Synchronous adenocarcinoma of the rectosigmoid and the perianum is exceptionally rare and we could only find 6 previously reported cases [5
]. Very frequently, the symptoms in such patients are only attributed to their perianal disease and the development of supervening carcinoma is missed. It is therefore necessary to restress that it remains advisable in all cases of fistula and anorectal abscess to assess histology[11
The presence of perianal malignancy of mucinous histology may have several aetiologies. One possibility is malignant degeneration within a long-standing anal fistula and this, although uncommon, has been well reported. The histology of such cases tends to be either squamous or mucinous, with Rosser first describing in 1931 the 3 basic criteria for definitional purposes which determine that a fistula has undergone malignant transformation[12
]. For this diagnosis, the fistula should have been present for a minimum of 10 years, there should be no evidence of tumour within the rectal or anal canal mucosa and the internal opening of the fistula should be devoid of malignancy. A further clue to this diagnosis is the presence of free globules of mucin which lie away from areas of granulation tissue recognizable within a fistulectomy specimen, as noted in our case[13
]. Of course, a rectal carcinoma may present as a fistula or with acute perirectal sepsis and it may be difficult in some cases to determine whether the tumour is a complication of a long-standing perianal fistula or whether the carcinoma itself has fistulated. Additionally, carcinoma may supervene within the rectum adjacent to a fistula in perianal Crohn's disease[14
]. True cutaneous metastases to the anal skin, (from colonic or other types of carcinomas), are exceptionally rare; most likely resulting from retrograde vascular dissemination or systemic haematogenous spread by tumours which are typically advanced at presentation[15
]. Equally, mucinous carcinoma may develop within an unrecognized rectal duplication and present as a perianal mass or discharge[17
Viable tumour cells which represent clones capable of transplantation and proliferation have been retrieved from the lumen of the large intestine distal to established rectal tumours[18
]. In this case it is believed that direct implantation of malignant cells occurred onto an area of anal granulation tissue within the fistula; a phenomenon which has previously been reported in patients with fresh haemorrhoidectomy wounds where a rectal carcinoma had been missed[19
] and following local anal canal trauma following insertion of an endorectal stapler during low anterior resection[20
]. Colorectal tumour implantation at the time of surgery is controversial and may possibly be diminished by the use of intraluminal cytocidal agents[21
]. It would seem most likely in our case, given the long-standing nature of the perianal fistula, the similarity of histology of the two tumours, the intervening normal rectal mucosa and the stage of the colorectal primary. Evidence for this hypothesis has come from Scott et al.
] where tumours shared DNA ploidy on flow cytometric analysis, suggesting a common clonal origin and from Hyman and Kida[10
] where the pattern of cytokeratins 7 and 20 immunostaining of the anal tumour matched that of the colonic cancer.