RÉSUMÉ
Objective:This study aimed to compare the dosimetric differences in unintended irradiation to the ipsilateral axillary region between intensity-modulated radiation therapy(IMRT)and intensity-modulated proton therapy(IMPT)in patients receiving whole breast irradiation(WBI). Methods:A total of 20 patients with early breast cancer who received WBI at our center between August and September 2022 were included in this study.One IMPT plan and one IMRT plan were formulated for each patient,with prescription dose of 4005 cGy(RBE)in 1 5 fractions.Dosimetric parameters of axillary lymph nodes(ALN)level Ⅰ,Ⅱ,Ⅲ,Rotter's lymph nodes(RN),and the axillary-lateral thoracic vascular junction(ALTJ)were compared between IMPT and IMRT plans. Results:All plans met the criteria of CTV V95％Dose≥95％.IMPT showed significantly better conformity index(0.97 vs 0.95,P=0.0003)and homogeneity index(0.05 vs 0.07,P=0.0301)compared to IMRT.The mean dose of the heart[27.48 vs 114.74 cGy(RBE),P＜0.0001]and ipsilateral lung[356.66 vs 498.89 cGy(RBE),P＜0.0001]were significantly lower in the IMPT plan compared to the IMRT plan.The mean dose,V50％Dose,V80％Dose,V90％Dose,and V95％Dose of ALNⅠ,ALN Ⅱ,ALN Ⅲ and RN in the IMPT plan were significantly lower than those in the IMRT plan(all P＜0.01),with the most significant difference observed in the dosimetric parameters of the axillary region inferior to the axillary vein[mean dose:79.75 vs 995.31 cGy(RBE),P＜0.0001].The mean dose and serial dosimetric parameters(V5,V10,V15,V20,V25,V30,and V35)of the ALTJ were also significantly lower in IMPT plans compared to IMRT plans. Conclusion:IMPT demonstrates lower unintended irradiation dose in the inferior axillary region and reduces dose volume in the ALTJ region compared to IMRT.When employing IMPT,the clinical target volume(CTV)should be delineated based on the individual locoregional recurrence risk for patients with positive sentinel lymph nodes who omitted axillary lymph node dissection.For high-risk patients,the axillary region should be included in the CTV to ensure efficacy,while for low-risk patients,excluding the axillary region can help mitigate the risk of breast cancer-related lymphedema.