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

Abstract Background & Purpose Mean heart dose (MHD) correlates with late cardiac toxicity among survivors of lymphoma receiving involved-field radiotherapy (IFRT). We investigated MHD and cardiac substructure dose across older and newer radiation fields and techniques to understand the value of evaluating MHD alone. Patients & Methods Following IRB approval, we developed a database of dosimetry plans for 40 mediastinal lymphoma patients, which included IFRT (AP/PA), involved-site radiotherapy (ISRT) + 3-dimensional conformal radiotherapy (3DCRT), ISRT + intensity-modulated radiotherapy (IMRT), and ISRT + 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, and aortic valves (TV, MV, AV); 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 > 0.8, 0.6-0.79, 0.4-0.59, and Results A strong correlation was observed between MHD and the mean cardiac substructure dose for each plan as follows: IFRT—LV, RV, and LAD; ISRT+3DCRT—LV, RV, and LA; ISRT+IMRT—LV and RV; ISRT+proton therapy—none. The following moderate correlations were observed: IFRT—LA and MV; ISRT+3DCRT—LAD, MV, and TV; ISRT+IMRT—LA, LAD, and MV; ISRT+proton therapy—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 radiotherapy that rely on MHD can be misleading when using modern treatment fields and techniques.