Popliteal fossa metastasis from rectal carcinoma in a young adult: A rare case report.

Signet-ring cell carcinoma (SRCC) of the rectum is a rare and aggressive subtype of colorectal cancer, typically metastasizing to regional lymph nodes, peritoneum, and liver. Metastasis to the popliteal fossa is exceedingly rare. We report a 27-year-old male with biopsy-proven rectal SRCC who developed a painful soft tissue mass in the left popliteal fossa within 1 year despite ongoing chemoradiotherapy. MRI demonstrated a heterogeneous mass with central necrosis, peripheral enhancement, and diffusion restriction. ¹⁸F-FDG PET/CT revealed peripheral FDG avidity (SUVmax 2.8). Ultrasound-guided core biopsy confirmed metastatic adenocarcinoma of colorectal origin. This case highlights an extremely rare metastatic pattern of rectal SRCC and emphasizes the importance of a multimodality imaging approach, including ultrasound, MRI, and PET/CT, in evaluating atypical soft tissue lesions in oncology patients.