Women with diabetes are more likely than those without the disease to be diagnosed with a more advanced and difficult to treat form of breast cancer, a Dutch study suggests.
The researchers examined data on 6,267 women diagnosed with breast cancer from 2002 to 2014, including 1,567 patients with type 2 diabetes.
Overall, women with diabetes were 28 percent more likely to be diagnosed with more advanced and aggressive types of tumors than women without diabetes, the study found. Whether women with diabetes used insulin didn't appear to influence the characteristics of their breast cancers.
"This finding matters for patients with type 2 diabetes who may be concerned about how insulin treatment may affect breast cancer risk," said Christina Dieli-Conwright, a researcher at the University of Southern California in Los Angeles who wasn't involved in the study.
Type 2 diabetes, the most common form, is linked to obesity and aging and happens when the body can't properly use or make enough of the hormone insulin to convert blood sugar into energy. Failure to manage the condition can result in complications like blindness, kidney failure, nerve damage and amputations.
Many people with type 2 diabetes can control their symptoms with prescription drugs designed to help lower blood sugar and with lifestyle modifications like eating healthier food and exercising more often. Some of these patients also need to inject insulin to help regulate their blood sugar.
Some previous research has linked insulin use to an increased risk of breast cancer in women with type 2 diabetes, but results have been mixed and often lacked detailed information about the exact types of tumors women developed, researchers note in Diabetes Care.
In the current study, women with diabetes were more likely to have more advanced tumors, that were larger and had spread to more lymph nodes surrounding the breast. With diabetes, women were also more likely to have tumors that were "graded" as more aggressive and more likely to grow and spread rapidly in the body.
Half of the 388 women taking insulin for diabetes had been on insulin for at least 3.4 years.
Researchers didn't find any connection between the duration of insulin use and breast cancer characteristics.
The study wasn't designed to prove whether or how diabetes or insulin use might directly impact the risk of developing breast cancer or getting more aggressive or hard to treat tumors.
Beyond its small size, another limitation of the study is that it lacked data on a range of individual patient characteristics including obesity and any history of abnormal mammogram findings, note the authors, Jetty Overbeek of VU University Medical Center in Amsterdam and colleagues. Overbeek didn't respond to requests for comment.
The results are surprising because of previous research suggesting that more insulin in the body might trigger processes that lead to tumor growth, Dieli-Conwright said by email.
"Perhaps this study did not find insulin to play a role in breast cancer progression because of the small sample size," she added. "This is an understudied area within diabetes and cancer risk, thus further larger studies are warranted to examine the impact of insulin treatment on cancer risk."
Patients should still understand that diabetes itself can pose a cancer risk, said Dr. Tahseen Chowdhury, a diabetes consultant at Royal London Hospital in the UK, who wasn't involved in the study.
"Women with diabetes appear to have not only a higher risk of breast cancer, but also a higher risk of poorer types of breast cancer," Chowdhury said by email.
In addition to taking prescribed medications and making lifestyle changes to lose weight, diabetics should also make sure to get regular screening mammograms and seek immediate medical attention when they detect pain or lumps in their breasts, Chowdhury advised.