Cardiac chamber and coronary artery doses associated with postmastectomy radiotherapy techniques to the chest wall and regional nodes.

PURPOSE: To compare the estimated radiation doses delivered to the cardiac chambers (CC) and coronary arteries (CA) for 5 postmastectomy radiotherapy (PMRT) techniques. METHODS AND MATERIALS: A dosimetry study of 20 left-sided PMRT cases was conducted. Cardiac chambers (left and right atria [LA/RA] and left and right ventricles [LV/RV]) and coronary vessels (left main [LM], left anterior descending [LAD], left circumflex [LCX], right coronary [RCA] and posterior descending [PDA] arteries) were contoured on contrast-enhanced CT scans and verified by a cardiologist (T.K.). Five PMRT techniques were applied to each case; 50 Gy in 2 Gy fractions was delivered to the chest wall +/- internal mammary node targets. The techniques were: (1) standard tangents (TAN); (2) cobalt (Co); (3) reverse hockey stick (RHS); (4) mixed photon/electron beam (20/80); and (5) partially wide tangent fields (PWTF). Three-dimensional dose calculations for 100 plans were performed for all structures. Plans were compared by using the mean dose (Dmean) and the volume that received more than 30 Gy (V(30)) and 45 Gy (V(45)) for each structure.
RESULTS: Cobalt and 20/80 techniques delivered higher Dmeans to the whole heart and individual cardiac chambers (RA, RV, LA, and LV) as compared with the other three techniques. The heart received a Dmean of 21.03 +/- 3.5 Gy from Co and 11.87 +/- 5.22 Gy from 20/80. The remaining techniques delivered heart Dmeans of 2.90-4.94 Gy. When V(30) was used as a metric, all techniques had comparably low V(30) to the heart, except for Co, which resulted in a significantly higher irradiated volume of right-sided cardiac chambers (59.06% +/- 30.7 for RA F-test < 0.0001; and 61.46% +/- 22.13 for RV, F-test < 0.0001). Dmean to the proximal LAD (LAD_p) was significantly higher for RHS (17.64 +/- 7.43 Gy) and 20/80 (20.52 +/- 8.36 Gy) and lowest for PWTF (9.5 +/- 4.16 Gy). The Dmean for the distal LAD (LAD_d) was significantly lower with PWTF (11.02 +/- 7.34 Gy) than with all other techniques, including TAN (p < 0.0001). Similar results for PWTF and TAN were observed when V30 and V45 were used.
CONCLUSIONS: Cardiac substructures receive the most radiation exposure after PMRT with CO, 20/80 or both and least exposure with PWTF. Although TAN resulted in significant sparing of the majority of the cardiac structures, a significantly higher dose and volume of LAD was exposed when compared with PWTF. Although the clinical relevance of these dose differences is not clearly understood, these dosimetric estimates can serve as a baseline in the development of new techniques for locoregional treatment that will further reduce cardiac exposure.