['Sparing of normal lung is best achieved in prone whole breast irradiation (WBI). However, exposure of the heart and coronary arteries might increase due to anterior movement of the heart in prone WBI.', 'Treatment plans of 46\xa0patients with large breasts irradiated for mammary cancer after breast-conserving surgery were retrospectively analyzed. The average treated breast volume of right-sided breasts (n\u2009=\u200933) was 1,804\xa0ccm and 1,500\xa0ccm for left-sided breasts (n\u2009=\u200913). The majority had invasive cancer (96\u2009%) of which 61\u2009% were pT1 and 39\u2009% pT2 tumors. All patients received radiation therapy to the breast only. For three-dimensional (3D) treatment planning, all patients underwent a noncontrast-enhanced CT in the supine position with a wingboard and a second CT in the prone position using a prone breastboard. Nontarget volumes of the lung, heart, and coronary arteries were contoured. A total dose of 50.4\xa0Gy was prescribed to the breast only. Differences were calculated for each patient and compared using the Wilcoxon signed-rank test.', 'Treatment of left-sided breasts resulted in similar average mean heart doses in prone versus supine WBI (4.16 vs. 4.01\xa0Gy; p\u2009=\u20090.70). The left anterior descending artery (LAD) had significantly higher dose exposure in left versus right WBI independent of position. Prone WBI always resulted in significantly higher exposures of the right circumflex artery (RCA) and LAD as compared to supine WBI. In left WBI, the mean LADprone was 33.5\xa0Gy vs. LADsupine of 25.6\xa0Gy (p\u2009=\u20090.0051). The V20prone of the LAD was 73.6\u2009% vs. V20supine 50.4\u2009% (p\u2009=\u20090.0006).'] Is incidental dose to coronary arteries higher in prone than in supine whole breast irradiation . A dosimetric comparison in adjuvant radiotherapy of early stage breast cancer?