SEATTLE - Most breast cancers develop in the ducts, or pathways, of the breast. But a less common type occurs in the lobes, deeper in the breast tissue.

"Lobular cancers have a very unique growth pattern where they grow as linear strains or sheets of cells, so they're difficult to see on a mammogram and more difficult to feel on a physical exam," said Dr. Christopher Li of the Fred Hutchinson Cancer Research Center.

These lobular cancers have increased since the 1980s. Now researchers at FHCRC have established a link to hormone use.

"We found that women who were using the combined hormones, the estrogen plus the progestin, while they had a 40 percent increase in their risk of the more common type, the ductal type. They had a nearly 300 percent increase in their risk of the more rare type of breast cancer, the lobular cancer," said Dr. Li.

And it can happen fairly quickly, after just three years of combined therapy, not five as previously thought.

Interestingly, estrogen alone doesn't seem to have the same effect.

"We actually found that those women had no increased risk of any type of breast cancer even if they had used it for 10 years or longer," said Dr. Li.

But estrogen has been linked to other cancers, so experts say women considering or currently using it should get good medical advice.

"Evidence, evidence, evidence is what we need before we make those really important decisions because it does affect our lives," said Christine Brunswick of the National Breast Cancer Coalition.

These findings do not change current recommendations for hormone therapy. If you're going to use HRT, take it at the lowest dose possible for the shortest period of time.

The study was funded by the national cancer institute. Researchers looked at more 1,500 postmenopausal women in the Puget Sound area.