DEVIL'S ADVOCATE #1: HOW MANY AUSTRALIANS WILL DEVELOP SKIN CANCER?

Author: Dr D Czarnecki, 2000

It has been stated that two out of three Australians will develop skin cancer during their lives. This projection arose out of surveys of skin cancer that were carried out in the 1980s. However, were these surveys reliable? The design of each study will be summarized because each has faults.

A. The first survey was carried out in 1985. A market research firm contacted Australian households at random and a series of questions was asked. The questions included what products were purchased, voting intentions, and had the interviewee been treated for skin cancer in the preceding year. The treating doctor was contacted to confirm the diagnosis and the incidences of basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) were calculated.A summary of the survey is set out in Table 1.

Table 1

30,976 were asked questions

1,179 said that they had been treated for skin cancer

928 gave enough information for the treating doctor to be contacted

823 doctors replied

795 records were found

253 medically confirmed skin cancers were recorded in 245 patients

The incidence of BCC was calculated, as was the incidence of SCC, but the two were added together to give an incidence of 555 per 100,000 nonmelanoma skin cancers. In other words, some patients were counted twice. The incidence was then multiplied by a correction factor of 1179/795 because more people said that they had been treated for skin cancer than records were found. This resulted in a claim that the incidence of nonmelanoma skin cancer was 823 per 100,000. From this figure it was projected that two out of three Australians will develop skin cancer during their lives.

Table 2: A summary of the findings:

Reported incidence	555 per 100,000
True incidence	<555 per 100,000
True incidence (histologically confirmed)	<444 per 100,000
Adjusted incidence	823 per 100,000

It should be noted that the incidence of a disease is the number of people with the disease per 100,000 of the population per year. It is not the number of diseases per 100,000 of the population per year. The researchers were wrong to count some people twice. They were also wrong to include unconfirmed cases of skin cancer because a clinical diagnosis is often wrong. This means that the incidence of histologically confirmed nonmelanoma skin cancer found by the researchers was less than 440 per 100,000. If the incidence is really less than 440 per 100,000, it means that the projected number of Australians who will get skin cancer is closer to one in three.

B. The second study was carried out in a Victorian country town. Residents were asked to attend for a free skin check and suspected skin cancers were removed. The residents were asked to attend again five years later and the incidence of skin cancer was calculated. This type of study is biased towards people with skin diseases because those who do not have a disease are less likely to attend for the free skin check. Researchers from Western Australia carried out a similar study and calculated the degree of bias. Compare the two studies which are summarized in Table 3.

Table 3 Two population-based studies of skin cancer (Victoria, Western Australia)

Both studies had similar percentages of the population attending for the first and the second visits. However, the researchers from Western Australia calculated that people who attended twice were 1.37 times as likely to have a skin cancer as those who attended once, and that they over-estimated the incidence of skin cancer by one third.

The Victorian study found an incidence of skin cancer of 666 per 100,000. From this it was predicted that two out of three people would develop a skin cancer by the age of 70 years. No adjustment was made for possible bias.

The major fault of both studies that claim that two out of three Australians will develop skin cancer is that they failed to ascertain if the people with skin cancer had previous skin cancers. This is critical because patients with skin cancer tend to develop more skin cancers and so may be counted more than once in surveys. This will result in an overestimation of the risk to the population over a lifetime.

Two prospective Australian studies found that two out of three patients (adjusted for losses) with skin cancer will develop another within five years. This has a dramatic influence on any projection with regard to the number of people who will develop skin cancer. If 1000 people develop skin cancer in one year, it is projected that 10,000 will develop one after ten years (10x1000). In reality, far fewer will develop skin cancer if patients with a previous skin cancer are excluded, as can be seen from Table 4.

Table 4: Theory (a) vs Reality (b) (based on Melbourne data)

New people: In the first year 1,000 were detected with skin cancer, but in subsequent years fewer new patients with skin cancer will be detected.

A prospective Melbourne study found that 36% of people with skin cancer develop another within one year, therefore there will only be 640 new people with skin cancer, not 1,000 as predicted. Within two years 49% develop another skin cancer, 60% within three years, and 67% within four years. This means that less half the number predicted will develop skin cancer after 10 years.

How accurate is the Victorian prediction that two out of three people will develop skin cancer by the age of 70 years? Dr D Czarnecki, a Victorian dermatologist, reviewed all patients that he saw in private practice. Between August 1982 and August 1998 he saw 2,987 patients aged 70 or more. There were 1,600 women and 1,387 men, and 45% had a biopsy taken of a skin lesion of some sort. A total of 850 patients had at least one histologically confirmed skin cancer during the 16 years, and another 85 said that they had had a skin cancer removed by another doctor. Patients with a history of skin cancer were included in the calculation even though not all cases were confirmed. By the age of 70 years, 31% of patients had had skin cancer. By the age of 80 years, 39% of patients had had skin cancer. Patients aged 90 or more were fewer in number but 41% had had skin cancer. This is approximately half the number predicted by the two surveys. However, it should be noted that those surveys failed to determine if people had previous skin cancers, therefore some people were counted twice.

In summary: there is no reliable evidence to predict that two out of three Australians will develop skin cancer. My best guess is that about one in three will.

References: 15, 21, 23, 29