For people with Li-Fraumeni syndrome, getting a full-body MRI every year can find early cancers in most cases where a cancer is spotted—but it still misses nearly half of all cancers, including many that show up between scans.

Every year, people with Li-Fraumeni syndrome get a full-body MRI to check for cancer, but almost 1 in 4 scans show harmless things that aren’t cancer—these false alarms lead to lots of extra tests, none of which find cancer, and that can cause stress and cost money.

If you have Li-Fraumeni syndrome and get a whole-body MRI that shows no cancer, there’s a 97.4% chance you really don’t have any right now — but it doesn’t mean you won’t get cancer before your next scan.

When doctors use whole-body MRI scans on people with Li-Fraumeni syndrome, about 7 out of 10 abnormal results turn out to be harmless — not cancer — so patients often get scared and go through more tests that aren’t needed.

Doctors checked the brains of 53 adults with a rare genetic condition using yearly MRI scans and found one hidden brain tumor that hadn’t caused any symptoms yet. This might mean the scans are helpful, but because so few people were checked and not many doctors do this yet, we can’t be sure it’s a good idea for everyone.