A Pulmonary Tumorlet with Caseous Granuloma Associated with Atypical Mycobacterium

: We encountered a case of pulmonary tumorlet with caseous granuloma associated with atypical mycobacterium. A 73-year-old woman was admitted to the hospital because a chest x-ray film showed enlargement of an abnormal shadow in the middle lobe of the right lung. Primary lung cancer was suspected and right middle lobectomy was performed. Acid-fast bacilli (Gaffky 1) were found in a caseous lesion and examination of intraoperatively obtained frozen specimens showed caseous granulomas. The bacilli were later identified as Mycobacterium avium complex. The permanent specimen showed a minute lesion consisting of small clusters of epithelial cells resembling carcinoid tumor in contact with granulomatous tissue. Histopathological examination revealed argyrophilia on Grimelius stain and immunoreactivity to chromogranin-A in the clusters of epithelial cells. Although these results are consistent with small cell carcinoma or peripheral carcinoid tumor, pulmonary tumorlet was diagnosed because of the lesion's small and minimal cytologic atypia, and because of chronic pulmonary damage around the lesion. Pulmonary tumorlets are minute, usually microscopic, tumor-like lesions mostly found in damaged lung tissue obtained at autopsy or during surgery. Morphological diagnosis is sometimes very difficult, but recently these lesions have been regarded as hyperplastic lesions arising in pulmonary neuroendocrine cells (Kultschitzky cells) and caused by chronic pulmonary damage, such as hypoxia and inflammation. Pulmonary tumorlets must be considered in the differential diagnosis of minute lesions suspected to be small cell carcinoma or peripheral carcinoid tumor.