Navigating the diagnostic maze: the challenge of sclerosing pneumocytoma in frozen sections.

Pulmonary Sclerosing Pneumocytoma (PSP) represents a rare benign tumor that exhibits a predisposition towards females. Often asymptomatic, its identification usually occurs incidentally through imaging modalities. Histologically, PSP demonstrates features consistent with pneumocytic differentiation and possesses a dual-cell population. However, in rare instances it may demonstrate pleural invasion or lymph node metastasis. Diagnosing PSP through small biopsy or frozen section presents considerable challenges attributed to its heterogeneous growth patterns and striking similarity to well-differentiated pulmonary adenocarcinoma. We report a case of PSP in a 57-year-old female smoker, presenting as a slow-growing 2.5 cm mass that recently exhibited enlargement, as noted on computed tomography (CT) scan. The recommendation for excising the mass prompted the patient to undergo a right robotic-assisted thoracoscopic procedure, which entailed wedge resection of the right lower lobe and an intraoperative consultation. A completion right lower lobectomy was performed, accompanied by lymph node dissection, following a frozen section diagnosis indicating at least adenocarcinoma in situ. The permanent section revealed bland cuboidal cells lining papillary and sclerotic areas, with occasional atypical features such as prominent nucleoli and scattered mitotic figures. Adjacent foci of atypical adenomatous hyperplasia (AAH) were noted. Immunohistochemical (IHC) staining revealed positive Napsin A, keratin AE1/3, and CK7 in surface cells but not in round cells. Both EMA and TTF1 immunostains highlighted surface cells and scattered round cells. Elastic stain highlighted visceral pleural involvement. The combined morphology and immunoprofile supported the diagnosis of PSP. This case underscores the critical importance of accurately diagnosing slow-growing pulmonary nodules, which are increasingly detected by the widespread use of imaging for various medical conditions.