A case of microscopic, multiple sclerosing pneumocytoma.

Sclerosing pneumocytoma is a rare tumor of the lung, commonly affecting middle-aged women, and is mostly isolated. Although this tumor is thought to be derived from primitive respiratory epithelial cells, the characteristics of the precursor cells are still unknown. A 19-year-old woman presented with multiple nodules in the right lung. Partial resection of the right middle lobe was performed, and seven sclerosing pneumocytomas, including four that were microscopic, were detected. The latter showed a simple papillary pattern, and three of them consisted of only round cell-like cells (single population). Interestingly, these round cell-like cells were positive for both p63 and TTF-1, but totally negative for SP-A. On the other hand, the tumor cells of the other four sclerosing pneumocytomas showing a papillary pattern with a dual population, were diffusely positive for TTF-1 and focally positive for SP-A (only in surface cells), but negative or very focally positive for p63. It has been reported that p63-positive stem cell-like cells are present in the distal airway and have potential to differentiate into type II pneumocytes. The immunohistochemical features of these multiple microscopic lesions suggest that the p63-TTF-1 double-positive cells are candidate precursor cells of sclerosing pneumocytoma.