When scientists try to figure out how fast breast cancer grows and how many tumors are slow-moving, their calculations get less accurate if the numbers they use for slow-growing tumors or fast-growing tumors are too low or too high.

Scientists have shown that if new breast cancers develop slower than existing ones grow, then a certain math model can tell the difference between slow-growing and fast-growing cancers.

Scientists have built a math model to understand how breast cancer grows in different ways — some fast, some slow — and they say they can tell the two apart as long as new cancers start slower than they grow big enough to be found, even if you only get one mammogram and don’t follow people for long.

Scientists can't accurately tell how many slow-growing breast cancers are in the Canadian breast screening data because the information isn't detailed enough—even with the best math tools.

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.