Teaching Case

We report a case of an 80-year-old woman who presented with an anal mass and underwent a haemorrhoidectomy. After that, the patient was sent to our hospital for further evaluation.

The digital rectal examination identified a scar and adjacent to it a hard, fixed and nodular mass.

On magnetic resonance imaging (MRI), we depicted a solid, intraluminal and lobulated tumour.  It originated in the posterior wall of the anorectal junction.  It was iso to hyperintense on T2WI, hyperintense on T1WI and highly restrictive on diffusion-weighted imaging (DWI). It was not possible to clearly define the integrity of muscularis propria on MRI. A rounded lymph node was also seen in the mesorectum.

Since the integrity of muscularis propria was critical for the surgical management, the patient underwent rectal ultrasound, which demonstrated that the tumour was contained to the submucosa. 

The pathological evaluation proved the lesion was an anal melanoma.

Anorectal melanoma (AM) accounts for 1% of anal cancers [1-3]. It has particularly aggressive biology compared with cutaneous melanoma [4]. Clinically, AM tends to be a polypoid tumour, ulcerated and amelanotic, with an uneven surface, with eventual dark spots [5].

AM usually affects women in the fifth or sixth decades of life [5]. Patients usually complain of bleeding, a mass or anorectal pain. Occasionally, melanoma is diagnosed as an incidental finding on the pathologic evaluation of a haemorrhoidectomy, like our case.

Although the diagnosis is not clear in an initial stage, most patients have a curable local disease with or without lymph node metastases [3,4]. Perineural invasion, deep depth of invasion and the amelanotic melanoma are associated with a poor prognosis [2].

The staging workup included a positron emission tomography (PET), a pelvic MRI (Fig. 1 and Fig. 2) and a rectal ultrasound (Fig. 3), which verified the integrity of muscularis propria layer.

Surgery is the primary curative choice, aiming a resection with negative margins and sparing the sphincter complex. Abdominoperineal resection is held for patients with bulky local disease and a few patients with local recurrence [6].

The five-year survival rates for patients with AM is <20% [3,5].

The patient refused surgical treatment, so electrochemotherapy was proposed as a palliative approach.

Written informed patient consent for publication has been obtained.