Comparison of incidental irradiation dose to the internal mammary nodes among I-IMRT, F-IMRT, and 3DCRT after breast-conserving surgery

Objective To explore the dosimetric variance in incidental irradiation to the internal mammary nodes among inverse intensity-modulated radiotherapy (I-IMRT), forward intensity-modulated radiotherapy (F-IMRT), and three-dimensional conformal radiotherapy (3DCRT) after breast-conserving surgery, and to provide a basis for deciding whether to spare the internal mammary nodes in clinical treatment. Methods A total of 84 patients undergoing breast-conserving surgery were enrolled as subjects. The internal mammary nodes in the first three intercostal spaces were contoured. Three radiotherapy plans were designed for each patient. The internal mammary nodes were not included in the planning target volume. Comparison was made among the three plans. The results were compared using Wilcoxon signed rank test. Results The I-IMRT, F-IMRT, and 3DCRT plans had similar median Dmean values for the internal mammary nodes, which were 2740.2, 2973.9, and 2951.4 cGy, respectively. The analyses of the three individual intercostal spaces showed that there was no difference in Dmean for the first intercostal space or the second intercostal space between the three plans; For the third intercostal space, however, I-IMRT had a significantly higher Dmean than 3DCRT and F-IMRT. The analyses of the three individual plans showed that for each plan, the Dmean was the highest in the third intercostal space, followed by the second intercostal space and the first intercostal space. Conclusions All the three plans fail to attain an adequate prescribed dose to cure subclinical disease, and there is no significant difference among the three plans. Therefore, it is risky to exclude the internal mammary nodes using any one of the three radiotherapy techniques for patients with clinical indications for internal mammary nodes radiation. In the combination therapy including chemotherapy, endocrine therapy, and targeted therapy, however, further follow-up is needed to determine whether the incidental irradiation dose to the internal mammary nodes could meet clinical requirement. Key words: Breast-conserving postoperative radiotherapy; Internal mammary lymph node region; Incidental irradiated dose; Treatment planning comparison