Primary Pulmonary Low-Grade Fibromyxoid Sarcoma in a 70-Year-Old Woman: A Diagnostic Challenge in Thoracic Spindle Cell Pathology

BackgroundLow-grade fibromyxoid sarcoma (LGFMS) is an uncommon malignant fibroblastic neoplasm characterized by deceptively bland histology and risk of late recurrence and metastasis. Primary pulmonary LGFMS is rare, with fewer than a dozen reported patients. Because the lung is also the most frequent metastatic site for soft-tissue LGFMS, distinguishing a primary pulmonary tumor from metastatic disease requires careful clinicopathologic correlation.Patient PresentationWe report a 70-year-old woman with a slowly enlarging right lower lobe pulmonary mass on imaging during a 2-year interval. A computed tomography (CT)-guided biopsy revealed a spindle cell neoplasm with a nonspecific immunoprofile, necessitating external consultation. Immunohistochemical studies were negative for epithelial, melanocytic, mesothelial, smooth muscle, and solitary fibrous tumor markers. Diffuse MUC4 expression supported the diagnosis of LGFMS. A comprehensive metastatic survey revealed no evidence of an alternative primary soft-tissue tumor, supporting designation of the lesion as a primary pulmonary LGFMS. The patient underwent right lower lobectomy, which showed a well-circumscribed, low-grade spindle cell neoplasm with negative margins and no lymph node involvement.ConclusionPrimary pulmonary LGFMS is a rare diagnostic entity that requires integration of morphology, immunohistochemistry, occasionally molecular testing, and thorough metastatic evaluation. Diffuse MUC4 expression remains a critical diagnostic marker. Although LGFMS is considered a low-grade sarcoma, its potential for late recurrence and metastasis necessitates long-term clinical surveillance. This patient adds to the limited literature on primary pulmonary LGFMS and highlights key diagnostic considerations for pathologists evaluating spindle cell tumors of the lung.