Background:
In recent decades, the burden of early-onset cancer (EOC, diagnosed at 20-49 years) has increased in Australia and many countries globally. The relative contributions of this population’s changing exposure to various cancer risk factors are not well understood.

Objectives:
We firstly reviewed trends in EOC rates for Australian men and women to identify priority cancers for analysis then, for those types, assessed trends in young people’s exposure to risk factors.

Methods: Priority EOC types for analysis comprised those with the highest-ranked sex-specific absolute increases in incidence rates between 2000-2004 and 2015-2019 as well as a high absolute incidence in 2015-2019. Risk factors assessed comprised risk factors for priority cancers as reported by the International Agency for Research on Cancer (IARC) and the World Cancer Research Fund (WCRF), plus potential emerging risk factors as identified in a systematic scoping review covering 2015-2025. For each risk factor examined, we sourced sex- and age-specific Australian exposure prevalence trend data for ages <50 as available from the National Health Survey, the National Drug Strategy Household Survey, the National Nutrition and Physical Activity Survey, the Australian Institute of Health and Welfare, the Australian Bureau of Statistics, journal articles and grey literature. Risk factors were classified as either: 1) Possible contributors to increases in EOC in Australia (increasing prevalence); 2) Unlikely contributors (decreasing/stable prevalence); or 3) Inconclusive (inconsistent/unclear/unavailable prevalence trend).

Results:
We identified 12 priority EOCs, namely: breast, cervical, colorectal, kidney, pancreatic, prostate, testicular, thyroid and uterine cancers, Hodgkin lymphoma, leukaemia and neuroendocrine neoplasms. The highest absolute increases in incidence rates from 2000-2004 to 2015-2019 were for thyroid (+7.2 cases/100,000 women; +2.7 cases/100,000 men), prostate (+4.2 cases/100,000 men) and colorectal cancer (+4.2 cases/100,000 women; +3.9 cases/100,000 men). A total of 116 unique risk factors were examined, comprising 98 risk factors reported by the WCRF or IARC, and a further 18 risk factors identified in the scoping review. Of these risk factors, 11 were classified as possible contributors to increases in EOC (e.g. increases in overweight and obesity, diabetes, non-alcoholic fatty liver disease, low parity, and various dietary changes including decreased consumption of dietary fibre, fruit, dairy, vitamin C and calcium), 18 were classified as unlikely contributors (e.g. alcohol, tobacco smoking, hyperlipidaemia, peptic ulcers), and the remaining 87 risk factors were considered inconclusive (e.g. many dietary, pharmacological and environmental exposures, inflammatory bowel disease, and several metabolic and hormonal factors).

Conclusions and implications:
We identified several risk factors that may partly explain the rise in EOC in Australia, and several risk factors which are unlikely to have contributed. We were unable to assess many risk factors due to lack of quality prevalence data. To monitor potential causes of the rising burden of EOC, there is a need for improved monitoring of the age- and sex-specific population prevalence of dietary, pharmacological and environmental risk factors in Australia, especially in younger people. This work can inform future public health interventions targeting exposure to relevant risk factors in younger people.
 
Funding: This work was funded by Cancer Australia.