The tumor consists of organoid nests of uniform cells with largely no necrosis. The cells are composed of a minimal to moderate amount of eosinophilic cytoplasm and finely granular nuclear chromatin. Scattered mitotic figures are not difficult to identify.
However, a few areas did show carcinoid nests with punctate foci of necrosis. Infarct like necrosis, a feature of large cell neuroendocrine carcinoma, is specifically lacking in this tumor.
Interface with the lung shows broad pushing nests. The cells show features cytologically similar to those of typical carcinoid when viewed at high power.
Up to 50% of atypical carcinoids show metastastic behavior. Here a tiny focus is present in a peribronchiolar node.
Atypical carcinoid is a problematic tumor to definitely classify. The classification of neuroendocrine lung tumors has evolved over the last decades. The current World Health Organization classification is based on a study by Travis et al. which uses a four diagnostic categories which reflect three grades: typical carcinoid (low grade neuroendocrine carcinoma), atypical carcinoid (intermediate), large cell neuroendocrine carcinoma (high), and small cell lung carcinoma (high). Travis et al suggests that based on clinical behavior, the definition of atypical carcinoma should be modified to include tumors of lower histologic grade and that the mitotic threshold should be decreased to fewer than 5 mitoses per 2 mm2 (10 HPF). Additionally, the use of the term "neuroendocrine carcinoma" is preferred as even typical carcinoid is a potentially malignant tumor, and the term "atypical carcinoid" may give the presumption of a benign tumor in entity with metastatic potential.
A 5-and 10-year survival rate of 56% and 35% for atypical carcinoid has been reported (Travis) for this entity. The survival rates are significantly worse for atypical carcinoid than for typical carcinoid, which is associated with 87% survival at both 5 and 10 years.
Travis et al. Survival Analysis of 200 Pulmonary Neuroendocrine Tumors With Clarification of Criteria for Atypical Carcinoid and Its Separation From Typical Carcinoid. Am J Surg Pathol, 1998, 22(8), 934-944