The number of cancer survivors in the United States has risen 20 percent over the past decade, with nearly 12 million Americans, or one out of 25 citizens, a cancer survivor, according to a new estimate from the Centers for Disease Control and Prevention.

The findings, published last week, show the number of cancer survivors has risen dramatically over the past forty years, from 3 million in 1971 (1.5 percent of the population) to 9.8 million in 2001 (3.5 percent) and to 11.7 million (3.9 percent) in 2007.

The CDC credited the increase in survivors not only to earlier detection and better treatment but also to an aging population. Most survivors were over 65, the CDC said, and had been diagnosed with breast, prostate or colorectal cancers.

Interestingly, the majority of survivors (about 57 percent) were women, and women also lived the longest with cancer. About 68 percent of survivors who had lived with a cancer diagnosis for at least 15 years were female. And of the 1.1 million survivors who lived more than 25 years with the disease, 75 percent were women.

One-fifth of the survivors, or 2.6 million, were breast cancer patients.

"Women are more likely to be survivors because cancers among women (e.g., breast or cervical cancer) usually occur at a younger age and can be detected early and treated successfully; in addition, women have a longer life expectancy than men," the CDC said.

The CDC acknowledged some limitations to the report. The data the group analyzed with the National Cancer Institute, nine Surveillance, Epidemiology, and End Results (SEER) programs, is more urban and foreign-born than the U.S. population as a whole, "which might limit generalizability."