A report from the National Cancer Registry has revealed the extent to which geography affects a person's risk of developing cancer.
The report, Atlas of Cancer in Ireland 1994 to 2003, links cancer levels in Ireland to deprived areas. It showed that for most cancers, there is significant geographical variation based on where you live.
Furthermore, more densely populated areas consistently had a higher risk of cancer than those that were sparsely populated.
The Atlas said that well-known risk factors such as smoking, alcohol, obesity, diet and sexual behaviour affected cancer incidence, but that many factors that can lead to cancer are not yet known or are poorly understood. Some of the patterns in cancer risk observed in the report could not be explained on the basis of what is currently known, it said.
According to the Atlas, evidence suggests that there are genetic differences in different parts of Ireland, but it is unlikely that variations in genetic make up alone can explain the variations in cancer incidence.
Each of the cancers analysed showed some association with deprivation, which is also evident in the UK and other countries.
The authors stressed that socio-economic variations occur for a number of reasons, including differences in screening uptake rates, smoking, alcohol, diet and obesity. Groups of lower socio-economic status have higher prevalence of smoking and obesity, and lower consumption of fruit and vegetables.
The Atlas concludes that while most diseases are a result of both environment and genetics, it is lifestyle factors that are of profound importance.
According to the Atlas, the higher incidence of lung cancer in cities and in the east of the country reflect geographical variations in smoking habits, as 90% of lung cancers are caused by smoking.
Smoking is also a major risk factor for cancers of the bladder and head and neck and, to a somewhat lesser extent, for cancers of the stomach, oesophagus and cervix. Therefore, some similarities in the geographical distributions of these cancers and lung cancer might have been expected, the authors of the Atlas said.
The research showed similarities in the distribution of lung cancer with that of bladder, head and neck cancer.
However, the distributions of cancers of the stomach, oesophagus and cervix were much less similar to those of lung cancer, pointing to the importance of other risk factors in these cancers.
There were some similarities in the patterns of breast and prostate cancer and of non-melanoma and melanoma cancer of the skin. Significantly, the detection of all these cancers is influenced by better health awareness and access to early detection or screening. Higher incidences of prostate cancer were seen around the major urban centres, with the notable exception of Limerick. In Dublin and Cork, incidence was higher in the more affluent areas of the cities.
Variations in GPs conducting PSA testing also appeared to drive incidence of prostate cancer. Similar patterns occurred in relation to breast cancer. The highest breast cancer incidence occurred around Dublin, where the national screening programme began. The distribution of private health insurance was also an influence.
The Atlas revealed striking variations in incidences of cervical cancer around the country. A distinct area of higher risk extends westwards from Dublin and south towards Wexford, and human papillomavirus (HPV), which can lead to cervical cancer, was also found to be more prevalent in the east than the west of the country.
There was a higher risk for stomach cancer in the northeast, while the south has a higher risk for oesophageal cancer. The incidence of colorectal cancer was more balanced, but there were areas of higher incidence in and around Cork and Dublin, and in the northwest for women and the northeast for men. According to the authors, variations could be due to the combined influence of geographical variations in obesity, levels of physical activity, diet, use of aspirin and other drugs.
With the exception of prostate cancer, all cancers were associated with population density. More densely populated areas consistently had a higher risk of cancer than those that were sparsely populated. According to the authors, this may come back to socio-economic factors, as many places classified as deprived are in urban areas.
Risk of cancer was highest in areas with the highest proportion of elderly people living on their own. It seems that this is because of lifestyle differences, but several different explanations are offered for this scenario. Firstly, elderly people living alone can be associated with deprivation. On the other hand, they may make greater use of health services and are consequently more likely to be diagnosed with cancer.
Furthermore, studies in the UK have shown that elderly people living alone have poorer diet, lower physical activity, more hazardous alcohol use and are more likely to be smokers than those who do not live alone.
Areas with a higher percentage of agricultural workers were found to have a lower risk of cancer. This was true for all cancers apart from prostate cancer. The researchers believe that this may be related to the fact that farmers tend to work in less densely populated areas.
The researchers concluded that cancer is arises as a result of prolonged exposure to a particular, or potentially several, risk factors.
“Therefore, what is relevant in terms of interpreting current patterns of cancer incidence are patterns of exposure to risk factors 20 or more years ago. However, the available data relate to current (or recent) patterns of exposure, and these may not reflect patterns in past years,” they said.
The Atlas stressed that much is unknown about cancer risk factors. A 2007 study estimated that 40% of cancers in France were attributable to known environmental or lifestyle risk factors including smoking, alcohol, obesity, lack of physical activity, exogenous hormones, and only a small proportion of the remainder were likely to be due solely to known genetic factors. This means that there are still major gaps in knowledge about cancer risks.
Further study is needed to address why certain areas have unexplained higher than average cancer incidence. There is little or no explanation as to why there is a higher incidence of stomach cancer in the northeast and far northwest of the country, a strip of higher incidence of bladder cancer down the east coast and an area of higher incidence in the northwest, a diagonal split across the country into areas of higher (south and east) and lower (north and west) incidence of oesophageal cancer, an increased risk of colorectal cancer around Cork, and various patches of higher incidence of melanoma and non-melanoma skin cancer around the coastline, particularly in the west of the country.
The Atlas was written by Anne-Elie Carsin, Linda Sharp and Dr Harry Comber, and involved an analysis, on an electoral division basis, of cancers diagnosed in the population of Ireland between 1994 and 2003 and registered with the National Cancer Registry.