26-year decline in cancer mortality includes largest single-year drop on record, American Cancer Society says

Deaths from cancer have gradually decreased over the last 26 years, with the largest single-year drop ever recorded in the last decade.

The death rate in the U.S. declined by 29% from 1991 to 2017, according to the annual statistics from the American Cancer Society, which the health organization said was "due to long-term drops in death rates in the 4 most common cancer types: lung, colorectal, breast, and prostate."

The record drop of 2.2% from 2016 to 2017 was driven by the decline in overall deaths from lung cancer.

The ACS said this steady drop "translates into an estimated 2.9 million fewer cancer deaths" compared to if the rate at its peak in 1991 had persisted.

"Deaths fell from about 3% per year from 2008 to 2013 to 5% from 2013 to 2017 in men and from 2% to almost 4% in women," the statistics showed. "However, lung cancer is still the leading cause of cancer death."

Wasif M. Saif, M.D., deputy physician-in-chief and medical director at Northwell Health Cancer Institute, told ABC News that in light of the new findings, there are more longterm implications.

"It is very important to understand that ... this decline is a good sign to report the progress of science and technology, but it also carries its own risks of longevity and concerns for metastases in uncommon sites or development of late-onset complications of treatment in these patients, including other malignancies," he explained. "Therefore, utmost attention needs to be maintained in monitoring these patients."

Additionally, Dr. Saif said that "some tumor types are rising in the order for death associated with them, such as pancreatic cancer."

Progress in reducing lung cancer deaths has improved due to declines in smoking and advances in early detection and treatment. However, progress in reducing colorectal, breast, and prostate cancers has slowed.

The amount of people who die from lung cancer dropped the most rapidly of all cancers to 51% fewer now than in the 1990s. The rapid drop began in 2008 and the ACS predicts that number to drop further.

"Lung cancer death rates declined by 51% from 1990 to 2017 among men and 26% from 2002 to 2017 among women," according to the ACS.

Researchers suggested that the decline is relative to an increase in people who have quit smoking along with advances in Immunotherapy.

Liver and breast cancer are forecast to rise in 2020, which the ACS thought to be due to the rise in obesity rates in the U.S.

The American Society of Clinical Oncology celebrated the news on Twitter and referenced its data driven cancer progress timeline that shows a historical overview and major advances in cancer treatment and research.

The numbers reported in "Cancer Statistics, 2020," published by the American Cancer Society’s peer-reviewed journal CA: A Cancer Journal for Clinicians, was based on past data and trends that show an overall decrease in deaths related to cancer.

The estimated forecasts of future cancer trends are based on a mathematical calculation that the researchers used that only included data from 2016 onward.

Imran Ali, MD MS MPH, from the ABC News medical unit contributed to this report.