by: Gil Lederman, M. D.

Epidemiologic studies are often important because of trends that may be observed. Associations between diseases in studies of large groups of a population, may lead to the determination of risk factors - factors that may lead to finding causation of disease or more importantly, the prevention of disease.
A recent report in the Journal of Clinical Oncology by Ahsan et al evaluated other cancers that occur in patients with malignant brain tumors. As the authors noted, this separate epidemiologic study can help determine whether there are "shared risk factors, effective treatment for one primary tumor on the risk of another cancer or genetic predisposition to multiple cancers."
This is especially critical as there is very little known about the etiology or cause of primary malignant brain tumors. In that effort, the SEER Program was used to collect this data. The SEER Program is run by the National Cancer Institute and has been collecting cancer-related information for more than two decades through major cancer registries. These registries document disease in 10% of the American population. More than 1-1/2 million Americans with cancer were evaluated between 1973 and 1990.
The authors evaluated those patients who had other cancers followed by the development of malignant brain tumors. Furthermore, the brain tumor had to be diagnosed more than six months after the initial cancer as the authors desired to eliminate coincidental findings during staging or evaluation of the first cancer. Additionally the researchers broke out the observations into five year sequences.
So, which cancers were associated with a higher risk of developing subsequent brain tumors? There was an increased risk of brain tumors after the diagnosis of bladder cancer in both men and women - although in women this did not reach statistical significance. Thus, only in men was statistical significance reached to verify whether the observed phenomenon was a trend or an actual reality. In men, there were elevated risks of developing brain tumors in those who had sarcomas or leukemias.
In women, the only increased risk of developing brain tumors was in those who had prior colon or rectum cancer. There was a slight risk of developing brain tumors after breast cancer and endometrial cancer, but those associations were not statistically significant. The role of hormonal factors was felt to be "further implicated by our finding of elevated relative risks of brain tumor after both breast cancer and endometrial cancer."
Of interest was that the authors found no increased risk of malignant brain tumors after the development of tobacco-related cancers such as those afflicting the lung or head and neck area. In fact, the opposite was true in that there was significantly lower risk of brain tumors after lung cancers in men. This would suggest that smoking is not related to the development of brain tumors. Finally, a negative disclaimer the tobacco industry can place on cigarette packages!!
This study is important for a variety of reasons. It is the first that evaluated the development of brain cancers after other cancers. An earlier evaluation suggested the risk of brain tumors was greater after only the diagnosis of melanoma but it was not confirmed.
The authors speculated that the association of brain tumors and bladder cancers might suggest that causes of bladder malignancies such as chemicals and dyes have similar effect on the brain. Another possibility is that bladder cancers and brain tumors might have a genetic link, predisposing certain individuals to both.
This data should help other investigators search for the cause of brain and other cancers. Important medical observations are made on the shoulders of giants.