Pulmonary neuroendocrine tumors with nuclear inclusion.

Nuclear inclusion or pseudoinclusion is a peculiar cytological feature, and its recognition in appropriate clinicopathological settings can aid in the diagnosis of several disease entities. To the best of our knowledge, only 1 case of pulmonary neuroendocrine tumor (NET) with nuclear pseudoinclusion has been reported. A review of 227 patients who had undergone surgical resection for pulmonary NETs revealed 2 tumors with different mechanisms of nuclear inclusion. To explore the cause of nuclear inclusion, NET with nuclear inclusion was characterized immunohistochemically and ultrastructurally. Nuclear inclusions were observed in 2 of the 227 (0.9%) patients with pulmonary NETs. The first patient was a 46-year-old woman with small cell carcinoma. Tumor cells with nuclear inclusions were distributed focally. Ultrastructural analysis showed that these inclusions were pseudoinclusions. The second patient was a 62-year-old man with large-cell neuroendocrine carcinoma. Nuclear inclusions were observed in the focal area of the tumor. Immunohistochemical analysis revealed that the intra-nuclear materials consisted of biotin and aberrant cytoplasmic and nuclear accumulation of β-catenin. Mutational analysis revealed a CTNNB1 gene mutation. Although very rare, diagnostic errors may be observed in cases of pulmonary NETs with nuclear inclusions. The mechanisms of nuclear inclusion differed, with one due to herniation of the cytoplasm into the nucleus (pseudoinclusion) and the other due to accumulation of biotin resulting from a CTNNB1 gene mutation.