# A Diagnostic Pitfall: Retrocaval Nerve Sheath Tumor

## TL;DR

A patient with a history of colorectal cancer had a suspicious lesion that was ultimately diagnosed as a rare nerve sheath tumor, not a cancer recurrence.

## Contribution

This case emphasizes the diagnostic challenge of FDG-avid lesions in cancer survivors and the need for tissue confirmation.

## Key findings

- The lesion was FDG-avid and initially suspected to be metastatic cancer.
- Immunohistochemistry confirmed it was a primary nerve sheath tumor, not metastatic disease.
- The case illustrates the importance of biopsy when clinical behavior is atypical.

## Abstract

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.