Answer: Large cell neuroendocrine carcinoma

Histology: none provided

Discussion: The tumor exhibits a predominantly solid and focally trabecular growth pattern with areas of necrosis. The necrotic areas are well demarcated and are surrounded by a vaguely granulomatous response and fibrosis. The tumor cells have round to oval to irregular nuclei with fine granular chromatin and small inconspicuous nucleoli. There is increased mitotic activity (28 mitoses per 10 hpf) and 30-40% of tumor cells label with Ki67 by immunohistochemistry. The residual thymic tissue is cystic.

Immunostains demonstrated that the tumor cells were diffusely positive for CD56, chromogranin, synaptophysin, CD5 and c-kit, and negative for TTF-1.

Positive CD5 and C-kit staining is seen in the majority (75-80%) of thymic carcinomas (WHO type C thymoma), whereas they are typically negative in thymomas. This tumor exhibits neuroendocrine differentiation both morphologically and by immunohistochemistry. The differential diagnosis includes atypical carcinoid tumor and small cell carcinoma of the thymus and metastatic neuroendocrine lung tumors. Lung primaries can primarily be excluded by imaging (as was the case in this tumor). Atypical carcinoid tumors of the thymus should have a mitotic index that is less than 10/10 HPF and usually also lack necrosis. Thymic small cell carcinomas, like their counterparts in other organs, exhibit a markedly increased mitotic activity including a very high Ki67 proliferation index.