Cancer rates have dropped 22 percent in two decades, the American Cancer Society said, and credit goes to screenings, declines in tobacco use and advances in cancer prevention and treatment.

Death rates from four major types of cancer — lung, breast, prostate and colon — drove the decrease, the society reported.

“It’s wonderful news,” said Dr. Richard Wender, chief cancer control officer for the society in Atlanta. “There’s no mystery about how this happened. The biggest factor has been reduction in tobacco use — first in men and then in women.”

While rates dropped largely because more people are putting out their cigarettes for good, cancer experts said advances in screening are also helping save more lives.

Screenings for breast and colon cancer are key, said Cam Scott, senior director of government relations in Texas for the American Cancer Society Cancer Action Network.

“Basically, breast cancer is a curable form of cancer if it is caught early,” Scott said. “If breast cancer is caught at an early stage there is a 98 percent survival rate.”

The survival rate drops to 24 percent if caught later.

The American Cancer Society’s annual statistics report states that the 22 percent drop in cancer mortality helped avoid more that 1.5 million cancer deaths.

Lung cancer deaths among men rose during most of the 20th century, peaking in 1991. Since then, a steady decline in the cancer death rate is credited to fewer Americans smoking.

Lung cancer deaths have plummeted in men and are dropping in women too. Those deaths declined 36 percent from 1990 to 2011 among men and 11 percent from 2002 to 2011 for women.