But when it comes to less prevalent cancers, such as liver cancer, rates are still on the rise. Study co-author Jemal said that for liver cancer, the underlying cause could be rising obesity rates. Alternatively, higher rates of infection with the hepatitis virus could be a factor.
As for kidney, thyroid and other cancers, Jemal said, "I don't really know why there is an increase."
Even the decrease seen in certain cancers may not be entirely good news, Texas' Zwelling warned.
"Scarily, fewer people could be going to the doctor due to increased uninsurance rates," he said, adding that doctors might also be less vigilant about checking for certain cancers. He noted, however, that this last scenario is unlikely.
The uncertainty involved with interpreting the results could make it difficult for physicians to predict whether these trends will continue in the years to come -- or whether this year's report is a one-off bit of good news.
Still, most doctors remained optimistic.
"I think these downturns are sustainable for several reasons," Wake Forest's Spangler said. "I think people are more aware about cancer screening in general, such as mammograms for women and colonoscopy exams for everyone over the age of 50.
As more and more communities adopt smoke-free policies, there will be continued decreases in cancer rates from secondhand smoke exposure."
And Lichtenfeld of the American Cancer Society noted that even if incidence rates do not continue their decline, there is still good news to be found.
"I believe the mortality rates will continue to decline over time," he said. "That is the more sustainable trend."
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