A new analysis by the state health department says that the rate of new cancer diagnoses in Silver Bow County is about the same as the rest of Montana.
But at a council of commissioners meeting Wednesday night in Butte, state cancer epidemiologist Heather Zimmerman said that's not the case for cancer mortalities.
“The mortality rate for all cancer sites combined was significantly higher from 2012-2016," said Zimmerman.
She also reported that death rates from liver cancer, specifically, are elevated in Butte-Silver Bow, and have been for a longer period of time.
There’s no easy answer for why the mortality rate, or number of people who die from cancer, is higher in Silver Bow County. Zimmerman said available data, from the state’s central tumor registry and public death records, can’t really answer why people are dying from cancer.
But she said mortality rates are strongly determined by overall access to health care, which can influence how early a person is diagnosed, what kind of treatment they can get, and what other health problems they have.
That’s why incidence, the rate at which new cancer cases are diagnosed, is a more useful measure.
“That’s the indicator that's most likely to show us whether or not there’s a problem with contaminants,” Zimmerman said.
Butte residents have been asking more questions about rates of cancer and other diseases as the EPA conducts its second mandated Superfund health study. Many are concerned about potential connections to exposure to heavy metals left in the environment from a century of copper mining and the ongoing Superfund cleanup.
Responding to a request from the county health department, state epidemiologists dug deeper into current data and science on cancer to see how Silver Bow compared to other counties across Montana from 2002 to 2016.
Liver, lung, bladder, kidney, prostate and skin cancers are all known to be associated with exposure to arsenic, lead and cadmium. Those are all contaminants of concern in Butte’s Superfund site.
But the incidence of those kinds of cancer in Silver Bow County is about the same as the rest of the state. That’s true for all cancer types here, too.
There’s an asterisk on the end of that conclusion. For the first time, epidemiologists were able to drill down into Silver Bow county’s geography and match cancer cases with where people live. They found one census tract near the airport had a higher than expected rate of new cancer cases. But, it’s also the tract where new cases without precise street addresses ended up, so that number could be inflated.
Overall, the state’s new report paints a more positive picture than a peer-reviewed study on cancer deaths recently published by an independent researcher from the University of South Carolina.
That researcher, Professor Suzanne McDermott, accessed the same mortality data as the state health department, but made different decisions about how to use it. State epidemiologist Laura Williamson said both studies are accurate, but can’t be compared apples to apples. But she sees a common theme.
“Both of us found that in recent years, mortality due to all types of cancer is higher in Butte than in Montana," said Williamson.
Several months ago, McDermott shared unpublished data showing that deaths from brain cancer specifically are elevated in Butte-Silver Bow and Anaconda Deer Lodge Counties. Williamson said Montana’s Department of Public Health and Human Services (DPHHS) considers the jury to still be out on the link between brain cancer and heavy metal exposure.
"But I think that Dr. McDermott’s findings of brain cancer certainly have caused DPHHS to have further conversations, like this is something we should look into because we haven’t exactly looked at it this way before. Let’s see what’s going on," Williamson said.
Importantly, McDermott’s study also combined cancer death rates from both Silver Bow and Anaconda-Deer Lodge Counties, used different time periods, and counted cancer deaths even when it wasn’t the primary cause.
The state health department’s Williamson says they believe there’s value in analyzing the counties separately, and will continue to only count cancer mortalities that are listed as the underlying cause of death, to be consistent with the way other public health agencies report on cancer.
She said DPHHS is in conversations with Anaconda health officials about doing a similar study on cancer there, but there is no timeline for it yet.
See the state’s full report called "Cancer in Silver Bow County" here.