WASHINGTON (Reuters) – Colorectal cancer diagnoses and deaths have fallen in the United States this decade, but the gap in progress between whites and blacks is widening, the American Cancer Society said this week.

Improvement has come about chiefly due to prevention and early detection through colonoscopy and other screening methods recommended starting at age 50, the group said in a report.

Colorectal cancer, which includes cancer of the colon and rectum, ranks third among all types of cancer in the United States both in the number of people who get it and in the number who die, but rates have been falling since the 1980s.

While death rates were dropping in all groups, blacks were roughly 45 percent more likely to die from it than whites in the five-year period ending in 2005, the report said. The gap was about 40 percent for the five-year period ending in 2001.

In addition, in the period ending in 2005 blacks were 20 percent more likely than whites to be diagnosed with it.

Before 1980, colorectal cancer death rates were lower in black men than white men and similar in black and white women.

"Colorectal cancer incidence and mortality rates have been declining among the general population in the United States for some time," said Elizabeth Ward, one of the report's authors.

"We are seeing much greater declines among whites than among African Americans. And we think the primary reason for that is different access to and utilisation of colorectal cancer screening tests," Ward said in a telephone interview.

Blacks are less likely to be screened and more likely to be diagnosed after the cancer has spread beyond the colon, making it far deadlier. Blacks also are less likely to get the recommended surgical treatment and therapy, the report said.

Caught early on, colorectal cancer is often curable.

Leading screening tools include the colonoscopy, in which doctors use a flexible tube inserted through the rectum to check the inside of the rectum and entire colon for signs of cancer. It can find precancerous polyps that, when removed, can prevent the onset of colorectal cancer.

Other screening methods include the flexible sigmoidoscopy in which the inside of the rectum and a third of the colon are inspected, and the faecal occult blood test, which checks for hidden blood in stool samples that might indicate cancer.