Exploring geographical differences in the incidence of colorectal cancer in the Norwegian Women and Cancer Study: a population-based prospective study.

Sunday Oluwafemi Oyeyemi,Kristin Benjaminsen Borch,Edoardo Botteri,Tonje Braaten,Paula Berstad

doi:10.2147/clep.s207413

Sunday Oluwafemi Oyeyemi, Kristin Benjaminsen Borch + Show 3 more

Open Access

https://doi.org/10.2147/clep.s207413

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Abstract

PurposeNorway has experienced an unexplained, steep increase in colorectal cancer (CRC) incidence in the last half-century, with large differences across its counties. We aimed to determine whether geographical distribution of lifestyle-related CRC risk factors can explain these geographical differences in CRC incidence in Norwegian women.MethodsWe followed a nationally representative cohort of 96,898 women with self-reported information on lifestyle-related CRC risk factors at baseline and at follow-up 6–8 years later in the Norwegian Women and Cancer Study. We categorized Norwegian counties into four county groups according to CRC incidence and used Cox proportional hazard models to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for risk factors. We used the Karlson, Holm, and Breen (KHB) method of mediation analysis to investigate the extent to which the risk factors accounted for the observed differences in CRC incidence between counties.ResultsDuring an average of 15.5 years of follow-up, 1875 CRC cases were diagnosed. Height (HR=1.12; 95% CI 1.08, 1.17 per 5 cm increase); being a former smoker who smoked ≥10 years (HR=1.34; 95% CI 1.15, 1.57); or being a current smoker who has smoked for ≥10 years (HR=1.28; 95% CI 1.12, 1.46) relative to never smokers was associated with increased CRC risk. Duration of education >12 years (HR=0.78; 95% CI 0.69, 0.87) vs ≤12 years, and intake of vegetables and fruits >300 g (HR=0.90; 95% CI 0.80, 0.99) vs ≤300 g per day were associated with reduced CRC risk. However, these risk factors did not account for the differences in CRC risk between geographical areas of low and high CRC incidence. This was further confirmed by the KHB method using baseline and follow-up measurements (b=0.02, 95% CI −0.02, 0.06, p=0.26).ConclusionLifestyle-related CRC risk factors did not explain the geographical variations in CRC incidence among Norwegian women. Possible residual explanations may lie in heritable factors.

Highlights

Colorectal cancer (CRC) is the second most common malignancy in women globally,[1] and the second leading cause of cancer-related death in high-income countries.[2]Norway has experienced an unexplained, steep increase in the incidence of CRC in both men and women in the last half-century.[3,4] From 1957–61 to 2012–16, incidence rates among Norwegian women increased from 21 to 54 per 100,000 person-years for colon cancer, and from 9 to 20 per 100,000 person-years for rectal cancer.[5]
We conducted a sensitivity analysis using the 19 counties individually, which showed that the combined effects of the risk factors did not significantly mediate the variations in CRC incidence across counties. In this large cohort of Norwegian women, we found that county-level differences in CRC incidence were not explained by differences in lifestyle-related CRC risk factors
Our results showed that these factors, together with other CRC risk factors, did not significantly explain the differences in the CRC incidence between the counties

Summary

Introduction

Colorectal cancer (CRC) is the second most common malignancy in women globally,[1] and the second leading cause of cancer-related death in high-income countries.[2]Norway has experienced an unexplained, steep increase in the incidence of CRC in both men and women in the last half-century.[3,4] From 1957–61 to 2012–16, incidence rates among Norwegian women increased from 21 to 54 per 100,000 person-years for colon cancer, and from 9 to 20 per 100,000 person-years for rectal cancer.[5]. Colorectal cancer (CRC) is the second most common malignancy in women globally,[1] and the second leading cause of cancer-related death in high-income countries.[2]. Differences in CRC incidence vary over 10-fold across countries,[7] which may be ascribed to variations in dietary and environmental exposures, coupled with genetic susceptibility.[8] CRC incidence varies within Norway, with a more than 20 per 100,000 person-years difference between areas of high and low CRC incidence.[9,10] The factors responsible for this geographical heterogeneity are yet to be determined, and knowledge of these factors could be useful to guide screening strategies and health policy

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