Summary of results
There are several important features of the recent CMM incidence trends for New South Wales. First, the trends differ significantly according to gender: female incidence increased only very slightly over the 14 year study period (0.45% on average per year) whereas male incidence increased by 3.17% per year. The result is a substantial male preponderance of CMM, where previously there was approximate equality (though rates were higher in women at younger ages and men at older ages). Second, there are substantial differences in the trends according to age: incidence increased most in the oldest age groups, particularly in men, but actually fell significantly in young women (age groups 15–34 and 35–49) and non-significantly in the youngest men (age 15–34). Third, the patterns of change with age and time are suggestive of a birth cohort effect, with cohorts born between 1935 and 1950 experiencing rising incidence, and subsequent cohorts experiencing falling (females) or stable (males) incidence. Fourth, for each gender, there were similar rising trends across body sites, with the exception of the lower limb in females where incidence fell slightly. Finally, incidence rates were similar for thin (≤0.75 mm) and thick (>0.75 mm) melanomas within each gender, but their trends differed somewhat. Incidence of thin melanoma rose in all except the youngest age groups whereas incidence of thick melanoma rose only in the oldest age groups.
There are few other reports of recent (i.e., since the mid-1980s) melanoma incidence trends. Results from data in populations of mainly European origin suggest 2 major patterns: one of falling or stable incidence in recent years, seen first in the youngest age groups, and the other of continuing increases across all age groups.
The first pattern, as observed in New South Wales, was evident in Hawaiian whites as early as the beginning of the 1980s.1 It has also become apparent more recently in several other North American populations: Canada, since the mid-1980s4 and the U.S. areas in the SEER (Surveillance, Epidemiology and End Results) program since the early 1990s.13 In these populations, increases have generally been greater in males than in females (or decreases greater in females than in males), so that the male to female incidence rate ratio has risen. In both North America and Australasia, there is good evidence that the secular trends are the result of cohort patterns, with more recently born cohorts at lower risk.2–4, 14–16
The second pattern is evident in most populations of Europe. In regions with a large enough population base to produce stable age-specific incidence rates (in the age groups used here), rates were rising up to the time of the most recent data from Cancer Incidence in Five Continents (generally 1988–92) for both males and females.6, 9, 17 There was no consistency across regions in terms of which gender or age groups showed the greatest rate of increase.
There are even fewer data concerning recent trends according to body site and tumour thickness. For body site, 2 reports have findings similar to ours. For the U.S. SEER areas, Hall et al.13 reported proportional increases in incidence of 12% to 25% across sites for males and 3% to 15% for females between 1985–89 and 1990–94; the lowest rate of increase was for lower limb melanomas in females (3% rise). Gaudette and Gao4 noted a fall in the incidence of leg melanomas in women in recent years in Canada, whereas rates for other subsites in women seemed to be level. The actual size of these incidence changes cannot be determined because data were presented only graphically.
The incidence of both thin (≤0.75 mm) and thicker (>0.75 mm) melanomas increased in SEER areas between 1988 and 1994,13 but with much greater increases in males than females; in females, increases were greater for thin melanomas, whereas in males thicker melanomas increased more. Using different cut points on the same data set, Dennis18 reported slightly larger increases for lesions 1.0 mm or more thick than for thinner ones, for both genders combined.
The major cause of cutaneous malignant melanoma is exposure to the sun,19 with an intermittent pattern of exposure considered particularly harmful. The trend of continuously increasing incidence over several decades in populations of European origin, largely in a cohort fashion (with those born before the middle of the century at ever-increasing risk), has been attributed to changing sun exposure. There are several aspects of sun-related behavior that may have been responsible for increases. They include adoption of briefer clothing styles, including less hat use, and increasing recreational exposure to sun, both on vacations and at other times, with increasing affluence and correspondingly, a reduction in work-related sun exposure; the latter has been associated with lower than average risks of melanoma.20, 21
In New South Wales, there is evidence that the stabilization or fall in incidence has, like the increase, happened in a cohort-wise fashion, with those born since about 1950 at the same or lower risk compared with earlier cohorts. What changes have occurred to turn around the pattern of increase, particularly among women? It would seem reasonable to postulate either reduction in amount of sun exposure without a countervailing increase in intermittency, or a fall in intermittency of exposure without an increase in amount. Australia has had programs to increase public awareness about the hazards of sun exposure and to educate the public concerning use of effective sun-protective behaviors for nearly 20 years.22 There is evidence from repeated surveys starting in the late 1980s in the Australian state of Victoria that important shifts in behavior have occurred toward increased use of sun protection, including sunscreens,23 and reduced levels of sunburn.24 The latter may be an indication that people are either reducing their amount of sun exposure or moving away from the more hazardous intermittent pattern of exposure. Further support comes from the systematic observation of attendees at two major sporting events in Melbourne, Australia that noted that a high percentage of people were covered or had applied sunscreen; few, however, were considered to be optimally protected from the sun.25, 26 Of some interest is the anecdotal observation by Borland and Theobold that the Swedes in attendance at one of the sporting events (at least those who were obviously Swedes by their flags or face paint) had considerably less sun protection than the crowd average.26
It may be argued that it is too early to be seeing falls in melanoma incidence in people born after 1950 resulting from sun protection programs that started only in the 1970s or 80s, especially given the apparent importance of early life sun exposure on melanoma risk. There are a number of lines of evidence, however, pointing toward later stage effects of sun exposure on melanoma risk.27 We could postulate, perhaps, that early life exposure to the sun determines the lifetime potential for melanoma and later exposure determines how much of that potential is realized. The trends are consistent with that possibility but direct empirical data are lacking.
Why do women seem to have fared better than men? Empirically, changes in sunburn and protective behaviors have been greater in women than in men.24 It is also relevant to note that women have experienced major social changes in Australia and many other countries over the past 50 years. In particular, their participation in the workforce has greatly increased. Data on labour force participation in Australia between 1978 and 199528 show a substantial increase in participation by women 15–64 years of age (13.4 percentage points) beginning in 1984, with the largest increases being in women 25–54 years of age (between 15.2 and 20.4 percentage points). In relative terms, participation in these age groups increased by between 27% and 43%. Over the same period, men aged 15–64 years showed a small fall in participation (−2.8 percentage points); this fall was greatest in men 55–59 and 60–64 (−7.9 and −14.4 percentage points respectively). Both men and women 15–19 years of age showed a fall in participation over the period, greater in men (−6 percentage points) than in women (−0.6 percentage points). These data suggest an appreciable fall in opportunity for recreational sun exposure in women over the period of our study, due to competing work commitments, but a small increase in men, particularly in their 50s and 60s.
Additionally, special tabulations of data from the Australian Time Use Surveys of 1992 and 1997,29 supplied by the Australian Bureau of Statistics, indicate that at every age, men spent more time outdoors, during both employment and other activities. Somewhat trivially, perhaps, it may be noted that the fall in incidence of melanoma on the legs in women is consistent with the trend to much greater wearing of trousers by women over the past couple of decades.
Our results beg two other questions. First, why is the incidence of thick melanoma now increasing only in older people (65 years and older)? Presumably, this is a reflection of the cohort nature of changing incidence. Those who were recently aged 65 and over were born well before 1950 and so are now experiencing the higher rates of their cohort, for both thin and thick melanomas.
Second, do these data for New South Wales as a whole support the hypothesis, based on data from one part of the State, that increased and persisting high incidence rates in the late 1980s suggest increasing detection of a clinically inconspicuous form of melanoma with low metastatic potential?30 In the age-gender subgroups with the greatest increase in incidence, both thin and thick melanomas have been rising, whereas in those subgroups with declining incidence, both thin and thick melanomas have been falling. This does not support either a medical surveillance effect or a permanent increase in thin melanomas. The apparent persistence noted by Burton et al.30 may have been due to the effects of extended early detection, particularly after the airing of “Good-bye Sunshine”.31 “Good-bye Sunshine” was a television production that aired initially in late 1987 and featured a young man dying from melanoma.