Less than 1.0% of malignant lymphomas are primary cardiac lymphoma (PCL), a rare malignant lymphoma. Due to its infrequency, the metabolic dynamics of the treatment have not been completely analyzed. A 62-year-old man who had been complaining of exertional dyspnea for a month arrived at our emergency room. He developed right cardiac failure as a result of a mass in the right atrium, according to a computed tomography (CT) scan. According to an echocardiogram, the mass was obstructing his blood flow and affecting how his heart worked. The lump was pathologically determined to be diffuse large B-cell lymphoma after he underwent urgent heart surgery. The lesion was only localized in the heart, according to a postoperative 18F-fluorodeoxyglucose positron emission tomography (18F-FDG-PET)/CT scan, indicating that the disease was in clinical stage IE. An 18F-FDG-PET/CT scan showed a thickness of the right atrial wall as residual disease despite the majority of the cardiac lymphomatous mass being removed during surgery; it also showed that the usual uptake of 18F-FDG in healthy myocardium had diminished. Following chemotherapy, 18F-FDG uptake recovered in the patient's normal myocardium of the heart in remission. In conclusion, a sort of "metabolic steal phenomenon" that may be connected to PCL is the difference in uptake between tumor-involved and healthy myocardium.