Carcinoid tumor of the thymus with divergent sarcomatoid differentiation: report of a case with histogenetic consideration.

An anterior mediastinal tumor resected from a 54-year-old man without paraneoplastic symptoms was found to be an unusual biphasic thymic tumor composed of carcinoid tumor in sarcomatous stroma characterized by fibrosarcoma-like spindle cells with areas of chondroid and osseous differentiation. Immunohistochemical and ultrastructural studies verified that the tumor described was a carcinoid tumor with divergent sarcomatoid differentiation rather than a mixed carcinoid tumor and sarcoma. The chondroid and osseous differentiations observed were unique features. This tumor is rare and provides an opportunity for understanding tumors with divergent components. The sarcomatoid transformation of thymic carcinoid tumor might denote highly malignant clinical behavior as illustrated by this case.