The news

Cancer deaths in the United States are falling, with four million deaths prevented since 1991, according to the American Cancer Society’s annual report.

At the same time, the society reported that the number of new cancer cases had ticked up to more than two million in 2023, from 1.9 million in 2022. Cancer remains the second leading cause of death in the United States, after heart disease. Doctors believe that it is urgent to understand changes in the death rate, as well as changes in cancer diagnoses.

Background: Treatment improvements help reduce cancer deaths.

The cancer society highlighted three chief factors in reduced cancer deaths: declines in smoking, early detection and greatly improved treatments.

Breast cancer mortality is one area where treatment had a significant impact.

In the 1980s and 1990s, metastatic breast cancer “was regarded as a death sentence,” said Donald Berry, a statistician at the University of Texas MD Anderson Cancer Center and an author of a new paper on breast cancer with Sylvia K. Plevritis of Stanford University and other researchers (several authors of the paper reported receiving payments from companies involved in cancer therapies).

The paper, published Tuesday in JAMA, found that the death rate from breast cancer had fallen to 27 per 100,000 women in 2019 from 48 per 100,000 in 1975. That includes metastatic cancer, which counted for nearly 30 percent of the reduction in the breast cancer death rate.

Breast cancer treatment has improved so much that it has become a bigger factor than screening in saving lives, said Ruth Etzioni, a biostatistician at the Fred Hutchinson Cancer Center.

Death rates have even declined among women in their 40s, who generally did not have regular mammograms, said Dr. Mette Kalager, a professor of medicine at the University of Oslo and Oslo University Hospital, “indicating a substantial effect of treatment,” she said.

“The biggest untold story in breast cancer is how much treatment has improved,” said Dr. H. Gilbert Welch, a cancer epidemiologist at Brigham and Women’s Hospital. “This is unambiguous good news.”

What We Don’t Know: The cause of new cancer cases.

The American Cancer Society found increases in the incidences of many cancers, including cancers of the breast, the prostate, the uterus, the oral cavity, the liver (in women but not men), the kidney, and the colon and rectum in middle-aged adults. Melanoma incidence also increased. The numbers were adjusted for changes in the size of the population.

Dr. William Dahut, chief scientific officer of the cancer society, said that while the overall rate of colorectal cancer had continued to decline, he was concerned about an increase in one group: people under age 55. In those younger people, the society reports, the incidence is now 18.5 per 100,000 and has been rising by 1 percent to 2 percent a year since the mid-1990s, with 30,500 people expected to be diagnosed this year.

In the late 1990s, colorectal cancer was the fourth leading cause of death for people younger than 50. Now it is the leading cause in men under 50 and the second leading cause in women. Doctors cannot say why.

“We don’t have a good explanation,” Dr. Dahut said. “We do a lot of hand waving. Is it diet? Is it obesity? Is it something in the environment? Is it in utero exposure?”

But colorectal cancer remains overwhelmingly a cancer of older people — among whom, in those over age 65, it has been declining by 3 percent a year, the cancer society says. Its incidence is now 155.4 per 100,000, with 87,500 people expected to be diagnosed this year.

Facts to Keep in Mind: Cancer incidence can be difficult to interpret.

Cancer researchers say that the more you look for cancer, the more you find. As screening becomes increasingly sensitive, doctors are discovering more and more cancers.

That sounds like a good thing — wouldn’t it be best to remove cancers before they become dangerous? The problem is that sometimes treatment might be unnecessary, because not every cancer will be life-threatening or even noticeable. Some cancers never spread. Others actually go away. Others might eventually have had a fatal result, but a person dies of something else first. But it can be impossible to tell the harmless cancers from the deadly ones, so all are treated.

The situation is called overdiagnosis, but no one can precisely say how often it occurs. With mammography, Dr. Berry said, estimates of overdiagnosis range from 0 to 50 percent.

“Increases in incidence are always concerning at first glance, but we need to understand why they are occurring because they may be an artifact,” Dr. Etzioni said.

That is the challenge facing cancer researchers now.