Malignant mesothelioma with heterologous elements: clinicopathological correlation of 27 cases and literature review.

Only a small number of malignant mesotheliomas with heterologous elements have been described. There are currently no criteria for diagnosis and little data regarding prognosis. We suggest that the term heterologous mesothelioma should be reserved for tumours that show malignant heterologous elements, notably osteosarcomatous, chondrosarcomatous, or rhabdomyoblastic elements but have immunohistochemical and clinical characteristics of mesothelioma. We identified 27 such cases and characterized the clinical and pathological characteristics of these tumours. In our series, 89% originated in the pleura, and 11% from the peritoneal cavity. The median age at diagnosis was 68 years, ranging from 27 to 85 years. Of these cases, 93% occurred in males and 7% in women. Of the 27 mesothelioma cases 16 (59%) were sarcomatoid, 10 (37%) were biphasic, and one was reported as epithelioid; 40% (11 cases) showed osteosarcomatous elements only, 19% showed areas of rhabdomyosarcoma only, 19% contained areas of chondrosarcoma only, and 22% exhibited osteochondromatous elements. Immunohistochemical labelling for cytokeratins was present in the majority of cases. Exposure to asbestos was identified in all the 17 cases for which an exposure history was available (63%). Median survival was 6 months after diagnosis, similar to the survival seen in sarcomatoid mesotheliomas. The differential diagnosis includes primary and secondary pleural sarcomas, including osteosarcomas and chondrosarcomas. Immunohistochemical labelling for cytokeratins is helpful in the distinction, but lack of labelling for cytokeratins in a spindle cell/sarcomatoid tumour does not exclude the diagnosis of mesothelioma, irrespective of the presence of heterologous elements. We suggest that if the anatomical distribution conforms to that of mesothelioma, a diagnosis of heterologous mesothelioma should be made in preference to a diagnosis of primary pleural osteosarcoma or chondrosarcoma, regardless of cytokeratin positivity, as for conventional non-heterologous sarcomatoid mesothelioma.