Perinuclear dot-like cytokeratin 20 staining in small cell neuroendocrine carcinoma of the ovary (pulmonary-type).

Primary ovarian small cell carcinomas are rare tumors that are subclassified into the hypercalcemic (HT) and the pulmonary type (PT), and each tumor type has different clinicopathologic characteristics. However, overlapping morphologic features can make the distinction between these lesions difficult. The PT small cell carcinoma is very rare, with only one series and a few case reports published. The authors report two additional cases of this entity and also report for the first time the immunohistochemical finding of perinuclear dot-like staining with cytokeratin 20. This staining pattern has been reported only for Merkel cell carcinoma and salivary gland small cell carcinoma. The authors suggest that the punctate CK20 staining is a useful tool in the diagnostic distinction of PT small cell carcinoma from HT small cell carcinoma, from other primary ovarian tumors such as granulosa cell tumors, and from metastatic neuroendocrine tumors originating at primary sites such as the lung. Since the authors' conclusions are based on two cases of this rare entity, additional cases will need to be studied in the future.