Heart-sparing effect of postmastectomy radiotherapy for breast cancer patients: A dosimetric study of cardiac substructures.

We investigated to what extent can the dose-volumes of the coronary artery and the cardiac substructures be reduced by using IMRT technique in left-sided breast cancer patients. We chose 40 pN2M0 patients treated with postmastectomy IMRT. The original treatment plans were retrieved and the (internal mammary nodes) IMNs and cardiac substructure delineations were added. Three sets of dose-volume parameters including the original plans without internal mammary irradiation (IMNI), the plans with IMNI, and the plans with dose constraints to the heart, were derived. In left-sided patients, when IMNI was included, the V30 for right ventricle (RV), left ventricle (LV), pulmonic valve (PV), and left anterior descending artery (LADA) were 56.37% ± 7.9%, 25.3% ± 7.3%, 48.3% ± 6.3%, and 69.7% ± 6.4%, respectively. Of the 4 main coronary arteries, LADA had the highest dose followed by the left main coronary artery (LMCA). LADA had a V40 of 62% ± 9.7% vs 13.5% ± 3.5%, and a V50 of 27.5% ± 4.7% vs 0, with and without IMNI. For the right-sided patients, the V30s for all the heart substructures were 0 with or without IMNI. When we set a dose constraint of V40 < 10% for the LADA in the left-sided patients, the PTV volumes covered by 50 Gy decreased by only 1%. IMNI increased the V30 of the right and left ventricle and significantly increased the V40 and V50 to the LADA of left-sided breast cancer patients. IMRT markedly reduces the dose to the main coronary arteries and the right and left ventricle.