Oral Cavity Cancer and the Subsequent Incidence of Other Cancer Diagnoses: A Retrospective Cohort Study in German General Practices

Background/Objectives: Subsequent cancer diagnoses are well documented after oral cavity cancer, particularly within the upper aerodigestive tract. However, evidence from routine primary care is limited, although such data may provide complementary insights into the long-term documentation of subsequent cancer diagnoses. This study examined subsequent cancer diagnoses after oral cavity cancer in German general practices and assessed the influence of anatomically adjacent cancer codes. Methods: This retrospective cohort study used the IQVIA Disease Analyzer database. Adults with a first documented oral cavity cancer (International Classification of Diseases, 10th Revision [ICD-10]: C02–C06) recorded in German general practices between 2005 and 2024, with at least 12 months of prior observation and no earlier cancer, were included. A landmark design was applied to both cohorts: follow-up began 365 days after the index date, and individuals not observed beyond that date or with a cancer documented in the first year were excluded. Patients were matched 1:10 on age, sex, index year, consultation frequency in the 12 months before the index date and Charlson Comorbidity Index. The outcome definition was identical in both cohorts (ICD-10 C00–C97, excluding C02–C06 and C77–C80). Cox regression with robust variance estimation was used. Results: A total of 1043 patients with oral cavity cancer and 10,430 matched individuals were analyzed. A cancer diagnosis was documented in 16.8% and 6.9% of individuals, respectively, corresponding to 38.0 and 15.6 per 1000 person-years (hazard ratio [HR] 2.43; 95% confidence interval [CI] 2.06–2.87). The HR was highest among patients aged ≤60 years (HR 4.07) and lowest among those aged >70 years (HR 1.48; p for interaction <0.001). Subsequent diagnoses involving the upper aerodigestive and respiratory tracts were disproportionately represented in the oral cavity cancer cohort. Excluding anatomically adjacent cancer codes attenuated the association (HR 1.81; 95% CI 1.51–2.17). Conclusions: Oral cavity cancer was associated with a higher incidence of subsequently documented cancer diagnoses. The association was attenuated but persisted after excluding anatomically adjacent and potentially ambiguous cancer codes; the data do not establish whether these diagnoses represent confirmed second primary malignancies.