# Age-specific trends in colorectal, appendiceal, and anal tumour incidence by histological subtype in Australia from 1990 to 2020: a population-based time-series analysis

## TL;DR

Bowel cancer rates in Australia have risen, especially among younger people, with different patterns across cancer types and age groups.

## Contribution

This study provides detailed age-specific and histology-specific trends in bowel cancer incidence in Australia from 1990 to 2020.

## Key findings

- Early-onset bowel cancer incidence increased by 5–9% annually, with the highest rise in rectal cancers.
- Appendiceal mucinous adenocarcinoma and anal squamous cell carcinoma showed the greatest subtype-specific increases.
- Colorectal neuroendocrine neoplasms and other subtypes increased in both early-onset and later-onset age groups.

## Abstract

Early-onset bowel cancer incidence (age <50 years) has increased worldwide and is highest in Australia, but how trends vary across age, histology, and anatomical site remains unclear. We investigated appendiceal, proximal colon, distal colon, rectal, and anal cancer incidence trends by age and histology in Australia.

Cancer incidence rate data were obtained from nation-wide cancer registries (1990–2020). Birth cohort-specific incidence rate ratios (IRRs) and annual percentage change were estimated using age-period-cohort modelling and joinpoint regression.

Combining all malignant tumour histologies, early-onset incidence rose 5–9% annually, yielding 4072 excess cases (1·5 per 100,000 person-years; 15% appendix, 28% colon, 48% rectum, 9% anus). Trends varied by site, period, and age: appendiceal cancer rose from 1990 to 2020 in 30–49-year-olds; colorectal cancers rose from around 1990–2010 in 20–29-year-olds and from 2010 to 2020 in 30–39-year-olds; anal cancer rose from 1990 to 2009 in 40–49-year-olds. Across sites, IRRs increased with successive birth cohorts since 1960. Adenocarcinoma incidence in the 1990s versus 1950s cohort was 2–3-fold for colorectum and 7-fold for appendix. The greatest subtype-specific increases occurred for appendiceal mucinous adenocarcinoma, colorectal non-mucinous adenocarcinoma, and anal squamous cell carcinoma. Colorectal neuroendocrine neoplasms, squamous cell carcinomas, and signet ring cell carcinomas rose across early-onset and later-onset strata.

Appendiceal, colorectal, and anal cancer incidence is rising in Australia, with variation across age and histology. These generational shifts suggest evolving risk factor profiles and early-life exposures, highlighting the need to identify drivers of this growing burden.

Australian Government.