Volume 224 - Issue 2
Interval Cancer Characteristics, Staging and Survival Among National Bowel Cancer Screening Program Participants, Western Australia, 2018: A Retrospective Observational Cohort Study
Authors: Shantelle J. Smith, Rachael Moorin, Dagmawi Tadesse, Kathleen O'Connor, Thi Ninh Ha
Correspondence: shantelle.j.smith@curtin.edu.au
Med J Aust 2026 || doi: 10.5694/mja2.70138
Published online: 2 February 2026
Published online: 2 February 2026
Abstract
Objective
To examine the features of interval colorectal cancer (interval CRC) in Western Australia in the context of the National Bowel Cancer Screening Program (NBCSP), including incidence, characteristics and survival by NBCSP participant characteristics.
Study Design
Retrospective observational cohort study, analysis of linked National Cancer Screening Register and Western Australian Cancer Registry data.
Participants, Setting
Participants in the Western Australian NBCSP (50–74years of age) with negative immunochemical faecal occult blood test (iFOBT) results during the 2018 screening round (1 January 2018–31 December 2018) were followed up for interval CRC diagnoses until 31 December 2020, and for death until 30 September 2022.
Main Outcome Measures
Crude and adjusted incidence rates of interval CRC were analysed overall and by sex, age group and residential socio-economic and remoteness categories. Survival outcomes for people with interval CRC were also assessed.
Results
Of 122,851 NBCSP participants with negative screening results in 2018, 51 people were diagnosed with interval CRC during follow-up (crude incidence rate, 21 per 100,000 person-years; 95% confidence interval [CI], 16–27). The adjusted incidence rate ratio of interval CRC was higher for men than women (adjusted incidence rate ratio [aIRR], 5; 95% CI, 3–11) and for people aged 70–74years than for those aged 50–59years (aIRR, 3; 95% CI, 1–6). Nineteen of 51 interval CRCs were diagnosed 19–24months after negative iFOBT results, 25 were located on the right side of the colon and 34 were adenocarcinomas. Only 13 interval CRCs were stage I tumours at diagnosis. During follow-up (median, 33months; interquartile range, 28–42months), the all-cause mortality rate among the 51 people with interval CRC was 41 per 1000 person-years (95% CI, 18–92), and the colorectal cancer mortality rate was 35 per 1000 person-years (95% CI, 14–83).
Conclusions
We provide a comprehensive analysis of interval CRC staging and clinical characteristics in the context of the NBCSP in Western Australia, facilitating the definition of benchmarks for monitoring programme performance.
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