Background
Perianal fistula mucinous adenocarcinoma (FMA) usually presents at an advanced stage, necessitating extensive surgical resection. Symptoms of perianal pain and discharge are indistinguishable from fistula sepsis. Absence of defined features on magnetic resonance imaging (MRI) precludes early diagnosis. This study aims to validate MRI features that should increase suspicion of early mucinous transformation, prompting urgent examination and targeted biopsies.

Methods
Retrospective review of MRI studies was conducted in 9 patients with FMA in Crohn’s perianal and non-Crohn’s ileoanal pouch fistula between 2015-2019. Radiological features were assessed. Fine T2 weighted high signal lobulation of the fistula tract on MRI, described as ‘budding’, was retrospectively noted on historic studies in all cases of FMA and was determined a feature distinct from expected T2 weighted high signal appearance of bland fistula sepsis. The significance of these features in early diagnosis of FMA was assessed using a case control study.

Results
‘Budding’, mass-like expansion of the tract, and septation of T2 weighted high signal components of the fistula were significantly associated with FMA using Fishers exact test (p <0.001). The presence of T2 weighted high signal ‘‘budding’’ predated the histological confirmation of FMA by a median 36 months (range 12-156 months). One control patient was diagnosed with FMA during the study as ‘budding’ was retrospectively detected, triggering urgent targeted biopsy.

Conclusions
Radiological awareness of early features of FMA may improve outcomes by reducing the morbidity of exenterative surgery with delayed diagnosis. The presence of T2 weighted high signal ‘budding’ on MRI should prompt urgent targeted biopsy.