Familial Adenomatous Polyposis (FAP)

📋 Key Information Summary

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Familial adenomatous polyposis (FAP) is an autosomal dominant condition caused by germline pathogenic variants in the APC tumour-suppressor gene on chromosome 5q21–22; approximately 70–80 % of cases are inherited and 20–30 % arise from de novo mutations.
Attenuated FAP (AFAP), associated with mutations at the extreme 5′ or 3′ ends of APC, presents with fewer polyps (<100), later onset, and CRC risk around 70 % by age 80.
The genotype–phenotype correlation guides clinical management: codon 1250–1464 mutations predict profuse polyposis and earlier need for surgery; codon 1–157 and >1595 mutations correlate with AFAP.
Extracolonic manifestations are common and include upper-gastrointestinal polyps (fundic gland, adenomatous), desmoid tumours, hepatoblastoma, thyroid carcinoma, CNS tumours (Turcot variant), and congenital hypertrophy of the retinal pigment epithelium (CHRPE).
Screening for APC mutations should be offered to all first-degree relatives of known FAP patients; predictive genetic testing is recommended from age 10–12 years in classic FAP families.
Colectomy remains the cornerstone of CRC prevention in classic FAP — options include total proctocolectomy with ileal pouch–anal anastomosis (IPAA), or total abdominal colectomy with ileorectal anastomosis (IRA) in selected patients with low rectal polyp burden.
Chemoprevention with sulindac or celecoxib can reduce polyp number and size but has NOT demonstrated prevention of CRC and is adjunctive only; celecoxib 400 mg BD is PBS Authority Required for FAP.
Lifelong endoscopic surveillance of the upper and lower gastrointestinal tract is mandatory — annual flexible sigmoidoscopy after IRA, ileal-pouch surveillance after IPAA, and upper-GI endoscopy every 1–3 years from age 25.
Desmoid tumours are the leading cause of morbidity and second leading cause of mortality in FAP; management ranges from observation (asymptomatic) to sulindac plus tamoxifen, or cytotoxic chemotherapy for aggressive disease.
Aboriginal and Torres Strait Islander Australians have lower rates of genetic testing and referral for hereditary CRC syndromes; culturally safe outreach and community-based genetic counselling can improve uptake of surveillance and prophylactic surgery.
A multidisciplinary team involving gastroenterology, colorectal surgery, clinical genetics, oncology, endocrinology (thyroid screening), ophthalmology, and psychology is essential for optimal FAP care.

Introduction & Australian Epidemiology

Familial adenomatous polyposis (FAP) is the most common hereditary colorectal cancer syndrome, accounting for approximately 1 % of all colorectal carcinomas worldwide. It is an autosomal dominant condition caused by inactivating germline pathogenic variants in the APC (adenomatous polyposis coli) tumour-suppressor gene located on chromosome 5q21–22. In the classic form, hundreds to thousands of adenomatous polyps carpet the colorectal mucosa beginning in adolescence; without prophylactic colectomy, progression to CRC is virtually inevitable, with a median age at diagnosis of approximately 39 years.

In Australia, FAP affects an estimated 1 in 7,000–10,000 live births, with approximately 250–400 affected individuals identified at any given time. The Australian Institute of Health and Welfare (AIHW) reports that hereditary non-polyposis colorectal cancer (HNPCC/Lynch syndrome) and FAP together account for 5–10 % of CRC cases nationally. However, under-diagnosis remains a significant issue, particularly in regional, rural, and remote communities, and among Aboriginal and Torres Strait Islander peoples, where access to genetic testing and specialist gastroenterology services is limited.

The Cancer Genetics Clinic network — including services at the Peter MacCallum Cancer Centre (Melbourne), Familial Cancer Service (Sydney), Genetic Health Queensland, and the South Australian Clinical Genetics Service — coordinates most Australian FAP registries. National guidelines from Cancer Australia and the eviQ resource provide evidence-based management pathways. This guideline synthesises current evidence on genetics, clinical features, extracolonic manifestations, and surveillance/management strategies applicable to the Australian healthcare setting.

Genetics & Pathogenesis

The APC Gene and the Wnt Signalling Pathway

The APC gene encodes a 2,843-amino-acid protein that functions as a critical negative regulator of the canonical Wnt/β-catenin signalling pathway. In normal colonic epithelium, the APC protein forms a destruction complex with Axin, glycogen synthase kinase-3β (GSK-3β), and casein kinase 1α, promoting phosphorylation and proteasomal degradation of β-catenin. When APC is inactivated by germline mutation, β-catenin accumulates in the nucleus, constitutively activating TCF/LEF transcription factors and driving uncontrolled expression of pro-proliferative genes such as MYC and Cyclin D1.

Germline Mutations

Approximately 70–80 % of FAP patients carry a germline pathogenic variant in APC. The mutational spectrum includes:

Nonsense and frameshift variants (~70 %): Premature termination codons leading to truncated, non-functional protein; the majority cluster in exon 15 (codons 1,250–1,464).
Splice-site variants (~10 %): Aberrant mRNA splicing with variable phenotypic severity.
Large deletions / rearrangements (~5–10 %): Detected by MLPA (multiplex ligation-dependent probe amplification) when sequencing is negative; may account for a higher proportion of apparent de novo cases.

In approximately 20–30 % of patients, the pathogenic variant is de novo (no family history), underscoring the importance of genetic testing even in apparently sporadic polyposis.

Genotype–Phenotype Correlations

Mutation Region	Codon Range	Phenotype
5′ end (exon 4–9)	Codon 1–157	Attenuated FAP; fewer polyps, later onset
Central cluster (exon 15)	Codon 158–495	Classic FAP; CHRPE; desmoid risk may be lower
Mutation cluster region	Codon 1,250–1,330	Profuse polyposis (>5,000 polyps); earliest CRC risk
Codon 1,309	Codon 1,309	Most severe; CRC ~10 years earlier than average
3′ end	Codon >1,595	Attenuated FAP; higher desmoid risk
Desmoid-associated	Codon 1,395–2,000	50–80 % desmoid risk; "Turcot" overlap

The Multi-Hit (Knudson) Model

FAP exemplifies the Knudson two-hit hypothesis: the inherited first hit (germline APC mutation) is followed by somatic loss of the remaining wild-type allele (loss of heterozygosity, LOH) in individual colonic epithelial cells. Additional somatic mutations in KRAS, SMAD4, and TP53 then drive the adenoma→carcinoma sequence at an accelerated rate. The APC mutation is termed the "gatekeeper" of colorectal tumorigenesis.

MYH-Associated Polyposis (MAP) — Key Differential

MUTYH-associated polyposis (MAP) is an autosomal recessive condition caused by biallelic pathogenic variants in MUTYH, a base-excision repair gene. It phenotypically mimics attenuated FAP but is distinguished by autosomal recessive inheritance, absence of APC mutations, and characteristic somatic APC G:C→T:A transversions. All patients with multiple adenomas and negative APC testing should undergo MUTYH analysis.

Classic & Attenuated FAP Features

Classic FAP

Classic FAP is defined by the presence of ≥100 colorectal adenomatous polyps detectable by flexible sigmoidoscopy or colonoscopy, typically manifesting by age 15–20 years. If untreated, virtually 100 % of patients develop CRC, with a median age of CRC onset of 39 years (range 34–43). Key clinical features include:

Hundreds to thousands of 2–10 mm adenomatous polyps carpeting the colorectum, with density greatest in the rectum and sigmoid.
Symptomatic presentation may include rectal bleeding, diarrhoea, mucus discharge, abdominal pain, or iron-deficiency anaemia.
Polyps begin as early as age 10; CRC has been reported as early as age 15 in high-risk genotypes (codon 1309).
Without intervention, almost all patients will require colectomy by their mid-20s.

Attenuated FAP (AFAP)

Attenuated FAP is characterised by a milder phenotype with fewer colorectal polyps (typically 10–99), later onset (mean age at diagnosis ~35–45 years), and a CRC risk of approximately 70 % by age 80 if not managed. AFAP is associated with mutations at the extreme 5′ end of APC (exons 4–9, codons 1–157) or at the 3′ end (codons >1,595). Key distinctions from classic FAP:

Feature	Classic FAP	Attenuated FAP
Polyp number	≥100 (often >1,000)	10–99 (sometimes <10)
Age at onset	10–20 years	20–45 years
CRC risk	~100 %	~70 % by age 80
Polyp distribution	Diffuse; rectal predominance	Right-sided predominance; sparse in rectum
Mutation site	Codon 158–1,464	Codon 1–157 or >1,595
Surgical approach	Prophylactic colectomy in late teens	Surveillance-guided; colectomy may be deferred
CHRPE	Common	Rare or absent
Desmoid risk	Moderate (10–15 %)	Lower (unless 3′ mutation)

Gardner Syndrome and Turcot Syndrome

Gardner syndrome and Turcot syndrome are phenotypic variants of FAP — not separate genetic entities:

Gardner syndrome: FAP plus extra-intestinal benign tumours — osteomas (mandible, skull), epidermoid cysts, desmoid tumours, dental abnormalities (unerupted teeth, supernumerary teeth), and CHRPE.
Turcot syndrome: FAP plus central nervous system tumours — most commonly medulloblastoma (APC-related) or glioblastoma (more often associated with mismatch repair gene mutations / Lynch syndrome). APC-related Turcot carries a median age of brain tumour diagnosis of ~14 years.

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Key clinical point: Patients with 10–99 adenomas or with <10 adenomas but a strong family history of CRC should still undergo APC and MUTYH genetic testing. Attenuated FAP can be misclassified as sporadic adenomas, leading to missed surveillance and preventable cancers.

Extracolonic Manifestations

FAP is a true multi-organ syndrome. Awareness of extracolonic manifestations is essential for comprehensive surveillance and early intervention. The following table summarises the major manifestations, estimated prevalence, and recommended screening approach.

Organ System	Manifestation	Prevalence in FAP	Screening / Management
Upper GI	Duodenal / periampullary adenomas	50–90 %	Upper GI EGD from age 25; Spigelman staging every 1–3 years
Upper GI	Fundic gland polyps	30–88 %	Biopsy if >10 mm or dysplastic
Gastric	Gastric adenomas	5–10 % (higher in Asian FAP)	Endoscopic resection; surveillance
Soft tissue	Desmoid tumours	10–25 % (up to 80 % with 3′ mutations)	MRI surveillance; surgical excision for symptomatic / enlarging lesions
Thyroid	Papillary thyroid carcinoma	2–12 % (female predominance)	Annual thyroid ultrasound from age 15
Hepatobiliary	Hepatoblastoma	1.6 % (mostly children <5 years)	Serum AFP every 6 months from birth to age 15
CNS	Medulloblastoma (Turcot)	<1 %	Low threshold for neuroimaging if neurological symptoms
Ophthalmic	Congenital hypertrophy of the retinal pigment epithelium (CHRPE)	60–80 % (classic FAP)	Baseline fundoscopy; no treatment needed (diagnostic marker)
Dental	Osteomas, supernumerary teeth, unerupted teeth	20–75 %	Panoramic dental X-ray; oral surgery review
Adrenal	Adrenal adenomas / carcinomas	7–13 %	Incidental on CT; functional work-up if >4 cm

Duodenal Adenomas and Spigelman Classification

Duodenal adenocarcinoma is the second most common malignancy in FAP after CRC, carrying a cumulative lifetime risk of 4–12 %. The Spigelman staging system (Stage 0–IV) guides surveillance intervals and intervention thresholds:

Stage 0–II

Spigelman 0–II

Number <5, size <10 mm, low-grade dysplasia, tubular histology. Total score 0–6.

Surveillance: EGD every 3–5 years

Stage III

Spigelman III

Score 7–8. Multiple adenomas, moderate size, tubulovillous or high-grade dysplasia.

Surveillance: EGD every 1–2 years; consider endoscopic resection

Stage IV

Spigelman IV

Score 9–12. ≥20 adenomas, >10 mm, villous histology, high-grade dysplasia.

Pancreas-sparing duodenectomy or Whipple procedure if cancer risk unacceptable

Desmoid Tumours

Desmoid tumours (aggressive fibromatosis) are the leading cause of morbidity and the second leading cause of death in FAP patients. They arise in approximately 10–25 % of FAP patients overall (up to 80 % in those with 3′ APC mutations) and are most common in the mesentery, abdominal wall, and extra-abdominal sites. Risk factors for desmoid development include prior abdominal surgery, female sex, and positive family history of desmoids.

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Sulindac

Clinoril® · Aclin® · NSAID (COX-1/2)

Adult dose 150–200 mg PO BD (desmoid: 200 mg BD)

Paediatric dose 3–6 mg/kg/day PO in 1–2 divided doses

Renal adjustment Avoid if eGFR <30 mL/min/1.73 m²

PBS status ✔ PBS General Benefit

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Tamoxifen (as adjunct for desmoids)

Nolvadex® · Generic · SERM

Adult dose 20–40 mg PO daily (combined with sulindac)

Notes Off-label for desmoid; response rate ~40–50 %

PBS status ✔ PBS General Benefit (for breast cancer indication)

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Celecoxib

Celebrex® · Generic · Selective COX-2 inhibitor

Adult dose 400 mg PO BD (FAP chemoprevention)

Renal adjustment Reduce dose if eGFR 30–60; avoid if <30

PBS status ⚠ PBS Authority Required (FAP)

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Post-surgical desmoid warning: Abdominal surgery itself can trigger desmoid growth in FAP patients. Prophylactic colectomy decisions should weigh the risk of post-operative desmoid development, especially in patients with a family history of desmoid disease or 3′ APC mutations. Delaying surgery and intensive endoscopic surveillance may be appropriate in these patients.

Surveillance & Management

Genetic Testing and Cascade Screening

Genetic testing for the known familial APC pathogenic variant should be offered to all first-degree relatives of confirmed FAP patients. Predictive genetic testing in children is recommended from age 10–12 years (classic FAP) or 18–20 years (AFAP), preceded by age-appropriate genetic counselling. Mutation-negative relatives can be discharged from intensive surveillance.

ℹ️

Medicare MBS item 73293: Genetic testing for APC mutation is available under MBS for individuals with a first-degree relative with a known pathogenic variant or individuals meeting clinical criteria for FAP (≥10 colorectal adenomas). Refer to a familial cancer service for testing and counselling.

Colorectal Surveillance

1

Mutation carriers (gene-positive)

Annual flexible sigmoidoscopy from age 10–12. Progress to full colonoscopy if polyps detected. Transition to annual colonoscopy from age 18 or when adenomas identified.

2

At-risk family members (untested)

Annual sigmoidoscopy from age 12–14 until genetic status confirmed or polyps found.

3

Post-IRA (ileorectal anastomosis)

Flexible sigmoidoscopy of the rectal remnant every 6–12 months for life. Biopsy suspicious lesions; proctectomy if unmanageable polyps or high-grade dysplasia.

4

Post-IPAA (ileal pouch–anal anastomosis)

Pouchoscopy with biopsies every 1–3 years from 1 year post-surgery. Adenomas can develop in the pouch and at the anal transition zone (ATZ).

Surgical Options

Procedure	Description	Advantages	Disadvantages	Indication
TAC + IRA	Total abdominal colectomy with ileorectal anastomosis	Better bowel function; lower surgical morbidity; no permanent stoma	Rectal remnant retains CRC risk (0.7–1 %/year); requires lifelong surveillance	Classic FAP with <20 rectal polyps and no rectal cancer
TPC + IPAA	Total proctocolectomy with ileal pouch–anal anastomosis (J-pouch)	Removes virtually all at-risk mucosa; eliminates rectal cancer risk	Higher morbidity; pouchitis (up to 50 %); reduced fertility in women; worse stool frequency	Classic FAP with >20 rectal polyps; rectal cancer; AFAP with severe rectal involvement
TPC + IRA / end ileostomy	Total proctocolectomy with end ileostomy	Lowest residual cancer risk	Permanent stoma; significant QoL impact	When IPAA not feasible (low rectal cancer, sphincter incompetence, severe desmoid)

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Timing of colectomy: Surgery is recommended when polyps become unmanageable endoscopically (high-grade dysplasia, polyps >10 mm, or polyp number increasing despite endoscopic therapy). For classic FAP, this typically occurs in the late teens to early twenties. In codon 1309 mutations, surgery may be needed as early as age 15. AFAP patients may defer surgery into their 30s–40s with intensive colonoscopic surveillance.

Upper Gastrointestinal Surveillance

All FAP patients should commence upper GI endoscopy (EGD) from age 25 (or 20 in high-risk genotypes). The Spigelman classification guides ongoing intervals:

Spigelman Stage 0–I: EGD every 4–5 years
Spigelman Stage II: EGD every 2–3 years
Spigelman Stage III: EGD every 1–2 years; consider endoscopic mucosal resection (EMR) or argon plasma coagulation
Spigelman Stage IV: EGD every 6–12 months; surgical consultation for pancreas-sparing duodenectomy or Whipple procedure

Thyroid Surveillance

Thyroid ultrasound is recommended annually from age 15 (some guidelines suggest from age 10). Papillary thyroid carcinoma in FAP is more common in women, tends to be multifocal, and may present at a younger age than sporadic thyroid cancer. Baseline thyroid function tests and neck ultrasound should be performed at the time of FAP diagnosis.

Hepatoblastoma Screening

For children with a known APC mutation or at 50 % risk (untested children of FAP patients): serum alpha-fetoprotein (AFP) and abdominal ultrasound every 6 months from birth until age 15. Hepatoblastoma is rare (~1.6 %) but treatable if detected early.

Chemoprevention

NSAIDs and COX-2 inhibitors can reduce the number and size of colorectal adenomas in FAP but have NOT been shown to prevent CRC or replace surgery:

💊

Sulindac

Clinoril® · FAP polyp reduction

Adult dose 150–200 mg PO BD

Efficacy Polyp regression 40–60 %; no evidence of CRC prevention

Adverse effects GI bleeding, renal impairment, cardiovascular risk

PBS status ✔ PBS General Benefit

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Celecoxib

Celebrex® · FAP polyp reduction

Adult dose 400 mg PO BD

Efficacy 28–30 % reduction in polyp number (Familial Adenomatous Polyposis Study)

Adverse effects CV thrombotic events, GI ulceration

PBS status ⚠ PBS Authority Required

⚠️

Chemoprevention caveat: Chemoprevention with NSAIDs or COX-2 inhibitors should NOT delay indicated surgery. It may be useful as adjunctive therapy post-colectomy (reducing rectal polyp burden after IRA) or while awaiting surgery. Ongoing cardiovascular and renal risk must be weighed.

Emerging Therapies

Several novel approaches are under investigation for FAP, including:

Eflornithine ± sulindac: Ornithine decarboxylase inhibitor combined with sulindac has shown significant reduction in duodenal polyp progression in recent Phase III trials (Sulindac and Eflornithine in Treating Patients with Familial Adenomatous Polyposis — NCT01483144).
Wnt pathway inhibitors: Porcupine inhibitors (e.g., ETC-159, WNT974) target Wnt ligand secretion; early-phase trials ongoing.
Encapsulated faecal microbiota transplantation: Preliminary data suggest microbiome modulation may influence polyp growth.

None of these therapies are currently PBS-listed or approved by the TGA for FAP. Patients should be referred to clinical trials where appropriate.

Special Populations

🤰 Pregnancy

Timing of surgery

Prophylactic colectomy should ideally be completed before pregnancy. If diagnosed during pregnancy, surveillance colonoscopy can be safely performed in the second trimester; surgery deferred to post-partum unless cancer is suspected.

Sulindac / Celecoxib

NSAIDs are contraindicated in the third trimester (premature ductus arteriosus closure). Discontinue from 28 weeks gestation. First- and second-trimester use requires risk–benefit discussion.

Desmoid risk

Oestrogen may promote desmoid growth. Monitor for abdominal wall or mesenteric desmoids during and after pregnancy.

Fertility post-IPAA

IPAA is associated with reduced female fertility (increased adhesion-related subfertility). Counselling before surgery is essential; consider IRA if fertility preservation is a priority.

👶 Paediatrics

Genetic testing

Predictive testing for APC mutation from age 10 (classic FAP) with genetic counselling. Psychological support is critical for the child and family.

Surveillance onset

Annual flexible sigmoidoscopy from age 10–12 in gene-positive children. If polyps are found, transition to full colonoscopy.

Hepatoblastoma

Screen with serum AFP and abdominal USS every 6 months from birth to age 15. Hepatoblastoma is rare (~1.6 %) but aggressive; early detection improves survival.

Surgery timing

Colectomy is typically deferred until late teens (unless high-risk genotype, high-grade dysplasia, or symptomatic). IPAA is generally preferred in paediatric patients to avoid lifelong rectal surveillance.

👴 Elderly

Surgical risk

Comorbidities may preclude major surgery. In patients with AFAP or low polyp burden, intensive endoscopic surveillance may be appropriate over colectomy. Shared decision-making is essential.

NSAID use

Increased GI bleeding and cardiovascular risk with sulindac/celecoxib in the elderly. Use lowest effective dose; add PPI gastroprotection; monitor renal function.

🫘 Renal Impairment

NSAIDs (sulindac, celecoxib)

Contraindicated if eGFR <30 mL/min/1.73 m². Use with caution if eGFR 30–60. Monitor serum creatinine and electrolytes at baseline and 2-weekly during initiation.

Chemoprevention alternatives

In patients unable to tolerate NSAIDs, endoscopic management is the primary approach. Eflornithine is not yet approved but may offer future options.

🫁 Hepatic Impairment

NSAIDs

Use with caution in hepatic impairment (Child–Pugh A/B); avoid in severe liver disease due to bleeding risk and altered drug metabolism.

Hepatoblastoma surveillance

AFP is a tumour marker — elevated levels in chronic liver disease may complicate interpretation. Coordinate with hepatology.

🛡️ Immunocompromised

Post-transplant FAP

FAP is occasionally diagnosed incidentally in transplant recipients. Immunosuppression does not directly alter FAP progression but may increase surgical infection risk. Coordinate management with transplant team.

NSAIDs and immunosuppression

NSAID use with calcineurin inhibitors increases nephrotoxicity risk. Avoid concurrent use if possible; monitor renal function closely.

Aboriginal and Torres Strait Islander Health Considerations

Aboriginal and Torres Strait Islander Health

Epidemiology

Aboriginal and Torres Strait Islander Australians experience colorectal cancer at comparable or slightly lower incidence than non-Indigenous Australians, but present at a later stage with worse outcomes. Hereditary CRC syndromes, including FAP, are likely under-diagnosed in this population due to lower access to genetic testing and specialist services.

Access to genetic services

There is a documented disparity in referral rates for genetic counselling and testing among Indigenous Australians. Remote and very remote communities have limited or no access to familial cancer services. Telehealth genetic counselling (available via the Australian Genomics Telehealth framework) is a critical enabler but requires reliable internet connectivity.

Surveillance adherence

Endoscopic surveillance for FAP requires access to a colonoscopy-capable facility, which is limited in rural and remote Australia. Faecal immunochemical testing (FIT) is NOT a substitute for colonoscopic surveillance in known FAP patients. Outreach endoscopy programmes and patient-assisted travel schemes (PATS) can improve access.

Surgical access

Prophylactic colectomy and complex upper GI surgery for duodenal disease require tertiary surgical centres. Aboriginal and Torres Strait Islander patients may face delays due to travel, accommodation, and cultural factors. Aboriginal Hospital Liaison Officers (AHLOs) and Indigenous health workers can facilitate navigation and culturally safe perioperative care.

Cultural safety

Genetic testing discussions must be culturally appropriate, involving family/community as desired by the patient. Concepts of hereditary risk and predictive testing may require explanation using culturally relevant frameworks. Avoid assumptions about family structure or kinship systems.

Data sovereignty

Genetic information is sensitive. Ensure data management complies with principles of Indigenous data sovereignty (CARE principles). Genetic testing results should be managed in accordance with patient consent and community expectations.

✅

Recommended actions: (1) Proactively offer cascade genetic testing to ATSI first-degree relatives of FAP patients via outreach and telehealth. (2) Engage Aboriginal Community Controlled Health Organisations (ACCHOs) for bowel cancer screening education. (3) Fund patient-assisted travel for endoscopic surveillance and surgery. (4) Employ Aboriginal Health Workers in familial cancer services.

📚 References

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