Lung parenchymal invasion in pulmonary carcinoid tumor: an important histologic feature suggesting the diagnosis of atypical carcinoid and poor prognosis.

The majority of previous studies on pulmonary carcinoid tumor have usually focused on clinical behavior or outcome, seldom considering histopathologic features. We retrospectively collected 63 cases of resected pulmonary carcinoid tumors from 1995 to 2011 at Samsung Medical Center, Seoul, Korea. The clinical and pathological features were correlated and survival analyses were performed. Forty cases (63.5%) were classified as typical carcinoid (TC) and 23 cases (36.5%) were classified as atypical carcinoid (AC) according to WHO classification criteria. AC patients showed a higher frequency of current smoking status and a higher stage of the tumor by the American Joint Committee on Cancer than TC patients. The disease was associated with death and recurrence in five and seven patients, respectively, with almost all of the associations found in AC patients. The five-year survival rate of TC and AC were 100% and 83.5%, respectively, with AC showing poorer prognosis than TC in overall survival (OS) and disease free survival (DFS) (p=0.005 and p=0.002). Lung parenchymal invasion was observed more commonly in AC than in TC (39.1% vs 12.5%, p=0.01) and was a poor prognostic factor in OS and DFS. Rosette-like arrangements were found only in six cases of AC, while abundant basophilic cytoplasm mimicking paraganglioma and ossification were found only in TC. Through the comprehensive study of pulmonary carcinoid tumor in Korea, we suggest that lung parenchymal invasion could be a useful histologic feature to suspect the diagnosis of AC in daily practice as well as to predict the prognosis of carcinoid tumor.