Why do BRCA mutation carrier probabilities sometimes increase abruptly when I increase an age at breast cancer diagnosis from 19 to 20 years in my input pedigree?

Breast cancers diagnosed under 20 years of age are uncommon, and so when the BOADICEA model used by the CanRisk Tool was initially developed, there were insufficient data to determine the risk of breast cancer for affected BRCA mutation carriers in this age group. As a result, the BOADICEA model assumes that the risk of breast cancer for BRCA mutation carriers under 20 years of age is the same as that in the general population.

In practice, this means that:

(1) BRCA mutation carrier probabilities predicted by the program may increase abruptly when you increase an age at breast cancer diagnosis from 19 years (or younger) to 20 years (or older) in your input pedigree;

(2) The canRisk Tool should be used with caution if the input pedigree includes breast cancer diagnoses under 20 years of age.