Perianal mucinous adenocarcinoma arising from chronic perianal fistula are rare tumours and only accounts for 3-11% of perianal cancers [6]. It may arise de novo and present as a fistula, or it may arise from a long-standing perianal fistula or abscess cavity [6]. Most often it becomes a diagnosis dilemma in view of the absence of tumour within the lumen of bowel and slow growth of the lesion hidden within the ischioanal fossa or perineum [6,7]. Usually, it mimics benign presentation and most often clinicians fail to recognize it. The pathogenesis of the disease is still controversial [7]. It is usually associated with long standing fistula in ano and turns malignant secondary to chronic inflammatory changes [7]. Rosser postulated three possibilities describing the pathophysiology of this unusual entity; the fistula may precede the carcinoma by at least 10 years, the fistula opens inside the anal canal and outside the tumour, and any anorectal cancer should be an extension from the fistula harboring the carcinoma [7].

There are few key highlights points in our case which reflects the difficulty in diagnosing perianal mucinous adenocarcinoma. Firstly, the short duration of history of fistula which does not favor the arbitrary time period of ten years as suggested by Skir in 1948 [8]. The prior biopsies which were obtained were all inconclusive which resulted in depriving the patient of early neoadjuvant therapy. The decision to operate was based solely on clinical manifestations and not supported by negative biopsy reports.

We should have high index of suspicion in patients who have non healing chronic fistula in ano especially with mucinous discharge which fail to heal in spite of multiple attempts of surgery. Multiple biopsies of the suspected lesions are critical to early diagnosis and treatment due to the isolated areas of malignant transformation separated by inflammatory zones. Presence of extracellular mucinous lakes surrounded by well differentiated dilated tortuous glands, nerves and vessels in histopathology confirms the diagnosis [5]. However, in many cases definitive diagnosis is only verified by the final histopathological examination of the resected specimen [2].

MRI of pelvis plays an important role to establish the disease extension and to plan for the surgical treatment [2,4]. Abundance of mucin in the tumours gives rise to significant hyperintense signal on T2 weighted MRI images [2]. Despite that the interpretation of MRI occasionally can be challenging.

Once diagnosed, treatment should be radical. A proper multidisciplinary team approach should be undertaken prior to surgery to improve outcome. Patients usually will require neoadjuvant chemoradiotherapy for local control of disease before proceeding with definitive surgery. Hongo et al., in an 11-patient study, reported higher disease-free survival in patients who were given chemoradiotherapy prior to surgery [2]. Abdominal perineal resection with wide local excision is the preferred option as it guarantees a R0 resection and thus reduces the chances of local recurrence [9]. In selected cases pelvic floor reconstruction will be needed to cover the wide perineal defect after an ischioanal or extralevator APR [2,9].

In conclusion, perianal mucinous adenocarcinoma arising from an anorectal fistula is an uncommon malignant entity. Surgical decision should be made on clinical manifestation even if biopsy fail to provide conclusive findings. An aggressive and multimodal approach is key to achieving optimum patient outcome.