Cardiac dosimetry for adjuvant left-sided breast radiotherapy; patterns with 2D- versus 3D-era planning and correlates of coronary dose with maximum depth of myocardial exposure

The purpose of this study was to evaluate the cardiac dosimetry delivered before and after routine 3D CT whole-breast radiotherapy planning, including cardiac contouring and the relevance of a 15-mm maximum myocardial depth (MMD) planning tolerance threshold. The PULp FICTion study permitted cardiac dosimetry comparisons for 140 patients (70 in the 'before-contouring era' (BC) and 70 in the 'post-contouring era' (PC)). Comparisons were made of MMD and dosimetry for whole heart, anterior myocardium and left anterior descending (LAD)/coronary artery (overall, superior and inferior) by contouring era. The MMD mean was 15.6mm (range 1-40). If the internal mammary chain (IMC) was treated, the MMD increased from 15 to 27.7mm (P<0.0001). Excluding IMC patients revealed no difference in MMD between conservation and mastectomy (14.9 vs 17.4, P = 0.3), boost and no boost (14.5 vs 15.5, P = 0.41) or BC and PC (15.7 vs 14.5. P = 0.33) and no differences related to treating clinician, anthracycline use or age. Only in potentially low-risk patients defined as not requiring IMC or boost treatment was there a difference in MMD by era, with 17.7mm (1-40) in the BC era and 13.9mm (1-31) mm in the PC era (P = 0.013). Inferior LAD mean doses increased from 49% to 84% of the prescribed breast dose when MMD was >15mm, and the proportion of patients with a mean dose <40% of the prescribed breast dose fell from 48% to 8%. Changes in cardiac dosimetry associated with routine cardiac contouring have initially been minor and restricted to low-risk patients. A 15-mm MMD reasonably represents a transition from low mean distal LAD doses to substantial doses.