Primary malignant mixed mesodermal tumor of the gallbladder. Report of a case and critical review of diagnostic criteria.

A rare case of primary malignant mixed mesodermal tumor of the gallbladder arising in a 75 year old woman is reported. The previously published cases of similar tumors were reviewed in order to outline the histological features and the histogenesis. In diagnosing a malignant mixed mesodermal tumor of the gallbladder it was imperative that we excluded malignant neoplasms with multiple histological patterns. The diffuse and close intermingling of the epithelial and mesenchymal component ruled out a collision tumor. The high mitotic rate, the typical reticulin pattern and the obviously malignant osteoblasts excluded a spindle cell carcinoma with osseous metaplasia. The authors conclude that this is the first case of malignant mixed mesodermal tumor with evident osteosarcomatous areas, described in the gallbladder.