Objective To compare the dosimetric characteristics of thyroid between intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) in patients with left-sided breast cancer after modified radical mastectomy.

Methods Thirty patients with left-sided breast cancer who underwent adjuvant radiotherapy after modified radical mastectomy at the Huangpu Branch of the Ninth People’s Hospital, Shanghai Jiao Tong University School of Medicine, from December 2021 to May 2023, were selected as the study subjects, and IMRT and VMAT were used with a target prescription dose of 50 Gy/25 times. The target dose parameters of the two plans, including the mean dose (D_mean) and the dose-volume limit for organs at risk (V_x, x represents dose), were compared.

Results The conformity index of VMAT was higher than that of IMRT (P＜0.001). Compared with VMAT, IMRT plan significantly reduced the V₅、V₁₀、D_mean of heart, the V₅ of right lung and the D_mean of right breast, while it significantly increased the V₂₀ of left lung (P＜0.001). There was no statistically significant difference in V₅ and V₁₀ of the left thyroid between IMRT and VMAT. However, the V₂₀, V₃₀, V₄₀, and D_mean of left and whole thyroid, and the V₁₀ and D_mean of right thyroid were significantly lower in IMRT than those in VMAT (P＜0.001).

Conclusions Both IMRT and VMAT plans can meet the clinical dosimetric requirements, among which IMRT has lower thyroid exposure dose and is more suitable for patients with high cardiac requirements.