Objective:To investigate the dosimetric differences in volumetric-modulated arc therapy (VMAT) and intensity-modulated radiation therapy (IMRT) in patients receiving adjuvant radiotherapy and internal lymph node irradiation after left-sided modified radical mastectomy.Methods:VMAT and IMRT radiotherapy plans were established for 20 patients undergoing left-sided modified radical mastectomy. The dosimetric parameters of the target area and organs at risk were calculated by the dose volume histogram. The categorical variables were tested by χ2 or Fisher′ s exact probability test. The continuous variables with normal distribution were analyzed by paired-t test or rank-sum test. Results:Among the two radiotherapy techniques, the homogeneity index of IMRT was significantly higher than that of VMAT ( P<0.05). The time of VMAT treatment was significantly shorter than that of IMRT ( P<0.01). VMAT was superior to IMRT in V 20Gy and V 30Gy of the affected lung (both P<0.05). VMAT was superior to IMRT in the left anterior descending coronary artery D mean, D max, and heart V 30Gy, V 40Gy, D mean and D max(all P<0.01). The esophageal D mean in the VMAT group was superior to that in the IMRT group ( P<0.05). The V 5Gy and V 10Gy of the contralateral lung and the D max of the esophagus in the IMRT group were significantly better compared with those in the VMAT group (all P<0.05). Conclusions:VMAT can significantly reduce the dose of the heart, contralateral lung, spinal cord, esophagus and other vital organs, and shorten the treatment time. For patients who need adjuvant radiotherapy and internal mammary lymph node irradiation after left-sided modified radical mastectomy, VMAT technology can better protect normal tissues than IMRT.