Background
The incidence of early-onset cancers, diagnosed between 20 and 49 years of age, is increasing globally and has emerged as a significant public health challenge. A recent study reported that the overall incidence of early-onset cancers has increased by nearly 80% since 1990. Some cancer types, such as prostate and thyroid cancers, have shown increasing trends, while others, including liver cancer, have declined. In addition, trends in early-onset cancers have been reported to vary by cancer type and across countries.

Objectives
To better understand these age-specific patterns, this study aimed to examine trends in early-onset cancer incidence in Korea compared with late-onset cancer, using data from the Korea Central Cancer Registry.

Methods
We used cancer incidence data from the Korea Central Cancer Registry covering the period from 2003 to 2022. The study population was categorized into early-onset (diagnosed at 20-49 years) and late-onset (diagnosed at ≥50 years) groups. Incidence trends were examined for all cancers combined and for 11 major cancer types (stomach, colorectal, liver, gallbladder, pancreas, lung, breast, prostate, kidney, thyroid, and cervical cancer). We calculated age-standardized incidence rates (ASR) per 100,000 population, using the 2020 mid-year resident registration population as the standard. Sex-specific incidence rates were estimated for breast, cervical, and prostate cancers using the corresponding female or male population. Incidence trends were analyzed using the Joinpoint regression program to estimate the annual percent change (APC) and average annual percent change (AAPC) with 95% confidence intervals (CI).

Results
Over the 20-year study period, the incidence of early-onset cancer showed a significant increasing trend (AAPC = 3.1%, 95% CI: 1.8 to 4.5), whereas late-onset cancer remained relatively stable (0.5%). In the early-onset group, thyroid cancer exhibited the highest increase (8.3% [5.9 to 10.7]), followed by prostate cancer (6.2%) and kidney cancer (5.3%). In contrast, liver (-5.2%), stomach (-3.1%), gallbladder (-2.5%), and cervical (-1.5%) cancers significantly decreased. For most cancer types, incidence trends were directionally similar between the early-onset and late-onset groups. However, colorectal and lung cancers showed a difference between age groups. Colorectal cancer incidence increased significantly in the early-onset group (4.1% [2.3 to 5.8]) but decreased in the late-onset group (-0.2%). Lung cancer also showed an increasing trend in the early-onset group, contrasting with a sustained decline in the late-onset group. Specifically, early-onset colorectal surged from 2018 to 2022 (APC = 17.7%), whereas incidence in the late-onset group has declined since 2010. In addition, early-onset lung cancer increased from 2015 to 2022 (APC = 1.9%), compared with a decreasing trend in late-onset cancer since 2003.

Conclusions
This study demonstrates a rising burden of cancer among young adults in Korea over the past two decades, in contrast to trends observed in older adults. In particular, colorectal and lung cancers showed marked increases in the early-onset group, while decreasing in the late-onset group. These findings provide population-based evidence of shifting cancer incidence patterns among young adults in Korea. Together, they highlight early-onset cancer as an emerging important epidemiological phenomenon in Korea that warrants continued surveillance.