Introduction: The incidence of colorectal cancer in adults younger than 50 years has increased, but the trends of incidence in Switzerland has not been yet described. We assessed long-term incidence, stage distribution, and survival of early-onset colorectal cancer (EOCRC) in Switzerland.

Materials and methods: We conducted a nationwide, population-based study including all patients aged 20 years or older who were diagnosed with a first colorectal adenocarcinoma between 1980 and 2021, based on data from the National Cancer Dataset provided from the National Agency for Cancer Registration (NACR). Age-standardized incidence rates per 100'000 person-years were calculated using the 1976 European Standard Population. Temporal trends were assessed using joinpoint regression, expressed as annual percent change (APC). Stage distribution was analysed for 2010-21, when harmonized TNM data were available. Five-year net survival was estimated using the Pohar-Perme estimator.

Results: Among 96'410 patients, 5'928 (6.1%) were below 50 years old. Rectal cancers comprised 31.8% of EOCRC, versus 22.9% in those aged ≥ 75 years, whereas proximal colon cancers were less common (30.5% vs 42.7%, respectively). From 2010 to 2021, 27.7% of EOCRC were UICC stage IV, compared with 22.1% at ages 50-74 and 19.4% at ≥ 75. EOCRC incidence rose steadily (men: APC +0.46% [95% CI 0.17-0.76]; women: +0.49% [0.17-0.81]), driven by rectal cancers in both sexes and proximal colon cancers in young women. In contrast, incidence declined in older adults after the mid-2000s. Five-year net survival improved and reached a peak of 76.9% in 2010 for young men and 78.2% in 2007 for young women.

Discussion: EOCRC incidence in Switzerland has risen continuously, with advanced stage at diagnosis and heterogeneous survival gains, underscoring the need for adapted prevention, awareness, and screening strategies.