* Imaging Findings

- On MRI, there is an irregular large perirectal/perianal mass with markedly hyperintense T2 signal representing abundant mucin pools. Solid tumor components are intermixed, showing restricted diffusion on DWI and heterogeneous enhancement after contrast. The lesion demonstrates continuity with a chronic perianal fistulous tract, suggesting malignant transformation within a long-standing fistula rather than a primary intraluminal rectal tumor.

* Clinical and Pathologic Correlation

- Fistula-associated carcinoma is a rare but well-recognized complication of chronic perianal fistula, typically after more than 10 years of disease. Most cases are mucinous adenocarcinomas. Prognosis is generally poor due to delayed diagnosis, local invasion, and limited response to neoadjuvant therapy.

* Differential Diagnosis

- Primary rectal mucinous adenocarcinoma: Typically arises from the rectal mucosa, presenting as an intraluminal mass without clear fistula tract involvement.

- GIST with cystic degeneration: May appear heterogeneous on T2, but usually has stronger enhancing solid portions and no fistulous tract association.

- Rectal duplication cyst / benign mucinous lesion: Typically thin-walled, cystic, and non-infiltrative, without solid enhancing tissue or restricted diffusion.

* Key Teaching Point

- In patients with a history of long-standing perianal fistula, the development of a mucinous mass with solid enhancing components and diffusion restriction should strongly suggest fistula-associated mucinous adenocarcinoma. Careful evaluation of continuity with fistulous tracts is essential for correct diagnosis and surgical planning.