Primary Cardiac Myxofibrosarcoma of the Left Atrium with Heterologous Elements Mimicking a Cardiac Myxoma

BACKGROUND Primary cardiac malignancies are extremely rare, with an incidence of 0.07% on autopsy series. Primary sarcomas represent up to 95% of malignant neoplasms, with myxofibrosarcomas accounting for only 10%. Around 90% of patients present with unspecific symptoms and a tumor with polypoid appearance on imaging, thus frequently receiving a misdiagnosis of myxoma. CASE REPORT A 65-year-old man presented with abrupt chest pain, blood pressure of 130/80 mmHg, and heart rate of 180 beats/min. Electrocardiogram showed atrial fibrillation, and imaging revealed a polypoid tumor on the atrioventricular septum obstructing the mitral valve. The tumor was removed and sent for histopathological evaluation, revealing a multinodular pattern with spindled hypocellular areas and hypercellular areas featuring pleomorphic cells. The mitotic count was 11 in 10 high-power fields, and necrosis was present in less than 50% of the tumor area. Tumor cells were calretinin and MDM2 negative and CD34 positive. Heterologous elements, necrosis and hemorrhage, were noted. Considering these findings, this tumor was classified as intermediate-grade myxofibrosarcoma. CONCLUSIONS Due to the rarity of myxofibrosarcomas, evidence for optimal diagnostic and therapeutic management is limited. Despite being frequently polypoid, seemingly benign tumors on imaging, the extent of infiltration at their base is usually deep. Their innocent appearance can hinder adequate presurgical planning, leading to suboptimal resections. We present the example of a seemingly benign tumor as a potential pitfall in evaluating cardiac neoplasms, highlighting the importance of histopathological and immunohistochemical evaluation in their correct characterization, in order to offer the best follow-up and adjuvant treatment, when needed.