Abstract
Pulmonary sclerosing pneumocytoma (PSP) is a tumor of pneumocytic origin that is classified as a benign neoplasm. To date, aggressive behavior of this tumor has rarely been reported. Here, we describe a case of a 56-year-old woman with a huge, 19-cm PSP that resulted in mediastinal shift and showed microscopic endobronchial invasion and necrosis. The differential diagnosis included malignant mesenchymal tumors, such as solitary fibrous tumor; however, PSP was confirmed based on the characteristic thyroid transcription factor 1 positivity and membranous expression of Ki-67 on immunohistochemical staining of tumor cells.