Feasibility of the new WHO classification of pulmonary neuroendocrine tumours.

Primary pulmonary neuroendocrine tumours present a heterogeneous group of tumours causing problems in diagnosis and treatment. The new WHO classification of lung tumours was published in 1999 in order to improve this situation by combining morphology, immunohistochemistry and clinical background for diagnosis. The aim of our study was to evaluate the feasibility of this classification and to discuss the consequences of modified diagnostic criteria. 50 cases of neuroendocrine tumours and 50 poorly differentiated lung tumours diagnosed in the years 1981-1994 were independently evaluated by three pathologists. The diagnosis of all 27 typical carcinoids (TC) was given by all authors, however, no unanimous agreement was achieved in one of three atypical carcinoids (AC) and two of four large cell neuroendocrine carcinomas (LCNEC). While typical and atypical carcinoids can be distinguished by the number of mitoses or presence of necrosis it was found that the most difficult diagnostic factor for large cell neuroendocrine carcinoma is the recognition of its light-microscopic neuroendocrine features. In consequence it must be distinguished not only from atypical carcinoid or small cell lung carcinoma (SCLC), but also from poorly differentiated carcinoma. Immunohistochemistry is important for the diagnosis of this entity but also for nonsmall cell lung carcinoma with neuroendocrine differentiation (of which 1 case was detected in our series) There was agreement on the diagnosis of small cell carcinomas in all but one case. The results indicate the excellent reproducibility of the WHO classification.