Scar adenocarcinoma of the lung: a light and electron microscopic study.

Five well differentiated peripheral adenocarcinomas of the lung were investigated, using light and electron microscopy. Each tumour contained a central nidus of fibrous tissue and fulfilled the criteria for "scar cancer." One tumour also had a focus of lamellated collagenous tissue, suggestive of an old tuberculous granuloma. Electron microscopy showed the features of Clara cells, with characteristic dense bodies in the apical cytoplasm and scattered microvilli on the luminal surface. It was concluded that this variant of scar cancer was a carcinoma of Clara cells, which was sufficiently distinctive in appearance to be recognised on light microscopy alone. It remains uncertain, however, whether the central fibrous area is a desmoplastic response to tumour growth or a pre-existing scar.