Five years ago, when actor Chadwick Boseman died at age 43 from colon cancer, it was a genuine shock. Last summer, when Catherine, Princess of Wales, was diagnosed with cancer at age 42, it was tragic, but it landed a bit differently. At that point, it was clear her diagnosis was part of a clear narrative: More and more relatively young adults are developing cancer.

But now, a more complicated story is emerging.

According to a new study published in JAMA Internal Medicine, some of the rise in early-onset cancer is a diagnostic mirage: Doctors are finding cases that would never have led to serious illness anyway. Younger adults — those under 50 — are indeed being diagnosed with cancer at nearly twice the rate they had been in 1990: 60 cases per 100,000 people, up from 30. But the number o

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