Atypical carcinoid with prominent mucinous stroma: a hitherto unreported variant of pulmonary neuroendocrine tumor.

Carcinoid tumors of the lung may show several growth patterns and cell types. We report here a previously undescribed case of primary pulmonary atypical carcinoid with prominent mucinous stroma, an exceedingly rare variant of carcinoid tumor so far reported only in the thymus. The tumor was discovered in a 50-year-old, non-smoker woman, and pursued an aggressive behavior. The lack of TTF-1 expression and the positive staining with hormonal receptors made the diagnosis very challenging. Differential diagnosis with other primary and metastatic mucin-rich tumors is also discussed.