Abstract
The histologic zones of the anal canal include the colorectal zone, anal transitional zone (ATZ), non–hair-bearing squamous mucosa, and perianal skin. The dentate line is the distal end of the ATZ and marks the division of the vascular, lymphatic, and neural supplies. Understanding the gross and histologic anatomy of the anal canal is important for understanding the pattern of neoplastic development and spread in this complex site. Tumors of the anal canal constitute 2% of all large bowel tumors. Most anal cancers are squamous carcinomas, with a distribution of the major histotypes as 82% squamous cell carcinoma, 12% adenocarcinoma, and 1.6% melanoma. Most anal squamous cell carcinomas are caused by human papillomavirus (HPV) and have precursor lesions analogous to those for other HPV-related lesions in the lower anogenital tract. The LAST project has recommended a two-tiered nomenclature for HPV-associated squamous intraepithelial lesions: low-grade squamous intraepithelial lesion (LSIL) and high-grade squamous intraepithelial lesion (HSIL). HIV infection is a significant risk factor for the development of anal (pre)cancer. Highly active antiretroviral therapy (HAART) has had little impact on the natural history and outcome of preinvasive anal squamous neoplasia. The incidence of anal cancer in HIV-positive men who have sex with men (MSM) is alarmingly high at 131/100,000; however, there is currently no national guideline for anal cancer screening. The ANCHOR study is a multicenter phase III clinical trial aimed to demonstrate the efficacy of treating HSIL to reduce the incidence of anal cancer. Screening and treatment of anal HSIL is expected to become the standard of care with the success of this trial.
Keywords
anal dysplasia, ANCHOR trial, high-grade squamous intraepithelial lesion, human immunodeficiency virus, human papilloma virus, low-grade squamous intraepithelial lesion, squamous cell carcinoma
Introduction
Embryology
A portion of the distal hindgut forms an expansion called the cloaca, which is a common chamber for the developing intestinal, urinary, and reproductive tracts. The division of the cloaca into separate conduits for these three functioning systems begins in the fifth week of embryogenesis. The urorectal septum, a coronal ridge of mesenchyme, extends caudally to join the urogenital membrane, thereby creating two separate tracts—anteriorly, the urogenital sinus and, posteriorly, the rectum by the end of the eighth week ( Fig. 10.1 ; see Chapter 1 ). Concurrently, the müllerian ducts extend caudally toward the cloaca and merge with the posterior wall of the urogenital sinus to form the müllerian tubercle. The ducts fuse and join with the ligamentum inguinale to differentiate into the vagina and uterus distally and the fallopian tubes proximally. The superior portion of the primitive urogenital sinus, on exstrophy of the mesonephric and ureteric ducts, forms the bladder. The anal canal forms by the fusion of the endodermal hindgut (primitive rectum) with the ectodermal proctodeum (anal pit). This dual embryologic origin of the anal canal results in a dual blood supply, drainage, and innervation (described in detail later in this chapter). Whether or not the dentate line represents the fusion site of the two embryologic derivatives remains controversial, but this anatomic landmark is generally regarded as the region in which the shift in blood supply and drainage and innervation occur. This developmental segmentation of the anal canal is clinically relevant when evaluating congenital malformations and patterns of metastases of malignant neoplasms.
Anorectal Anomalies
Defects of anorectal development are rare; they range from 1 in 3000 births for anorectal anomalies to 1 in 50,000 for cloacal malformations. Overall, these developmental disturbances constitute approximately 10% of all anogenital malformations in females. The spectrum of anorectal malformations ranges from the mildly stenotic anus to an imperforate anus, with a fistula between the urinary and intestinal tracts, to the most severe form, persistent cloaca. Cloacal defects may affect the gastrointestinal, urinary, and reproductive tracts; cases of associated anomalies also have involved the respiratory tract, neural tube, abdominal wall, cardiac anomalies, hemivertebrae, and digits. These anomalies often present in the newborn period, but they may not be detected until adulthood. A common urogenital sinus is often present in these patients, which may be the common site of exit for the rectum, urethra, and vagina. In approximately 30% of cases, the anal opening is present in the perineum but is displaced anteriorly. The urethra may exit normally or drain into the urogenital sinus or vagina or is entirely absent. Variably, a neurogenic bladder may be present. The external genitalia in chromosomally female patients often consist of an empty skin sac or phallus-like structure, occasionally with a draining urethra. Vaginal structures may be bifid or duplexed, and they lead into the urogenital sinus or into the urethra or bladder. Abnormal sacral development is also frequently present.
Anorectal anomalies are less rare and may occur in 1 in 3000 to 5000 births. They are divided into three types, defined by the location of the defect—high (supralevator), low (translevator), and intermediate. A miscellaneous group also exists. High defects are characterized as anorectal agenesis, with fistulization of the rectum to the bladder, urethra, or vagina. Intermediate defects include anal agenesis, anorectal stenosis, and anorectal membrane. Low defects predominate in female children and include ectopic anus, anal stenosis, and membrane-covered anus.
The pathogenesis of many of these anomalies lies in defects in the completion of the urorectal septum, which in turn may reflect abnormalities in signaling, documented in many cases by genetic mutation. One proposed mechanism includes defects in the expression of sonic hedgehog, of which experimental mutations lead to a dose-dependent range of anorectal malformations, including the cloaca. Mice null for p63, a p53 homologue expressed in basal epithelial cells and critical to epithelial development, fail to complete the urorectal septum, leading to a cloaca ( Fig. 10.2 ). Humans with gain of function mutations in this gene also exhibit milder urogenital anomalies (see Chapter 1 ).
Anatomy
The anal canal is a grossly and histologically defined anatomic area joining the distal intestinal tract with the external perianal skin. Conceptually, the anal canal is the transition from an absorptive epithelium, which samples materials from the external environment, to a protective epithelium, which provides a barrier between the body and external environment ( Fig. 10.3 ). Grossly, the surgical anal canal is defined as the distal 3- to 4-cm segment of the alimentary tract that begins at the apex of the anal sphincter complex (palpable anorectal ring) and ends at the mucocutaneous junction of the squamous mucosa and perianal skin, which approximately coincides with the outermost boundary of the internal sphincter muscle (palpable intersphincteric groove). The proximal anal canal is composed of anal columns, which are vertical mucosal folds separated by anal sinuses. The distal edges of the anal columns are demarcated by horizontal mucosal folds, termed anal valves. The line formed by the anal valves and sinuses is the dentate line and, as mentioned earlier, marks the division of the vascular, lymphatic, and neural supply and divides the anal canal into cranial and caudal segments.
The histologic zones of the anal canal are divided into four components, defined loosely by the types of mucosal epithelium present. These include the colorectal zone, anal transitional zone, squamous zone, and perianal skin.
Colorectal Zone
The colorectal zone ( Fig. 10.4A ) is the most cephalad segment of the anal canal and the most caudad portion of the colonic mucosa. It is a continuation of the rectal mucosa, lined by an uninterrupted absorptive columnar epithelium that extends to the proximal extent of the anal columns. There is no clear histologic demarcation between the rectal mucosa and colorectal zone of the anal canal, although the crypts tend to be shorter and more irregular in the anal canal. The epithelial cells are CK20-positive, CDX-2 positive, and CK-7 negative, an expected immunophenotype of colorectal-type mucosa. No anal transitional zone (ATZ) mucosa, anal glands, or squamous mucosa is present in this histologic zone.
Anal Transitional Zone
The ATZ ( Fig. 10.5 ; see Fig. 10.4 ) is a region of squamous metaplasia that extends from the dentate line (old squamocolumnar junction [SCJ]) to the new SCJ. On high-resolution anoscopy (HRA), the new SCJ is the most important and readily identifiable landmark, typically visible as a clear line of tissue that is paler than the adjacent proximal rectal epithelium (see Fig. 10.5 ). The dentate line, in contrast, is not as easily visualized on HRA. Histologically, the ATZ is flanked by colonic mucosa above and squamous mucosa below. The ATZ is defined by its relative location to the adjacent histologic zones because the epithelial types present here are variable. The prototypic epithelial type of this zone is the ATZ epithelium (see Fig. 10.4B ), which consists of multilayered columnar to cuboidal, and sometimes flattened, cells, ranging from one to nine cell layers, with variable mucin production—scant intracytoplasmic mucin to rare goblet cell metaplasia. This cell population is positive for CK7 and negative for CK20, CDX2, and p63. Scattered CK7-positive ATZ cells extend into the proximal adjacent colorectal epithelium and over the surface of the distal stratified squamous epithelium; colorectal and squamous epithelial cells are negative for CK7. There is an underlying basal cell layer that is usually one or two cells thick, with cells staining positive for p63 and negative for CK7. The cells are present as a continuation of the basal cell layer of the perianal skin and squamous zone and terminate at the ATZ and colorectal junction. The adult and fetal ATZ are remarkably similar, although the adult ATZ demonstrates occasional islands of mature squamous epithelium, as well as scattered crypts of colonic-type epithelium (see Fig. 10.4C ). These epithelial heterogeneities are relatively rare in the fetal ATZ (see Fig. 10.4A ).The overall microanatomy of the adult anal canal is established at least by 9 weeks’ gestational age. The length of the ATZ is highly variable, ranging from abrupt transition (0 mm; direct squamous to colorectal epithelial transition, without intervening ATZ epithelium) to 16 mm on a longitudinal histologic section (average, 9.5 mm).
Anal glands and ducts lined by ATZ-type epithelium are present in the submucosa (see Fig. 10.4A ) and internal sphincter muscle. Scattered melanocytes and endocrine cells are present. Lymphoid follicles may also be present in the lamina propria and submucosa, but in smaller quantities than in the colorectal zone.
Squamous Zone
The squamous zone (see Fig. 10.4A ) is composed of uninterrupted, nonkeratinizing squamous epithelium. The transition from the ATZ to the squamous zone generally takes place at the level of the dentate line. Melanocytes may be present in this zone, but are present in greater number toward the perianal skin. Basal and parabasal cells demonstrate strong nuclear positivity for p63, but staining gradually diminishes with epithelial maturation toward the surface. By definition, ATZ epithelium, anal glands, and skin appendages are not present.
Perianal Skin
The perianal skin is composed of keratinizing squamous epithelium with hair, sebaceous glands, sweat glands, and apocrine glands. In addition, there are anogenital mammary-like glands characterized by simple columnar epithelium with cytoplasmic snouts and an outer layer of myoepithelial cells. These glands may give rise to epithelial and fibroepithelial neoplasms, analogous to those arising in the breast. Melanocytes may be present at a greater density than in the squamous zone. Scattered Langerhans cells and Merkel cells may be present. The transition from the squamous zone to perianal skin is gradual, without any abrupt histologic landmarks.
In addition to the major epithelial types listed above, components of the anal wall include smooth (internal sphincter) and striated muscle (external sphincter), nerves, blood vessels, and lymphatics. The supply and drainage of these components differ significantly above and below the dentate line, as discussed below.
Understanding the gross and histologic anatomy of the anal canal is important for understanding the pattern of neoplastic development in this complex site. Squamous carcinoma commonly arises in the canal and is linked to human papillomavirus (HPV) infection. Adenocarcinoma of the anal canal may arise from the distal rectal mucosa and, more rarely, from anal ducts. Melanomas, lymphomas, leiomyomas, leiomyosarcomas, and neural tumors such as schwannomas have also been reported.
The anatomy of the anal canal is also key to understanding the spread of neoplastic disease based on its origin in the upper or lower anal canal. The upper two-thirds of the anal canal is supplied with blood mainly from the superior rectal artery (a branch of the inferior mesenteric artery) and drains into the superior rectal veins. The venous drainage of the superior rectal vein empties into the inferior mesenteric vein, but branches of the superior mesenteric and middle veins may also anastomose in that area. Drainage from the upper two-thirds of the canal leads to the inferior mesenteric lymph nodes. The autonomic nervous system innervates the anal canal above the dentate line, whereas the somatic nervous system via the pudendal nerve and sacral plexus innervates the canal below the dentate line. The lower third is supplied by the inferior rectal arteries, branches of the internal pudendal arteries. Venous drainage is provided by the inferior rectal veins leading to the internal iliac veins via the internal pudendal veins. Lymphatics for the lower third of the canal lead to the superficial inguinal lymph nodes. The somatic nervous system supplies that area via the inferior rectal nerve. Understanding the histologic zones and gross anatomy of the anal canal can provide important clues for clinical disease. For example, iliac and periaortic lymph node metastases from tumors originating above the dentate line are frequently seen in adenocarcinomas, whereas lymphadenopathy of the inguinal and femoral nodes more likely reflects spread from tumors below the dentate line and are more likely to be squamous cell carcinoma.
Non-Neoplastic Lesions
Perianal Cysts
A variety of developmental anomalies may lead to perianal or anal cysts. It is believed that cysts in these areas may arise from remnants of the neuroenteric canal, tailgut, or hindgut, but in practice these mucus-producing cysts may be difficult to distinguish from anal duct cysts or anal gland cysts. The cyst lining typically is composed of mucus-producing cuboidal to columnar epithelium. Clinically, these cysts come to attention because of infection and perianal abscess formation.
Hemorrhoids
Clinical
Hemorrhoids are characterized by dilations of superficial mucosal vessels accompanied by enlargement or prolapse of connective tissue. Traditionally, it was believed that hemorrhoids were caused by the pathologic development of superficial varicosities of the submucosal plexus of venules. However, hemorrhoids are now presumed to result from enlarged or prolapsed fibrovascular and connective cushions in the submucosa, which perform a physiologic protective role during defecation. Portal hypertension does not cause hemorrhoids but may precipitate bleeding ( Fig. 10.6 ).
Risk factors for hemorrhoids include a low-fiber diet, constipation, and prolonged intra-abdominal pressure (straining). Other factors implicated in their development include diarrhea, obesity, hypertension, and hereditary tendency. Increased age also predisposes to symptomatic hemorrhoids because loss of elastic tissue and replacement of muscle with collagen fibers cause instability and venous stasis. Patients with concomitant portal hypertension or coagulopathy are then susceptible to bleeding and prolapse.
Hemorrhoids can occur caudal (external) or cephalad (internal) to the sphincter (external), and the location will determine the symptoms. Internal hemorrhoids primarily cause painless bleeding, whereas external hemorrhoids are associated with pain, particularly when thrombosed, strangulated, or inflamed. Erosion of the surface of a prolapsed hemorrhoid may also cause pain or infection.
Histology
The hemorrhoidal tissue is composed of thick-walled, medium-sized vessels in the submucosa and background stroma containing connective tissue and smooth muscle fibers ( Fig. 10.7 ). Additional features include thromboses, neuronal hyperplasia, and erosion, with inflammatory changes of the mucosal surface. Incidental anal intraepithelial neoplasia (AIN) has been reported in 2% of excised hemorrhoids.
Anal Tags and Fibroepithelial Polyps
Fibroepithelial stromal polyps (FSPs) are polypoid projections of the anal mucosa with stromal hyperplasia that occur in response to mucosal trauma. FSPs are composed of two components, including squamous mucosa with minimal acanthosis and underlying loose to compact stromal tissue, often with distinctive stellate or multinucleate cells showing fibroblastic and myofibroblastic differentiation. Various degrees of inflammation are present, depending on the degree of trauma to the polyp. Fibroepithelial polyps lack the dilated submucosal vessels of a hemorrhoid and are thought to be the equivalent of acrochordons found in other sites of the body (see Fig. 10.7 ).
Inflammatory Polyps
Background
Inflammatory polyps, or inflammatory cloacogenic polyps, are prolapsed mucosal projections with histologic features suggesting an ischemic or reactive origin. Inflammatory polyps are believed to occur as a component of the solitary rectal ulcer syndrome or mucosal prolapse syndrome. These polyps occur most often in middle-aged patients, although they also occur in children and often present with rectal bleeding. A few case reports of dysplasia found in these polyps are present in the literature.
Clinical and Gross Examination
Grossly, the lesions are pink or tan polypoid excrescences of the anal canal mucosa, which are varied in size and are typically sessile, with a tubulovillous architecture.
Histology
The lesions are lined by a mixture of squamous and colorectal epithelium, often with surface ulceration or erosion. In addition, fibrosis of the lamina propria and thickening of the muscularis propria are present, with some extension of smooth muscle into the lamina propria. Mucosal gland hypertrophy and surface telangiectasia may also be seen ( Fig. 10.8 ).
Management and Outcome
Treatment of the lesions is simple excision, usually with curative results.
Anal Fissures and Ulcers
Clinical
Anal fissures are traumatic erosions of the mucosal surface, primarily caused by the passage of hard stool. These so-called stercoral fissures usually occur in the posterior midline of the anal canal and may develop into chronic ulcers with repeated trauma. A reactive stromal hyperplasia, histologically similar to a fibroepithelial polyp, can form in chronic cases at the proximal end of the lesion and is termed a sentinel tag . Secondary causes of anal fissures include inflammatory bowel disease, particularly Crohn disease, neoplasms, or infectious processes. Trauma and anal intercourse are also implicated in the formation of fissures.
Histology
The fissures or ulcers show mucosal erosion, with fibrinoid ulcer formation and underlying granulation tissue. Foreign body giant cells may also be present and should not be mistaken for the granulomas of Crohn disease.
Anal Abscesses and Fistulas
Clinical
Most anal fistulous disease is believed to arise from infection of the anal ducts and subsequent tracking into the anal glands. Because of the position of the anal glands deep to the wall of the internal anal sphincter muscle, deep abscesses may result acutely. Fistulous tracts may form in the chronic inflammatory phase. Hidradenitis suppurativa is an associated condition, which is a chronic infection of the apocrine glands and surrounding connective tissue, occurring most often in males ( Fig. 10.9 ). Patients present with complaints of pain, purulent discharge, and pruritus. Inflammation may involve the distal anus as well as the anal canal and rectum, with the formation of fistulas ( Fig. 10.10 ). Associated conditions include oily skin, acne, obesity, smoking, and diabetes mellitus. Crohn disease may also result in perianal and anal fistulous disease.
Histology
The characteristic features include ulceration and granulation tissue in the tracts, with foreign body giant cells and fibrosis.
Inflammatory Bowel Disease
Inflammatory bowel disease can manifest in the anal canal in several ways.
Ulcerative Colitis
Ulcerative colitis uncommonly involves the anal region (ulcerative proctitis) and produces nonspecific inflammatory changes in the anal canal, restricted to the mucosa of the colorectal zone. Fissures and fistulas are more rare. Of note, however, one study has found that the columnar epithelium affected by ulcerative colitis often extends in irregular tongues into the transitional epithelium, occasionally reaching the dentate line. Thus, surgical intervention for ulcerative colitis is a balance between removing all the mucosa of the colorectal zone and preserving sphincter function.
Female fecundity decreases following surgical procedures for ulcerative colitis, specifically following an ileal pouch–anal anastomosis (IPAA). Studies have shown that fecundity before surgery in patients with ulcerative colitis is equivalent to a control population. However, following bowel excision and revision, women had more difficulty getting pregnant. A small study of hysterosalpingography in 21 women after IPAA found abnormalities in two-thirds of patients. Abnormalities included fibrous bands, fallopian tubes adherent to the pelvic side wall, hydrosalpinx, and tubal occlusion.
Crohn Disease
Crohn disease affects the anus more frequently and with a wider variety of symptoms and pathology. Approximately 25% of patients with small bowel Crohn disease have anal canal involvement, compared with involvement in 50% to 75% of patients with small and large bowel disease. Anal tags, fissures, ulcers, fistulas, and abscesses may form. Granulomatous inflammation affecting the anal canal mucosa and wall are seen histologically.
Intraepithelial neoplasia of the ATZ may also occur following IPAA for ulcerative colitis or Crohn disease. A study from the Cleveland Clinic has shown that although no patients in their series (total of 210 patients) developed carcinoma of the transitional zone, LSILs and HSILs developed in six patients and one patient, respectively. All patients who developed a lesion had a prior history of intraepithelial neoplasia or carcinoma, either in the colon or rectum.
Endometriosis
Endometriosis rarely involves the anus and perianal area. The patients typically report cyclic pain that correlated with menses, occasionally with perianal nodule formation. Many cases are associated with an episiotomy scar and occasionally may involve the sphincter muscles. The histologic correlate is an endometrioma, composed of a blood-filled cyst containing endometrial-type glands and stroma and hemosiderin deposition.
Infections
The anus may be involved in a wide range of infections, including tuberculosis, amebiasis, and fungal and yeast infections. Other infections typically associated with the vulva may also involve the anus, including herpes, syphilis, and, as discussed later, papillomaviruses (see Chapter 4 ).
Iatrogenic Lesions
Various iatrogenic insults to the anal region have been reported in the literature. Irradiation of the anal region following abdominoperineal resection for anal cancer is one of the most common causes of iatrogenic injury. Histologically, the anal sphincter muscle shows increased fibrosis and nerve density. Ergotamine-induced strictures from ergotamine suppositories for migraine treatment have also been reported. In addition to trauma from repeated suppository use, ergotamine toxicity results from the drug’s vasoconstrictive properties, producing local ischemia. Rectal ulcerations are the most common early manifestation of this disorder, leading to progressive fibrosis and stricture formation.
Neoplasms of the Anus
The most recent World Health Organization (WHO) classification (2010) divides tumors of the anal canal into epithelial tumors, mesenchymal tumors, and secondary tumors. Epithelial tumors are subdivided into premalignant lesions of the anal canal mucosa and perianal skin and carcinoma. The carcinoma group consists of squamous cell carcinoma, adenocarcinoma, and neuroendocrine neoplasms.
For practical purposes, neoplastic lesions can be divided into three categories—benign adnexal lesions, squamous intraepithelial lesions (SILs) and carcinoma, and other malignancies. Squamous lesions are the most commonly encountered neoplasms and will occupy the bulk of the discussion that follows.
Benign Adnexal Lesions
Hidradenoma Papilliferum or Papillary Hidradenoma
Clinical Presentation
This is a rare neoplasm of the vulvovaginal and perianal skin, primarily in women. A few cases have been reported in men and in other sites (e.g., breast, eyelid, ear canal) in women. Most patients are asymptomatic, although some have reported fluctuation in tumor size during the menstrual cycle.
Histopathology
The tumor is present in the dermis as a well-circumscribed solid or cystic nodule with papillae and glands, similar what is seen in the vulva (see Chapter 5 , Fig. 5.11 , Fig. 5.12 , Fig. 5.13 ). The papillae are arranged in fronds or complex anastomotic patterns, usually completely enclosed in the dermis and without epidermal connection. Two cell layers line the papillae, an epithelial and a myoepithelial layer. The epithelial layer is composed of tall cuboidal to columnar cells with pale eosinophilic cytoplasm, with regular nuclei. In about 30% of tumors, the epithelium may exhibit brightly eosinophilic cytoplasm, and snouting with decapitation secretion may be seen. Occasionally, nuclear atypia and mitoses may be present. The myoepithelial cells are small cells with round to ovoid nuclei and clear to pale eosinophilic cytoplasm. Periodic acid–Schiff (PAS)–positive, diastase-resistant granules are present in the larger epithelial cells. Immunohistochemical staining has shown the epithelial cells to stain for a variety of receptors, including estrogen and progesterone. The myoepithelial layer has been shown to be immunoreactive with S-100 protein and smooth muscle actin. It is speculated that these tumors arise from sweat glands or anogenital mammary-like glands.
Differential Diagnosis
The differential diagnosis of these tumors includes syringocystadenoma papilliferum and tubular apocrine adenoma. Hidradenoma papilliferum is distinctive for its occurrence in the anogenital region of women almost exclusively and its disconnection from the epidermis.
Granular Cell Tumors
Granular cell tumors are relatively common benign tumors that occur at a variety of sites and in a wide age range. They rarely involve the anus. The cells are thought to be of Schwann cell origin. Histologically, the tumors are characterized on low power as a circumscribed nodule of eosinophilic cells in the dermis, with overlying pseudoepitheliomatous hyperplasia (see Chapter 9 , Fig. 9.31 ). The lesion may have a slightly infiltrative appearance, particularly at the edges, which may extend into the papillary dermis. The cells are plump, with abundant granular pink cytoplasm, and contain small hyperchromatic round to slightly irregular nuclei. The granular cytoplasm is PAS diastase-resistant and may also be stained immunohistochemically with NKIC3. These tumors have a very low malignant potential. Sometimes, marked epithelial hyperplasia may be present, giving the false impression of an invasive malignant lesion (see Chapter 9 ).
Squamous Neoplasia of the Anal Canal
Epidemiology and Variables Influencing Anal Cancer Risk
Squamous cell carcinoma is the most common anal cancer—85% of total. The incidence of anal carcinoma in the general population is low; however, this figure has been rapidly increasing over the past several decades, from 0.35/100,000 persons in the 1970s to 1.5/100,000 persons in the early 2000s. The average age of patients is in the sixth to seventh decades, and the white population has the highest incidence rate, followed by the black, Hispanic, and Asian populations. Women have a higher rate of anal cancer than men; however, increasingly larger proportions of anal precancer are detected in men. Although referral bias needs to be taken into consideration, in a cohort of the San Francisco–Oakland area, the incidence of anal HSIL (AIN2/3) in 2009 was 10-fold higher in men (12.5/100,000) than in women (1.3/100,000). The overall increase in the incidence of anal squamous neoplasia and possible shift in demographics may be best explained in the context of the two major causative risk factors for anal squamous carcinoma—HPV infection and immunosuppression, largely due to human immunodeficiency virus (HIV) infection.
Epidemiologic and molecular studies have shown that HPV is the causative agent in most anal squamous cell carcinomas, with HPV detected in 88% of anal cancer and 95% of anal HSILs (AIN2/3). HPV infection, particularly with HPV-16 and HPV-18, is frequently found in anal squamous cell carcinomas, both of the anal canal and perianal skin. Receptive anal intercourse is an evident mechanism of HPV transmission and accordingly, the prevalence of anal HPV infection is highest in HIV-positive men who have sex with men (MSM). However, the perception that anal intercourse is limited to men having sex with men is now obsolete, with increasing reported rates of heterosexual anal intercourse. One study has shown that the risk of anal cancer increases as much as 18-fold for individuals (men or women) who frequently engage in receptive anal intercourse. However, anal intercourse does not appear to be a necessary behavioral cofactor of anal HPV infection because other studies have shown the frequent detection of HPV in the anal canal of women who do not engage in anal intercourse ( Table 10.1 ). The prevalence of anal HPV infection is at least as high as cervical HPV infection in women regardless of a history of anal intercourse, and women with HPV-related cervical or vulvar lesions are at a higher risk of developing anal lesions. Autoinoculation is a probable mechanism of HPV exposure, supported by studies that have shown that concurrent anal and cervical HPV infections have a high concordance of HPV genotype. Further supporting autoinoculation is a recent study showing that post-toilet behavior of front to back wiping is associated with an increased prevalence of abnormal anal cytology, histology, and high-risk HPV infection. This underscores the importance of an anal examination for high-risk individuals, irrespective of gender and sexual practices. Cervical examinations in patients with anal lesions are particularly important (and vice versa) in at-risk populations, given the high frequency of concurrent anal and cervical HPV infection in these populations. Other risk factors for anal HPV infection include younger age (<45 years), intravenous drug use (17-fold risk increase), and white race. Risk factors for anal carcinoma are summarized in Table 10.1 .
| Reference(s) | Variable | Risk |
|---|---|---|
| Female gender | 2–4× | |
| Anal intercourse | RR = 1.8–2.4; OR = 6.9 | |
| HIV-positive | RR = 3.2 | |
| Abnormal cervical cytology | RR = 1.8–4.13 | |
| Smoking | RR = 7.7 | |
| Anal condyloma | SIR = 8.5 | |
| High-risk HPV | Detected in 95% of HSILs and 88% of invasive squamous cell carcinomas |
In addition to HPV exposure and infection, the HPV-driven cancers require persistence of infection for malignant transformation. There are known behavioral cofactors, such as smoking, which are thought to promote persistent infection. However, in this regard, immunosuppression by way of HIV infection and acquired immunodeficiency syndrome (AIDS) plays a much more vital role in the development of anal squamous cell carcinoma. Historically, the introduction of HIV into the human population and the development of AIDS may have resulted in a population of patients who are at high risk of developing HPV-related anogenital neoplasms. A study from the AIDS-Cancer Match Registry from 1978 to 1996 has found a relative risk of anal cancer for women and men diagnosed with AIDS of 7.8 and 60.1, respectively, for the development of anal HSIL (AIN2/3). In particular, the risk for women diagnosed with AIDS between the ages of 30 and 39 years was increased 21-fold compared with controls. It is presumed that the detrimental impact of HIV on cell-mediated immunity prevents clearance of HPV infection. A low CD4 count is the strongest independent predictor of high-risk HPV infection, and generalized immunosuppression from other causes has also been shown to be associated with a higher risk of anal HPV-related disease.
Unlike other cancers associated with HIV, the incidence of HPV-related cancers does not appear to be declining with the introduction of antiretroviral therapy (ART). In the case of anal cancer, the incidence appears to be increasing. This is most likely due to increased ART-associated longevity in the setting of a high incidence of anal HSIL (AIN2/3) without established screening guidelines. This would have the effect of affording individuals increased time to progress from HSIL (AIN2/3) to invasive cancer. Conversely, HPV infection may also increase the risk of HIV acquisition by eliciting a local immune response that attracts vulnerable lymphocytes and macrophages to the site of HIV infection. Given this feed-forward cycle of HPV and HIV infection, the population at highest risk for the development of anal cancer is HIV-positive MSM; the incidence of anal carcinoma in this specific population is alarmingly high, at a reported rate of 131/100,000. Hence, the increase in incidence of anal (pre)cancer and the shift in demographics over the past few decades may be explained, at least in part, by changes in sexual practices at the societal level, the AIDS epidemic of the 1980s, and the pathogenesis of this HPV-driven disease, with HIV infection as a significant cofactor.
An additional variable influencing anal cancer risk is the microanatomy of the anal canal. Given the topographic preference for cervical neoplasia to occur at the SCJ, one might expect that the anus, which also harbors an SCJ, to be similarly susceptible to HPV-driven neoplasia. However, despite its fairly recent increase in incidence, anal cancer is still much less frequent than cervical cancer (incidence of 8/100,000). It is tempting to attribute this disparity entirely to differences in sexual practices and HPV exposure; however, as mentioned earlier, HPV DNA is detected at least as frequently in the anus as in the cervix. Therefore, despite high rates of anal HPV exposure, there is an inexplicably low rate of anal cancer compared to cervical cancer.
Recent studies have suggested that this discrepancy may be explained by subtle differences in the microanatomy of the cervical and anal SCJs ( Fig. 10.11 ). A population of residual embryonic cells was recently identified at the cervical SCJ that are presumed to be the cells of origin for most HSILs and squamous cell carcinomas based on their microanatomic location and shared immunophenotype with squamous dysplasia (see Chapter 13 ). Traditional models of HPV-driven carcinogenesis require epithelial injury and exposure of the basal cells for viral infection to take place; however, these SCJ cells are arranged as a single layer, directly exposed to the environment (see Fig. 10.11A ). Therefore, it has been postulated that this topographic arrangement of the SCJ cells, at least in part, makes them distinctly susceptible to HPV exposure and infection. The ATZ is composed of a multilayered ATZ epithelium, with an underlying layer of basal cells (see Fig. 10.11B ); this configuration is functionally equivalent to stratified squamous epithelium in that epithelial injury is needed for HPV infection of basal cells. This would make the anal SCJ less susceptible to HPV infection than the cervical SCJ. In fact, the incidence of anal carcinoma is similar to that of vulvar carcinoma, supporting the idea that site-specific microanatomy may influence the rate of HPV-driven carcinogenesis.
Screening and Prevention
Screening
Despite the increasing incidence of anal cancer, and an unacceptably high incidence specifically in HIV-positive MSM, there is no standardized screening program implemented for the prevention of anal squamous cell carcinoma. There are several reasons for this :
- 1.
The treatment of HSIL (AIN2/3) is not proven to reduce the risk of anal cancer.
To date, there has been no evidence demonstrating the efficacy of treating HSIL to reduce the incidence of anal cancer. To address this, an investigation known as the ANCHOR study ( A nal C ancer/ H SIL O utcomes R esearch) has been initiated. In this multicenter phase III clinical trial, HIV-positive men and women with HSIL (AIN2/3) have been randomized into two groups:
Treatment group: This group will be treated with topical agents such as imiquimod and 5-fluorouracil or with excisional surgical procedures.
Monitored group: This group will undergo active monitoring every 6 months for 5 years or more. If the trial results show that the treatment of HSIL leads to a meaningful reduction in the incidence of anal cancer, it is expected that screening and treatment of anal HSIL will become the standard of care.
Because the results of the ANCHOR study will not be available for at least another 5 years, some practices approach the management of high-risk patients with the assumption that early screening is beneficial until this trial indicates otherwise. Another complicating factor here is that the treatment of anal HSIL is not optimized. Although there are multiple topical treatments and ablative therapies available, total clearance is difficult, and retreatment is often required, particularly in patients with large multifocal lesions.
- 2.
Screening methods to identify HSIL (AIN2/3) are not optimized.
Given the biologic similarities of anal and cervical carcinoma and the highly successful cervical screening program, proposed models of anal screening also incorporate the triad of cytology, colposcopy (HRA), and biopsy. However, significant differences in epidemiology, risk factors, and natural history preclude the direct translation of cervical screening protocols to the anus. The uncertain utility of HPV testing in anal cytology represents a prime example of an epidemiologic difference posing such a challenge. HPV testing, particularly of HIV-infected individuals, yields positive rates in as many as 85% or more MSM, and the presence of HPV does not necessarily indicate a coexisting lesion. This results in low specificity and low positive predictive value of HPV testing in the high-risk population targeted for anal cancer screening, limiting its value in primary screening and triage. Instead, HPV genotyping, especially of high-risk types such as HPV-16, might be of value, as might other tests such as HPV RNA or protein-based tests and the use of nonviral markers such as p16. In addition, anal cytology has limited sensitivity to detect high-grade SILs, and the grade of disease is often underestimated on cytology compared with HRA-guided biopsy. Therefore, taken together with the limited utility of the HPV test, it is currently recommended for patients with atypical squamous cells–undetermined significance (ASC-US) or higher on anal cytology to be triaged to HRA. Such adjustments to the cervical screening protocol based on data specific to anal SILs and cancer are necessary steps for establishing a nationally standardized anal cancer screening program.
Human Papillomavirus Vaccination
Given that most anal squamous cell carcinomas are HPV-associated, HPV vaccination provides a promising opportunity for primary prevention. Currently, there are three HPV vaccines approved by the US Food and Drug Administration (FDA) for routine vaccination: (1) bivalent HPV vaccine (against HPV types 16 and 18—Cervarix, Glaxo-SmithKline Biologicals); (2) quadrivalent HPV vaccine (against HPV types 6, 11, 16, and 18—Gardasil, Merck); and (3) nonavalent HPV vaccine (against HPV types 6, 11, 16, 18, 31, 33, 45, 52, and 58—Gardasil 9, Merck).
Clinical trials have demonstrated that these vaccines are effective for the prevention of vaccine-type, HPV-related cervical lesions in women, but data pertaining to the efficacy of HPV vaccination for the prevention of HPV-associated anal lesions, particularly in men, are comparatively scant. In a multicenter, randomized, double-blind study (Merck 020 protocol), Palefsky et al. have demonstrated that the quadrivalent HPV vaccine is effective against vaccine-type HPV-related AIN grades 1 to 3 and persistent anal HPV infections in healthy MSM 16 to 26 years of age. A cost-effectiveness model based on this study has shown that routine vaccination of MSM aged 9 to 26 years is cost-effective. A randomized double-blind trial by Kreimer et al. has demonstrated that the bivalent HPV vaccine is effective against anal HPV16/18 infection in women, although with lower efficacy than for cervical infection. An important observation of this study was that vaccination is more effective in younger, HPV-naive women. Vaccine efficacy declines with age and an increasing number of sexual partners, likely because it works predominantly through the prevention of initial infection. A notable challenge here is that (self) identification and vaccination of MSM at a young age may be difficult, and vaccination may not be pursued until after sexual debut. Interestingly, a retrospective chart review study by Swedish et al. has demonstrated that the quadrivalent HPV (qHPV) vaccine may have some efficacy in reducing the risk of recurrent high-grade AIN in older, HIV-negative MSM. Therefore, HPV vaccination may be useful not only as a primary prevention tool in young patients, but also as a post-treatment adjuvant approach to reduce the risk of recurrence after treatment in patients who have already been exposed to HPV. Further investigation is needed for the development of HPV vaccines that are concurrently preventive and therapeutic.
The current recommendations for the use of HPV vaccines from the Advisory Committee on Immunization Practices (ACIP) are as follows :
- 1.
Routine HPV vaccination initiated at age 11 to 12 years (can be started at 9 years)
- 2.
Vaccination for females aged 13 to 26 years and males aged 13 to 21 who have not previously been vaccinated or have not completed all recommended doses; males aged 22 to 26 years may be vaccinated
- 3.
Vaccination for MSM and immunocompromised persons through the age of 26 years
Currently, two doses are recommended for those initiating vaccination at ages 9 to 14 years, and three doses are recommended to those initiating vaccination at ages 15 to 26 years. Three doses are recommended for immunocompromised persons, regardless of age of initiation.
Without a national guideline for anal cancer screening, vaccination is the only anal cancer prevention program currently available. Efforts must be made to maximize vaccine uptake in eligible men and women.
Examination Methods
The clinical examination should begin with a visual inspection of the perianal region for signs of condyloma acuminatum or plaquelike lesions suggestive of Bowen disease. Fissures and fistulas should also be sought because these are also risk factors for carcinoma. The perianal and inguinal regions should be palpated for masses.
Digital Anal Rectal Examination
A digital anal rectal examination (DARE) is a low-cost, low-tech technique that may be considered as the primary anal cancer screening test. Invasive cancer can be palpable on this examination, even in the absence of clinical symptoms. The lesion may feel hard, thickened, and indurated. A digital examination is especially important because cytology and direct visualization of lesions using HRA may not always identify submucosal lesions. In addition, important information, such as the degree of firmness of the lesion and its mobility, may be obtained with a digital examination and may alter the index of suspicion for cancer. In particular, DARE should be part of routine clinical care of HIV-positive MSM.
Cytologic Collection
The goal of cytologic collection is to sample the entire anal canal thoroughly, including the transformation zone, because this area is also at risk of development of squamous neoplasia. A synthetic fiber swab (rayon or polyester) moistened with water is inserted into the anal canal until the distal rectum is reached. Gentle pressure is applied to the anal canal wall while the swab is rotated 360 degrees and then gradually withdrawn. The harvested cells may be placed in a liquid-based cytology medium or smeared on a glass slide. Either method is acceptable, but liquid-based cytology increases cell yield while decreasing fecal contamination and air-drying artifact. Although collection for cytology is generally performed by a clinician in the context of an office visit, a patient self-collected, anal cytologic specimen has been found to have an acceptable yield, especially given its advantages (convenience and privacy to the patients).
High-Resolution Anoscopy
HRA-guided biopsy is the gold standard for the diagnosis of an anal SIL. The technique is performed in an outpatient setting in a manner similar to that for cervical colposcopy. After insertion of an anoscope and the application of 5% acetic acid, with or without Lugol’s iodine, the transformation zone, squamous zone, and perianal skin are visualized under magnification, and abnormal areas are biopsied. Abnormal colposcopic features include acetowhitening, irregular margins, surface contour, and vascularity. One study that correlated gross and histologic findings has found some differences between low-grade and high-grade lesions. High-grade lesions were significantly more likely to have a flat, smooth, nonpapillary surface compared with low-grade lesions. Low-grade lesions were also more likely to have warty vessels, punctation, and a vascular mosaic pattern. Anal SILs may also be discovered incidentally on specimens excised for unrelated reasons, such as hemorrhoidectomy specimens.
The efficacy of HRA as a diagnostic test depends largely on the colposcopist. Unfortunately, as a technique, HRA is considered by some to be more difficult than cervicovaginal colposcopy, and there is a steep learning curve for someone to become competent in HRA. The current shortage of providers skilled in HRA is an additional obstacle to establishing anal cancer screening.
Anal Cytology
Anal cytology was first included in the 2001 Bethesda system atlas and has been thoroughly updated in the third edition of the atlas ( The Bethesda System for Reporting Cervical Cytology, 2015) to incorporate the most recent epidemiologic literature and photomicrographs of findings unique to anal cytology. Anal cytology has been gaining acceptance as a screening tool in conjunction with HRA and directed biopsy. If the ANCHOR study demonstrates that the treatment of HSIL leads to a meaningful reduction in the anal cancer rate, it is expected that anal cancer screening will become the standard of care, with anal cytology as a core component.
The target of cytologic sampling includes the entire anal canal. Although there is a paucity of literature assessing the adequacy of anal samples, it is generally accepted that adequate sampling constitutes 2000 to 3000 nucleated squamous cells (approximately one or two nucleated squamous cells/high power field [hpf] for ThinPrep and three to six nucleated squamous cells/hpf for SurePath). The presence of columnar cells (colorectal zone–transitional zone sampling) is reported as a quality indicator. Samples that lack this minimum number of nucleated squamous cells are considered unsatisfactory for evaluation. The unsatisfactory designation may also be given to samples that are obscured by blood, bacteria, or fecal matter, as well as samples that are predominantly composed of anucleate squames (insufficient sampling of anal canal mucosa beyond the perianal skin). Organisms seen in cervical and anal cytology tests include candida and herpes virus. Organisms unique to anal cytology are intestinal parasites, such as ameba and pinworms (and their eggs).
The terminology and cytomorphologic criteria for squamous lesions in anal cytology are analogous to those of cervical cytology. Squamous abnormalities are divided into four main categories.
Atypical Squamous Cells—Undetermined Significance
These cells, ASC-US, have borderline changes suggestive but not diagnostic of LSIL, such as mild nuclear enlargement, perinuclear clearing without significant atypical nuclear features, and atypical keratinized cells. Differential diagnoses to consider include pseudokoilocytosis (reactive changes, glycogenation), herpes cytopathic effect, repair, and radiation changes.
Atypical Squamous Cells–Cannot Exclude High-Grade Squamous Intraepithelial Lesion
The atypical squamous cells–cannot exclude HSIL (ASC-H) category is reserved for specimens specifically worrisome for HSIL; all other forms of squamous atypia should be categorized under ASC-US. Differential diagnoses to consider include immature squamous metaplasia, atrophy, reparative changes, colonic epithelial cells, anal transitional zone cells, and inflammatory cells.
Low-Grade Squamous Intraepithelial Lesion
The lesional cells are characterized by enlarged nuclei (three times or more the size of intermediate squamous cell nucleus), with hyperchromasia, irregular nuclear contours, and occasional binucleation. Abundant dense cytoplasm with sharply defined perinuclear cavitation, known as koilocytosis, may be seen. Cells are arranged singly or as small clusters.
High-Grade Squamous Intraepithelial Lesion
The lesional cells are characterized by enlarged hyperchromatic nuclei with irregular nuclear contours and anisonucleosis. The nuclear-to-cytoplasmic (N/C) ratio is high, and cells may be arranged singly or as crowded syncytial clusters. Keratinizing lesions are more frequently seen in anal cytology due to the juxtaposition of keratinized and nonkeratinized portions of the distal anal canal. Also, fecal matter and abundant degenerative changes are difficult to differentiate from tumor diathesis that accompanies invasive squamous cell carcinoma. According to the College of American Pathologists (CAP) Interlaboratory Comparison program for nongynecologic pathology (2006–2011), performance on challenges with target diagnoses of HSIL and squamous cell carcinoma was poor, with concordance rates of 57.1% and 56.2%, respectively.
Squamous Cell Carcinoma
The lesional cells demonstrate marked nuclear atypia, macronucleoli, cytoplasmic keratinization, and background tumor diathesis. As noted, there are many overlapping cytomorphologic features with HSIL, and differentiation may be difficult.
Unlike cervical cytology, the primary target population of anal cancer screening (HIV-positive MSM) has a high rate of high-risk HPV infection, limiting the utility of HPV testing as a triaging tool. Also, as mentioned earlier, anal cytology frequently underestimates the severity of lesions that are found on biopsy ( Table 10.2 ). Therefore, in some practices, patients with ASC-US or higher are referred directly for HRA. Cytology screening is not recommended if the resources for proper cytologic interpretation—HRA and biopsy—and treatment are not available. In these situations, at a minimum, annual DARE examinations are recommended for populations at risk of anal cancer.
| Reference(s) | Baseline | Variable | End Point | Comment |
|---|---|---|---|---|
| Normal cytology | HIV-positive | Abnormal cytology | Risk = 22/100 person-years | |
| + CD < 500 | RH = 4.11 | |||
| + HR HPV | RH = 2.54 | |||
| + Smoker | RH = 3.88 | |||
| LSIL cytology | HIV-positive | HSIL | 62% | |
| HIV-negative | HSIL | 36% | ||
| H/o anal condyloma or benign disease | Homosexual males | HSIL | 60% at initial biopsy | |
| Cancer | 3% at initial biopsy |
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