Cancer of unknown primary (CUP) is a malignancy without an identified primary site. The definition of CUP in cancer registry reports appears inconsistent, potentially impeding an assessment of the true burden of this poor prognosis malignancy.
The aim of the study was to document and compare the current CUP registration and reporting practices by population-based cancer registries in Australia, the four countries of the United Kingdom (UK), and the Republic of Ireland.
Cancer registry directors were asked to complete a survey about the procedures and coding rules followed in the registration of CUP, and the reporting of CUP statistics. A total of 20 registries in Australia (n=8), the UK (n=11) and Ireland (n=1) were approached and agreed to participate.
The survey findings show no evidence of consistent national or international coding guidance for registering and reporting CUP, resulting in varied cancer registration practices. The variation in practice includes differing interpretations of ICDO3 and ICD10 codes for unknown primary, the investigation of death certificate only notifications, electronic notifications, consideration of prior registrations of site-specific cancers, and the types of notifiers approached for additional information. In addition, there is variation in coding practices for tumours with non-epithelial morphologies such as melanoma and sarcoma, and the use of ill-defined primary site codes such as 'gastrointestinal' cancer. Reporting practices also vary, with some registries using ICDO3 codes and others using different ICD10 codes to represent CUP.
Inconsistencies in the registration practices for CUP impact on CUP incidence reporting and hinder comparisons between jurisdictions. This obscures an accurate understanding of the burden of the disease which is important for its management, and creates challenges for CUP research based on registry data. The survey results will be used to better understand current incidence data and inform future national and international registration guidance.