Gastric cancer completeness in Finnish Cancer Registry and Finnish Patient Registry: a population-based nationwide retrospective cohort study

BACKGROUND: Gastric cancer is the fourth-leading cause of cancer-related deaths worldwide. The only curative treatment options of gastric cancer are perioperative chemotherapy and surgical resection. Many nationwide registries have high validity and provide vast range of opportunities for registry-based research. Cancer diagnoses in the Finnish Cancer Registry (FCR) are reported by pathology laboratories and clinician forms, while discharge diagnosis codes are reported to the Finnish Patient Registry (HILMO) automatically. Finland is known for complete registries but the completeness of gastric cancer in FCR and HILMO remains unclear. OBJECTIVES: The aim of this study is to assess the registry coverage for gastric cancer in FCR and HILMO and to explore potential reasons for possible differences between these registries. DESIGN: Population-based nationwide retrospective cohort study. PARTICIPANTS: All patients diagnosed with gastric cancer in Finland during 1990 to 2014, with follow-up until 31 December 2019. RESULTS: Out of 21 468 total gastric cancers reported to either registry, 17 107 (79.7%) had a gastric cancer diagnosis in both registries. A substantial decrease from 88.3% to 83.4% was observed in the proportion of cases reported to FCR over time. The completeness of FCR was estimated at 87%. For HILMO, the completeness was 92.7%. Death due to gastric cancer was most common in those with gastric cancer in both registries (80.8%), and less common in those reported to only FCR (36.3%), followed by those reported to only HILMO (9.3%). CONCLUSIONS: The study indicates that gastric cancer is well captured by both FCR and HILMO but there is an alarming decrease in the proportion of cases captured by the FCR over time. Some gastric cancer diagnoses in HILMO might, however, be misclassified due to cancer diagnoses being assigned based on clinical suspicion.