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Introduction

Malignant melanoma is the least common but most serious type of skin cancer. Early diagnosis usually results in timely surgical treatment and cure. Education and prevention are the most important aspects in the management of melanoma.

There has been a world-wide increase in the incidence of melanoma over the last four decades. In New Zealand, the incidence and mortality rates have increased dramatically since 1960. Until recently Australia was considered to have the highest incidence of melanoma in the world with the highest rates being recorded in Queensland at 50/100,000/y (50 new cases per 100,000 population per year).

Auckland is in the northern part of New Zealand at latitude 37¬? S and receives on average 2100 sunshine hours per year. The region contains one third of the population of the country. The true incidence of malignant melanoma in Auckland is not known, although it is thought to be high. The aim of our study was to document the incidence of melanoma in the Auckland Caucasian population. This disease is rare in Maori, Polynesian and Asian races and mainly affects people of European descent.

Methods

A list of all cases of invasive cutaneous malignant melanoma registered in 1995 from the Auckland region was obtained from the New Zealand Cancer Registry. Cases of in-situ (non-invasive) melanoma were excluded.

This data was combined with cases from 1995 collected by the Auckland Melanoma Unit. The AMU database consists of clinical, pathology and treatment details of 2500 patients with melanoma managed from 1970 to 1999.

Statistics from the 1991 and 1996 New Zealand Census were used to derive a population estimate for the Auckland region based on domicile in 1995. People identified as non-Caucasian (NZ Maori, Pacific Islanders and Asians) were excluded from the census figures and from the melanoma data-set. The resultant incidence figures were calculated for the Caucasian population only.

Results

There were 525 reported new cases of invasive malignant melanoma in 1995 in the Auckland region. The total population for Auckland in 1995 was calculated as 1,055,813. Of these, 64% of residents were Caucasian. Therefore, the overall crude incidence of melanoma amongst the Caucasians was 77.7/100,000/y. The rate was higher in males at 78.5 than females at 76.8/100,000/y.

The cumulative risk of developing melanoma, over a lifetime from age 0 to 74, assuming no other causes of death are in operation, was 5.9% for males and 5.4% for females.

The average age at diagnosis for males was 59 years and for females, 54 years. The rates for melanoma steadily increase with advancing age in both sexes. Melanoma is rare under 15 years of age, however in the eighth decade of life the age specific rate approaches 200/100,000/y in females and 300/100,000/y in males.

With regard to body site; the trunk is the most common site, followed by the lower limbs, upper limbs and then the head and neck. There are significant gender differences, with the trunk and head and neck being more common sites in males, and the lower limbs more common in females.

Discussion

The high incidence of melanoma in Auckland is probably due to out-door recreational activities and inadequate sun protection. Intermittent sun exposure and 'sunburn', particularly in childhood, is now well established as a causative factor in the development of melanoma.

The recent increase in incidence of melanoma in Scotland, Australia and New Zealand has been largely due to the diagnosis of more thin lesions. This is encouraging and suggests earlier diagnosis as a result of improved surveillance and education. Despite the high proportion of thin lesions, there are still thick melanomas being diagnosed, particularly in the head and neck region and in the elderly.

It is hoped that with public education and improved sun exposure behaviour, the rates should decrease in the longer term. However, with the effect of local stratospheric ozone depletion and the time lag from sun exposure to melanoma development, the incidence of melanoma in this part of the world may continue to rise well into the 21st century.

In conclusion, Auckland, New Zealand has the highest documented incidence of invasive malignant melanoma in the world, with a crude rate of 77.7/100,000/y. The cumulative risk of developing melanoma in a lifetime is a staggering 5.7%.

References

The full text of this article was published in the July 1999 issue of the World Journal of Surgery:

Jones WO, Harman CR, Ng AKT, Shaw JHF. Incidence of malignant melanoma in Auckland, New Zealand: The highest rates in the world. World J Surg 1999; 23(7): 732-5.