The Meaningless Meaning of Mean Heart Dose in Mediastinal Lymphoma in the Modern Radiation Therapy Era

Purpose: Mean heart dose (MHD) correlates with late cardiac toxicity among survivors of lymphoma receiving involved-field radiation therapy (IFRT). We investigated MHD and cardiac substructure dose across older and newer radiation fields and techniques to understand the value of evaluating MHD alone. Methods and Materials: After institutional review board approval, we developed a database of dosimetry plans for 40 patients with mediastinal lymphoma, which included IFRT (anterior-posterior and posterior-anterior), involved-site radiation therapy (ISRT) thorn 3-dimensional conformal radiation therapy (3DCRT), ISRT thorn intensity modulated radiation therapy, and ISRT thorn proton therapy plans for each patient. Each plan was evaluated for dose to the heart and cardiac substructures, including the right and left ventricles (RV, LV) and atria (RA, LA); tricuspid, mitral (MV), and aortic valves; and left anterior descending coronary artery (LAD). Correlation between MHD and cardiac substructure dose was assessed with linear regression. A correlation was considered very strong, strong, moderate, or weak if the r was >=similar to 0.8, 0.6-0.79, 0.4-0.59, or <0.4, respectively. Results: A very strong correlation was observed between MHD and the mean cardiac substructure dose for each plan as follows: IFRT-LV, RV, LA, MV and LAD; ISRT thorn 3DCRT-LV, RV, MV, TV, and LA; ISRT thorn intensity modulated radiation therapy-LV and RV; ISRT thorn proton therapy-none. The following strong correlations were observed: IFRT-RA; ISRT + 3DCRT-LAD, RA, AV; ISRT thorn IMRT-LA, RA, LAD, AV, TV, and MV; ISRT thorn proton-LV only. Conclusions: In the management of mediastinal lymphoma, more conformal treatment techniques can lead to more heterogeneous dose distributions across the heart, which translate into weaker relationships between mean heart dose and mean cardiac substructure doses. Consequently, models for assessing the risk of cardiac toxicity after radiation therapy that rely on MHD can be misleading when using modern treatment fields and techniques. Contouring the cardiac substructures and evaluating their dose is important when using contemporary RT. (C) 2019 The Author(s). Published by Elsevier Inc. on behalf of American Society for Radiation Oncology.