Each year approximately 1,000 cases of possible cancer clusters are reported. Of these cases only 5% to 15% are investigated after the initial report. The bulk of the cases are clearly not cancer clusters and researchers can eliminate the possibility with just a bit of research.
Out of the 5% to 15% of cases that are investigated most are still definitively classified as cancer clusters. Epidemiologists are rarely able to provide definitive answers however a set of specific questions helps guide the initial investigation. The following questions from the Cancer Council of New South Wales are similar to those utilized by many local and state health organizations and include:
- Have all the cases been identified?
- Are all the cases of the same (or similar) type?
- Are both sexes affected? (If environmental factors, such as chemical contamination or radiation leakage, are suspected to be a cause then a rise in cancer amongst both sexes would be expected.)
- Is there statistical evidence to suggest that the number of cases exceeds what would be expected in this population?
- Do the cancer types have a known common cause, whether occupational or non-occupational?
- Did the persons diagnosed with cancer have a common occupational (or non-occupational) exposure?
- Are there known workplace exposures that could have contributed to the occurrence of the cancers?
- Did the cancers occur at an appropriate time in relation to the possible workplace exposures? (Some cancers can take many years to develop.)
- On the balance of probabilities, is it likely that the identified cancers occurred as a result of occupational exposures?
- Are there any plausible non-occupational causes for the apparent cluster?
Many epidemiologists feel that identifying cancer clusters is so difficult because of the lack of accurate cancer reporting from state to state. Although the federal department of the Center for Disease Control (CDC) controls the cancer data registry this department must rely on the information provided by each state. Each state may have different reporting systems making collecting and compiling accurate information difficult.
Further complicating the process is the lack of information on the local and state level regarding environmental risk factors and the existence of possible carcinogens. Some local governments collect insufficient information and some don’t collect any information at all.
In recent years there have been investigations of nuclear facilities, hazardous dumping grounds, schools, offices and power lines. None of these investigations have provided proof that carcinogens exist in the environment nor have they provided definitive evidence of cancer clusters. Individuals in areas surrounding these sites have reported cancer clusters, however the rates are in line with the expected rate of cancer in these areas.
Many states have a central registry of cancer statistics for the state. This data often includes the number of newly reported cancer cases as well as death rates due to cancer. Epidemiologists use this data to compare expected cancer rates to the actual reported rates. This information helps in the initial decision when investigating a cancer cluster.
Some of the most common reports of cancer clusters come from individuals who suspect occupational exposure to carcinogens. This information is often directed to the National Institute for Occupational Safety and Health (NIOSH). NIOSH will investigate occupational hazards and assist with determining if a cancer cluster does exist.