| Summary: | Katja Maretty-Nielsen,1,2 Ninna Aggerholm-Pedersen,1–3 Johnny Keller,1,4 Akmal Safwat,1,3 Steen Baerentzen,1,5 Alma B Pedersen6 1Sarcoma Centre of Aarhus University Hospital, 2Department of Experimental Clinical Oncology, 3Department of Oncology, 4Department of Orthopedic Surgery, 5Department of Pathology, 6Department of Clinical Epidemiology, Aarhus University Hospital, Aarhus, Denmark Background: The aim of the present study was to validate the data in the Aarhus Sarcoma Registry (ASR), to determine if this registry is population-based for western Denmark, and to examine the incidence of sarcomas using validated, population-based registry data. Methods: This study was based on patients with bone and soft tissue sarcoma treated at the Sarcoma Centre of Aarhus University Hospital between January 1, 1979 and December 31, 2008. The validation process included a review of all medical files by two researchers using a standardized form. The Danish Cancer Registry was used as a reference to assess the completeness of registration of patients in the ASR. Crude and World Health Organization age-standardized incidence, as well as age-, gender-, and year-specific incidences were estimated. Results: The validation process added 385 to the 1442 patients who were registered in the ASR. Before validation, on average, 70.5% of the data for the variables was correct. Validation improved the average completeness of the registered variables from 83.7% to 99.3%. The 1827 patients in the ASR after validation include 85.3% of the patients registered in the Danish Cancer Registry. The overall World Health Organization age-standardized incidence of sarcoma in the trunk or extremities in western Denmark in the period 1979–2008 was 2.2 per 100,000, being 0.8 for bone sarcomas and 1.4 for soft tissue sarcomas. Conclusion: The validation process significantly improved the completeness of the variables and the quality of the ASR data. ASR is now a valuable population-based tool for epidemiological research and quality improvement in the treatment of sarcoma. It is our recommendation that documented validation of registries should be a prerequisite for publishing studies derived from them. Keywords: clinical databases, cancer registry, sarcoma, incidence, completeness, validity 
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