Last week, Sen. John McCain had surgery to remove a blood clot from above his eye. On Wednesday, we learned that tests had revealed the blood clot was linked to a previously undiagnosed brain tumor.

“The Senator and his family are reviewing further treatment options with his Mayo Clinic care team,” his office said in a statement on Wednesday night. “Treatment options may include a combination of chemotherapy and radiation.”

The tumor, known as a glioblastoma, is the most common type of malignant brain cancer among adults, and the prognosis for this kind of cancer is generally very poor. The median survival is 15 months, and fewer than 5 percent of patients live beyond five years of their diagnosis.

But McCain and his family can take comfort in one truth about glioblastoma and many other kinds of cancer: Patients who have private health insurance or Medicare like McCain tend to outlive the uninsured and patients on Medicaid. This fact is particularly poignant at a time when McCain’s party has been scrambling to repeal and replace the Affordable Care Act, which the Arizona senator had said he would support despite concerns about the proposed legislation. If passed, the GOP plan is expected to leave millions more people — some of them current and future cancer patients — without insurance compared with Obamacare.

Cancer patients with insurance outlive those without insurance and those on Medicaid

In a 2016 study, published in the journal Cancer, researchers tried to tease out the effects of insurance on survival among Americans diagnosed with glioblastoma between 2007 and 2012, using the National Cancer Institute's Surveillance, Epidemiology, and End Results Program, which collects data on cancers in the US population.

Among the more than 13,600 glioblastoma patients in the study, those who were uninsured and those on Medicaid died sooner than patients with private insurance, regardless of age, sex, race, marital status, tumor size, or even treatment approaches.

Researchers crunched the same data set for another 2016 study in the journal Cancer to see how health insurance affected the survival of patients with testicular cancer. Unlike glioblastoma, 95 percent of patients with testicular cancer live past five years. In that study, the researchers found the same trend.

Among the more than 10,000 American adults diagnosed with testicular cancer between 2007 and 2011 included in the study, the researchers saw the same insurance survival advantage: The uninsured patients had an 88 percent greater chance of dying of testis cancer compared with the insured, while Medicaid patients had a 58 percent greater chance of dying than the privately insured.

There are a few reasons Medicaid patients may fare worse, despite their access to health care. People with Medicaid are poor, and tend to have patchier access to health care throughout their lives. Christopher Sweeney, a Brigham and Women's Cancer Center researcher who authored the testicular cancer study, said Medicaid patients generally had larger tumors and were more likely to have metastatic disease when they were diagnosed, which “suggests patients who do not have insurance present later because of fear of costs of going to a doctor.”

What’s more, many patients with Medicaid seek out coverage when they have a major illness, which may help explain why people on Medicaid don’t fare as well as those with private insurance. “This is why preventive and early access to care is key to decreasing deaths from treatable [cancers],” he added.

These studies don’t come in isolation, though. Other researchers have found a survival advantage for patients who have private insurance compared with the uninsured and those on Medicaid. They’ve found this trend among breast, colorectal, lung, prostate, bladder cancer, and non-Hodgkin’s lymphoma patients.

You can see what the survival curves look like for several other cancers, from a study of New Jersey residents:

In breast, colorectal, lung, prostate, bladder cancer, or non-Hodgkin’s lymphoma, people without health insurance “had a significantly higher risk of death within 5 years of diagnosis (41% to 97%) than those with private insurance even after adjustment for important prognostic factors such as gender, age, race/ethnicity, marital status, [socio economic status], and stage.”

A 2010 systematic review brought together 23 studies on lung cancer survival and insurance status. The conclusions were similar once again: “Patients with Medicaid or no insurance consistently had worse outcomes than other patients with lung cancer.” The reason, the researchers said, was because lung cancer patients without insurance “do poorly because access to care is limited and/or they present with more advanced disease that is less amenable to treatment.”

A word of caution in interpreting these studies: Like many health insurance studies, they were observational by design, meaning researchers were looking at associations among phenomena in the population (in this case, cancer survival and insurance status), instead of running an experiment. Observational research can’t tell us about cause and effect.

There’s not much in the way of more definitive experimental studies on cancer survival comparing people who have been randomly allocated to insurance compared with those who haven’t, so these observational studies are among the best evidence we have. A more recent study used Tennessee’s shrinking Medicaid program in 2005 — and termination of coverage among enrollees — as a natural experiment to look at what impact kicking people off health insurance had on breast cancer diagnosis. The researchers found Medicaid dis-enrollment was associated with women being diagnosed with breast cancer at a later stage in their disease.

The Tennessee natural experiment didn’t look at cancer mortality but other quasi-experimental health insurance research tells us that people who get coverage fare better than their counterparts who don’t: They tend to have better access to preventive care and primary care, more cancer screenings, better mental health outcomes, better financial health ... and they generally live longer. (For a good review of that research, see here.)

The bottom line: Insurance matters for the life expectancy of the 1.6 million Americans diagnosed with cancer each year. If Sen. McCain were uninsured, he’d likely live a shorter life after his cancer diagnosis.

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