Biological and immunological aspects of small cell carcinoma of the lung in relation to ectopic hormone production.

Small cell carcinoma share morphological and functional characteristics with normal Kulchitsky cells (K-cells) of the bronchial tree. This tumor is thought to reflect a neoplastic transformation of the pulmonary APUD cells because of the morphological similarities with K-cells, the association with paraneoplastic syndromes and the high incidence of ectopic hormone production. The peptide hormones produced and secreted by small cell carcinoma can be used as tumor markers to guide diagnosis and to monitor chemotherapy. They may be of particular value in following the course of a patient and in detecting early recurrences. Some of the hormonal substances produced by the tumor have the biochemical characteristics of prohormones. The difference between the abnormal hormones and their physiological counterparts probably will lead to the development of more specific marker assays. On the other hand, some of the atypical hormones have autoantigenic properties, as they occur in circulating immune complexes of patients with small cell carcinoma. The correlation between ectopic hormone production and host immunity might be of importance for the development of future treatment modalities.