[A mesenchymal malignant mesothelioma that changed from mixed type to purely sarcomatous type].

A 65-year-old man was admitted to the hospital because of an abnormal shadow on a chest roentgenogram. A diagnosis of mixed-type malignant mesothelioma was made after transcutaneous needle biopsy and thoracoscopic biopsy. The tumor was considered to be inoperable because it had diffusely invaded surrounding tissue, and therefore the patient was treated with chemotherapy only. About one year later, he died of acute pneumonia. At autopsy, a mesenchymal malignant mesothelioma that did not have an epithelial component was found. We know of no previous report of a case in which a tumor with a biopsy-proven epithelial component apparently changed to a purely sarcomatous type. No satisfactory explanation for this phenomenon has been offered.