Objective: To describe a data artefact in degree-of-spread at first presentation in the New South Wales Central Cancer Registry (NSW CCR), the only Australian cancer registry that records degree-of-spread data for all solid tumours.
Method: Trends in the proportions of cancer cases diagnosed annually over 1972-2004 by degree-of-spread categories of localised, regional, distant and unknown were calculated for each major cancer type.
Results: Excepting breast cancer and melanoma, the proportion of localised cancer cases reported from 1993-1998 was approximately 5% lower than expected, and was mirrored by an artefactual increase in unknown degree-of-spread cases.
Conclusion: This artefact was caused by the introduction of the Electronic Notification System and cannot easily be remedied retrospectively. However, regional and distant categories of degree-of-spread in the NSW CCR data are reliably recorded for the 1972-2004 period.
Implications: It is important that past and present cancer data users are notified and understand the quality issues with NSW CCR degree-of-spread data, and use it as recommended to avoid anomalous results or conclusions.