A Diagnostic Pitfall: Retrocaval Nerve Sheath Tumor.

A 78-year-old female with a history of resected colorectal adenocarcinoma presented with progressively rising serum carcinoembryonic antigen (CEA) levels since 2022. An August 2023Positron Emission Tomography/Computed Tomography (PET-CT) revealed a 1.4 cm fluorodeoxyglucose (FDG)-avid retrocaval lesion, initially raising suspicion for metastatic lymphadenopathy given her cancer history. Despite receiving stereotactic body radiotherapy and remaining asymptomatic, her CEA levels continued to climb, and the lesion mildly enlarged to 1.8 cm by November 2024. This persistent elevation and lesion growth necessitated a definitive tissue diagnosis. Subsequent biopsy showed a spindle cell neoplasm. Immunohistochemical staining, crucial for characterizing soft-tissue tumor differentiation, revealed diffuse positivity for SOX10 and S100 protein, consistent with a primary nerve sheath tumor rather than metastatic disease. This case highlights the diagnostic challenge posed by FDG-avid lesions in patients with a history of malignancy, underscoring the importance of considering rare differential diagnoses and pursuing tissue diagnosis when imaging is equivocal or clinical presentation deviates from expected metastatic behavior.