Primary ano-rectal malignant melanomas within a population-based national patient series in Sweden during 40 years

To analyze 251 patients (101 males and 150 females) diagnosed with ano-rectal malignant melanoma (ARMM) reported to the Swedish National Cancer Registry during 1960-1999.

Incidence, gender and age profiles, primary anatomical sites and density of the melanomas along with geographic distribution, and prognosis were investigated.

The age-standardized incidence of ARMM was significantly higher for females (1.0 per 10(6) females) than for males (0.7 per 10(6) males) throughout the 40-year-period. The incidence increased with age peaking at 75-84 years in both genders. 54% of the tumours were primary in the anal canal, 24% engaged the whole ano-rectal unit and 10% were located at the anal verge (11% unknown primary site). Although ARMM were rare in absolute numbers, their density (number of tumours/square unit) was higher than that of cutaneous malignant melanomas (CMM) on average. No linkage between the geographic distribution of ARMM and population density was found. The prognosis was very poor albeit with a significant gender difference with a five-year survival rate of 10.6% for males and 15.7% for females. The survival rates for both genders improved during the 40-year-period but significantly more for females than males.

The reason(s) for the difference in incidence and prognosis according to gender is unknown. The majority of ARMM emerged primary in the anal canal and a primary location exclusively in the colonic mucosa of the rectum is questionable. The higher density of ARMM as compared to the average density of CMM tallies with the result of our previous studies on vulvar melanoma and might be instrumental in exploring non-UV light associated factors in melanoma genesis. The concentration of patients with anal squamous cell carcinoma to population-dense urban areas, as previously reported, was not found in cases of ARMM.