According to recent American Cancer Society statistics, the death rate from cancer has declined over the last twenty years. And it dropped another 1.7 percent last year.

The rate of new cancer diagnoses also declined by 2 percent in the last decade for men, but not for women.

To find out what’s behind this good news, I asked Dr. Otis Brawley, M.D., the chief medical officer for the American Cancer Society. Here is what he told me:

“The biggest reason for the decline in the death rate is that people have stopped smoking,” he said.

Smoking in men started to decline in late 1950s and continued to decrease well into the 1980s. After a period of levelling off, it declined again during the last 15 years.

When men stopped smoking, they stopped stop getting smoking-related cancers. Women continued to increase their smoking rates throughout the 1960s, and the more recent decline has been slower than in men. As a result, we’ve seen less of a decline in cancer death rates in women.

Another problem is that there are huge geographical variations in smoking rates. In West Virginia and Kentucky, approximately 30 percent of adults still smoke. In Utah and California, it’s down to around 10 percent.

Advances in diagnosis and treatment have also contributed to decreased death rates in a number of common cancers. Here are some of them:

• Lung cancer. Lung CT screening of older smokers decreases the risk of death by about 20 percent. However, for every 5.4 lives saved by screening, one life is lost related to the bronchoscopy and biopsies that resulted from screening. Dr. Brawley counsels that anyone who is considering screening needs to balance the benefit and risk and then make their own choice. 

New treatments are now available for certain types of lung cancers. For example, people with non-small cell lung cancer associated with the ALK gene rearrangement treated new targeted therapies have been alive with metastatic disease for 5 to 6 years. Less than ten years ago the life expectancy was only a year or two.

• Breast cancer. Much of the 40 percent decline in breast cancer death rates over the last 30 years has been due to improvements in treatment. But nearly half of it is due to screening. The problem is there are a substantial number of Americans who are diagnosed with breast cancer who get less than adequate care. Even though could improve rates of screening, the fact is we could save many lives if we just correctly treated everyone that we diagnose now.

• Prostate cancer. The decline in mortality from prostate cancer is close to 50 percent. Even though the answer to the question of whether a man should be screened for prostate cancer is still controversial, it has changed recently. Dr. Brawley said there are some men that he would not have recommended screening just five years ago but would now. Men, he added, need to understand the risks and potential benefits related to screening and make the decision for themselves.

Treatment has changed as well. In the past, almost every man was treated almost immediately. Now, more than half of all men who are diagnosed through screening are told their cancer should be watched. Many of those men will never be treated because we are better at distinguishing cancers that need to be treated and those that can be watched.

• Colorectal cancer. Death rates from this form of cancer have declined dramatically since the 1970s. The decline is linked to highly effective screenings, including colonoscopy and sigmoidoscopy. Even stool blood testing that triggers a colonoscopy has been shown to find polyps that reduces the risk of cancer.

Our ability to treat the disease, including in stages 3 and 4, has also improved. That’s also part of the reason for the decline in mortality. The sad thing is there is geographic variability in outcomes. There are 12 states that have not had a 10 percent decline in colorectal cancer death rates, even though the United States as a whole has had close to a 45 percent decline.

So the good news is that cancer rates overall have been declining, but unfortunately the good results are not spread evenly across the population.

You can read the transcript of the full conversation with Dr. Brawly on The Doctor Weighs In.