A method to adjust for stage coding changes in the National Program of Cancer Registries illustrated for colorectal cancer.

We describe a simple statistical model that allows for a comparison of staging data from the Centers for Disease Control and Prevention's (CDC's) National Program of Cancer Registries during 1998-2008. In this program, cancers diagnosed during 1998-2000 were coded according to Summary Stage 1977, those diagnosed during 2001-2003 according to Summary Stage 2000, and those diagnosed during 2004-2008 according to the Collaborative Stage system. These changes in stage coding systems were associated with an abrupt shift in the distribution of extent of disease for colorectal cancer, particularly changes in the proportion of local vs regional stage disease, in some states. The method described here adjusts for the use of different staging systems over time so that temporal trends in the distribution of extent of disease can be evaluated. The method is applied to the proportion of localized stage colorectal cancer, but should be applicable to other cancers.