The proportion of breast cancer tumors that were hormone receptor–positive rose in the 1990s as the proportion of hormone receptor–negative tumors declined, according to researchers at the Fred Hutchinson Cancer Research Center in Seattle.

While the researchers did not have a definitive explanation for this change, they said "hormonal factors may account for this trend.

The researchers analyzed breast cancer incidence rates by estrogen receptor (ER) and progesterone receptor (PR) status from 1992 to 1998 obtained from cancer registries that participate in the National Cancer Institute’s Surveillance, Epidemiology, and End Results (SEER) Program.

From 1992 to1998, the overall proportion of breast cancers that were ER-positive and PR-positive increased from 75.4 percent to 77.5 percent and from 65.0 percent to 67.7 percent, the researchers reported in the Journal of Clinical Oncology.

The proportions of ER-positive/PR-positive tumors increased from 56.7 percent to 62.3 percent among 40- to 49-year-olds, from 58.0 percent to 63.2 percent among 50- to 59-year-olds, and from 63.2 percent to 67.9 percent among 60- to 69-year-olds.

"Because the incidence rates of hormone receptor–negative tumors remained fairly constant over these years, the overall rise in breast cancer incidence rates in the United States seems to be primarily a result of the increase in the incidence of hormone receptor–positive tumors," the researchers concluded.

Other sources: Journal of Clinical Oncology