When computing risks with the CanRisk Tool, it is possible for the breast/ovarian cancer risks computed for the index to be lower than the equivalent population cancer risks in the cancer risk graphs. In the CanRisk Tool, the index’s breast/ovarian cancer risks are computed using information for all family members in the input pedigree for whom an ‘Age or age at death’ and ‘Year of birth’ have been specified. In contrast, the equivalent population breast cancer risks are computed using only the following information for the index:
Age or age at death
Year of birth
Age at first breast cancer diagnosis if applicable
In general, if you add unaffected family members (who have not developed cancer) to a pedigree, this will tend to lower the mutation carrier probabilities and breast/ovarian cancer risks predicted by the CanRisk Tool (other things being equal).
If your pedigree has a weak family history of cancer and includes many unaffected family members (relatives of the index), these unaffected family members will be taken into account when the index’s breast/ovarian cancer risks are computed. However, these unaffected family members will not be taken into account when the program computes the equivalent population breast/ovarian cancer risks. Under these circumstances, the index’s breast/ovarian cancer risks can be lower than the equivalent population breast/ovarian cancer risks.