Multiple primary melanomas: an analysis of cancer registry data from Victoria and New South Wales.

All primary invasive cutaneous malignant melanomas (CMM) diagnosed in Victoria and New South Wales from 1985 to 1989 were obtained from the population-based cancer registries. Altogether 14,590 people with first CMMs were followed for at least 2 years, during which time 496 multiple primary CMMs were identified. Of the study population, 3.4% developed a second primary CMM and 0.3% developed three or more. It was estimated that 4.5% of people would develop a second CMM within 5 years of the first and that the risk was higher in males, particularly in men aged over 70 years. With regard to metachronous primaries, only age and thickness of the first primary were significant predictors of the thickness of the second: older people tended to have thicker CMMs and second CMMs were generally thinner than the first. Body site concordance was higher than expected by chance, particularly for synchronous diagnoses. The high degree of site concordance of metachronous primaries lent support to the hypothesis that skin adjacent to the first CMM might have undergone a 'field effect', rendering it at increased susceptibility to malignancy.