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25
Polyps
Marcus J. Burnstein and Terry C. Hicks
The word polyp refers to a macroscopically visible lesion or mass projecting from an epithelial surface. Polyp is a descriptive and nonspecific term—the specific diagnosis of a polyp is made by histopathologic examination. Polyps may be classified as neoplastic or nonneoplastic. Neoplastic polyps encompass epithelial tumors such as adenomas, poly- poid adenocarcinomas, and carcinoid tumors, as well as nonepithelial lesions such as lipomas, leiomyomas, and lym- phomatous polyps. Nonneoplastic polyps include hamar- tomas, hyperplastic polyps, and inflammatory polyps.
Colorectal polyps may be further classified on the basis of clinical information as sporadic or hereditary, the latter cate- gory making up the polyposis syndromes discussed in Chapter 26.
Adenomas
The adenoma, a benign neoplasm of the epithelium, is the most common and most important colorectal polyp.
Adenomas may be single or multiple, sporadic or hereditary.
Adenomas are dysplastic and premalignant. Most adenocarci- nomas arise from adenomas, and the removal of adenomas has been shown to be effective in decreasing the incidence of colorectal cancer.1 It is the relationship between adenomas and adenocarcinomas that confers upon adenomas their tremendous clinical significance.
Clinical Presentation
Most adenomas are clinically silent and are found by screen- ing or by investigation of symptoms unrelated to the ade- noma. Large colonic adenomas may cause gross bleeding or may cause anemia secondary to occult blood loss. Large rec- tal adenomas, in addition to bleeding, may cause mucus dis- charge, tenesmus, and urgency. Mucus production in sufficient volume to cause electrolyte disturbances has been described. Distal rectal adenomas may rarely prolapse through the anus.
Colonoscopy is the most accurate test for polyps. The US National Polyp Study showed conclusively that colonoscopy is more accurate than double contrast barium enema for the diagnosis of colorectal polyps.2 Barium enema detected a polyp in only 39% of cases in which one was subsequently found at colonoscopy. Even when a patient had a polyp ≥1 cm in diameter, the barium enema was negative in 52% of cases.
The false-positive rate for barium enema was 14%. Computed tomography (CT) colonography (virtual colonoscopy) has a sensitivity for adenomas >1 cm of approximately 90% and for adenomas 0.6–0.9 cm of approximately 80%. The false-posi- tive rate is 17%. CT colonography is an evolving technology and its role in screening is being defined. Virtual colonoscopy may be particularly useful in the evaluation of patients with incomplete colonoscopy.3,4
Pathology
Small adenomas are usually sessile (broad-based) and redder than the background mucosa. As polyps enlarge, some become pedunculated (attached to the bowel wall by a stalk of submucosa lined by normal mucosa) and some remain sessile, diffusely carpeting the bowel wall. The distribution of adeno- mas in the National Polyp Study was cecum 8%, ascending colon 9%, hepatic flexure 5%, transverse colon 10%, splenic flexure 4%, descending colon 14%, sigmoid 43%, and rectum 8%.5The likelihood of synchronous sporadic adenomas when one adenoma was found approached 40%.6
There are three main histologic subtypes of adenomas:
tubular, villous, and tubulovillous. Tubular adenomas exhibit dysplastic tubules in ≥80% of the lesion. Villous adenomas have dysplastic villous fronds in ≥80% of the lesion. The fin- ger-like villi are actually elongated crypts with a length that is more than twice the length of normal crypts. Tubulovillous adenomas have > 20% tubular and < 80% villous formation.7 In the National Polyp Study, of 3358 sporadic colorectal ade- nomas, 87% were tubular, 5% villous, and 8% tubulovillous.5 In adenomas, cellular proliferation is not limited to the lower half of the tubule, as in normal colonic epithelium, and
the normal process of cellular maturation and differentiation from the basal zone of the crypt to the surface of the lesion does not occur. Adenomas can be graded by the degree to which epithelial growth is disturbed. Mild dysplasia is char- acterized by tubules that are lined from top to bottom by epithelium that is morphologically similar to the normal basal proliferative zone. The nuclei are enlarged, oval, hyperchro- matic, and normally oriented. There is a slight excess of mitotic figures but the architecture is not disrupted. In moder- ate dysplasia, the nuclear features are more advanced, cellular polarity is less preserved, there is nuclear stratification, and glands are more crowded. In severe dysplasia, there are large vesicular nuclei, irregular, and conspicuous nucleoli, scal- loped nuclear membranes, and increased nuclear to cytoplas- mic ratio. Nuclear polarity is disrupted and marked cellular pleomorphism and aberrant mitoses are present. Structural alterations include budding and branching tubules, back-to- back arrangement of glands, and cribriform growth of epithe- lial cells in clusters and sheets; mitotic figures are numerous.
The terms carcinoma in situ and intramucosal carcinoma are often used to describe these severely dysplastic adenomas, but these terms are potentially misleading because these lesions do not have metastatic potential. Although the lymphatics of the colon and rectum are closely associated with the muscu- laris mucosa, only lesions that have invaded through the muscularis mucosa have the potential to metastasize. The dominant risk factors for invasive carcinoma, that is, cancer cells invading beyond the muscularis mucosa, are polyp size and villous histology.8,9
Dysplastic epithelium may become misplaced within the submucosa of a polyp and may mimic invasive cancer. This situation, called pseudo-invasion, is usually seen in peduncu- lated sigmoid polyps and is believed to be the result of tor- sion, ischemia, and architectural distortion. Pseudo-invasion is distinguished from invasive cancer by retention of lamina propria around the displaced glands, a lack of morphologic features of malignancy in the epithelium, and the presence of hemosiderin, indicating mucosal ischemia and hemorrhage.10
Epidemiology
Adenoma prevalence, the percentage of the population bear- ing one or more adenomas at a given point in time, is largely a function of age, gender, and family history.11Colonoscopy- determined prevalence rates in asymptomatic average risk individuals older than 50 years range from 24% to 47%.12–15 Prevalence rates determined by colonoscopy are approxi- mately double the rates determined by flexible sigmoi- doscopy.11 The prevalence rate approximately doubles from age 50 to 60, but does not clearly continue to increase with age, unlike the incidence of colorectal cancer. Higher preva- lence rates have been identified in men, with a 1.5 relative risk compared with age-matched women.12,13,15 A multicentered screening colonoscopy study examined the risk of colorectal adenomas in a cohort of individuals with one affected
first-degree relative with sporadic colorectal cancer and found the odds ratio to be 1.5 for adenomas, 2.5 for large adenomas, 1.2 for small adenomas, and 2.6 for high-risk adenomas (see below).16The prevalence of adenomas is higher in relatives of individuals with colorectal cancer or adenoma at a young age, and in individuals with multiple relatives with cancer or adenomas.17
The incidence of adenomas is the rate at which individuals develop colorectal adenomas over a specified time interval.11 The incidence of adenomas at intervals ranging from 6 months to 4 years in postpolypectomy surveillance colonoscopy studies varies from 30% to 50%.18–21 Most incident polyps are small, and a higher incidence has been associated with polyp multiplicity at the index colonoscopy, increased size of the index polyp, older age, and a family his- tory of a parent with colorectal cancer.1,18,22–24
The incidence rate after a clearing colonoscopy is actually the sum of the true incidence rate of new adenoma formation plus the miss rate at the initial colonoscopy, plus the recur- rence rate of incompletely removed polyps.11 Judged by repeat endoscopy, including studies with same-day back-to- back colonoscopies, the miss rate for adenomas ≥1 cm is approximately 5%, for adenomas 6–9 mm is approximately 10%, and for adenomas ≤5 mm approaches 30%.25–28 The high miss rates for small lesions suggest that most adenomas detected on surveillance colonoscopy are actually lesions that were missed during the index examination. Incident polyps are distributed more proximally, consistent with the observa- tion that miss rates for adenomas are higher in the proximal colon.25The miss rates must be kept in perspective; in post- polypectomy surveillance studies, the cancer incidence is low and in the National Polyp Study, colonoscopic surveillance was associated with a 76%–90% reduction in the cancer inci- dence compared with reference populations.1
More important than the overall incidence rate is the inci- dence rate for advanced adenomas, defined as polyps ≥1 cm in size or containing high-grade dysplasia, or containing appreciable villous tissue.29The incidence rate for advanced adenomas ranges from 6% to 9%29and is closely related to the findings at initial colonoscopy. Three or more polyps at the initial colonoscopy has been shown to increase the risk of subsequent advanced adenomas, and in the National Polyp Study, age > 60 years plus a family history of a parent with colorectal cancer was also a predictor of incident advanced adenomas.30The cumulative incidence of advanced adenomas at 3 and 6 years of follow-up in the National Polyp Study in the highest risk group (three or more adenomas at baseline, or age ≥60 years plus a parent with colorectal cancer) were 10%
and 20%, respectively.30The lowest risk group (only one ade- noma and age < 60 years at baseline) had an incidence of advanced adenomas of <1% at both 3- and 6-year follow-up.
The 5-year incidence of advanced adenomas in individuals with a previously negative colonoscopy is also <1%.31
The appearance of incident cancers at short intervals in patients who have had a clearing colonoscopy suggests that
either a neoplasm was missed or that cancer developed rap- idly. Aggressive adenomas have been recognized in heredi- tary non-polyposis colorectal cancer, a condition in which patients may go from a normal colonoscopy to an established cancer in 1–3 years.18,32,33These cancers show a phenotype called microsatellite instability (MSI), a feature that is also present in 15% of sporadic cancers. Sporadic cancers with high-frequency MSI (MSI-H) may have developed rapidly.11
Adenoma-carcinoma Sequence
The prevalence of sporadic adenomas in the general popula- tion ≥60 years is approximately 30%–40% but the lifetime risk of developing colorectal cancer in Western countries is 6% by age 85; this observation suggests that only a few ade- nomas become adenocarcinomas. The likelihood that a diminutive tubular adenoma will progress to become an ade- nocarcinoma is difficult to determine. One longitudinal study showed that over a 3- to 5-year period, only 4% of 213 ade- nomas measuring 2–15 mm increased in size.34Slow trans- formation of adenomas is suggested by the fact that the mean age of adenoma patients precedes the mean age of cancer patients by 7 years. A mathematical model suggested that it takes 2–3 years for an adenoma ≤5 mm to grow to 1 cm, and another 2–5 years for the 1-cm adenoma to progress to can- cer.35For a lesion ≥1 cm, the cancer probability is 3%, 8%, and 24% after 5, 10, and 20 years, respectively.36Overall, the yearly rate of conversion from adenoma to carcinoma has been estimated to be 0.25%, but the risk is higher for polyps
>1 cm (3%) for villous adenomas (17%) and for adenomas with high-grade dysplasia (37%).37
In a study that analyzed 7590 adenomatous polyps to deter- mine risk factors for high-grade dysplasia or invasion, size was the strongest predictor.9The percent of adenomas with high-grade dysplasia or invasive cancer based on the size of the polyp was: < 5 mm, 3.4%; 5–10 mm, 13.5%; and >10 mm, 38.5%. Villous change, left-sided lesions, and age ≥60 years were also associated with advanced histologic features; no invasive cancer was found in polyps ≤5 mm.
On a molecular level, a simplified view of the traditional pathway from adenoma to adenocarcinoma is as follows: ade- noma development is dependent on an individual epithelial cell having both copies of the APC gene deactivated.38This feature seems to allow for mutations in additional oncogenes, the key targets being K-ras, DCC, P53.39The accumulation of molecular abnormalities is associated with the development of invasive cancer. This pathway is the predominant pathway of colorectal carcinogenesis and is what is seen in familial adenomatous polyposis. Not all colorectal cancer follows the adenoma-carcinoma pathway, and alternate pathways to col- orectal cancer are increasingly recognized. These pathways may involve polypoid lesions such as the hyperplastic polyp, the mixed polyp, and the serrated adenoma.
In hereditary nonpolyposis colorectal cancer (HNPCC), patients inherit a mutated copy of a DNA mismatch repair
(MMR) gene. When the second copy is inactivated, loss of MMR function results in the development of mutations throughout the gene. The accumulation of mutations is asso- ciated with the rapid evolution of adenocarcinoma, often without a recognizable precursor lesion. When there is a precursor lesion for an HNPCC cancer, it is often a typical adenoma, but hyperplastic polyps and mixed polyps with distinct components of hyperplastic and adenomatous polyps have been implicated in some HNPCC cancers.40–42
Management
When an adenoma is found, every effort should be made to do a complete colonoscopy to the cecum because of the high rate of synchronous neoplasms in patients with adenomas and adenocarcinomas. However, the significance of a single, small (<1 cm) tubular adenoma on a screening flexible sig- moidoscopy is controversial. Most studies of screening flexi- ble sigmoidoscopy suggest that patients with no distal polyps, distal hyperplastic polyps, or single small tubular adenomas have a similar low risk of proximal advanced adenomas, in the range of 0%–4%. Multiple studies support the recommen- dation that villous polyps regardless of size and adenomas >1 cm are important markers for the presence of advanced ade- nomas and carcinoma in the proximal colon.43
The majority of colorectal polyps are treated by endo- scopic snare polypectomy; polyp removal is performed using electrocautery snare. As current is delivered to the snare, heat is generated in the encircled tissue which is cut and coagu- lated. The polyp is transected as the snare is tightened. The degree of thermal damage must balance the need for vessel coagulation with the need to avoid full-thickness injury to the bowel wall. The colon wall is thin, varying from 1.7 to 2.2 mm.44The mucosa, submucosa, and muscularis propria each contribute approximately one-third to the thickness of the wall. Injection of saline solution (with or without epineph- rine) into the submucosa, increases the distance between the mucosa and the muscularis propria and increases the safety of endoscopic polypectomy.45Submucosal injection is most often used for sessile polyps in the right colon, especially those >1.5 cm in diameter. The addition of dye, such as methylene blue, to the injected solution may make it easier to recognize the edges of the polyp. Injection volumes range from a few milliliters to 30 mL. Sessile polyps > 2 cm are often best dealt with by a piecemeal approach. Tissue is retrieved for histologic analysis which should include the histologic type of polyp, degree of dysplasia, and status of the margins. Even small polyps should be removed. For some small polyps, a specimen for histologic examination may be most efficiently obtained and the lesion most safely eradicated by cold biopsy or cold snare, that is, with- out current.
The technical aspects of colonoscopy, including the poten- tial complications of this procedure, are reviewed in Chapter 5. Some technical tips in polypectomy include45:
●Aspiration of gas and decreasing wall tension may facilitate placing the snare around the polyp.
●Position the polyp at 5–6 o’clock; the snare enters the field at this orientation and makes it easier to capture the polyp.
●Perform polypectomy on withdrawal when the scope is
“straight”—this method increases the effectiveness of tip deflection and scope rotation.
●Use submucosal injection of saline for large (>1.5–2 cm) sessile lesions; begin the injection on the proximal aspect of the polyp, thereby tilting the lesion toward the scope.
●In piecemeal polypectomy, start on the proximal aspect; a spike-tip snare allows the snare to be anchored so that push- ing the sheath causes the snare loop to widen for more effec- tive placement.
●Retrieval of proximal lesions is best accomplished with a device such as a Roth basket (US Endoscopy Group, Mentor, OH); smaller polyps may be suctioned into a trap;
distal lesions may be suctioned onto the end of the scope.
Almost all polyps can be safely endoscopically removed, but if the polyp appears to be malignant, snare polypectomy may not be possible, and is generally inadvisable. Malignancy should be suspected in the setting of irregular surface contour, ulceration, friability, firm or hard consistency, thickening of the stalk, and nonlifting with submucosal injection (a feature of submucosal invasion or fibrosis from previous attempts at polypectomy).45,46It is particularly important that if polypec- tomy is performed for a suspicious polyp, the site of the polyp should be precisely localized by tattooing the bowel wall with India ink or similar dye.
Some large polyps may not be amenable to polypectomy and are treated by colon resection; in these instances, a con- ventional oncologic resection should be done.
Rectal Adenomas
Sessile villous adenomas are usually encountered in the rec- tum and the larger lesions may not be amenable to snare polypectomy. Local excision or rectal resection may be required. For lesions in the lower half of the rectum, endoanal excision is generally performed. For more proximal lesions, transanal endoscopic microsurgery (TEM) may be appropriate. Larger lesions that extend too proximally for endoanal excision will usually be best managed by ante- rior resection. If the lesion extends into the anal canal, anterior resection with endoanal mucosectomy and hand- sewn coloanal anastomosis may be needed to restore intestinal continuity.
Preoperatively, the lesion should be evaluated with respect to the risk of containing invasive cancer. The best clinical clue to the presence of invasion is firmness on digital rectal exam- ination, although previous attempts at removal can produce fibrosis.47Endoanal ultrasound and magnetic resonance imag- ing may be helpful, but sensitivity with respect to a small focus of submucosal invasion is low.
When invasive cancer is present, treatment will be deter- mined primarily by the level and size of the lesion, the depth of mural penetration, and by any evidence of lymphatic metastases. More proximal rectal lesions will usually be treated by anterior resection. Selected small proximal lesions may be managed by TEM. More distal lesions will need either resection or local excision. If biopsy of a hard area reveals a poorly differentiated cancer, a resectional approach is recom- mended. Even small rectal cancers that are not poorly differ- entiated carry a risk of lymph node metastases. A study from Memorial Sloan-Kettering Cancer Center evaluated whether standard pathologic factors predicted lymph node metastases from small rectal cancers.48 Of 318 patients with T1 or T2 rectal cancers who underwent rectal resection, 159 patients were considered potentially eligible for local excision. Even in the absence of poor pathologic factors (advanced T stage, poorly differentiated histology, lymphatic or vascular inva- sion), 15% of patients had lymph node metastases.
The technical aspects of TEM and endoanal excision are discussed in Chapter 30.
Surveillance
After polypectomy of large or multiple adenomas (three or more) or advanced adenomas, cancer risk is increased three- to fivefold.49The risk of subsequent cancer is not measurably increased in patients with only one or two small tubular ade- nomas.22,50 The National Polyp Study determined that colonoscopy performed 3 years after initial polypectomy pro- tects patients as well as more frequent examinations.1 Recommendations for surveillance postpolypectomy are based on the estimated risk of metachronous neoplasia51,52:
● After colon clearance, first follow-up colonoscopy in 3 years (for most patients).
● First follow-up colonoscopy in 5 years for low-risk patients [fewer than three small (<1 cm) tubular adenomas, no sig- nificant family history of colorectal cancer or adenomas].
● If first follow-up examination is negative, second follow-up colonoscopy in 5 years.
● Earlier follow-up colonoscopy for selected patients with multiple or large sessile adenomas.
● Individualize for age and comorbidity. (After removal of a small tubular adenoma, no follow-up may be indicated in elderly patients, or for those individuals with significant comorbidity, or the first follow-up can be delayed for 5 years.)
For large sessile polyps (>3 cm), there is a significant recur- rence rate after endoscopic polypectomy. Even when the endoscopist believes that the entire polyp has been removed, follow-up examination reveals residual polyp in approxi- mately 50%. There should be close follow-up, for example, every 3–6 months in the first year, every 6–12 months in the second year, and yearly to the fifth year. Treating the base and edge of the polypectomy defect with the argon plasma
coagulator has been shown to decrease the incidence of residual polyp.53
The Malignant Polyp
Polyps with cancer cells penetrating the muscularis mucosa are malignant polyps. Invasion is invariably limited to the submucosa. In terms of TNM classification, these are T1NxMx lesions. Malignant polyps (T1 lesions) account for 2%–12% of polyps in colonoscopic polypectomy series.54–56 The risk of malignancy related to adenoma size in one large series was 0.6–1.5 cm, 2%; 1.6–2.5 cm, 19%; 2.6–3.5 cm, 43%; and > 3.5 cm, 76%.54
The clinical decision to proceed with further treatment, such as resection or local excision, depends on the estimated risk of lymph node metastases and the patient’s general con- dition.57 The main determinant of the risk of lymph node metastasis is the depth or level of invasion of cancer within the polyp. Haggitt’s classification58 of malignant polyps (Figure 25-1) is based on the level of invasion:
Level 0: noninvasive (severe dysplasia)
Level 1: cancer invading through the muscularis mucosa but limited to the head of a pedunculated polyp
Level 2: cancer invading the neck of a pedunculated polyp
Level 3: cancer invading the stalk of a pedunculated polyp Level 4: cancer invading into the submucosa of the bowel wall
below the stalk of a pedunculated polyp. All sessile polyps with invasive cancer are level 4.
The stalk of a pedunculated polyp is covered by normal mucosa and has a central core of submucosa. A line drawn at the junction of normal and adenomatous epithelium is the transition between the stalk and the head of the polyp. The junction zone is called the neck.
Kudo59 has stratified the depth of submucosal invasion into three levels (Figure 25-2):
SM1: invasion into the upper third of the submucosa SM2: invasion into the middle third of the submucosa SM3: invasion into the lower third of the submucosa
Haggitt levels 1, 2, and 3 are SM1; Haggitt level 4 may be SM1, SM2, or SM3.
The risk of lymph node metastases is <1% for peduncu- lated polyps with Haggitt level 1, 2, or 3.58,60,61 The risk of lymph node metastases for Haggitt level 4 lesions, peduncu- lated or sessile, ranges from 12% to 25%.62–64 Factors reported to be associated with an increased risk of lymph node metastases include lymphovascular invasion,48,63 poor differentiation,48,65,66 gender,48 extensive budding, microaci-
FIGURE 25-1. Anatomic landmarks of pedunculated and sessile malignant polyps. (Reprinted from Haggitt et al.,58 copyright 1985, with permission from the American Gastroenterological Association.)
nar structure,67 flat or depressed lesions,66 and SM3 level of invasion.57,62,68 In a series of 353 T1 sessile cancers, the risk factors for lymph node metastasis that were statistically sig- nificant on multivariate analysis were SM3 level of invasion, lymphovascular invasion, and location in the lower third of the rectum.62 In another study, only SM3 invasion was an independent risk factor for lymph node metastases.68
The rate of lymph node metastases from rectal lesions is not different than from colon lesions. However, T1 lesions in the distal third of the rectum have been found to have a higher risk of lymph node metastases than more proximal rectal lesions.62 This finding is consistent with the high local recur- rence rates, in the range of 5% to 28%, which have been observed after full-thickness local excision of T1 lesions of the distal rectum.48,65,69,70
A positive polypectomy margin should not be considered an adverse risk factor for recurrence—in general, this should be regarded as inadequate treatment. A distance of 2 mm beyond the deepest level of invasion is needed to consider the margin clear.57
In view of the risk of lymph node metastases of <1%, pedunculated polyps with invasion to Haggitt levels 1 to 3 are safely treated by snare polypectomy. Level 4 pedunculated lesions are treated as sessile lesions. Sessile lesions that are snared in one piece and have a margin of at least 2 mm are considered adequately treated.57 If a piecemeal polypectomy was performed, margins cannot be adequately assessed and further treatment is needed. High-risk sessile lesions such as lesions with SM3, lymphovascular invasion, poor differentia- tion, and those in the lower third of the rectum should have an oncologic resection. For lower rectal lesions, full-thickness endoanal excision with adjuvant chemoradiation is an alterna- tive approach. Nivatvongs’s summary of the indications for oncologic resection are presented in Table 25-1.
Close endoscopic follow-up is required to look for local recurrence. A reasonable schedule is to examine the polypec- tomy site in 2–3 months and then every 6 months for the first 2 years; complete colonoscopy is done in the third year, and then at 3-year intervals.
Chemoprevention after polypectomy to inhibit adenoma recurrence cannot yet be recommended. Prospective random- ized intervention trials have not been supportive of this approach. The role of chemoprevention is reviewed in Chapter 23.
Specific Adenomas
Flat and Depressed Adenomas
Some adenomas display a flat or depressed growth pattern and are not elevated above the mucosal surface; they are not
“true polyps.”71 These lesions are recognized by color and textural changes, and by interruption of the capillary network pattern.71,72 They are most readily identified by chromoen- doscopy, a technique in which the mucosa is sprayed with indigo carmine dye.73 The incidence of flat and depressed adenomas in three Western population studies was approxi- mately 20%, and these lesions contained cancer more often Sm1 Sm2 Sm3
Submucosa
D.F MAYO C 1996
FIGURE 25-2. Depth of submucosal invasion in sessile malignant polyps. Sm1: invasion into upper third; Sm2: invasion into middle third; Sm3: invasion into lower third. (Reprinted from Nivatvongs S. Surgical management of early colorectal cancer. Surg Clin North Am 2000;82:1052–1055, copyright 2000, with permission from Elsevier.)
TABLE25-1. Summary of malignant colorectal polyps that should have an oncologic bowel resection
A. Lesions in colon
a) Pedunculated Haggitt level 4 with invasion into distal third of submu- cosa, or pedunculated lesions with lymphovascular invasion
b) Lesions removed with margin < 2 mm c) Sessile lesions removed piecemeal
d) Sessile lesions with depth of invasion into distal third of submucosa (Sm3)
e) Sessile lesions with lymphovascular invasion B. Lesions in middle third and upper third rectum
Same as lesions in colon C. Lesions in distal third rectum
a) Pedunculated Haggitt level 4 with invasion into distal third of submu- cosa, or pedunculated lesions with lymphovascular invasion
b) All sessile lesions
An alternative may be a per anal full-thickness excision plus chemoradiation.
Source: Reprinted from Nivatvongs S. Surgical management of early colo- rectal cancer. Surg Clin North Am 2000;82:1052–1055, copyright 2000, with permission from Elsevier.
than polypoid adenomas.74–76 In a large United Kingdom study of 1000 patients in which chromoendoscopy was used to search for small flat lesions, 36% of the 321 detected ade- nomas were flat or depressed.74The overall risk of a polypoid lesion containing early cancer was 8%, but was 14% for the flat lesions. Flat or depressed lesions that were >1 cm were about twice as likely as protruding lesions of a similar size to contain high-grade dysplasia or cancer. Twenty-nine percent of flat lesions >1 cm contained either high-grade dysplasia or cancer. The average size of advanced flat and depressed ade- nomas is smaller than that of their polypoid counterparts.
Because of the risk of cancer, these small lesions should be removed, either by endoscopic polypectomy or by operative resection. It has been suggested that using special dyes and magnifying colonoscopy should be incorporated into general endoscopic practice.71
Serrated Adenomas
The serrated adenoma is a more recently recognized histo- logic phenotype of sporadic adenoma.77 Serrated adenomas are uncommon, accounting for approximately 0.5%–1.3% of colorectal polyps.78,79 Initially, serrated adenomas were described as hyperplastic polyps that contained adenomatous features. The serrated adenoma has serrated crypts that are longer and broader than in hyperplastic polyps. The crypts contain cells with enlarged hyperchromatic and stratified nuclei (as in adenomas) as well as cells with normally arranged, small, basal nuclei (as in hyperplastic polyps).80,81
It is unclear whether serrated adenomas develop in associ- ation with hyperplastic polyps or develop de novo. Both types of polyps are primarily found in the rectosigmoid region, and they have similar mucin characteristics. Endoscopically, most serrated adenoma look like hyperplastic polyps—pale, slightly protruding lesions, and most are in the range of 0.2–7.5 mm in diameter. Some serrated adenomas are larger and may resemble villous adenomas.
A pathway to colorectal cancer called the “serrated path- way” has been postulated, in which colorectal cancer devel- ops through precursor lesions that have serrated glandular architecture. The serrated polyps include the mixed polyp, in addition to the hyperplastic and serrated adenoma. The mixed polyp is recognized by distinct areas of hyperplastic and adenomatous morphology.
The relationship between hyperplastic polyps, serrated adenomas, and cancer is not clear. In one report, 5.8% of col- orectal cancers were associated with an adjacent serrated ade- noma and some serrated adenomas harbor high-grade dysplasia.79,82 A review concluded that the risk of high- grade dysplasia was the same in serrated adenomas as in the more common adenomatous phenotypes.79The serrated ade- noma pathway may be a separate route to colon cancer char- acterized by methylation of promoter regions leading to switching off MMR genes, resulting in replication errors and ultimately to cancer. Hyperplastic polyps and serrated
adenomas show MSI in the absence of APC mutations.
Individuals with sporadic colorectal cancer with high-level MSI cancers (MSI-H) are 4 times more likely to harbor at least one serrated polyp than individuals with low MSI cancers.74
Nonneoplastic Polyps
Hyperplastic Polyps
Hyperplastic polyps are the result of a failure of programmed cell death.33,38,80,81 The epithelial cells differentiate and mature normally but accumulate on the mucosal surface pro- ducing small sessile elevations. Cellular crowding results in the characteristic microscopic appearance of tubules with a saw-tooth or serrated pattern. Mature goblet cells are the main cellular component of hyperplastic polyps, whereas adeno- matous crypts have reduced numbers of goblet cells. The cytoplasm of hyperplastic polyps is eosinophilic on hema- toxylin and eosin staining and the nuclei remain at the base with minimal stratification.
Hyperplastic polyps are the most common polyps found on flexible sigmoidoscopy. The true ratio of hyperplastic polyps to adenomas approximates 1:1. In a study of 1964 diminutive (≤5 mm) polyps, 41% were adenomas, 37% hyperplastic polyps, and 18% nonneoplastic. Combining histologic data from several studies of diminutive polyps revealed that 53%
were adenomas and that approximately 0.5% had high-grade dysplasia; < 0.1% contained invasive cancer.43
Most hyperplastic polyps are small (< 3–5 mm in diame- ter), pale, usually located in the rectosigmoid region, and are almost always asymptomatic. Hyperplastic polyps are often multiple. Although hyperplastic and adenomatous polyps have characteristic appearances, biopsy is needed to make a diagnosis. Endoscopic diagnosis has a sensitivity of 80% and a specificity of 71%.82 Chromoendoscopy can improve the endoscopist’s ability to distinguish hyperplastic from adeno- matous polyps. Hyperplastic polyps have a characteristic star- like pit pattern when stained with indigo carmine and assessed with a magnifying colonoscopy. Adenomatous polyps have surface grooves. The sensitivity and specificity of this technique in discriminating between adenomatous and nonadenomatous polyps was found to be 93% and 95%, respectively.83
Data conflict as to whether hyperplastic polyps found on a screening examination represent an increased risk of future neoplasia. Whereas some authors have suggested that left- sided hyperplastic polyps are predictors of proximal adeno- mas, the National Polyp Study found no association between left-sided hyperplastic polyps and synchronous adenomas. A report using data from two large chemoprevention studies demonstrated that hyperplastic polyps were not predictive of an increased risk of developing adenomatous polyps on fol- low-up colonoscopy.84 The American College of Gastro- enterology states that hyperplastic polyps found on flexible
sigmoidoscopy are not an indication for colonoscopy.85There are no specific recommendations for treatment and follow-up of right-sided hyperplastic polyps.
Hyperplastic polyps have been implicated in some colorec- tal cancers, especially in HNPCC. HNPCC cancers arise through mutations in one of several MMR genes; sporadic colon cancer with defective MMR occurs as a result of deac- tivation of a single gene—hMLH1. Inactivation occurs through methylation of the hMLH1 gene promoter. The molecular phenotype of abnormal MMR is MSI. MSI has been used to distinguish these high-level MSI cancers (MSI- H) from cancers with low or no level of MSI, as seen in APC- related cancers.86A strong association of MSI-H cancers with residual adenomas has not been observed.87Sporadic adeno- carcinomas arising through the defective MMR pathway (MSI-H adenocarcinoma) occur in older patients (> 70 years), have a female gender bias, and are predominantly located in the right colon.88 The vast majority lack APC mutations.
These MSI-H cancers can be associated with large hyperplas- tic polyps, and analysis of these combined lesions demon- strates that both the cancer component and the hyperplastic polyp epithelial cells lack hMLH1 expression. This suggests that hyperplastic polyps may serve as fertile soil for gene- specific hypermethylation leading to knockout of hMLH1 and loss of DNA MMR function.33,38
Carriers of hyperplastic polyps clearly at increased risk for colorectal cancer are those with the hereditary syndrome of hyperplastic polyposis. This syndrome is diagnosed if multi- ple (> 30) or large (>10 mm) hyperplastic polyps are found, especially in the proximal colon, with a positive family his- tory of hyperplastic polyposis. The syndrome is thought to be very rare. Reports of patients with hyperplastic polyposis syn- drome showed an average age of 52 years, >100 polyps in half the cases, and an average polyp diameter of 16 mm (range, 5–45 mm), and half the reported patients also had a cancer (half of these in the right colon).There are no general guide- lines for the management of hyperplastic polyps or the hyper- plastic polyposis syndrome.80,81(see Chapter 26).
Hamartomas
Juvenile polyps are hamartomas, localized overgrowths of normal mature cells. Most juvenile polyps are round, pink, smooth, and pedunculated, although some are small and ses- sile. Juvenile polyps are composed of dilated mucus-filled cystic spaces surrounded by lamina propria which has a mes- enchymal appearance with inflammatory cells and eosinophils. The muscularis mucosa does not participate in the formation of a juvenile polyp, and the unique potential of these lesions to twist and “auto-amputate” has been ascribed to the absence of supporting muscle fibers. Most juvenile polyps present in childhood, but they may present in infancy or in adulthood. Usually, only one or two polyps are found. Up to three polyps may be seen in nonfamilial conditions. Symptoms include rectal bleeding, mucus discharge, diarrhea, and
abdominal pain, intussusception and prolapse of a polyp through the anus. Treatment is snare polypectomy.
Numerous hamartomatous polyps are present in juvenile polyposis syndrome, Peutz-Jeghers syndrome, Cowden dis- ease, and Cronkhite-Canada Syndrome. Although sporadic hamartomatous polyps are not dysplastic and are not believed to be premalignant, the syndromes of hamartomatous polypo- sis have a significant rate of cancer development. These syn- dromes are discussed in Chapter 26.
Inflammatory Polyps
Inflammatory or pseudopolyps (a misnomer) are associated with colitis, most often ulcerative colitis and Crohn’s disease, but can result from any form of severe colonic inflammation.
The inflammatory polyp is a remnant or island of normal or minimally inflamed mucosa. The presence of inflammatory polyps in inflammatory bowel disease is not associated with dysplasia or cancer risk. Inflammatory polyps are almost always multiple. Treatment is directed at the underlying bowel disease.
Lymphoid Polyps
Benign enlargements of lymphoid follicles may produce polyps that are usually seen in the rectum. The overlying mucosa is normal. The lesions are typically multiple; their cause is unknown. Histologic criteria for establishing the benign nature of lymphoid polyps have been described: lym- phoid tissue is entirely within the mucosa and submucosa;
there is no invasion of the muscularis propria; at least two ger- minal centers must be seen; if the specimen does not include the muscle coat and no germinal centers are seen, then the diagnosis of lymphoid polyp cannot be made.
Lipomas, leiomyomas, lymphoma, hemangiomas, and car- cinoid tumors are discussed in Chapter 37.
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