Assessment of intensity modulated radiation therapy in left breast cancer including regional nodes without the internal mammary node: secondary cancer risks on thyroid and stomach

Background: There is no clear guideline regarding the optimum intensity modulated radiation therapy (IMRT) technique for patients with breast cancer (BC) requiring radiotherapy (RT) treatment of the regional node area but not of the internal mammary node (IMN). We evaluated the IMRT technique with a focus on secondary cancers of stomach and thyroid. Materials and Methods: Eight patients with left BC treated with RT after breast conserving surgery at a single institution in 2017 were enrolled. Three-dimensional conformal radiation therapy (3D-CRT) consisting of two opposed half tangential breast fields and IMRT plans was performed. Normal organ dosimetric parameters were compared. Excess absolute risks, excess relative risks, and lifetime attributable risks (LAR) were calculated. Results: Stomach V30 values were 10.27 and 1.31 for tangential 3D-CRT and IMRT, respectively, and corresponding V40 values were 7.46 and 0.2, whereas V5 values were 21.15 and 49.62, respectively. Thyroid values were similar; V30 26.53 and 7.93, V40 22.37 and 2.63, and V5 40.93 and 88.86, respectively. LAR values of stomach were 1.76 (per 100 persons) and 2.31 and for thyroid were 5.3 and 9.5, respectively. LAR values of contralateral breasts were 0.35 and 0.99, of ipsilateral lungs were 1.68 and 2.39, and of contralateral lungs were 0.58 and 1.73. All values weresignificantly different (p<0.05). Conclusion: LAR values of stomach and thyroid were higher for IMRT than 3D-CRT in left BC patients requiring regional node treatment without including IMN. Consensus on the priority among disease control rate, secondary cancer risk, and toxicity is required.