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The aim of this study was to analyse the impact of internal mammary chain (IMC) irradiation on coronary arteries and cardiac subvolumes exposure in breast cancer (BC) RT.

One-hundred and twenty-five patients planned for adjuvant breast cancer RT with indication of IMC irradiation were evaluated: 75 patients with left-sided BC and 50 patients with right-sided BC. All patients were treated with hypofractionated 3D conformal RT, by tangential fields +/- supraclavicular field, using a mono-isocentric technique. The prescription dose was 40 Gy delivered in 15 fractions +/- an additional boost of 13.35 Gy. Cardiac subvolumes were contoured based on the Duane’s cardiac contouring atlas. For each patient, DVHs for coronary arteries and cardiac chambers were generated with and without including IMC in the tangential fields. DVHs were statistically compared and the contribution of IMC irradiation to cardiac substructures mean and maximum doses was assessed.

RT was delivered to the chest wall in 58% and to the remaining breast in 42% of patients. Higher doses to the heart and cardiac subvolumes were reported in the left-sided BC RT when compared to the right-sided BC RT (p<0.01). Regardless of BC laterality, IMC RT was strongly correlated with MHD, Dmean and Dmax to coronary arteries and cardiac chambers with Pearson coefficients ranging between 0.73 and 0.86. The MHD was significantly higher with IMC irradiation with mean values of 3.83 Gy vs 2.49 Gy (p<0.01) in the left-sided BC group, and 1.56 Gy vs 0.83 Gy (p=0.02) in the right-sided BC group. IMC RT contributed to significant increase in Dmean and Dmax to all cardiac chambers and coronary arteries in both left and right-sided BC RT. The greatest contribution of IMC RT to cardiac subvolumes doses was found for the left-anterior descending artery (LAD)/left ventricle (LV) in the left-sided BC RT and for the right coronary artery (RCA) /right atrium (RA) in the right-sided BC RT. IMC RT yielded, on average, 23% increase in Dmax LAD and 29.5% in Dmean LV. For the right-sided BC RT, IMC RT increased on average the Dmax RCA by 17% and Dmean RA by 24.6%.

IMC RT was related to higher exposure of coronary arteries and cardiac chambers in both right and left-sided BC RT. The exclusive contribution of IMC RT to heart subvolumes doses was not negligible, which may translate into an excessive risk of late cardiovascular morbidity. Heart sparing techniques should be considered when IMC RT is indicated.