Monday August 9 2010
The research does not support a cancer risk for asthma pumps
Asthma pumps “may increase the risk of prostate cancer,” says the Daily Mail. The newspaper says that men with asthma who regularly use an inhaler to ease their symptoms could have a 40% higher risk of the cancer than men without asthma. According to The Daily Telegraph, just having asthma increases the risk of prostate cancer by 26%.
This large study followed 17,000 Australian men for an average of 13 years to assess the link between reported asthma, particular medications and the risk of developing prostate cancer. The research did produce a few interesting results, and may inspire further study into the association between asthma and risk of cancer. However, this study provides no evidence that taking medication to treat asthma increases the risk of prostate cancer. The researchers also note that it is difficult to separate the effects of medications for asthma from the effects of asthma itself, further complicating the issue. This is early work in this area and more research is needed.
Where did the story come from?
The study was carried out by researchers from the Cancer Epidemiology Centre in Melbourne and other academic institutions in Australia. The study was funded by the VicHealth health promotion foundation, the Cancer Council Victoria and by grants from the Australia’s National Health and Medical Research Council. It was published in the peer-reviewed medical journal Cancer Epidemiology, Biomarkers and Prevention.
The headlines featured in news reports may be misleading because the study found no evidence that taking medication increased the risk of prostate cancer in asthmatics.
What kind of research was this?
This cohort study followed nearly 17,000 Australian men for an average of 13.4 years to assess whether a report of asthma at the beginning of the study was linked with the development of prostate cancer during the follow-up period.
What did the research involve?
The study involved the male participants of the Melbourne Collaborative Cohort Study. A total of 17,045 men were recruited between 1990 and 1994 from the Melbourne area. All were aged between 27 and 81 years old at the point they entered the study, known as the “baseline”. Those with missing data on asthma status or with known prostate cancer at baseline were excluded from this analysis.
The baseline questionnaire, as well as asking about previous medical conditions, age, smoking habits, education and country of birth, asked whether a participant’s doctor had ever told them they had “asthma or wheezy breathing”. Participants who said they had were also asked their age at diagnosis and whether they took any medication for this condition. The researchers used a dietary questionnaire to determine nutrient intake and calculated each participant’s BMI. Any medication being taken was also assessed at baseline.
Over an average follow-up of 13 years, cancer cases were identified through State Cancer Registries in Australia and the severity of the disease was noted. The researchers then analysed whether presence of asthma at baseline, or use of asthma medications (categorised into four groups: antihistamines, bronchodilators, inhaled glucocorticoids and oral glucocorticoids), was associated with the occurrence of prostate cancer. To do this, they ran a number of analyses, some of which adjusted for possible confounding factors including BMI, smoking, education, alcohol consumption, total energy intake and country of birth.
What were the basic results?
Over the course of the follow-up, 1,179 men in the sample developed prostate cancer, equating to 7% of the population. A report of asthma at baseline was associated with a “small increase” in prostate cancer risk, with men reporting asthma at the study start being 1.25 times (HR 1.25, 95% CI 1.05 to 1.49) more likely to develop the disease over follow-up than those who did not have asthma at baseline. When they limited their analysis to only those men who said they had asthma and who responded to the medication audit (82% of the sample), there was no longer a significant association between asthma and prostate cancer.
When assessing only these men who provided a complete record of their medications they found that:
- using bronchodilator drugs was associated with 1.36 times greater risk of prostate cancer (HR 1.36, 95% CI 1.05 to 1.76)
- inhaled steroids (glucocorticoids) with 1.39 times greater risk (95% CI 1.03 to 1.88)
- systemic steroids with 1.71 times greater risk (95% CI 1.08 to 2.69)
When they adjusted these results for whether the person also said that they had asthma (i.e. considering asthma as the confounder), the only significant association between prostate cancer and medication, independent of asthma, was with inhaled glucocorticoids.
Separately, the researchers report that the risk of cancer in men using medications to control asthma was not different from that in men who did not use medications to control asthma. However, in their discussion they say they found “suggestive evidence that asthmatic men who reported taking medications for their asthma have a slightly higher risk of prostate cancer than those asthmatic men who reported not taking medications specifically for asthma”.
How did the researchers interpret the results?
According to the researchers, a history of asthma as well as use of asthma medications, particularly systemic [oral] glucocorticoids, is associated with an increased risk of prostate cancer. They note that it is difficult to disentangle the effects of medication to treat asthma from the effects of asthma itself.
This cohort study has found an association between reports of asthma and later development of prostate cancer. Some of the findings are hard to interpret, and the researchers acknowledge that it is difficult to separate the effects of asthma medications from the asthma diagnosis itself.
All cohort studies have a potential weakness in that they cannot control for all confounding factors that may influence the relationship being studied. While this research took into account some factors, including age, alcohol and smoking, it did not adjust for other known risk factors, including family history of disease and physical activity. It is not clear what effects these would have had on the results.
Another study limitation raised by the authors is that their questions did not distinguish between types of asthma and whether there were co-existing allergies. In addition, participants were asked if a doctor had ever told them that they had asthma or “wheezy breathing”, therefore it is highly possible that many men responding to this latter question were categorised as having asthma when they did not. Many things can cause wheezing, including acute respiratory infections and chronic bronchitis.
A further problem arises with the overlap between the medications used in asthma and in bronchitis (both of which can be treated with bronchodilators and steroids), which may have led to some people being wrongly considered as asthmatic. However, there are some strengths, too: notably, the prospective design and the large sample.
The strongest associations noted were with oral (systemic) glucocorticoids, although the researchers highlight that “it is premature to propose that systemic glucocorticoids are responsible for the observed associations” with prostate cancer. Instead, they say the drugs may suppress the immune system and therefore increase the risk of the disease.
This research may inspire further study into the association between asthma and the risk of cancer, but the bottom line is that there is no evidence from this study that using asthma medication increases the risk of prostate cancer in people with asthma.