Malignancies have demonstrated the ability to metastasize to cardiac tissue. However, an optimal diagnostic algorithm for cardiac tumors has not yet been established, due at least in part to the scarcity of symptomatic cases. Several case reports describe how the usage of 18F-labeled fluorodeoxyglucose positron emission tomography (18F-FDG PET) incidentally revealed cardiac neoplasia. This modality, which indicates uptake sites of the radioisotope 18F-FDG, allows for whole-body imaging and is often used for preoperative determination of malignant metastasis or for assessing response to therapy over time. However, findings of false positivity are often reported due to increased FDG avidity caused by a range of other, nonmetastatic processes, most notably inflammation and infection. In this case report, an 84-year-old male with stage IV non-small cell lung cancer presented a clinical course, echocardiogram, and 18F-FDG PET-CT findings that were suggestive of endocardial metastasis. Nine months into therapy, after extensive consultation, the patient finally consented to a more complete workup using cardiac MRI (CMRI), which showed no evidence of cardiac metastasis. This case report supports the utility of CMRI as a means of further interpreting intracardiac, localized FDG uptake foci in PET-CT findings, in order to avoid false positivity and further refine proposed cardiac differential diagnoses in cancer patients.