by Sharon Beder
Professor Bill McCarthy is head of the Sydney Melanoma Unit at Royal Prince Alfred Hospital where a skin cancer called melanoma is studied and treated. The Melanoma Unit is the largest treatment centre for melanoma in the world. Two studies into the causes of melanoma are being carried out by the Unit. One is a study of the relationship between mole formation in children and their exposure to sunlight. The other is a study into the genetic causes of melanoma.
Melanoma is one of the three major forms of skin cancer and the only one that is potentially fatal. The only cause of melanoma known so far is exposure to sunlight. The disease is far more prevalent in white races and the degree of this prevalence is related quite closely to where people live and how much sunlight they get. One of the research projects that Bill McCarthy is conducting is a comparative study of mole formation in children living in Melbourne, Sydney and the North Queensland city of Townsville to see whether moles are related to the differing amounts of ultra violet light in these separate locations.
Ultra violet or UV light is a part of the sunlight that humans are exposed to when they go outside. Technically it should be called UV radiation rather than light since it is not visible to the eye. It occurs naturally but there are fears that the amounts that humans are being exposed to may be increasing as the ozone layer is depleted. UV light causes sunburn and skin cancer through damage to the skin cells.
The researchers at the Melanoma Unit have a strong interest in how and why UV light damages the cells of the skin, particularly those cells that produce benign moles (the ordinary brown pigment spots people have on their skin) and how UV light goes on to cause melanoma. A lot of researchers all round the world are working in the same area. The Melanoma Unit provides human material (such as samples of melanoma, blood and skin) for a number of other people who are also working in similar areas related to UV light. The fact that UV light causes cancer in the skin has been well known for 50 years but research on how it does it and why we are so sensitive is a relatively new field. Work is well developed in Australia because we have a higher incidence of melanoma than the rest of the world and so there is more scientific interest in it.
McCarthy's team wants to know how the amount of UV light a person is exposed to affects the development of moles - that's the benign condition, not the melanoma itself. The number and type of moles a person has are very accurate indicators of the likelihood of their getting a melanoma later in life and research is showing that sunlight or UV damage to the skin, is the basic cause of moles.
"One of the interesting projects that we are doing at the moment," says McCarthy, "is to look at groups of children between the ages of 6 and 15 living in cities that have different levels of UV light. We are looking at a group of children in Melbourne, a group in Sydney and a group in Townsville. The sunlight is much more intense in Townsville than it is in Melbourne. We have assembled groups of 100 six year olds, 100 nine year olds, 100 twelve year olds and 100 fifteen year olds in each of these three cities. We start by counting their moles and then we compare the number of moles on children of the same age groups in the different cities. The children selected for the survey are all of fair skinned genetic origin."
The project involves monitoring the general levels of UV light in each city while at the same time a subgroup of the children are actually monitoring their own exposure to sunlight. UV light meters have been positioned in each of these cities in various places close to where the schools are. The children are specifically monitored by asking them to fill in a questionnaire for three weeks of their life. It asks what they do every day during a typical period in the middle of the school term. Some of them are asked to wear UV monitors which clip on their shoulder or their hat which measure the direct UV light. The object of this research is to find out what dose these kids are actually getting in relation to the overall dose that is hitting the area and how does this information correlate with how many moles they have.
This is a longitudinal study which means that it will be looking at changes over time. It is proposed to go back at least every three years, depending on the availability of funds and researchers, and look at that same group of children again as they progress from 6 year olds to 9, from 9 to 12 and from 12 to 15. Although it is a long-term programme the researchers are already finding that they are getting significant results after an initial counting of moles.
There are 6 times more moles on children in Townsville, for instance, than there are in Melbourne. This is a big difference which is unlikely to be due to chance. There is no doubt that the incidence of exposure to sunlight is much greater in 6 year old children living in Townsville than for similar children living in Melbourne. The results seem to confirm that childhood exposure to sunlight is responsible for the formation of moles.
McCarthy's hypothesis is that the more moles a person has, the higher their risk of getting melanoma, and because children get a lot more moles in the sunlight in Townsville and the incidence of melanoma in adults is much higher in Townsville than it is in Melbourne, then early exposure to sunlight is probably the vital factor in the risk of developing melanoma.
In March 1992 the Sydney Morning Herald reported that the number of melanoma cases in NSW had doubled in less than seven years. Queensland levels has also risen sharply and were 50% higher than NSW levels. (graph from Sydney Morning Herald 27/3/92)
Most people simply assume that because a person lives closer to the equator the incidence of UV induced disease is likely to be much higher than it would have been if they lived further south. It has also been known for some time that ozone depleted atmosphere does exist over southern Australia in December/January when the ozone hole over the Antarctic breaks up. It is assumed that ozone depletion will also cause more UV induced disease because of the increased levels of UV light coming through.
The team is testing these common assumptions by monitoring the actual levels of UV light in each city to see whether they are different from that which they expected and whether they vary much over time. "These monitors, sitting on the rooves which are giving continuous monitoring of the UV levels, may tell us much more accurately when there are clouds of ozone depleted air coming over the city. For instance, we may find that the monitor suddenly shows that the UV light goes up even though there is no sunspot activity. This would indicate that something else must have happened" says McCarthy, whose interest in these matters has been spurred on by his interest in the ozone layer.
These monitors will
initially be doing general monitoring and perhaps later more specific monitoring.
"They will probably show that our current understanding of what is actually
happening at skin level is very incomplete. It may change the whole picture
if we find they are looking at a more complex relationship between sunlight,
the skin and the development of malignancy."