More than half of all patients with cancer receive radiation therapy. While radiation is a critical component of cancer treatment for many malignancies, incidental cardiac radiation exposure can cause cardiac morbidity and mortality. This has been well defined in malignancies, such as breast cancer and lymphomas, in which cardiac toxicity can manifest several years following exposure.1,2 More recently, the importance of cardiac radiation exposure in outcomes for patients with non–small cell lung cancer (NSCLC) has been highlighted. Studies in NSCLC have found that incidental cardiac radiation is associated with cardiac morbidity and decreased survival.3-6 Grade 3 or higher cardiac events can occur at significant rates within 2 years of NSCLC treatment,3,6,7 and the competing risk of severe cardiac events has become more important, with newer treatments leading to improved cancer outcomes in NSCLC. For breast cancer–associated radiation-induced heart disease (RIHD), strong associations have been seen between cardiac events and mean heart dose (MHD).1 However, in NSCLC, the associations between whole heart doses and cardiac events and survival are not as clear and reproducible across studies.8 The cardiac radiation dose distribution in most patients with left-sided breast cancer is fairly reproducible, with the highest dose region deposited on the anterior myocardial wall. In contrast, cardiac radiation dose distributions in patients with lung cancer can be much higher and variable in their locations.8 Thus, in breast cancer, the MHD may act as an adequate surrogate for doses to critical cardiac substructures, while the MHD in patients with NSCLC may not be a suitable surrogate because of the potential for heterogeneous doses in different cardiac substructures based on radiation target locations. Likewise, less conformal radiation treatments for lymphoma had stronger associations between MHD and doses to cardiac substructures than more conformal modern treatments, in which increased variations in cardiac substructure doses were seen.9 Patients may have differences in RIHD risk based on whether doses are deposited in more critical regions for cardiac function. In addition, several studies have highlighted that the presence of baseline cardiac risk increases the risk of RIHD,1,3,6 and patients with NSCLC typically have a higher incidence of baseline cardiac risk factors than patients with breast cancer.3,6