If doctors keep checking on women longer after their last breast cancer screening, they can better tell which tumors would never cause harm and which ones are likely to grow quickly—especially when cancers tend to develop slowly.

If you stop screening for breast cancer, you need to keep checking patients for longer than the average time it takes for dangerous tumors to grow, or else you won’t be able to tell which cancers would never harm someone and how fast the bad ones progress.

If doctors don't keep an eye on breast cancer patients long enough after their last screening, they can't tell how fast the cancer is likely to grow — it's like trying to guess how long a candle will burn by only watching it for a few seconds.

When doctors stop screening for breast cancer, they can't tell exactly how many of the tumors found would never have caused harm, even with fancy math, because the data they have just isn't detailed enough to give clear answers.