interval cancer

Screening runs on a cycle. A woman is invited, her images are read, and if nothing is found she waits for the next round. An interval cancer is one diagnosed in that gap, after a screen that did not pick it up. The MASAI trial used the standard definition: a primary breast cancer diagnosed between two screening rounds, or within two years of the last scheduled screening, that was not detected at screening.

The rate matters because it measures the thing a screening programme most wants to know and can least easily see, which is what the screen missed. A detection rate on its own cannot answer that question, because finding more cancers at the screen does not by itself prove that fewer are appearing between screens. This is why the MASAI investigators made the interval cancer rate the trial's primary outcome rather than the number of cancers detected: 1.55 per 1,000 participants in the group screened with artificial intelligence support against 1.76 in the group read the standard way. They also recorded what kind of cancers appeared in the interval, counting fewer invasive, fewer large and fewer aggressive-subtype cancers in the AI-supported group.