CHICAGO – The U.S. has passed an important milestone in the fight against cancer, marking for the first time simultaneous declines in new cancer cases and cancer death rates for both men and women.

Between 1999 and 2006, cancer incidence – the rate at which new illnesses are diagnosed – dropped 0.8 percent annually, a small but statistically significant reduction, according to a report published online Tuesday in the Journal of the National Cancer Institute.

Experts at leading cancer organizations heralded the development while noting that new data need to be interpreted with caution. In particular, they observe that fewer men and women are being screened for prostate and breast cancer and that can result in fewer tumors being identified.

Although it’s not clear yet what role screening may have played, “the drop in incidence seen in this year’s Annual Report is something we’ve been waiting to see for a long time,” said Dr. Otis Brawley, chief medical officer of the American Cancer Society, in a statement.

Cancer death rates continued to fall, as they have since 1993, but the rate of the decline was accelerated, approaching nearly 2 percent a year between 2002 and 2005.

“What we’re seeing is clear evidence that cancer prevention is working,” said Dr. Therese Bevers, medical director of clinical cancer prevention at M.D. Anderson Cancer Center in Houston.

The falloff in newly diagnosed cancer cases cuts across both sexes and four major races and ethnicities – whites, blacks, Asian/Pacific islands and Hispanics – according to the new report by scientists at the American Cancer Society, the National Cancer Institute and the U.S. Centers for Disease Control and Prevention.

The downward trend, which spans several years and has been teased out through careful statistical analysis, underscores some clear victories. Notably, as more men and women have given up smoking, newly diagnosed lung cancer cases have skidded to their lowest level in more than 30 years for both sexes.

Also, as more adults 50 and older were screened for colon cancer, annual disease rates fell 2.8 percent for men and 2.2 percent for women between 1998 and 2005, the latest year for which data are available. By then, half of all adults 50 and older were getting stool samples, sigmoidoscopies and colonoscopies, up from 27 percent in 1987.

Less clear is the story surrounding prostate cancer, the No. 1 cancer diagnosis for men, which plunged 4.4 percent annually between 2001 and 2005 after incident rates rose 2.1 percent in each of the previous six years.