Validation, calibration and refitting of a familial breast cancer model in sisterships: a case study in the Swedish sisters data.

In Sweden, a unique data set has been compiled with breast cancer incidence in all sisterships with at least two sisters born between 1932 and 2001, and the effect of family history has been analyzed by standard epidemiological methods. Such data are ideal to explore the validity of existing models for familial breast cancer. This paper explores the validity of the Jonker model that adds a hypothetical gene to the well-known BRCA1 and BRCA2 genes. The validity of the model for the Swedish data is checked by using a calibration model for breast cancer incidence given the (retrospective) family history as assessed at the end of the study period. This enables the validity of the overall incidence and the effect of family history to be assessed in the same model. The conclusion is that the existing model does reasonably well for the effect of family history but is seriously wrong for the early incidence rate. Therefore, the model is refitted in the Swedish data. Finally, the calibration of the refitted model is checked when using current family history as used in the epidemiological studies. The refitted Jonker model fits the data well and shows good agreement with the epidemiological findings.