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Purpose To compare the dosimetric differences between intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) in the treatment of male and female thymoma. Material and methods This single-institutional analysis included 20 patients with thymoma treated with RT between January 2017 and December 2020. Twenty patients were retrospectively planned for IMRT (IMRT1 with an average field angle of 216°, 288°, 0°, 72°,and 144°; IMRT2 with fan-shaped field angles of 280°, 320°, 0°, 40°, and 80°) and VMAT (VMAT1 with two arcs ranging form 280° to 80°,clockwise and then counterclockwise; VMAT2 with two 360° arcs). The plans for all investigated RT modalities were optimized for a prescriptional dose of 50 Gy and fractional dose of 2.0 Gy. Planning target volume (PTV) and organs-at-risk (OARs: heart, breasts, lungs, spinal cord, and esophagus) dosimetric parameters were compared. Results All plans met the preparation aims for all the included metrics. There was little difference in the median values of PTV parameters (D2%, D98%, Dmean, homogeneity index[HI], and conformity index [CI]). The CI of the VMAT2 plan was the closest to 1 in both therapy groups. The monitor unit (MU) of IMRT2 and the estimated total delivery time of VMAT1 were the lowest in both therapy groups and were statistically significant. In the male group, the lung parameters (Dmean, V5Gy, V10Gy, and V20Gy) for VMAT1 were the lowest and showed statistical significance. In the female group, the lung parameters (Dmean, V5Gy, V10Gy, and V20Gy) and bilateral breast parameters (Dmean, V5Gy, V10Gy, and V20Gy) of IMRT2 were the lowest and showed statistical significance. Conclusions In male thymoma patients undergoing postoperative RT (PORT) treatment, the choice of fan-shaped VMAT may be a better option for protecting the lungs. For female thymoma patients receiving PORT, fan-shaped IMRT can better protect the lungs and breasts. The fan-shaped field performed better than the average and the full arc fields in PORT for thymoma.