When it comes to lung cancer, we know of more than 30 substances that are carcinogenic. But when researchers began investigating why there is a higher incidence of prostate cancer in firefighters, they had little to go on.

Prostate cancer is the most common form of cancer among men, yet up until now, no substance has been classified as a definite carcinogenic agent for this type of cancer by the World Health Organization’s cancer research agency IARC. IARC defines an agent as substances, microorganisms, exposure situations or professions that can cause cancer.

When it comes to lung cancer, which is the second most common form of cancer among men, there are more than 30 different agents that the IARC classifies as carcinogenic, including smoking and asbestos. For bladder cancer, there are 17 agents, and for breast cancer, five.

“But there are no known carcinogens for prostate cancer. It is surprising that we know so little about which substances are risk factors for this common type of cancer,” says Niki Marjerrison, a researcher in the Department of Biostatistics at the Institute of Basic Medical Sciences, UiO.

Is the amount of smoke and gases that firefighters inhale partly to blame?

Previous research from the Cancer Registry of Norway has found that firefighters are diagnosed with prostate cancer more often than the general population. But the reasons for this have been unclear. Marjerrison therefore wanted to study how factors such as exposure to smoke and gases at work might affect the risk of getting prostate cancer.

Among the 4,251 firefighters in the study, 268 were diagnosed with prostate cancer. The researchers then checked how long the firefighters had been in the profession and whether they had had a position that involved a number of other tasks such as administration and fire prevention, and estimated how many fires they had helped to put out. The aim was to find out whether being surrounded by smoke, various gases, chemicals and other substances often present at fire sites affected the risk of getting prostate cancer.

“When we compared the incidence of prostate cancer among those who had been most involved with firefighting with those who had been the least involved, we found little evidence of a connection between firefighting and the risk of getting prostate cancer, “says Marjerrison.

The study was recently published in the Scandinavian Journal of Work, Environment & Health.

PSA tests may have affected the incidence of prostate cancer

According to Marjerrison, there are many factors that complicate studies of how environmental and occupational factors may affect the risk of prostate cancer.

It was important that the researchers investigated one of these factors more thoroughly. They point out that something significant happened in the 1990s: PSA tests came into use. This test measures the amount of a protein in the blood. This protein normally increases when there is cancer of the prostate gland, but can also increase when there are benign conditions of the prostate.

“The use of PSA tests has meant that more cases of prostate cancer have been discovered than would have otherwise been the case. The PSA test is highly sensitive, which can be an advantage, but also results in positive test results at an early stage of conditions that would not cause problems in the future or require further treatment,” explains Marjerrison.

This sensitive test may therefore have been one of the reasons for the increased number of registered cases of prostate cancer among firefighters. In order to get around the problem caused by PSA testing, the researchers isolated and studied the aggressive cases of prostate cancer separately, because these were considered less likely to have been detected due to random testing.

“In total, 61 cases were classified as aggressive. This low number provides too weak statistical evidence to be able to point to a definite connection,” explains Marjerrison.

The researchers found that firefighters were younger and more often had cancers with better prognostic factors (less aggressive). This suggests more frequent PSA testing and increased diagnoses. But information on PSA testing was not available for each individual firefighter, so this could not be controlled for directly in the analyses.

Upcoming report on cancer among firefighters

Another difficulty encountered by the researchers has been that firefighters work in many different, acute situations. This makes it hard to determine and specifically measure which factors firemen are in fact exposed to during a fire, says Marjerrison.

The research team therefore developed so-called indicators for the substances the firefighters were exposed to, but they still found no evidence to show that increasing values of these indicators also increased the risk of prostate cancer.

The research into cancer among firefighters in Norway began about 10 years ago and was started by the Cancer Registry of Norway, where the work established a cohort and several articles were published based on this data. Now, Marjerrison and other scientists in the EpiStat group at UiO, and the Cancer Registry Norway, a part of the Norwegian Institute of Public Health, are working to complete a report that summarizes the studies that have been carried out over the past 10 years.