Where the sun never shines: anorectal melanoma

Melanoma occurring in the anorectal region is recognized to be an uncommon tumour with a poor prognosis. Queensland has the highest incidence rate of cutaneous melanoma in the world. The purpose of the present study was to determine if the very high incidence of cutaneous melanoma in Queensland has any bearing on the incidence of anorectal melanoma, and to compare this with published studies from other parts of the world. An assessment of the authors' results in the treatment of anorectal melanoma was made by a review of patients presenting during the last decade throughout Queensland.

All cases of melanoma occurring in Queensland are accumulated in the Melanoma Register, administered by the Queensland Melanoma Project, and data from this were used to derive the results in the present study. Queensland population data were provided by the Australian Bureau of Statistics.

During the study period 1985-95 in Queensland there were 21,391 cases of cutaneous melanoma which occurred in an average population, during that time, of 2.89 million. During the same time there were nine cases of anorectal melanoma, giving a relative incidence therefore of 0.04%. This contrasts with published relative incidences of 0.2% for white people in North America, 11% for Southwestern Native Americans, and 14.2% for Northern Pakistan (P < 0.0001). However, the population-based incidence rates of anorectal melanoma for Queensland, United States white people, Southwestern Native Americans and Northern Pakistan are not significantly different from each other. All patients in the present series presented with large tumours, and none survived beyond 25 months despite standard surgical and adjuvant therapy.

The protection against cutaneous melanoma afforded by skin pigmentation is not apparent in the anal area. Anorectal melanoma is no more or less frequent in Queensland, even with the very high incidence rate of cutaneous melanoma. Sun exposure appears to be neither protective nor a risk factor. The outcome of therapy for this condition remains disappointing in the authors' experience.