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Colorectal Cancer Screening Riskの包括的な教育ガイドを準備中です。ステップバイステップの解説、数式、実例、専門家のヒントをお届けしますので、もうしばらくお待ちください。
Colorectal cancer (CRC) risk stratification is a systematic clinical process used to categorise individuals by their probability of developing colorectal cancer over their lifetime, enabling targeted and cost-effective screening strategies. Colorectal cancer is the third most commonly diagnosed cancer and the second leading cause of cancer death worldwide, yet it is highly preventable through colonoscopic polypectomy and detectable at curable stages through organised screening. Risk stratification divides individuals into three principal categories: average risk (approximately 75% of the population), increased risk, and high risk. Average-risk individuals — those aged 45–75 with no personal or family history of CRC or advanced adenomas — are recommended to undergo screening colonoscopy starting at age 45 (per 2021 USPSTF and ACG guidelines) or age 50 (per many European guidelines), or alternatively to use non-invasive tests such as the faecal immunochemical test (FIT) annually or a multi-target stool DNA test every 1–3 years. Increased-risk individuals — those with one first-degree relative diagnosed with CRC or advanced adenoma (villous adenoma ≥1 cm or high-grade dysplasia) at any age — should begin colonoscopy screening 10 years before the earliest relative's diagnosis or at age 40, whichever comes first. High-risk individuals include those with hereditary syndromes: familial adenomatous polyposis (FAP), attenuated FAP (AFAP), MUTYH-associated polyposis (MAP), Lynch syndrome (hereditary non-polyposis CRC), serrated polyposis syndrome, and Peutz-Jeghers syndrome — each requiring dedicated surveillance protocols, often beginning in adolescence or early adulthood.
CRC Screening Start Age = max(40, index case age − 10) for increased-risk individuals with 1 first-degree relative <60; OR Age 40 if 1 FDR diagnosed at any age; OR Age 45 for average risk (US guidelines); Lynch syndrome: colonoscopy every 1–2 years from age 20–25 or 10 years before youngest affected relative; FAP: annual flexible sigmoidoscopy from age 10–12
- 1Take a structured family history covering at least three generations: identify all first-degree relatives (parents, siblings, children) and second-degree relatives (grandparents, aunts, uncles) with colorectal cancer or advanced adenomas, recording their ages at diagnosis.
- 2Identify any personal risk factors: personal history of adenomatous polyps (number, size, histology), inflammatory bowel disease (ulcerative colitis or Crohn's colitis — risk increases with extent and duration), prior radiation to the pelvis, acromegaly, or type 2 diabetes.
- 3Classify the individual into average, increased, or high-risk category based on family history, personal history, and genetic test results.
- 4For average-risk individuals aged 45–75: offer a choice of colonoscopy (every 10 years), annual FIT, multi-target stool DNA (every 1–3 years), CT colonography (every 5 years), or flexible sigmoidoscopy (every 5 years).
- 5For increased-risk individuals: recommend colonoscopy starting at age 40 or 10 years before the earliest first-degree relative's diagnosis, repeated every 5 years if the first colonoscopy is negative for advanced adenomas.
- 6For high-risk (hereditary syndrome) individuals: refer to a dedicated hereditary cancer clinic for genetic counselling, germline testing, and a syndrome-specific surveillance protocol — FAP requires annual endoscopy from age 10–12; Lynch syndrome requires colonoscopy every 1–2 years from age 20–25.
- 7After any colonoscopy, assign post-polypectomy surveillance intervals based on the number, size, and histology of removed polyps: no polyps or 1–2 small tubular adenomas (<10 mm) — repeat at 7–10 years; 3–4 adenomas or any high-risk adenoma — repeat at 3 years; 5–10 adenomas — repeat at 1 year.
The USPSTF lowered the recommended screening age from 50 to 45 in 2021 to address rising CRC incidence in younger adults.
In the absence of risk factors, the population-level lifetime risk of CRC is approximately 4–5% — colonoscopy or FIT screening reduces CRC mortality by 40–60%.
Having one first-degree relative with CRC doubles the lifetime risk to approximately 8–10%.
Guidelines recommend starting screening at 40 or 10 years before the youngest affected relative's age — whichever is earlier — for those with an FDR aged ≥60 at diagnosis.
MLH1 and MSH2 Lynch syndrome carriers have 40–80% lifetime CRC risk.
Lynch syndrome is the most common hereditary CRC syndrome, accounting for 3–5% of all CRC cases; colonoscopic surveillance every 1–2 years reduces CRC mortality by approximately 63%.
A villous adenoma ≥10 mm (advanced adenoma) doubles the risk of CRC at follow-up.
Tubulovillous adenomas ≥10 mm are classified as advanced adenomas; finding ≥3 adenomas or any advanced adenoma mandates 3-year follow-up colonoscopy per ACG/ESGE guidelines.
Gastroenterology practices use CRC risk stratification to assign appropriate surveillance colonoscopy intervals from post-polypectomy pathology reports, reducing unnecessary procedures, enabling practitioners to make well-informed quantitative decisions based on validated computational methods and industry-standard approaches
Primary care physicians use family history tools and risk stratification to identify patients who need earlier-than-standard colonoscopy screening referrals, helping analysts produce accurate results that support strategic planning, resource allocation, and performance benchmarking across organizations
Public health programmes use population-level FIT screening to reach average-risk individuals who would not otherwise present for colonoscopy, allowing professionals to quantify outcomes systematically and compare scenarios using reliable mathematical frameworks and established formulas
Hereditary cancer clinics use pedigree analysis and MMR testing to identify Lynch syndrome families and offer cascade genetic testing to at-risk relatives, supporting data-driven evaluation processes where numerical precision is essential for compliance, reporting, and optimization objectives
NHS Bowel Cancer Screening Programme in the UK uses biennial FIT in adults aged 50–74 to identify high-risk individuals for follow-on colonoscopy — a model that has reduced CRC mortality by approximately 25% in invited participants.
Familial adenomatous polyposis (FAP)
FAP is caused by APC gene pathogenic variants and characterised by hundreds to thousands of colorectal adenomatous polyps. Without surgical intervention, the lifetime risk of CRC is virtually 100% by age 40. Annual flexible sigmoidoscopy begins at age 10–12; prophylactic colectomy is recommended once polyposis is extensive. Attenuated FAP (AFAP) presents with fewer polyps (10–100) and a delayed onset, allowing a less aggressive surgical timeline.
MUTYH-associated polyposis (MAP)
MAP is an autosomal recessive condition caused by biallelic pathogenic variants in the MUTYH base-excision repair gene, resulting in 10–100 adenomatous polyps and a significantly elevated CRC risk (approximately 43% lifetime). Surveillance colonoscopy every 1–2 years from age 25–30 is recommended; prophylactic colectomy may be considered in extensive cases.
Serrated polyposis syndrome
Defined by WHO 2019 criteria as ≥5 serrated lesions/polyps proximal to the sigmoid colon (all ≥5 mm, with at least 2 ≥10 mm) OR any number of serrated polyps proximal to the sigmoid colon in an individual who has a first-degree relative with serrated polyposis. Risk of CRC is elevated; annual colonoscopy is recommended until polyp burden is controlled, then every 1–3 years.
Prior pelvic radiation
Individuals who received pelvic radiation for prior cancers (prostate, cervical, rectal) have an increased risk of radiation-induced secondary colorectal malignancies, typically arising 10–20 years post-treatment. Enhanced surveillance colonoscopy is recommended from 10 years post-radiation treatment completion.
| Risk Category | Criteria | Screening Start Age | Interval |
|---|---|---|---|
| Average | No personal/family history, no IBD | 45 (US) / 50 (EU) | Colonoscopy 10yr; FIT annual |
| Increased — 1 FDR ≥60 | One FDR with CRC/advanced adenoma at ≥60 | 40 | Colonoscopy every 5 years |
| Increased — 1 FDR <60 | One FDR with CRC at <60 OR ≥2 FDRs at any age | 40 or 10yr before index | Colonoscopy every 5 years |
| High — Lynch syndrome | MMR gene pathogenic variant confirmed | 20–25 (or 2–5yr before FDR) | Colonoscopy every 1–2 years |
| High — FAP | APC pathogenic variant or >100 adenomas | 10–12 | Annual flexible sigmoidoscopy; colectomy if extensive polyposis |
| High — IBD (extensive colitis) | UC/Crohn's >8–10yr disease duration | 8–10yr after diagnosis | Colonoscopy + biopsies every 1–3yr |
What is the faecal immunochemical test (FIT) and how does it compare to colonoscopy?
FIT detects human haemoglobin in stool using antibodies, offering greater specificity than older guaiac-based FOBT. Annual FIT reduces CRC mortality by approximately 40% in population-based trials. It is non-invasive and requires no bowel preparation, but a positive FIT must always be followed up with diagnostic colonoscopy. Sensitivity for cancer is 79% and for advanced adenomas 24–40%.
At what age should CRC screening stop?
For average-risk individuals, most guidelines recommend stopping routine CRC screening at age 75. Between 76–85, the decision is individualised based on health status, life expectancy, prior screening history, and patient preference. Routine screening is not recommended above age 85. High-risk individuals (hereditary syndromes) may continue surveillance beyond 75. Understanding this aspect of colorectal cancer risk is important for obtaining accurate and meaningful results in both clinical and analytical contexts.
Does inflammatory bowel disease (IBD) increase CRC risk?
Yes. Patients with extensive ulcerative colitis or Crohn's colitis have a significantly elevated CRC risk, beginning to accumulate after 8–10 years of disease. Surveillance colonoscopy with chromoendoscopy or random biopsies is recommended every 1–3 years depending on disease extent, activity, and additional risk factors (primary sclerosing cholangitis doubles CRC risk in UC patients).
What is Lynch syndrome and how is it identified?
Lynch syndrome (formerly HNPCC) is caused by germline pathogenic variants in mismatch repair (MMR) genes: MLH1, MSH2, MSH6, PMS2, or EPCAM. It accounts for 3–5% of CRC and significantly elevates risk of endometrial, ovarian, gastric, and other cancers. It is identified through universal tumour MMR testing (IHC or MSI-PCR) of all CRC resections, followed by germline testing for MMR variants when tumour testing is abnormal.
How many adenomas found at colonoscopy require repeat colonoscopy in 3 years versus 10 years?
1–2 small tubular adenomas (<10 mm, low-grade dysplasia): repeat at 7–10 years. 3–4 adenomas OR any advanced adenoma (≥10 mm, villous/tubulovillous, high-grade dysplasia): repeat at 3 years. ≥5 adenomas or any adenoma with high-grade dysplasia: repeat at 1 year. These thresholds reflect the 2020 US Multi-Society Task Force (MSTF) guidelines.
Is CRC risk higher in men than women?
Yes. Men have approximately a 1.3× higher age-adjusted incidence of CRC than women, and adenomas are more prevalent in men at equivalent ages. Some guidelines recommend men begin screening slightly earlier. Hormonal differences and oestrogen's protective effect in premenopausal women may partially explain this disparity. Understanding this aspect of colorectal cancer risk is important for obtaining accurate and meaningful results in both clinical and analytical contexts.
What role does diet play in CRC risk?
Red and processed meat consumption, alcohol intake, obesity, and low dietary fibre are established CRC risk factors. Physical inactivity and type 2 diabetes independently increase risk. Aspirin (75–300 mg/day) reduces CRC risk by approximately 25% with prolonged use but is only recommended for CRC chemoprevention in Lynch syndrome or selected high-risk groups due to bleeding risk.
Can a negative colonoscopy guarantee no CRC will develop?
A high-quality negative colonoscopy (adequate bowel preparation, caecal intubation confirmed, adenoma detection rate ≥25%) provides approximately 10 years of effective CRC protection for average-risk individuals. However, it does not eliminate risk — interval cancers do occur, most often from missed lesions, incompletely resected polyps, or rapidly progressing de novo tumours (particularly in Lynch syndrome).
プロのヒント
The Amsterdam II criteria (3-2-1 rule) are a useful bedside tool for identifying Lynch syndrome-suspicious families: 3 or more relatives with a Lynch-associated cancer (CRC, endometrial, urinary tract, small bowel), spanning 2 or more generations, with at least 1 diagnosis before age 50 and with FAP excluded. Families meeting these criteria should be referred to genetics even without tumour MMR testing results.
ご存知でしたか？
The human colorectum contains approximately 1 metre of large bowel and over 10 trillion microbial organisms in the gut microbiome. Research increasingly links microbiome composition — particularly the abundance of Fusobacterium nucleatum — to adenoma development and CRC risk. Several CRC biomarker tests now include Fusobacterium DNA detection alongside FIT to improve sensitivity.
参考文献
- ›USPSTF — Colorectal Cancer Screening Recommendation (2021)
- ›ACG Clinical Guideline — Colorectal Cancer Screening (Rex DK et al., 2017)
- ›US Multi-Society Task Force — Post-polypectomy surveillance (2020)
- ›NICE NG151 — Colorectal cancer (2020)
- ›Bonadies DC et al. — Lynch Syndrome: Current knowledge and future directions, Genet Med 2011