Small cell carcinoma of the lung is considerably more responsive to cytotoxic chemotherapy and radiotherapy than the other histologic types of lung carcinoma. The therapeutic implications of making an accurate pathologic distinction between small cell carcinoma and other types of lung cancer stimulated this assessment of the pathologic homogeneity of small cell carcinoma. The records of 40 autopsy patients with biopsy proved small cell carcinoma seen at the The Johns Hopkins Hospital between 1970 and 1978 were reviewed. In five of the 40 patients, carcinoma (squamous carcinoma in three, adenocarcinoma in one and large cell undifferentiated carcinoma in one), but not small cell carcinoma, was seen at autopsy. Six cases were identified in which other histologic patterns of lung cancer were present in addition to the small cell carcinoma in the autopsy specimens. In order to determine whether the tumor at autopsy was biochemically as well as morphologically distinct from small cell carcinoma, histaminase and L-dopa decarboxylase activity were measured in the lung tumor and in mediastinal metastases in four cases in which no small cell carcinoma was present at autopsy. The levels of these enzymes in the tumors were markedly lower than the levels found in small cell carcinoma in previous studies. These changes in the pathologic and biochemical characteristics of these lung cancers could reflect the emergence of another tumor which was present from the outset, the development of a second tumor, differentiation of the initial tumor or an effect of cytotoxic therapy on the morphology of small cell carcinoma. Determining the exact mechanisms for these morphologic and biochemical findings may provide the basis for more rationale therapy of small cell carcinoma and also has implications for studies of the biology and histogenesis of this tumor.
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