The primary causes of deaths for individuals with rare cancers can be difficult to diagnose clinically. Often, the symptoms implicate a variety of factors, and an autopsy is thus required to obtain the correct diagnosis. This study analyzes the death of a 45-year-old woman who reportedly died from an acute pulmonary dysfunction. The patient had been treated with antibiotics for three months for intractable pneumonia. Suspicious coin lesions detected by chest X-ray prompted a clinical clarification; however, no final diagnosis was made. The autopsy revealed a bulky thyroid tumor with venous invasion, leading to a massive pulmonary tumor embolism. Furthermore, microscopy identified the tumor as a rare pleomorphic myxoid sarcoma. Thus, the patient died of a large pulmonary tumor embolism originating from this rare sarcoma, and not of acute pulmonary dysfunction of any other means.