Egan, A.

Abstract Introduction: Pulmonary sclerosing pneumocytoma, formerly known as sclerosing hemangioma, is an uncommon benign neoplasm most recently classified by the World Health Organization as an adenoma. It occurs most commonly in middle-aged women of East Asian descent, in whom it is most often discovered incidentally on imaging. It typically appears as a single, well-circumscribed lesion on CT that lacks specific defining characteristics and can appear similar to malignancies such as pulmonary adenocarcinoma. Therefore, diagnosis relies on histopathology. The tumor is derived from respiratory epithelial cells expressing TTF-1 in both round and surface cells, and immunohistochemistry (IHC) staining is positive for both TTF-1 and EMA. Description of Case: A 52-year-old female from Southeast Asia presented to primary care clinic reporting atypical chest pain. She denied shortness of breath, cough, fever, night sweats, or weight loss. She had no prior smoking history, but there was a remote history of chewing tobacco. Chest x-ray revealed a retrocardiac left lower lobe pulmonary nodule. CT chest was obtained and revealed a solid left lower lobe pulmonary nodule measuring 2.2 cm in diameter. The nodule was mildly FDG avid on subsequent PET-CT with SUVmax 2.8, noted to be suspicious for a lesion on the pulmonary adenocarcinoma spectrum. She opted for surgical removal and underwent thoracoscopic left lower lobe wedge resection. Pathology demonstrated sclerosing pneumocytoma with IHC stains positive for TTF1, EMA, and keratin AE1/AE3. The follow-up CT chest showed no evidence of local recurrence or residual tumor. Discussion: Pulmonary sclerosing pneumocytoma is an uncommon benign neoplasm of the lung that can be easily mistaken for malignancy such as pulmonary adenocarcinoma given its imaging characteristics and FDG avidity on PET-CT. It must be considered in the differential for lung nodules, particularly in middle-aged women of East Asian descent. Diagnosis is made with biopsy showing both cuboidal surface cells and stromal round cells, with IHC staining positively for TTF-1 and EMA. However, these tumors remain difficult to diagnose in frozen section, small biopsies, and cytology, because they are easily mistaken for adenocarcinoma or carcinoid tumors. The clinical course is generally benign, but very rarely there is metastasis or recurrence.