Abstract： Objective:To explore the dosimetry difference between intensity-modulated radiotherapy (IMRT) and volumetric intensity modulated arc therapy (VMAT) based on the inner edge tangent field (IETF) after left-sided breast conserving surgery.Methods:The localization CT and target organ at risk (OAR) data of 35 patients with left-sided breast cancer treated with IMRT after breast conserving surgery at Department of Radiotherapy in Yunnan Cancer Hospital from June 2022 to June 2023 were selected. The IETF-IMRT and the IETF-VMAT plans were designed for the same patient based on IETF, the dosimetry differences of target areas and OAR, as well as the planned execution time were compared between the two groups.Results:Dosimetry of target areas: for IETF-IMRT and IETF-VMAT, the D 98% of the planning target volume were (47.92±0.51) and (48.21±0.33) Gy, respectively, while the D 50% were (52.04±0.22) and (51.91±0.26) Gy, respectively, and the D 2% were (53.93±0.36) and (53.62±0.41) Gy, respectively, the conformity index were 0.84±0.03 and 0.87±0.02, respectively, while the homogeneity index were 0.12±0.01 and 0.10±0.01, respectively, with statistically significant differences ( t=-3.87, P<0.001; t=3.53, P=0.001; t=5.30, P<0.001; t=-13.60, P<0.001; t=6.24, P<0.001). Dosimetry of OAR: for IETF-IMRT and IETF-VMAT, the left lung V 5 were (31.91±6.28) % and (33.99±6.31) %, respectively, and the V 20 were (11.71±2.06) % and (9.73±2.12) %, respectively, with statistically significant differences ( t=-4.18, P<0.001; t=12.40, P<0.001). The right lung V 5 were (0.11±0.08) % and (7.13±3.12) %, respectively, and the D mean were (1.05±0.12) and (2.71±0.27) Gy, respectively, with statistically significant differences ( t=-33.62, P<0.001; t=-13.30, P<0.001). The spinal cord D 2% were (1.08±0.11) and (4.83±1.40) Gy, respectively, with a statistically significant difference ( t=-15.99, P<0.001). The left lung D mean were (7.45±1.08) and (7.37±1.03) Gy, the heart D mean were (4.21±0.96) and (4.41±0.48) Gy, and the right-sided breast D mean were (3.74±1.52) and (3.48±1.11) Gy, respectively, with no statistically significant difference ( t=1.16, P=0.253; t=-1.76, P=0.088; t=1.41, P=0.169). Planned execution time: the execution time of IETF-IMRT and IETF-VMAT was (10.73±1.21) and (2.18±0.17) min, respectively, with a statistically significant difference ( t=44.71, P<0.001) . Conclusion:Both IETF-IMRT and IETF-VMAT can meet clinical requirements, however the two techniques have their own characteristics. IETF-VMAT has better conformity and homogeneity of target region. The planned OAR dosimetry in both plans are significantly lower than the dose limit of postoperative radiotherapy for breast cancer, among which the left lung V 5, the right lung V 5, D mean and spinal cord D 2% of IETF-IMRT are slightly lower, the left lung V 20 of IETF-VMAT is slightly lower. IETF-VMAT significantly reduces the planned execution time compared with IETF-IMRT, thus can greatly reduce the dose deviation caused by patient position change, and significantly improve patients experience and comfort of radiotherapy. Taken together, IETF-VMAT has advantages over IETF-IMRT in radiotherapy after breast conserving surgery of left-sided breast cancer.