Left Atrial Intimal Sarcoma Masquerading as Intracardiac Thrombus.

Primary cardiac malignancies are rare and often present with non-specific clinical and imaging findings, leading to diagnostic delays. We report a case of a 76-year-old woman with progressive dyspnea who was found to have a left atrial mass initially suspected to be a thrombus or benign lesion because of the absence of significant uptake on fluorodeoxyglucose positron emission tomography (FDG-PET). However, progressive enlargement and infiltrative imaging features raised concern for malignancy. Histopathological evaluation ultimately confirmed undifferentiated pleomorphic sarcoma with MDM2 amplification, consistent with intimal sarcoma. This case highlights the limitations of FDG-PET in excluding cardiac malignancy and underscores the importance of integrating multimodal imaging with clinical suspicion to avoid diagnostic anchoring.