Overall, cancer death rates in the United States fell 29 percent from 1991 to 2017, driven by steady drops in deaths from lung, colorectal, breast and prostate cancers, according to the Society’s annual report on cancer rates and trends published in CA: A Cancer Journal for Clinicians.

“The report shows a continued striking decline in overall cancer mortality in the United States,” Dr. Ned Sharpless, director of the National Cancer Institute, said in a telephone interview. “That trend shows no sign of abating.”

Study coauthor Rebecca Siegel, scientific director of surveillance research at the ACS, attributes that to a doubling in improvement in deaths from lung cancer, which fell 4% in the study period. Lung cancer is the second leading cause of death in the United States behind heart disease.

“If you take lung cancer out of the mix, the drop was only 1.4 percent. It is truly the acceleration for lung cancer that is driving the record drop that we saw,” she said in a telephone interview.

Sharpless, who in November returned to the NCI after a stint as acting commissioner of the Food and Drug Administration, believes some of the gains in lung cancer are related to better therapies, but given that the report only goes through 2017, many of the new drugs for lung cancer, such as Merck’s Keytruda, have yet to be reflected in the mortality data.

“We think the lung cancer data are going to continue to improve for a few years,” he said.

Typically, mortality data in the United States is about three years behind the current year, due in large part to the need confirm that deaths were actually linked with a cancer. Even so, Sharpless said, data on cancer deaths is the most important metric used by the National Cancer Institute for planning purposes.

To make up for the lag, the ACS uses computer models of cancer and population trends to project what’s currently happening in cancer.

In its latest report, the Society projects that in 2020, roughly 1.8 million new cancer cases and 606,520 cancer deaths will occur in the United States.

Nevertheless, progress has slowed for female breast and colorectal cancer, and has essentially leveled off for prostate cancer over the past decade.

The number of new cases of breast cancer has climbed by about 0.3 percent per year since 2004, a rise linked in part to lower rates of fertility and increases in obesity.

In prostate cancer, the number of new cases fell sharply from 2007 to 2012, linked in part to decreased use of blood tests for prostate-specific antigen (PSA) to screen for these cancers in the wake of guideline changes. But some studies suggest the declines in new cases may be masking increases in more severe cancers, the report said.

Sharpless said the increase in more advanced prostate cancers may reflect success in easier-to-treat cancers, but it also may reflect decreased PSA testing.

“It is concerning after seeing so much progress against prostate cancer to see it level off,” Sharpless said, adding that the NCI “has got to be open-minded about this and fund appropriate research, both clinical and basic science, to really tease this out.”

Sharpless also expressed concern that rapid declines in new cases of colorectal cancer are slowing in spite of the availability of effective screening tools, such as colonoscopy.

“There’s an increased incidence of mortality that we think is likely related to obesity,” Sharpless said.

Sharpless said the National Cancer Institute has noticed in particular an increase in colon cancer deaths in people under age 40, who would be too young for routine screening. “That’s a concerning trend,” he said.

Obesity may also be playing a role in the slight rise in new cases of breast cancer.

“We have all this progress against smoking-related cancers but obesity is something that we’re just probably seeing the tip of the iceberg now in terms of the influence on cancer,” Siegel said.

She said excess body weight currently accounts for about 7% of cancers. “I’m sure that proportion will continue to increase because it takes a decade or two before you see the influence on exposure reflected in cancer rates.”