Chapter 5.2
Anal Intraepithelial Neoplasia (AIN)
John H. Scholefield
Introduction
Squamous cell carcinoma (SCC) of the anus is the most common histolog- ical type of anal carcinoma, comprising over 80% of all anal cancers. They may arise from the perianal skin or from the anal canal itself, the other primary anal malignancies include anal gland adenocarcinoma, melanoma, and lymphoma (1). The variety of nomenclature used to describe histolog- ical subtypes of anal SCC reflects two facts: firstly, the changing epithelium of the anal region from the keratinized hair-bearing epithelium of the peri- anal skin through the modified squamous epithelium to the anal transition zone and rectal mucosa above, and secondly, the inconsistent use of a number of terms such as cloacogenic (2) and epidermoid (3) to refer to par- ticular histological subtypes of anal SCC thought to arise from the transi- tional epithelium of the anal canal.
The term anal intraepithelial neoplasia (AIN) was first used by Fenger (1978) to describe the histological features of dysplasia in squamous epithelium (4). Some ten years later, when the etiology of anal squamous cancer was shown to be etiologically linked with human papillomavirus type 16 (HPV 16), the potential parallels between AIN, cervical intraepithelial neoplasia (CIN), and vulvar intraepithelial neoplasia (VIN) became apparent and colposcopic techniques were applied to the anal epithelium to determine whether gynecological diagnostic principles could be applied to the anus. The identification of dysplastic anal lesions colposcopically (5) led to the suspicion that there may be a number of “at-risk” groups for AIN. Further work identified women with pre- vious genital intraepithelial and those individuals who were systemically immunosuppressed as being at increased risk for AIN (6). Despite the identification of “at-risk” groups, the natural history of the condition remains uncertain. The following account presents the development of our present state of knowledge about the etiology, pathogenesis and diagnosis of AIN, its relationship to CIN and VIN, the treatment options available, and some of the difficulties that arise in managing these patients. The
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role of cytology and “colposcopy” as it applies to advanced proctological practice is outlined.
Anatomy of the Anus
The anal canal is the most caudal part of the gastrointestinal tract; it is a short muscular tube about three centimeters in length situated between the rectum and perianal skin. Its boundaries are poorly defined; the upper border generally is quoted as the puborectalis sling, and the lower border is less well defined and merges with the perianal skin. The pectenate line lies between these areas and represents the fusion of the lower two-thirds of the anal canal (ectodermal in origin) and the upper one-third (endoder- mal in origin). The anal canal shares a common embryological origin with the endocervical canal from the proctodeum. The primary function of the anus is that of preserving continence, and to this end, the anal sphincter mechanisms are of great importance.
In the perianal skin, the epithelium is keratinized, hair-bearing squamous epithelium. In the lower anal canal, the keratinization is lost and the epithe- lium becomes thinner; further up the canal, at about the level of the pecte- nate line, the epithelium becomes transitional (cuboidal) and subsequently columnar (rectal epithelium).
The boundaries of the anal region are poorly defined, leading to confu- sion in the classification of anal tumors described by different authors.
Despite discrepancies in definition, it is apparent that anal canal carcinoma is nearly three times more common than cancer of the anal margin in the population of England and Wales (7). The age distribution is the same for both canal and margin cancers (average 57 years), but anal canal cancers are said to be more common in women than in men (3 : 2), whereas anal margin cancers are more common in men (4 : 1) (1).
Pathological Features of AIN (See Chapter 5.1)
Histologically, AIN is characterized by a loss of epithelial cellular matura- tion with associated nuclear hyperchromasia, pleomorphism, cellular crowding, and abnormal mitoses within the anal epithelium. These features are identical to those of similar cervical and vulval lesions, and AIN lesions are classified according to the nomenclature used for genital intraepithelial neoplasia (8). Thus, in AIN I, cellular and nuclear abnormalities are restricted to the lower third of the epithelium; in AIN II and AIN III, the lower two-thirds and the full thickness of the epithelium, respectively, are affected. The histological features of AIN III are consistent with it being the precursor lesion of invasive squamous cancer, although this progression is not as certain as that evident with the cervix. Anal intraepithelial neo-
plasia III is synonymous with carcinoma in situ of the anus—sometimes termed Bowen’s disease when applied to the perianal skin. Microinvasive carcinoma of the anus has been described and parallels similar lesions described on the cervix (i.e., a lesions in which the basement membrane has been breached by dysplastic epithelial cells). Careful orientation of the specimen in processing and examination must be enforced to avoid sections containing obliquely cut tissue, which can easily be misinterpreted as inva- sive. While the term AIN is used in most of the European literature on this subject, the terms used in the United States are slightly different. Thus, American authors describe HiSIL (or HSIL) and LoSil (or LSIL) (high- grade and low-grade squamous intraepithelial lesions). These are analogous to AIN III and AINI/II, respectively.
Natural History and Epidemiology of AIN
Anal intraepithelial neoplasia is a relatively recently described phenome- non, and at present, its natural history is unknown. In cervical cancer, rates of progression are well documented, but such data is lacking for anal dys- plasia. Etiological parallels with CIN and the histological appearances of AIN III suggest that AIN III may have malignant potential (9–11). The mol- ecular biological changes occurring with AIN and its comparison with CIN and VIN (vulvar intraepithelial neoplasia) are briefly outlined in Chapter 5.1 and covered in a recent review article by Zbar et al. (12). There also have been case reports of malignant change in anal condylomata (13–15).
In the original report of Fenger and Nielsen (16), AIN was found inciden- tally in 19 patients undergoing minor anal surgery; 17 of these patients were alive five years after initial diagnosis, and none developed invasive disease.
Nine of the lesions were graded as AIN III and were treated by local exci- sion (4 lesions recurred). Foust et al. (17) reported 19 cases of AIN III in the anal canal mucosa of routinely excised hemorroidal tissue. In a mean follow-up period of 6 years, only one patient developed recurrent disease and none developed invasive disease. These reports suggest that inciden- tally discovered AIN III is a non-aggressive lesion. However, Fenger and Nielsen (18) also have reported the presence of AIN III in 81% (13 of 16) of cases of anal squamous carcinoma assessing abdominoperineal resection specimens. The lesions were found to be situated at the border of the inva- sive lesions, as well as in areas separated from the tumors by normal mucosa.
In the Sheffield series, 32 patients with AIN III, including nine anal cancers in a field change of AIN III, were identified (6,19,20). Of these, 5 (15.5%) developed invasive anal disease within a median follow-up period of 18 months (range 0.5 to 2 years). This strongly suggests that AIN, par- ticularly AIN III, has malignant potential. This series of cases, however, was drawn from a unit with a reputation for its aggressive surgical approach to
genital intraepithelial disease and may not represent the generality of AIN.
The authors’ subsequent experience in Nottingham is that only 2 of 26 patients with AIN III have progressed to invasive anal cancer over an 8- year period. It is the author’s view that the latter series is more likely to represent the natural history of this condition. A figure of 10% progression for VIN III to invasive cancer is widely quoted, and it is the authors’ impres- sion that a similar progression rate for AIN III is likely. Nevertheless, more prospective cohort evidence is required to determine the proportion of AIN III lesions that progress to invasive disease or which indeed regress; the latter phenomenon having been quoted at about 5 percent (21).
Even less is known of the fate of low-grade AIN lesions (AIN I and II);
however, Palefsky et al. (22) observed that progression of low-grade lesions to high-grade AIN lesions occurred in 6 out of 37 homosexual men with human immunodeficiency virus (HIV) disease over an average follow-up period to 17 months. However, these patients were immunosuppressed and the natural history of the condition may have been accelerated. In the same series, it was observed that 10.8% of low-grade lesions regressed, and in the Sheffield series, regression of low-grade lesions was observed in 13% (3 of 23) of cases over a median follow-up period of 7 months. There is an urgent need for longitudinal studies to determine the rates of progression to high- grade disease in the various risk groups. In this respect, it is unknown whether an “altered immunology” may be responsible either for progres- sion (or regression) of AIN quite independently of HPV persistence or eradication. The immune responses in HIV-related AIN are complex and more studies are required to assess impaired lymphocytotoxic responsive- ness in vitro to HPV-related transforming proteins such as E6 and E7 (23).
The epidemiologic increase in anal cancer appears to predate the AIDS epi- demic where immunosuppressed patients with AIN appear to have more progressive disease with higher chemotherapy-related morbidity and mor- tality. The impact of highly active antiretroviral therapy (HAART) on this immune effect is unclear, although there is evidence to suggest that high- grade lesions, once established, do not regress during treatment (24).
Invasive anal cancer is at present an uncommon malignancy in which only approximately 400 cases are registered in England and Wales each year, where it is five times less common than cervical cancer (7,25). This would suggest that the risk of progression from intraepithelial neoplasia to inva- sive cancer is lower for the anus than for the cervix. However, balanced against this, there is some evidence that the incidence of AIN is increasing.
Wexner et al. (26) showed that the incidence of AIN and anal cancer, par- ticularly in the anal canal, is rising, where, in this study, a three-fold increase in the incidence of anal cancers in both sexes between 1959 and 1986 was observed. This is supported by further data from the United States and Scandinavia, and this may be due in part to the increasing prevalence of anogenital HPV infections (27). If the natural history of AIN is similar to that of CIN (and there are clinical assumptions here that are yet unproven),
then the development of anal cancer may occur several years after the diag- nosis of AIN. This may account for the fact that although homosexual men with HIV are at increased risk of AIN, the incidence of anal cancer in this group, although increasing, is still relatively low. As stated above, the intro- duction of HAART will affect the overall prognosis, as well as the inci- dence, of recognizable invasive anal carcinoma and its complications.
Although there has been an increase in anogenital HPV infections in the UK, Office of Population Censuses and Surveys (OPCS) data do not show a similar increase in the incidence of anal cancer, as has been demonstrated in the United States and in Scandinavia (28–30). This is most likely due to under-registration resulting from difficulties in the classification of squa- mous anal canal cancers—where tumors have been variously classified as cloacogenic, basaloid, and epidermoid—and where anal cancers often are misclassified as rectal tumors, whereas anal margin and perianal tumors typ- ically have been classified as skin tumors (18).
The Prevalence of AIN
The true prevalence of AIN is unknown. Although it initially was thought that AIN was a rare condition, the frequency of occurrence of unrecognized intraepithelial neoplasia encountered during minor surgery of the anal canal was reported as between 0.2% and 4.4% (16,31,32). However, in groups thought to be at risk—homosexual men who practice receptive anal intercourse, immunosuppressed HIV-positive individuals, renal allograft recipients, women with high-grade genital intraepithelial and invasive squamous neoplasia, and individuals of both sexes with anogenital con- dylomata—the prevalence of AIN is much higher. In these groups, the prevalence has been reported as between 15% to 53% in immunosup- pressed HIV-positive individuals, 19% to 47% in women with high-grade genital intraepithelial or invasive neoplasia, and 24% in renal allograft recipients (22,33–38). In contrast, the prevalence of AIN in control popu- lations is very low, being reported as less than 1%.
Etiology and Pathogenesis of AIN
Human papillomavirus 16 has been increasingly implicated as an etiologi- cal factor throughout the range of anogenital squamous cell carcinomas (SCCs). Human papillomavirus 16 is one of several sexually transmissible papillomavirus types (other common types are 6, 11, 18, and 31) associated with a spectrum of lesions from subclinical infection through condylomata to intraepithelial neoplasia and ultimately (and almost certainly involving co-carcinogens) to invasive SCC. Human papillomavirus 16 has been identified in 56% to 80% cases of SCC of the anus, and AIN lesions
(5,19,20,34,35,39,40). Anal intraepithelial neoplasia III lesions appear to contain HPV 16 more frequently than lower grades of AIN, with a similar relationship existing in cervical and vulval disease. A number of studies in gynecological and genitourinary medicine have shown that once one area of the anogenital epithelium becomes infected by human papillo- maviruses, the adjacent areas become infected by direct spread of the papil- lomavirus, creating a large potential field of epithelium susceptible to neoplastic change (41). In many of these cases, however, the papillomavirus infection may be asymptomatic and only detectable by colposcopic exami- nation or cytological sampling. Epidemiological studies of sexually trans- missible papillomavirus infection in Scandinavia have suggested that up to 70% of the sexually active adult population have at some time encountered genital HPV infection, and that the vast majority of these infections are completely asymptomatic (42). Although it is cited frequently that anal intercourse is a prerequisite in the pathogenesis of anal condylomata or AIN or anal SCC, this concept is essentially unfounded in the heterosexual population.
Despite the strong causal relationship between HPV and anogenital neo- plasia, it is unlikely that HPV on its own is sufficient for the production and maintenance of the neoplastic state. It is possible that HPV acts in a syn- ergistic fashion with other factors, including viruses such as HIV, herpes simplex virus 2 (HSV-2), Epstein-Barr virus (EBV), and cytomegalovirus (CMV). Herpes simplex virus 2 has long been suggested as a co-factor in the etiology of cervical cancer (43), and among individuals without genital warts, a history of seropositivity for HSV-2 has been found to be associated with anal cancer. In the only such published study to date, Palefsky et al.
(44), using the polymerase chain reaction, detected HSV-2 deoxyribonu- cleic acid (DNA) in anal cancer tissues of five of 13 patients and three of four patients with AIN III. This association between HSV-2 DNA detec- tion and high-grade anal disease suggests that HSV-2 infection could play a role in the pathogenesis of AIN and anal cancer.Although EBV and CMV have been detected in cervical epithelium (normal and neoplastic), similar data on anal neoplasia have yet to be reported. With regard to the rela- tionship between HIV and HPV, a number of reports suggest that HIV infection increases the risk of HPV infection and HPV related anogenital neoplasia (34,36,38). However, in a recent study, Surawicz et al. (45) observed no differences in the prevalence of anal HPV infection and AIN in immunosuppressed HIV-positive men compared with immunocompe- tent HIV-negative men.
Other factors that may be involved in the pathogenesis of anogenital neo- plasia in general and anal neoplasia in particular include smoking and the interaction between HPV, oncogenes, and tumor-suppressor genes.As in the cervix, recent epidemiological studies have shown a significant increase in the risk of anal neoplasia related to heavy or prolonged smoking (34,46,47);
however, the nature of this predisposition is unknown. Furthermore, there
have been numerous reports on the possible interactions of HPV with the p53 tumor-suppressor gene, as well as the c-myc oncogene in the patho- genesis of anogenital neoplasia (48–51). Some of these reports suggest that the expression of the products of these genes may act as prognostic indi- cators of disease progression (52,53). This issue is complex, for example, in the relationship between the oncogene c-myc and AIN. It is expected that proto-oncogenic growth factors are required for the development of inva- sive anogenital malignancy, but the c-myc oncogene product, which is over expressed in CIN and which is partially integrated with HPV gene sequences close to its locus in some cervical cancer lines, does not appear to play a major role in AIN progression (54). This is despite in vitro studies showing higher c-myc expression in HPV 16-transformed cell lines and dif- ferentiation resistance in HPV-immortalized keratinocytes over-expressing c-myc protein and c-myc gene sequences (55).
The association between neoplasia and systemic immunosuppression is well established. Studies based primarily on organ-transplant recipients have demonstrated an overall 5% to 6% risk of developing a neoplasm after transplantation (56–58). These tumors—lymphoid system, skin, uro- genital and anogenital tracts—are etiologically associated with oncogenic viruses, therefore non-Hodgkin’s lymphoma with EBV, squamous tumors of the skin and anogenital tracts with HPV, and primary liver cancer with the Hepatitis B virus. Although the nature of this predisposition is unknown, immunosuppression in this group may reduce immunosurveil- lance by the lymphoreticular system (59). Reports from Cancer Registry data and controlled studies suggest that transplant patients show a four- to 14-fold increase in the incidence and prevalence of HPV-associated high- grade intraepithelial and invasive tumors of the cervix (57,58,60,61). Reg- istry data also suggest a similar increase for the prevalence of invasive anal and vulval neoplasia. A prevalence rate of 24% (32 of 133) for anal HPV infection and associated AIN in renal transplant recipients has been reported (19,20). Of these, 18% (6 of 32) of patients had prior or synchro- nous high-grade intraepithelial or invasive genital lesions. Furthermore, a number of published reports on the prevalence of AIN in immunosup- pressed HIV-positive patients also have demonstrated that a high pre- valence of anal HPV infection and AIN is related to the degree of immunosuppression as determined by T-helper/T-suppressor cell ratios (34,35–38,62).
Systemic immunosuppression, however, does not appear to be a prereq- uisite for the development of multifocal HPV-associated neoplasia. In clin- ical studies on apparently immunocompetent women who had high-grade cervical or vulvar intraepithelial neoplasia or invasive vulval cancer, preva- lence rates of 20%, 19%, and 47%, respectively, for AIN were reported (6,20). It is possible that HPV infection of the anogenital area causes changes in the local cellular immunity that facilitate progression to neo- plastic change. Some evidence for this is available from a few studies on the
cervix, which have shown a reduction in local T-lymphocyte subset numbers and activity in women with CIN (63,64). In immunocompetent homosexual men—in whom prevalence rates for AIN of up to 8% have been reported—
it has been suggested that there may be subclinical immunosuppression, possibly due to the local immunosuppressive effects of seminal fluid, known to be rich in prostaglandins capabale of enhancing tumor promotion (34,65,66). Such immunosuppression might make homosexual men more susceptible to the development of anal HPV infection and neoplasia.
Although this hypothesis may have some validity, it is equally possible that, since some homosexual men engage in large numbers of sexual encounters, they are at high risk of developing sexually transmitted diseases such as syphilis and HIV, which also may permit de novo infection or reactivation of dormant anal HPV infection. The epithelial abnormalities that subse- quently occur may represent a consequence of actively replicating HPV with independent contributions from local or systemic immunosuppression, smoking, and other co-factors (67).
Diagnostic Techniques for AIN
The diagnostic techniques that may be used to identify AIN include histopathology, cytopathology, and “colposcopy.”
Histopathology remains the “gold standard” of diagnosis, but obtaining material for histopathological examination is invasive and uncomfortable for the patient, as biopsy of the anal canal and perianal area can only be performed under anesthesia. The histopathological features of AIN have already been described above and are generally robust (68) (see Chapter 5.1).
Cytopathological examination is minimally invasive, requires no anes- thesia, and is capable of repeatedly sampling a large area. Until recently, the application of this technique to the anus has yielded rather inconsistent results. Colposcopic examination of the anus is more invasive than cytopathology, although it does not require anesthesia and it can examine the entire anal canal. Anal colposcopy has a definite learning curve and requires considerable experience. Any abnormal areas must be biopsied for histopathological examination to confirm the diagnosis. Anal colposcopy has been used extensively in our practice to identify anal dysplastic lesions (69).
Cytology
The increasing clinical awareness of AIN and its association with genital intraepithelial neoplasia has provided an impetus for the development of a simple method of examining the malignant potential of the anal epithelium.
Anal cytology offers two main advantages over anal colposcopy—specimen collection is simple and sampling from a large area is possible. As such, a cytological test could play a major role in screening patients in high-risk groups for anal dysplasia.
Several studies have used anal cytology for the diagnosis of anal dyspla- sia. These studies reported a number of technical difficulties in obtaining adequate samples from the anus. Inadequate specimens were encountered in 15% to 30% of samples in published studies (70); however, there is a general trend in the manner of collection of anal cytological specimens that is producing results that are more consistent. Most early studies tried to sample the entire anal canal in every cytological preparation, and it was believed that, unless transitional type cells could be identified, the prepa- ration should be labeled as inadequate. However, trying to sample the whole anal canal and anal transitional zone is uncomfortable for the patient, as no lubricant can be used on the collection device—as this reduces cell yield dramatically—and fecal contamination of the preparation is almost inevitable. Because the vast majority of anal intraepithelial lesions occur on or near the anal verge—where AIN rarely involves the anal transitional zone in the absence of perianal skin involvement—sampling the perianal area is appropriate in most cases (Figure 5.2.1). This area is also easily accessed, reduces fecal contamination, and cell collection does not cause discomfort to the patient. In the author’s opinion, it is inappropriate to insist on sampling the anal transitional zone, particularly as attempting this greatly increases the likelihood of severe fecal contamination of the speci- men, thus rendering adequate cytopathological examination impossible.
The use of guidelines to provide a framework for the reporting of anal cyto- logical has provided a sound basis for the interpretation of anal cytological
Figure 5.2.1. Cytological appearances of the normal anal canal.
preparations with consistent results among experienced cytopathologists (69). Further developments along these lines may make anal cytology a useful test in both the diagnosis and follow-up of patients “at risk” for anal dysplasia.
The technique of preparation of anal cytological smears is similar to that used for the cervix and vulva, and it is quick and easy to perform after initial instruction. The author’s preferred technique is to use a small cotton swab moistened in 0.9% saline and then rubbed firmly against the perianal skin for 20 to 30 seconds, sampling a wide area of the perianal skin. The col- lected cells then are transferred to a labeled glass microscope slide and either air-dried of fixed according to the local cytopathologist’s preference.
If the cotton swab appears to be fecally contaminated, a second swab should be taken after wiping the perianal area to reduce the fecal load as much as possible.
It seems likely that the use of anal cytology probably will reduce the need for anal colposcopy, which has proved unpopular with both gynecologists and surgeons alike.
Anal Colposcopy Technique
In the author’s practice, patients usually are examined in a modified litho- tomy position, as this permits examination of the whole anogenital area without having to change the patient’s position. Genital colposcopy usually is performed prior to anal colposcopy—genital colposcopy for female patients is an essential part of the assessment of these patients, but does not necessarily need to be a part of the anal colposcopy procedure. Colorectal surgeons using colposcopy should be familiar with genital colposcopy, and the classical saline and topical acetic acid techniques in anal colposcopy use essentially the same techniques. A combined clinic with a gynecological col- poscopist is a good way of developing the necessary colposcopic skills.
Anal colposcopic examination as performed in the lithotomy position begins by parting the buttocks to examine the perianal skin. Firm traction on the perianal skin also permits examination of the lower anal canal. In general, relatively low magnification is required (6¥ is adequate). It is useful to begin anal colposcopy by examination of the perianal skin and lower anal canal before the application of 5% aqueous acetic acid. The perianal skin then is covered with swabs or cotton wool balls soaked in acetic acid for three minutes prior to further examination. Then the anal canal is exam- ined using an oblique viewing Graham Anderson proctoscope, before and after application of aqueous acetic acid spray. Using this instrument, views of the upper anal canal, the transition zone, and the lower rectal mucosa are readily obtained. Spraying the anal canal with acetic acid is adequate, as the epithelium in the anal canal is less heavily keratinized than the peri- anal skin and therefore responds rapidly to the acetic acid spray.
The appearance of the anal transition zone is similar to that of the cer- vical transformation zone in its natural state and takes on a pearly appear- ance when sprayed with acetic acid. Gland openings in the anal transition zone are analogous to those seen in the cervix (Figure 5.2.2).
Colposcopic Identification of AIN
The clinical diagnosis of AIN using colposcopy is still evolving. The range of abnormalities that may occur includes papillomavirus-associated lesions occurring throughout the anal area, paralleling those seen in the cervix and vulva (5,6,69).
Normal Colposcopic Appearances of the Perianal Skin and Anal Canal
Since the boundaries of the anal canal and perianal epithelium are clini- cally and histologically poorly defined, these terms are used in their broad- est sense, where the perianal skin is that which can readily be seen on parting the buttocks. The anal canal epithelium is that epithelium above this level and is limited above by the rectal epithelium, where this area is best examined using a proctoscope. As stated above, the author’s preference is for the oblique-ended Graham Anderson pattern proctoscope for colposcopy.
Normal perianal skin is unremarkable on colposcopy, both before and after the application of acetic acid. Immature normal squamous epithelium Figure 5.2.2. Colposcopic appearance of the normal cervical (left) and anal (right) transition zones after topical application of acetic acid.
tends to “acetowhiten,” necessitating careful interpretation of anal col- poscopy in patients who recently have undergone any anal surgery or who have anal symptoms such as pruritis ani.
The colposcopic features of AIN are described below.
Low Grade AIN and HPV Infection
The clinical features of low-grade AIN on the perianal skin are more diffi- cult to diagnose accurately than those in the anal canal. The keratinization of the perianal epithelium results in dense acetowhite change, obscuring vessel patterns and making differentiation of HPV infection from low- grade dysplasia difficult. In the anal canal, however, the vessel patterns are more obvious; hence, the diagnosis of low-grade dysplasia is more reliable than on the perianal skin. In the author’s experience, less than 10% patients with subclinical papillomaviral (SPI) or AIN I had anal canal disease in the absence of perianal SPI or AIN.
High Grade Anal Intraepithelial Neoplasia (AIN III)
High-grade perianal intraepithelial neoplasia (Figure 5.2.3) may be appar- ent on naked eye inspection as patchy, irregular pigmentation of the peri- anal skin. In the absence of pigmentation, a further clue to high-grade AIN may be thickening of the skin. Such areas may be single or multiple and may extend cephalad to the rectal mucosa or caudad onto the buttocks and thighs.
The normal perianal skin is heavily keratinized and responds slowly to acetic acid “soaks,” where hyperkeratotic perianal epithelium becomes ace- towhite very slowly. Capillary patterns are unusual on the perianal epithe-
Figure 5.2.3. Characteristic colposcopic appearances of high-grade AIN (see text).
lium, even in high-grade disease, differentiating the technique from cervi- cal colposcopy. In 3 of 6 cases in which perianal skin mosaicism was noted colposcopically (Figure 5.2.3), the histology was reported as early invasive squamous carcinoma in that area, suggesting that mosaicism in the perianal skin may be a marker of progression to invasive disease.
In the anal canal, dysplastic epithelium may appear hyperemic on exam- ination prior to the application of acetic acid. However, such areas become more obvious after the application of acetic acid spray. The colposcopic appearances of anal canal intraepithelial neoplasia are similar to those seen on the cervix, namely acetowhite areas with punctation and mosaic vessel patterns. In keeping with the lack of a clear anatomical distinction between the anal canal and perianal skin, the colposcopic features of papillomavirus infection and dysplasia in the lower anal canal and perianal skin merge into each other.
In the author’s experience, it is difficult to distinguish colposcopically invasive lesions from high-grade AIN.
Therapeutic Options
Possible treatment options for AIN III lesions include simple excision, exci- sion with grafting, laser ablation, cryoablation, the application of cytotoxic creams (such as 5-fluorouracil), or the use of newer disease modifiers such as the retinoids and Imiquimod. Skinner et al. (71) reported the frequent occurrence of involvement of hair shafts and sweat glands in AIN III lesions and suggested that these may not be destroyed by ablative techniques. Sur- gical excision also has the great advantage of providing a specimen for com- plete histological assessment.
For large areas of AIN III involving more than 50% of the anal margin or anal canal, the risk of anal stenosis following excision is a major concern.
Anal stenosis may be prevented by the application of split skin grafts to the anal canal and margin at the time of excision of the lesion. Scholefield et al. (72) reported their experience in local excision of AIN III lesions involv- ing less than 50% of the anal circumference from 9 patients without any sequelae. An additional 5 patients with circumferential AIN III extending from the perianal skin to the dentate line underwent excision of the affected epithelium and split skin grafting. In these patients, a defunctioning sigmoid loop colostomy was performed some weeks prior to anal skin excision and was rewarded by improved survival of the skin grafts. Treatment of this type necessitates multiple surgical procedures with an inherent morbidity;
such treatment can only be justifiable for a lesion with a high malignant potential.
Interestingly, treatment for invasive anal SCC is moving away from sur- gical intervention, sparing the patient a colostomy. This trend towards non- surgical treatment for invasive disease adds to the difficulties in managing
AIN III, where an observational policy for AIN III has many attractions, particularly as the malignant potential of AIN III is uncertain and AIN lesions are often asymptomatic. A policy of observation requires repeated examination, and because patients find anal colposcopy unpleas- ant, compliance is a major problem in such a policy (twice yearly as a minimum seems reasonable). An observational policy of this type has become our practice in Nottingham over the last 8 years. While it is too early to determine whether this less aggressive approach will prove effec- tive, our early results show only 2 invasive carcinomas developing in a group of 26 patients over eight years. One of the 2 patients whose AIN progressed to invasive disease was immunosuppressed for systemic lupus for over 5 years.
The retinoids and Imiquimod have been used to treat skin dysplasia and have been shown capable of impressive remission; however, once treatment ceases, there is a very high relapse rate. In addition, the retinoids have severe and unpleasant side effects, such as photosensitivity, liver dysfunc- tion, and gastrointestinal disturbances. Their use needs to be supervised by a dermatologist experienced in their use. While they are interesting devel- opments, treatments with these agents cannot be recommended for the treatment of AIN at present (73).
A Recommended Therapeutic Strategy
Low Grade AIN (AIN I and II)
In common with current gynecological practice, AIN I and II probably have minimal malignant potential, and therefore do not require definitive treat- ment. Biopsy of the visibly most abnormal area is essential for diagnosis with 6 monthly follow-ups for one year, with repeated colposcopic or cyto- logical examination. After one year, if the worst affected area has not pro- gressed, the patient is discharged from follow-up with a warning reminder to re-attend if the appearance of the lesions changes or if bleeding occurs.
Expectant treatment of low-grade AIN lesions seems appropriate for two reasons. Firstly, mild dysplasia is notoriously difficult to diagnose consis- tently and resolution of AIN I or II to subclinical papillomavirus infection may occur and therefore must be interpreted with some caution (72). Sec- ondly, many AIN I and AIN II lesions are likely to be due to HPV types other than HPV 16, and therefore are much less likely to possess inherent malignant potential.
High Grade AIN (AIN III)
Excision of AIN III lesions parallels the policy adopted by many gynecol- ogists for VIN III and CIN III along the lines of those described by Reynolds et al. (74), Kaplan et al. (75), and Schlaerth et al. (76). In the
Sheffield experience where the lesion occupied less than 50% of the area of the anal margin or canal, local excision with a margin of non-dysplastic tissue was performed. In those patients with extensive perianal and anal canal disease extending circumferentially and cephalad to the DL, complete excision of the affected epithelium with a margin required resurfacing of the anal canal and perianal area with split skin grafts taken from the medial aspect of the thigh. A sigmoid loop colostomy was performed several weeks before the skin grafting procedure. The loop colostomy was closed approx- imately two months after the grafting procedure.
The trend towards non-surgical treatment of invasive anal squamous cancer (77,78) and the lack of information concerning the rate of progres- sion of AIN III to invasive cancer may lead to over treatment of AIN III;
however, it may be difficult to exclude occult invasive disease in an area of AIN III, and histopathological assessment of the whole of such a high grade lesion is therefore desirable. Interestingly, by comparison, the incidence of occult invasion in extensive high-grade VIN treated by wide excision is only 7% to 10% (79).
Those who argue in favor of not treating AIN III do so on the basis that anal cancer is rare and that anal cancers may be treated non-surgically, avoiding the need for a stoma. Although OPCS figures do not show an increase in the incidence of anal cancer (7), the data may be incomplete due to the multitude of histological types reported and difficulties in defin- ing the morphology of the anal canal. Recent epidemiological data show that there is an increasing incidence of anal cancer over the last ten years, particularly in women (29)—a factor that may affect this conservative view.
In order to answer these uncertainties, further longitudinal information is required. A multicenter study could randomize patients with AIN III lesions to treatment or observation only, and although such a study would be costly given the rarity of invasive anal cancer, such a trial seems unlikely to be conducted at the present time.
Therapeutic Dilemmas
The uncertainties surrounding the natural history of AIN III lead to diffi- cult management decisions when faced with patients who have extensive areas of AIN III and appear to be at high risk of malignant progression.
Such patients are best managed by examination under anesthetic with multiple colposcopically guided punch biopsies, approximately 4 millime- ters in diameter (a disposable corneal punch is ideal for this purpose), taken from each quadrant of the anal margin and the lower and upper anal canal, with the site of each biopsy carefully recorded. In so doing, the extent of the AIN can be mapped histologically. Where facilities for, or expertise in, anal colposcopy are lacking, it is even more important to take a large number of biopsies to thoroughly assess the extent of the disease. Local-
ized segments of AIN III then can be excised, and where there are several adjacent lesions, several local excisional procedures with intraoperative mapping several months apart may allow healing of the previous sites of excision with healthy skin and thereby reduce the risk of anal stenosis. In patients with pan-anal disease, excisional treatment without grafting is likely to result in a severe anal stenosis, and thus is best managed by obser- vation with a low threshold for biopsy of any lesion that appears to undergo change. Access to an experienced gynecological cytopathologist may reduce the need for repeated anal colposcopy, but a low threshold for biopsy is still recommended. Excision and split skin grafting with a defunctioning loop colostomy is probably best reserved for those with pan-anal disease with a micro-invasive or small invasive squamous carcinoma.
Conclusion
The natural history of AIN is at present unclear, although high-risk groups—with or without HIV-related disease and possibly with high-grade CIN who are oncogenic HPV-positive with high-risk HPV subtypes—
should be regularly surveyed by “new” adapted cytologic and colposcopic techniques for the specific assessment of anal diseases using technology that has been translated from gynecologic practice. Such an approach requires the accrual of a multidisciplinary team experienced in anal “colposcopy”
and anal cytopathological and histopathological assessment of this very spe- cialized area.
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