Sarcomatoid mesothelioma as seen here consists of atypical elongated spindled cells and varying degrees of collagenization.
A closer view of malignant spindle cells reveals vesicular nuclei and moderate pleomorphism, set in a fibrous stroma.
Yet another area shows fascicles of spindled cells. Sarcomatoid mesotheliomas can be difficult to distinguish from sarcomas, even by IHC as sarcomatoid mesotheliomas can express actin and desmin. However, sarcomas should be negative for cytokeratin whereas meostheliomas are positive.
Other malignant areas of the same tumor are frankly epithelioid;, thus this is a biphasic tumor with both sarcomatoid and epithelioid histologic types.
Yet another area with epithelioid features consisting of abundant cytoplasm and sheet-like growth. This area might even look a bit deciduoid. An area of necrosis is seen on the left.
Malignant mesothelioma is subdivided into three main histologic subtypes:
(1) Epithelioid: Architectural patterns under this category is diverse and includes: tubopapillary (most common), acinar, microglandular (difficult to distinguish from pleural adenocarcinoma), deciduoid (seen more in young women), mxyoid and well-differentiated papillary mesothelioma (the latter two may have improved prognosis).
(2) Sarcomatoid: May mimic fibrosarcoma, malignant fibrous histocytoma. Occasional heterologous elements may be found such as cartilaginous and osseous metaplasia. The desmoplastic variant can be deceptively bland and mimic fibrous pleuritis.
(3) Biphasic (mixed): Contains a mixture of epithelioid and sarcomatoid features in varying proportions.
Fletcher CDM, ed. Diagnostic Histopathology of Tumors. 3rd Ed. Philadelphia, PA: Elsevier; 2007: 205-7.