Pulmonary neuroendocrine tumors: An entity in search of cytologic criteria

Abstract

Pulmonary neuroendocrine tumors (PNET) are histologically subclassified into typical carcinoid (TC), atypical carcinoid (AC), large cell neuroendocrine carcinoma (LCNEC), and small cell carcinoma (SCLC). The criteria for subclassification in cytological specimens are not well defined. In this study, we reviewed histologically confirmed 18 TC, 8 AC, 10 LCNEC, and 10 SCLC cytologic specimens from 45 patients. The following features were reviewed: small clusters, geographic sheets, trabecular structures, pseudo-rosettes, single cells, doublets, triplets or short cords, papillary-like structures, capillary vasculatures, necrosis, smear background, cell size, cell pleomorphism, amount of cytoplasm, plasmacytoid cells, spindle cells, nuclear atypia, molding, palisading and smearing, chromatin textures, nucleoli, and mitotic figure count. Based on our results, geographic clusters and necrosis were often seen in LCNEC and SCLC; while AC only showed scattered single cell necrosis. TC and AC commonly exhibited trabecular structures. Papillary-like structures and capillary vasculature were only present in TC, AC, and LCNEC. Cells forming doublets, triplets, and short cords were more commonly seen in SCLC and rarely seen in other entities. Plasmacytoid and spindle cells were only seen in TC and AC. Nuclear smearing was not identified in TC, rare in AC, focally present in LCNEC and obvious in SCLC. Mitotic figures were nearly absent in TC, ≤5/10 HPF in AT, and ≥10/10 HPF in SCLC. LCNEC showed a wide span of mitotic count ranging between 2 and 16/10 HPF. In this study, we propose a set of cytological features that are essential for subclassification of PNETs in cytologic specimens.