Objective  To analyze thyroid radiation dose of postoperative static intensity modulated radiation therapy (sIMRT) and volumetric intensity-modulated arc therapy(VMAT) for left breast cancer after left breast cancer surgery.

  Methods  A total of 68 patients with radiotherapy after radical mastectomy of left breast cancer were retrospectively selected. The sIMRT and VMAT plans were designed using Pinnacle³ 10.0 planning system, the dosages of organs at risk were compared between the two groups under the premise that the tumor target was normalized to the same dose.

  Results  The percentage of volumes of the left lung exposed to radiation dose of 5 Gy(V₅), 20 Gy(V₂₀), percentage of volume of the heart exposed to radiation dose of 30 Gy(V₃₀) of the heart, and mean radiation dose (D_mean) by VMAT were lower than those of the sIMRT(P < 0.05). V₅ of left thyroid, exposed to radiation dose of 10 Gy(V₁₀) and V₂₀ of both plans basically reached 100.00%, but there were no statistical differences (P > 0.05), while V₃₀, percentage of volume of left thyroid exposed to radiation dose of 40 Gy (V₄₀), exposed to radiation dose of 50 Gy(V₅₀) and D_mean of VMAT were significantly lower than sIMRT (P < 0.05); V₂₀ of the right thyroid and D_mean of VMAT were significantly lower than those of sIMRT (P < 0.05).

  Conclusion  Both of the two radiotherapy techniques can meet the clinical requirements. VMAT is superior to IMRT in thyroid protection and the radiation dose of the left thyroid is larger than that of the right thyroid. Further study is needed on how to minimize the thyroid exposure dose.