This is a highly aggressive, undifferentiated carcinoma which grows rapidly and metastasizes early. It accounts for about 20 - 30% of lung tumours.
The tumour contains dense core granules similar to the argentaffin cell granules of the gastrointestinal tract and actively secretes hormones and hormone like products. The granules bear close resemblance to secretory granules of the amine precursor uptake and decarboxylation - APUD - system.
A number of paraneoplastic syndromes are associated with these tumours, including the Eaton-Lambert myasthenic syndrome, SIADH, Cushing's syndrome due to excess ACTH, and carcinoid syndrome from biogenic amines.
Three types are distinguishable histologically:
- oat cell tumour - characterised by uniform small - to - medium sized, closely packed cells arranged in strands, clusters, or singly, with dark-staining nuclei and a high nuclear:cytoplasmic ratio. A crush artefact is commonly seen in biopsies.
- intermediate cell tumour - similar to the oat cell type but the cells are more fusiform in nature
- combined oat cell tumour - has definite areas of squamous and glandular differentiation