Benign clear cell tumor of the lung.

A 49-year-old woman presented with a solitary pulmonary nodule in the right lung. The tumor was well circumscribed and showed a reddish brown cut surface. It showed a diffuse growth pattern of polygonal cells with clear abundant cytoplasm and distinct cell border around thin-walled vascular spaces and sinusoid-type vessels. Based on morphological findings, the patient was diagnosed as having benign clear cell tumor of the lung. Silver impregnation and PAM stains showed fine reticular fibers continuously surrounding the vessels and individual neoplastic clear cells. Strong immunostaining for type IV collagen was observed surrounding all the individual clear cells. A few clear cells were positive for HMB-45 and NCAM 123C3 (CD56). Electron microscopically, clear cells had numerous membrane-bound glycogen granules and a large amount of non-membrane-bound glycogen. The neoplastic cells were surrounded by external lamina and cytoplasmic processes. The neoplastic clear cells showed morphologic features seen in pericytes, melanocytic cells, and neuroendocrine cells.