The cancer death rate in the United States fell 2.2 percent in 2017 — the biggest single-year drop ever reported — propelled by gains against lung cancer, the American Cancer Society said Wednesday.

Declines in the mortality rate for lung cancer have accelerated in recent years in response to new treatments and falling smoking rates, said Rebecca Siegel, lead author of Cancer Statistics 2020, the latest edition of the organization’s annual report on cancer trends.

The improvement in 2017, the most recent year for which data is available, is part of a long-term drop in cancer mortality that reflects, to a large extent, the smoking downturn. Since peaking in 1991, the cancer death rate has fallen 29 percent, which translates into 2.9 million fewer deaths.

Norman “Ned” Sharpless, director of the National Cancer Institute, which was not involved in the report, said the data reinforces that “we are making steady progress” on cancer. For lung cancer, he pointed to new immunotherapy treatments and so-called targeted therapies that stop the action of molecules key to cancer growth. He predicted that the mortality rate would continue to fall “as we get better at using these therapies.” Multiple clinical trials are exploring how to combine the new approaches with older ones, such as chemotherapy.

Sharpless expressed concern, however, that progress against cancer would be undermined by increased obesity, which is a risk factor for several malignancies.

The cancer society report projected 1.8 million new cases of cancer in the United States this year and more than 606,000 deaths. Nationally, cancer is the second-leading cause of death after heart disease in both men and women. It is the No. 1 cause in many states, and among Hispanic and Asian Americans and people younger than 80, the report said.

The cancer death rate is defined as deaths per 100,000 people. The cancer society has been reporting the rate since 1930.

Because lung cancer is the leading cause of cancer deaths, accounting for 1 in 4, any change in the mortality rate has a large effect on the overall cancer death rate, Siegel noted.

She described the gains against lung cancer, and against another often deadly cancer, melanoma, as “exciting.” But, she added, “the news this year is mixed” because of slower progress against colorectal, breast and prostate cancers. Those cancers often can be detected early by screening, she said.

The report said substantial racial and geographic disparities remain for highly preventable cancers, such as cervical cancer, and called for “the equitable application” of cancer control measures.

The five-year survival rate for all cancers diagnosed from 2009 through 2015 was 67 percent overall — 68 percent for whites and 62 percent for African Americans.

In recent years, melanoma has showed the biggest mortality-rate drop of any cancer. That’s largely a result of breakthrough treatments such as immunotherapy, which unleashes the patient’s own immune system to fight the cancer and was approved for advanced melanoma in 2011.