Alk positive histiocytosis presenting as a solitary pulmonary spindle cell tumor case report

Anaplastic lymphoma kinase-positive histiocytosis (ALK-PH) is a rare histiocytic neoplasm defined by ALK gene rearrangements and characterized by a broad spectrum of clinical and pathological manifestations. Primary involvement of the lung is exceedingly rare and poses significant diagnostic challenges. We report a case of a solitary pulmonary spindle cell tumor arising in the right middle lobe of a 54-year-old female patient. The lesion measured mm and was completely excised by surgical resection. Histologically, the tumor was composed of spindle-shaped cells within a myxoid stroma accompanied by prominent lymphoplasmacytic infiltration. No visceral pleural invasion, vascular tumor emboli, or perineural invasion was identified. All surgical margins were free of tumor, and no lymph node involvement was detected. Immunohistochemical analysis demonstrated positivity for ALK, vimentin, CD68 (partial), CD163, and cyclin D1, with a low proliferative index. Markers of epithelial, pulmonary, myogenic, and Langerhans cell differentiation were negative. Molecular analysis confirmed EML4-ALK fusion, supporting the diagnosis of ALK-positive histiocytosis. This case highlights a rare presentation of localized pulmonary ALK-positive histiocytosis mimicking a spindle cell neoplasm. Integrated histopathological, immunohistochemical, and molecular evaluation is essential for accurate diagnosis. Complete surgical resection may be a treatment for localized disease.