Mesotheliomas with crystalloid structures: report of nine cases, including one with oncocytic features.

Although the presence of crystalloids has historically been of largely academic interest or simply an intriguing curiosity, these structures have occasionally been useful in the differential diagnosis of some tumors. Crystalloids have only rarely been reported in mesotheliomas, and their presence in these tumors has not been sufficiently investigated, nor has their potential value as an ultrastructural marker for mesothelioma been established. The finding of a case of mesothelioma in which the vast majority of the neoplastic cells contained intracytoplasmic crystalloids prompted a search for these structures in 69 consecutive cases of mesothelioma (59 epithelioid, 7 sarcomatoid, 3 mixed-epithelioid sarcomatoid). Crystalloids were found in 9 (15%) of the 59 epithelioid mesotheliomas, indicating that these structures are not as rare as had been thought. That these inclusions were demonstrated in tumors exhibiting diverse histological patterns and were not confined to a single subtype of epithelioid mesothelioma indicates that, because of their unique morphology, when present, they can assist in the diagnosis of these tumors. In addition, oncocytic features were also seen in one of the cases with crystalloid inclusions. Pathologists should be aware of the fact that, even though uncommon, mesotheliomas can present oncocytic morphology and, therefore, these tumors should be included in the differential diagnosis of those neoplasms that display similar morphological features, and which can metastasize to the serosal membranes. To my knowledge, an oncocytic mesothelioma has not previously been reported.