The mortality rate linked to cancer is falling nationwide, but worrisome pockets of deadly malignancy persist — and in some places have worsened — in regions throughout the country, according to the first county-by-county analysis of cancer deaths across the United States.

The death rate attributed to various types of cancer declined 20 percent between 1980 and 2014, according to research published Tuesday in the Journal of the American Medical Association. During that time, the number of cancer deaths per 100,000 Americans dropped from 240.2 in 1980 to 192 in 2014.

Cancer, the No. 2 cause of death in the United States, has long been tracked by health officials. But existing databases have largely measured such statistics on state or national levels.

That can mask cancer trends that cross state borders, or that bubble up in geographically limited “hot spots.” It can also obscure associations with environmental exposures, ethnic settlement patterns or health behaviors like poor diet that may be unique to a single county or shared only with its near neighbors.

The new tally of close to 20 million cancer deaths over 35 years offers a more fine-grained view of cancer’s toll. It gives local and county officials — who design and carry out most of the nation’s public health campaigns — the data they need to detect or respond to trends within the populations for which they are responsible.

The steepest decreases in cancer mortality were seen in counties that hug the urban centers of both coasts and in pockets scattered throughout the Intermountain West.

In the heart of the Rockies just west of Denver, Colorado’s Summit County led the country in beating back cancer. Home to ski resorts and a single hospital, Summit County had the lowest rate of cancer deaths in both 1980 (130.6 deaths per 100,000 people) and 2014 (70.7 deaths per 100,000). That drop of close to 50 percent was bigger than just about anywhere else in the country.

From Imperial County in the south to Humboldt County up north, virtually all of California’s coastal counties saw steep and steady declines in deaths from cancer over the 35 years studied. Those improvements spread inland throughout the state, but were particularly impressive in many of the state’s northern and central counties.

The study also casts new light on a nationwide rise in deadly liver cancers; a worrying increase in fatal kidney cancers in Texas, Oklahoma and Louisiana; and a mysterious spate of testicular cancer deaths among men in California’s San Bernardino, Imperial and Stanislaus counties.

In Kentucky’s northeast corner, where coal mining and other industries have collapsed, the picture was particularly grim. More than 20 counties there, and scattered throughout Appalachia in neighboring West Virginia, Tennessee and Ohio, saw rates of cancer mortality rise between 20 percent and 46 percent, largely driven by cancers of the lung, colon, pancreas and liver.

Owsley County in Kentucky, the nation’s poorest, where roughly 42 percent live in poverty, saw the nation’s largest increase in cancers of the trachea, bronchus and lungs — a near doubling between 1980 and 2014. The rate at which men died of prostate cancer there also saw one of the nation’s steepest increases — 25 percent — in the same period.