Benign pulmonary lesions that may be misdiagnosed as malignant.

Five benign pulmonary lesions that may be misdiagnosed as malignant tumors are reviewed. In three lesions, diagnostic problems arise when a spindle cell component is dominant and obscures other characteristic histologic features. In the inflammatory pseudotumor, correct diagnosis relies on recognition of the benign cytology of the spindle cells and identifying a typical admixture of plasma cell-rich inflammatory cells. For spindle cell carcinoids, useful diagnostic features are the organoid pattern, benign cytology, and neuroendocrine differentiation features of the spindle cells. Localized pleural mesothelioma (fibroma) is composed of benign spindle cells in a fibrocollagenous background; mesothelial differentiation is not present by ultrastructural or immunocytochemical analysis. In sclerosing hemangioma, a complex histology may suggest a number of malignancies. Observation of solid and papillary areas of benign tumor cells, as well as sclerosis of vessel walls and intervening areas, will allow correct diagnosis. Pseudolymphoma, a nodular benign lymphoid infiltrate, is distinguished by its polymorphous and polyclonal composition and numerous germinal centers.