Women with recently diagnosed diabetes might be more likely to also get a breast cancer diagnosis than those without diabetes, suggests a new study from Canada.
It's not the first time diabetes has been linked to new cases of breast or other cancers. But the findings also hint that at least part of the reason why doctors find more breast cancer in diabetics is because they're looking harder – and not necessarily because diabetes itself raises a woman's cancer risk.
"The relationship that we see between diabetes and cancer, we wondered if it was something about the fact that people with diabetes go to the doctor's office more often," said co-author Dr Jeffrey Johnson from the University of Alberta in Edmonton.
Changes in insulin
It's also possible that changes in insulin and blood sugar levels that come with diabetes make it easier for breast tumours to grow, Johnson said.
While that could partly explain the increase in breast cancer researchers have noticed, extra doctor's visits and tests certainly seems to contribute to some extent.
Dr Johnson and his colleagues analysed data on nearly 170,000 women in British Columbia. Half the women had a recent type 2 diabetes diagnosis, and the other half were matched controls. During the four to five years after the diagnosis index date, about 2,400 women, or 1.4%, were diagnosed with breast cancer.
Rates of breast cancer were similar in both groups, the researchers reported online in Diabetes Care.
But when they stratified women by age and focused on the time shortly after the diabetes diagnosis, they found that older, post-menopausal women with diabetes were slightly more likely to be diagnosed with breast cancer than diabetes-free women.
Among those 55 and older, women with diabetes diagnosed in the last three months were roughly 30% more likely to be diagnosed with breast cancer compared to women in the control group - although the difference wasn't statistically significant (P=0.14).
After a few months – when the pace of appointments and tests after a diabetes diagnosis would have slowed – there was no difference in how often breast cancer was diagnosed in women with or without diabetes.
Dr Johnson said the finding doesn't rule out other explanations, such as common risk factors for diabetes and cancer or a hormone-driven increase in tumour growth.
Diabetic women more vigilant
"I think there are so many things going on in the relationship that this is maybe only one part of it," he told Reuters Health. We're really early on in understanding this relationship.
Dr Christos Mantzoros, a hormone researcher from Harvard Medical School in Boston, said the new findings could mean either that the higher rate of breast cancer diagnosis was due to more follow-up and screening in diabetics, or that the common roots of the diseases may lead both to develop within a short period of time.
"Women with diabetes need to be more vigilant and their doctors need to be screening them for malignancies associated with diabetes including, but not limited to breast cancer," Dr Mantzoros, who was not involved in the new study, told Reuters Health in an email.
The US Preventive Services Task Force, a government-funded expert panel, calls for mammograms every other year for women between ages 50 and 74.
(Reuters Health, October 2011)