Non-melanoma skin cancer in Australia: the 2002 national survey and trends since 1985

Australia has the highest incidence of skin cancer in the world,1,2 and non-melanoma skin cancer (NMSC) is the most commonly diagnosed cancer in Australia.3 It is Australia’s most expensive cancer, with more than $264 million (9% of the total costs for cancer) spent on diagnosis and treatment in 2000–01.4

Despite its frequency and cost implications, NMSC is not usually recorded by cancer registries in Australia. Incidence has been measured in defined communities at different locations across Australia,5-9 and nationally using a series of household surveys conducted in 1985, 1990 and 1995.2,10,11 These surveys showed increases in the incidence of both basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) between 1985 and 1995, but the increases were not consistent across age groups.

This report presents the results of the fourth national survey, conducted in 2002, and an analysis of trends since 1985.

The methods used were as in previous surveys.2,10-12 Face-to-face interviews were conducted by a market research company throughout 2002 to identify people who had been treated for skin cancer. Respondents who had been treated in the previous 12 months were asked for permission to confirm their diagnosis with the treatment provider.

During February, all respondents were asked about the reaction of their skin when it was repeatedly exposed, without any protection, to bright sunlight in summer. Three skin type categories were used: “Skin tans deeply”, “Skin tans moderately” and “Skin does not tan”. For the remainder of the year, only people reporting treatment for skin cancer in the past 12 months were asked the skin reaction question.

Age- and sex-specific rates were calculated by latitude, skin type, country of birth, and income and education levels. People with both BCC and SCC were counted for one occurrence of each cancer in the rates for each histological type and once only for the NMSC rates. Rates were adjusted for diagnoses we could not confirm by assuming that the skin cancer rate in respondents whose diagnoses could not be confirmed was the same as in those whose diagnoses could be confirmed. The adjustment factor used was the inverse of the overall response rate. To enable international comparisons and comparisons with previous surveys, the rates were standardised to the world standard population.1 For the analysis by latitude, Australia was divided into three zones at state borders that approximate the 29°S and 37°S parallels.

The body site distributions of BCC and SCC were compared using relative tumour density (RTD), calculated as the ratio of the proportion of tumours at a particular site to the proportion of total skin surface area of that site. The RTD adjusts the distribution of tumours at a site for its proportion of the total body surface area.13

Trends in rates were investigated by age, sex, latitude, and the sun sensitivity of the skin using data available from all four surveys. The number of cases was modelled as a Poisson random variable. Rate ratios (RRs) relative to rates in 1985 were estimated by age group, sex, latitude and skin type. The significance of trends in rates across the surveys was assessed from the significance of a continuous linear term for survey year in the model.

Approval for this study was obtained from the Human Research Ethics Committee of The Cancer Council Victoria.

In 2002, almost 2% of the whole population, 4% of those = 40 years and 8% of those = 70 years were treated for NMSC.

The highest age-standardised rates reported for NMSC outside Australia were almost an order of magnitude lower (men in Ireland 146.3; women in Graubünden, Switzerland, 102.7).1

Sun exposure (particularly during childhood), fair skin and poor tanning ability are established risk factors for skin cancer.15-17 The strong inverse association with latitude, the higher rates for Australian-born people and for people whose skin does not tan, all highlight the role of sun exposure in skin cancer risk.

Despite overall increases in the age-standardised rates of both BCC and SCC between 1985 and 2002, it is encouraging that the BCC rates for people younger than 60 years showed no substantial increase. The major determinant of lifetime potential for skin cancer is sun exposure in childhood and adolescence,18 although adult sun exposure also plays a role.19-21 Skin cancer prevention programs stressing moderation of sun exposure began about 35 years ago and were prominent in Australia from about 1980.22,23 Adults older than 60 years in 2002 were then 20 years old in 1962, and educational programs would not have affected their adolescent exposure. The stabilisation of BCC rates at ages less than 60 years is consistent with educational programs from the 1960s reducing childhood and teenage sun exposures, with or without changes in adult exposures. For SCC, a similar benefit in younger people may exist, but the numbers of cases surveyed in this age group were small. The trends in NMSC rates as younger people age into the high skin cancer risk age groups will also provide clearer evidence of the long-term effect of sun protection programs on skin cancer outcomes.

The relative increases in SCC rates between 1985 and 2002 were greater than those seen for BCC. Studies in other countries over similar periods have not found this difference24-28 and several have reported declining SCC rates.25,27 Cumulative sun exposure, such as might be acquired by fair skinned populations living in areas close to the equator, is associated with an increased risk of SCC. The fairer skinned populations in those studies lived further from the equator than Australia, while the studies reporting declining SCC rates were either predominantly Asian or had a substantial Hispanic population, which may account for some of the discrepancies.

There was no evidence that higher rates in the lower latitudes were approaching a plateau, as the magnitudes of the rate ratios were similar for each latitudinal zone. The caveat here is that the rates by latitude are based on the region of current residence, rather than where the respondent lived during childhood.

There is no simple method for monitoring the incidence of NMSC. For other cancers, Australia-wide population-based registries collate pathological and demographic data. To apply this technique to all NMSC is logistically impossible and would not account for skin cancers treated without pathological confirmation. The survey method we used was developed to overcome this lack of cancer registry data and to provide a reasonable estimate of the incidence of treated NMSC.

In the 2002 survey, about a quarter of people with confirmed NMSC had more than one skin cancer treated in the 12-month period. Multiple primary NMSCs are common, and up to 40% of patients could develop a second NMSC within 2 years.29-32 We counted people treated for confirmed skin cancers during a 12-month period, irrespective of whether they had skin cancer diagnosed in previous years. This differs from cancer registry reports of cancer incidence, which are based on the first ever occurrence of a primary cancer at a particular site.

Our survey method has several drawbacks. It relies on self-reported treatment for skin cancer and verification of those reports by the treatment provider. Most respondents reported no skin cancer, and we could not verify the accuracy of these negative reports. Diagnoses could not be verified if permission to contact the doctor was not given, or if the doctor did not respond to our enquiries. A high proportion of the doctors replied to our requests for confirmation, but only 33% of the self-reported skin cancers were confirmed. The main reasons for this were that many people reporting skin cancer had been diagnosed more than 12 months ago, or had had benign lesions other than skin cancer treated.

We have assumed that the skin cancer rate for respondents whose information we could not access was the same as for those we could, but cannot check the validity of this assumption. The methods were the same as in earlier surveys, and the proportion of unconfirmed reports was similar in all surveys (varying between 32% and 39%) and should not affect the trends.

A strength of our survey is that lesions treated with destructive techniques or topical agents such as imiquimod, but not histologically confirmed, would be counted. Standard cancer registry methods that depend on histological confirmation would not include these skin cancers.

This survey has documented the highest rates of a cancer in a whole population that have ever been recorded, and the high and increasing skin cancer rates highlight the importance of continuing to monitor their incidence. The stabilisation of NMSC rates, particularly for BCC, in people exposed to skin cancer prevention programs in their youth suggests that these programs have played a role, but the overall increases in NMSC rates seen since 1985 highlight the importance of maintaining and strengthening these initiatives for fair-skinned people.