.58 cGy (RBE), respectively. The mean V90% and V50% of ALN I was higher for IMRT (11.5 cm3, 22.9% of structure volume, p < 0.0001; 36.1 cm3, 66.6%, p < 0.0001) compared to IMPT (1.1 cm3, 2.0%; 8.1cm3, 15.9%). Similar finding was found in other low axilla. Mean dose to total ALN caudal to axillary vein was 936.6 cGy (RBE) in IMPT compared to 2407.3 cGy in IMRT (p < 0.0001). Within all substructures of axilla, mean dose to RN was the highest, but it remains lower in IMPT than in IMRT [2111.58 cGy (RBE) versus 3510.88 cGy, p < 0.0001]. WBI treated with IMPT leads to a significant reduction of axilla dose compared to IMRT. In clinical scenarios like Z0011 or likewise requiring incidental irradiation to the low axilla without additional RNI, contouring should be modified based on individual risk.