Colorectal Polyps and Familial Polyposis Syndromes

Overview

Polyp = growth protruding into lumen. Large>small intestine; rectosigmoid>proximal colon. 2 groups: NON-NEOPLASTIC (no true malignant potential per se) vs NEOPLASTIC/adenomas (genuine malignant potential).

A. Non-neoplastic polyps (4 types)

1. Hyperplastic (metaplastic) — COMMONEST epithelial polyp overall, esp. rectosigmoid, elderly (6th-7th decade). Gross: multiple, sessile, smooth, SMALL (<0.5cm). Micro: cystically dilated glands, SERRATED/saw-toothed luminal border. No malignant potential unless coexistent adenoma.

2. Hamartomatous (2 subtypes):

• Peutz-Jeghers polyps/polyposis — AD, hamartomatous polyposis + melanotic pigmentation (lips/mouth/genitalia). Commonest jejunum/ileum. Adolescence/childhood. Gross: large, multiple, pedunculated. Micro: TREE-LIKE muscularis mucosae branching (key feature). No malignant transformation unless coexistent adenoma — but ↑risk pancreas/lung/breast/ovary/uterus cancer
• Juvenile (retention) polyps — <5yrs. Solitary=rectum; polyposis=anywhere. Gross: spherical, smooth, ~2cm, pedunculated. Micro: cystic mucus-filled glands, normal epithelium. NOT precancerous

3. Inflammatory polyps (pseudopolyps) — from re-epithelialisation of undermined ulcers in IBD (mostly UC=“colitis polyposa”, sometimes Crohn’s). Gross: multiple, cylindrical-rounded. Micro: connective tissue core + inflammation, regenerating epithelium. NO malignant potential — UC cancers arise from DYSPLASIA areas, NOT these polyps.

4. Lymphoid polyps (“rectal tonsils”) — reactive lymphoid hyperplasia. Localised=rectum/elderly; diffuse=young/children. Gross: solitary/multiple tiny. Micro: lymphoid follicles+germinal centres. Benign — distinguish from lymphoma.

B. Neoplastic polyps (adenomas) — 3 varieties, same process different growth pattern

Tubular adenoma (adenomatous polyp) — COMMONEST (75%). >3rd decade, slight ♂. Distal colon/rectum. Sporadic OR familial (AD). Gross: single/multiple, sessile/pedunculated, <1cm→large. Micro: branching tubules in lamina propria, large-intestinal epithelium, ↓mucus, atypia spectrum (CIS→adenocarcinoma). Malignant transformation ~5% (higher if larger).

Villous adenoma — LESS common. 6th decade, equal sex. Distal colon/rectum. Gross: round-oval exophytic, sessile, 1-10cm+. Micro: finger-like villi from muscularis mucosae, fibrovascular core. ALWAYS symptomatic (bleeding/diarrhoea/mucus). Invasive carcinoma in 30% (highest risk).

Tubulovillous adenoma — intermediate pattern+behaviour. Same distribution as tubular. Gross: 0.5-5cm. Micro: vertical villi + deep tubular pattern.

Comparison table

C. Familial polyposis syndromes (all AD)

Familial polyposis coli (adenomatosis) — >100 adenomas (avg~1000; distinguishes from “multiple adenomas” ≤100). AD, germline APC mutation. 2nd-3rd decade, equal sex. Usually tubular-adenoma pattern. Malignant potential VERY HIGH — ~100% by age 50 if untreated → drives prophylactic colectomy.

Gardner’s syndrome = FAP + extracolonic: multiple osteomas (mandible/maxilla), sebaceous cysts, connective tissue tumours. Fewer polyps than FAP but IDENTICAL behaviour.

Turcot’s syndrome = FAP + CNS malignant neoplasms.

Juvenile polyposis syndrome = multiple juvenile polyps, colon+stomach+small intestine, fewer than FAP. AD in some, negative FH in others. Same age/morphology as solitary juvenile polyps. NO malignant potential.

Note: Peutz-Jeghers (hamartomatous) and Cronkhite-Canada (inflammatory) also cause multiple polyposis but lack familial basis — NOT classified as familial polyposis syndromes.

Other benign tumours

Rare non-epithelial: leiomyoma, leiomyoblastoma, neurilemmoma, lipoma, vascular tumours (haemangioma, lymphangioma).

Clinical relevance

Non-neoplastic vs neoplastic = the organising split for the whole topic. Tubular→tubulovillous→villous = ONE continuous risk gradient (5%→intermediate→30%) tracking rising villous component, not 3 unrelated numbers. FAP’s ~100%-by-50 transformation = the clinical rationale for prophylactic colectomy. Gardner’s/Turcot’s = FAP + one extra named finding each (bone/soft tissue tumours vs CNS malignancy) — colonic behaviour otherwise identical to FAP.

Overview

A polyp is any growth or mass protruding from a mucous membrane into the lumen. Polyps are much more common in the large intestine than the small intestine, and more common in the rectosigmoid colon than the proximal colon. They fall into two broad groups — non-neoplastic (several distinct-origin subtypes) and neoplastic (adenomas, with genuine malignant potential).

A. Non-Neoplastic Polyps

More common overall than neoplastic polyps; four subtypes.

1. Hyperplastic (Metaplastic) Polyps

The most common of all epithelial polyps, especially in the rectosigmoid. Called “hyperplastic” for epithelial hyperplasia at the crypt base, “metaplastic” for areas of cystic metaplasia. Any age, but more common in the elderly (6th–7th decade).

Gross: generally multiple, sessile, smooth-surfaced, small (<0.5 cm).

Micro: long, cystically dilated glands/crypts lined by normal epithelial cells; lining partly flat, partly papillary; luminal border characteristically serrated/saw-toothed.

Usually symptomless; no malignant potential unless a coexistent adenoma is present.

2. Hamartomatous Polyps

Tumour-like lesions composed of an abnormal mixture of tissues indigenous to the part. Two types:

Peutz-Jeghers polyps and polyposis — autosomal dominant, with hamartomatous intestinal polyposis plus melanotic pigmentation of lips, mouth, and genitalia. Polyps may occur in stomach, small intestine, or colon, but are commonest in jejunum and ileum. Most common in adolescence/early childhood.

• Gross: variable size, often large, multiple, pedunculated, more commonly in the small intestine.
• Micro: most characteristic feature is tree-like branching of the muscularis mucosae; lining epithelium appears normal; glands may show hyperplasia/cystic change.
• No malignant transformation unless a coexistent adenoma is present — but patients with the syndrome are more prone to cancers of pancreas, lung, breast, ovary, and uterus.

Juvenile (retention) polyps — occur more commonly in children under 5 years. Solitary juvenile polyps favour the rectum; juvenile polyposis may occur anywhere in the large bowel.

• Gross: spherical, smooth-surfaced, ~2 cm, often pedunculated.
• Micro: classic appearance is cystically dilated glands containing mucus, lined by normal mucus-secreting epithelium; stroma may show inflammatory infiltrate if the surface is chronically ulcerated.
• Usually removed once symptomatic (rectal bleeding); like other non-neoplastic polyps, not precancerous.

3. Inflammatory Polyps (Pseudopolyps)

Arise from re-epithelialisation of undermined ulcers and overhanging margins in inflammatory bowel disease, most often ulcerative colitis (“colitis polyposa”), sometimes Crohn’s disease.

Gross: usually multiple, cylindrical-to-rounded mucosal overgrowths, from minute nodules to several centimetres.

Micro: connective-tissue core with some inflammatory infiltrate, covered superficially by regenerating epithelial cells and some cystically dilated glands.

No malignant potential — carcinomas arising in longstanding ulcerative colitis develop from areas of epithelial dysplasia, not from these polyps themselves.

4. Lymphoid Polyps

Reactive hyperplasia of lymphoid tissue (normally more prominent in rectum and terminal ileum) gives rise to localised or diffuse lymphoid polyps, also called rectal tonsils. Localised form: more often rectal, in the elderly. Diffuse form: younger patients/children.

Gross: solitary or multiple, tiny elevated lesions.

Micro: prominent lymphoid follicles with germinal centres in submucosa/mucosa, covered by epithelium that may be inflamed.

Benign — must be distinguished from malignant lymphoma.

B. Neoplastic Polyps (Adenomas)

Colorectal adenomas carry genuine potential for malignant change (in contrast to non-neoplastic polyps). “Polypoid carcinoma” refers instead to invasive epithelial tumours. Three main varieties — differences in growth pattern of the same neoplastic process, with variable biologic behaviour.

Tubular Adenoma (Adenomatous Polyp)

The most common neoplastic polyp (75%). Common beyond the 3rd decade, slight male preponderance, most often in distal colon and rectum. May be sporadic (singly) or multiple as part of a familial polyposis syndrome (autosomal dominant). Often asymptomatic; may cause rectal bleeding.

Gross: single or multiple, sessile or pedunculated, <1 cm to large spherical masses with irregular surface; larger lesions usually have a recognisable stalk.

Micro: benign tumour overlying the muscularis mucosae, composed of branching tubules embedded in lamina propria; lining epithelium is large-intestinal type with diminished mucus capacity, large nuclei, increased mitoses. Cytologic atypia is variable — ranging from atypical epithelium confined within the glandular basement membrane (“carcinoma in situ”) to invasion into the fibrovascular stromal core (frank adenocarcinoma).

Malignant transformation in ~5%, higher in larger adenomas.

Villous Adenoma (Villous Papilloma)

Much less common than tubular adenoma. Mean age 6th decade, roughly equal sex incidence, most often distal colon/rectum.

Gross: round-to-oval exophytic masses, usually sessile, 1–10 cm or more; surface may be haemorrhagic or ulcerated.

Micro: characteristic slender, finger-like villi arising directly from the muscularis mucosae area; each papilla has a fibrovascular stromal core covered by epithelium ranging from apparently benign to anaplastic; excess mucus secretion sometimes seen.

Invariably symptomatic (rectal bleeding, diarrhoea, mucus). Severe atypia, carcinoma in situ, and invasive carcinoma are seen more frequently than in tubular adenoma — invasive carcinoma reported in 30% of villous adenomas.

Tubulovillous Adenoma (Papillary Adenoma, Villoglandular Adenoma)

Intermediate pattern between tubular and villous. Same distribution as tubular adenoma.

Gross: sessile or pedunculated, 0.5–5 cm.

Micro: mixed pattern — characteristic vertical villi with a deeper tubular pattern.

Malignant behaviour is intermediate between tubular and villous adenomas.

Non-Neoplastic vs Neoplastic Polyps — Comparison

C. Familial Polyposis Syndromes

A group of disorders with multiple colonic polyposis and autosomal dominant inheritance.

Familial Polyposis Coli (Adenomatosis)

Defined by >100 neoplastic polyps (adenomas) on the colonic mucosa; average count ~1000 — this distinguishes it from “multiple adenomas,” where the count stays ≤100. Autosomal dominant, due to germline APC gene mutation, resulting in hundreds of adenomas that progress to invasive cancer. Average diagnosis age is the 2nd–3rd decade, equal sex incidence. Gross/microscopic pattern is commonly that of adenomatous (tubular) polyps.

Malignant potential is very high — colorectal cancer develops in virtually 100% of untreated cases by age 50, making the adenoma-carcinoma sequence essentially inevitable here.

Gardner’s Syndrome

Familial polyposis coli combined with extra-colonic lesions: multiple osteomas (particularly mandible/maxilla), sebaceous cysts, and connective-tissue tumours. Polyp count is generally fewer than in familial polyposis coli, but clinical behaviour is identical.

Turcot’s Syndrome

Familial polyposis coli combined with malignant central nervous system neoplasms.

Juvenile Polyposis Syndrome

Multiple juvenile polyps appearing in colon, stomach, and small intestine, but in numbers lower than familial polyposis coli. Family history may show autosomal dominant inheritance in some cases, may be negative in others. Resembles typical juvenile polyps in age (<5 years), sex distribution, and morphology. Lacks malignant potential.

Note: Peutz-Jeghers syndrome (hamartomatous) and Cronkhite-Canada syndrome (inflammatory) also cause multiple colonic polyposis but are not classified as familial polyposis syndromes, since they lack this familial basis.

Other Benign Colorectal Tumours

Rare non-epithelial benign tumours occurring in the large intestine: leiomyoma, leiomyoblastoma, neurilemmoma, lipoma, and vascular tumours (haemangioma, lymphangioma).

Clinical Relevance

• The non-neoplastic-vs-neoplastic split is the single organising fact for this entire topic — everything about malignant potential, associated syndromes, and clinical urgency follows from which side of that split a given polyp falls on.
• Within adenomas, the tubular→tubulovillous→villous spectrum tracks a smooth gradient of rising malignant risk (5%→intermediate→30%) that correlates with rising villous component — worth remembering as one continuous biologic spectrum rather than three unrelated numbers.
• Familial polyposis coli’s near-100%-by-age-50 malignant transformation rate is the clinical rationale for prophylactic colectomy in these patients — this single fact drives the entire management approach for the syndrome.
• Gardner’s and Turcot’s syndromes are best remembered as familial polyposis coli plus one additional named finding each (osteomas/soft-tissue tumours vs CNS malignancy) — their colonic behaviour is otherwise identical to familial polyposis coli itself.

• Full polyp classification tree: non-neoplastic (4 types) vs neoplastic/adenomas (3 types) vs familial polyposis syndromes (4 conditions)
• The three adenoma types (tubular/villous/tubulovillous) side by side with their rising malignant-risk gradient

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