Anal and Perianal Masses: The Common, the Uncommon, and the Rare.

A stepwise algorithmic approach to diagnose anal and perianal lesions based on imaging is highlighted in Fig. 15 . Diagnostic algorithm for an imaging-based diagnosis of lesions in and around the anal canal.

Mesenchymal tumors rarely can develop in the ischioanal fossa, arising from vascular, neural, muscular, fibrous, and fatty elements. 38 Table 4 shows a simple classification of the ischioanal fossa tumors based on tissue of origin. Among them aggressive angiomyxoma is the most common primary tumor of ischioanal fossa and is discussed below. Lipomas and well-differentiated liposarcomas have predominant fatty components and can be diagnosed easily on imaging. 38 Most other tumors have nonspecific MRI features and may need biopsy for a definitive diagnosis. 38 Abbreviations: PEComa, perivascular epithelioid cell tumor. An aggressive angiomyxoma, although rare, is the most common tumor of the ischioanal fossa and has a predilection for females of reproductive age. 38 These tumors present as a painless mass that can sometimes cause local pressure on adjacent structures. They spread in multiple compartments without invading adjacent organs. 39 MRI features are very typical ( Fig. 10 ) and include iso- to hypointense signal intensity relative to the muscle on T1WI, and a characteristic “laminated” pattern on T2WI due to high signal intensity background myxoid stroma with low signal intensity parallel lines representing collagen fibrils. The lesion shows a heterogeneous postcontrast enhancement. 9 Aggressive angiomyxoma. T2-weighted image (T2WI) in ( A ) sagittal plane, ( B ) short tau inversion recovery (STIR) image in coronal plane, and ( C ) T1 image in axial plane show T2/STIR markedly hyperintense lobulated mass in the anal and perianal region with a characteristic laminated appearance. The lesion is hypointense on T1WI. ( D ) Hematoxylin and eosin–stained histology microphotograph, of original magnification 40x, shows myxoid stroma with scattered stellate to spindle cells. The treatment of choice is wide surgical resection; however, due to infiltrative growth and poor circumscription, it is difficult to excise this tumor completely, resulting in high risk of local recurrences. 39 Congenital cysts, which can occur in the ischioanal fossa, include tailgut cyst, epidermoid cyst, and dermoid cysts. Differentiation among them may be possible based on imaging. 40 Tailgut cysts are usually multicystic lesions with the locules having variable intensity due to the presence of mucin and blood ( Fig. 11 ). 40 The signal intensity of mucinous fluid is variable and depends on protein concentration. As protein concentration increases, T1 signal intensity changes from hypointense to hyperintense, while T2 signal intensity shows inverse trend from hyperintense to hypointense signals. Tailgut cysts can be complicated by infection and on rare occasions by malignant transformation. Malignant transformation is suspected when there is focal irregular wall thickening with enhancement. 40 Developmental cysts. ( A ) Tailgut cysts are multiloculated cysts in the retro-anal location, best demonstrated on T2 image. ( B ) The high protein content within the cyst can be seen as T1 fat-suppressed sequence (T1FS) hyperintensity. ( C ) Hematoxylin and eosin–stained histology microphotograph, of original magnification 40x, showing tailgut cyst lined by a columnar epithelium ( arrowhead ) and squamous epithelium ( arrow ). ( D ) Epidermoid cysts are unilocular on T2 images with the presence of T2 hypointense spheres within it representing keratin debris. Epidermoid and dermoid cysts are often unilocular cysts lined by a stratified squamous epithelium. 40 Epidermoid cysts contain a mixture of desquamated debris, cholesterol, keratin, and water, but it lacks the skin appendages such as sweat glands, hair follicles, and sebaceous glands, which are seen in dermoid cysts. On MRI, epidermoid cysts are unilocular thin-walled cysts with low signal intensity on T1WI and high signal intensity on T2WI, and may contain multiple T2 hypointense debris representing aggregates of keratin ( Fig. 11 ). 38 Dermoid cysts are similar to epidermoid cysts except that they can have fat components within. Loss of signal intensity in these cysts on out-phase images as opposed to in-phase images confirms their fatty nature. 40 These cysts, if symptomatic, may need surgical excision.

Bowen's disease is an intraepithelial SCC that occurs most frequently on the face, hands, and trunk, and less commonly in the perianal region. 41 In the perianal region, it probably represents severe anal intraepithelial neoplasia (AIN) and is associated with HPV-16 and HPV-18 infections. 40 Like anal canal SCCs, Bowen's disease is much more common in patients with HIV. 41 In 2 to 6% of cases, the disease progress to an invasive SCC. Bowen's disease has the highest incidence during the sixth and seventh decades of life. 41 Common symptoms include itching, burning, and occasionally bleeding, and on examination, the lesion appears as a well-defined erythematous, scaly plaque. 41 Diagnosis is made by obtaining several full-thickness biopsy from the central portion and edges of the lesion. Imaging is generally not done. The treatment of choice is wide local excision. 41 Perianal Paget's disease is quite an uncommon intraepithelial adenocarcinoma arising from the dermal apocrine sweat glands and can be associated with underlying apocrine or eccrine carcinoma, colorectal adenocarcinoma, and anal carcinoma in 38 to 69% of patients. 41 It is most commonly found in older patients, averaging 66 years, and shows a preponderance for women. The lesions are erythematous and crusty, eczematoid, or scaly appearing, and present with nonspecific symptoms like pain, bleeding, and itching. 41 Diagnosis is made with full-thickness biopsies of the affected skin. FDG-PET/CT or MRI is done before treatment to exclude associated underlying malignancy. 41 If the disease appears to be locally confined on preoperative workup and is noninvasive on biopsy, wide local excision is the treatment of choice. When the tumor is invasive or located close to the anal canal, and sufficient surgical margin cannot be achieved, a combined modality treatment with chemoradiation may be needed. 41 If underlying malignancy is identified, it will also require appropriate treatment. Perianal SCCs are at least five times less common than cancers of the anal canal and have a more favorable prognosis as they are typically well differentiated, slow growing, and without distant metastases. 41 The risk factors for perianal SCC are the same as for SCCs of the anal canal. 41 Most patients present after the fifth decade with an approximately equal male-to-female predominance. 41 Symptoms of perianal SCC are nonspecific and include bleeding, pain, discharge, and pruritus. On examination, they are usually hard, raised, and ulcerated skin lesions with rolled, everted edges. 41 The anal canal can become involved late in the disease, although the sphincter complex is rarely invaded. A VC is a rare histologic subtype of SCC characterized by highly differentiated squamous cells and appears as a large locally aggressive tumor with a cauliflowerlike appearance. 42 It is often associated and confused with a giant condyloma acuminatum (GCA), also called Buschke–Löwenstein tumor due to similar clinical and imaging appearance. A GCA has been linked to HPV, whereas a VC is negative for HPV. 42 On histological evaluation, VCs have no koilocytic atypia, whereas GCAs have koilocytic atypia. 42 On the other hand, distinguishing VCs from conventional SCCs is also important because, although large and in some cases locally aggressive, VCs are often amenable to wide local resection since nodal metastases are extremely rare. However, there are published reports showing that VCs and conventional SCCs can coexist in the same lesion. 41 Any patient with a suspicious lesion needs biopsy for definitive diagnosis. Pretreatment evaluation should include a full staging workup with imaging. 41 MRI of the pelvis can demonstrate the extent of lesion, anal canal involvement, and regional nodes ( Fig. 12 ). VCs on MRI appear as T2 hyperintense cauliflowerlike lesion from the perianal skin ( Fig. 13 ). 43 Perianal squamous cell carcinoma (SCC). T2- weighted image (T2WI) in ( A ) sagittal, ( B ) axial, and ( C ) coronal planes show T2 intermediate signal thickening in the posterior perianal skin, which reaches up to the anal verge ( arrowhead in A ). The diagnosis was done by biopsy and magnetic resonance imaging (MRI) was done as part of staging workup to evaluate anal canal involvement and to look for regional nodes. ( D ) Hematoxylin and eosin–stained histology microphotograph, of original magnification 40x, shows marked keratinization and minimal nuclear pleomorphism in a well-differentiated SCC. Verrucous carcinoma at perianal region. T2-weighted image (T2WI) in ( A ) sagittal and ( B ) coronal planes and ( C ) short tau inversion recovery (STIR) image in axial plane shows a cauliflower like T2/STIR hyperintense mass in the perianal region with infiltration of the superficial and subcutaneous bundles of external anal sphincter ( arrowhead ). ( D ) Hematoxylin and eosin–stained histology microphotograph, of original magnification 40x, shows massive hyperkeratosis and parakeratosis. Perianal SCCs are generally treated as skin cancers with wide local excision. 41 Lesions involving the anal canal needs combined modality of chemoradiation as initial treatment approach followed by salvage surgery if recurrent or residual disease is present. 41 VCs are also treated with wide local excision; however, high recurrence rates (as high as 66%) after excision have prompted the use of neoadjuvant or adjuvant chemoradiation therapy. 41

Secondary tumors arising in the vagina, vulva, urethra, and pelvic musculoskeletal system can involve the anal canal. Cancer of the vagina, if it involves the mucosa of the anorectum, is considered T4 according to the eighth edition of the TNM (2018) classification and stage IVa on the International Federation of Gynecology and Obstetrics (FIGO) staging. 8 Vulval cancer with extension to the anal canal is T2 according to the eighth edition of the TNM (2018) staging and stage II according to FIGO stage. 8 Urethral cancer (in both males and females) if fixed or invading the sphincter of the anal canal is T4 according to the eighth edition of the TNM (2018) staging. 8

Hemorrhoids are vascular cushions surrounding the anastomoses between the superior rectal artery and the superior, middle, and inferior rectal veins. 44 Internal hemorrhoids arise above the dentate line and external hemorrhoids arise below the dentate line. External hemorrhoids are innervated by somatic nerves and cause pain, whereas internal hemorrhoids are innervated by visceral nerve fibers and are painless. Most hemorrhoids are a combined type of internal and external hemorrhoids. 44 Hemorrhoidal disease is commonly diagnosed on clinical examination but can also be seen on MRI. On T2WI, hemorrhoids are seen as moderately hyperintense polypoidal lesion. They may also show increased signal on T1WI ( Fig. 14 ). 45 Tumorlike lesions. Rectal bleeding is the most common presentation in vascular malformation (VM) and can mimic neoplasms. Colonoscopy in such cases may reveal nodular submucosal tumorlike lesion in the anorectum with normal or edematous overlying mucosa. 46 They can also be divided into high- and low-flow lesions. 46 High-flow lesions should have an arterial component. MRI can help in diagnosing VM and help in defining its extent. 46 Low-flow malformations have cluster of cystic spaces with or without fluid debris layer ( Fig. 14 ), whereas high-flow malformations contain signal voids. On dynamic postcontrast MRI, high-flow lesions show rapid filling, whereas low-flow lesions show late filling. Calcification and phleboliths can be mistaken as flow void; however, they appear as focal signal void and are not tortuous as flow void. In case of doubt in differentiating them on MRI, a noncontrast CT can be done to identify calcification. Perianal endometriosis is extremely rare. It usually occurs in childbearing women following obstetric or gynecological procedures most commonly following delivery at episiotomy scar. 47 A patient typically presents with cyclical pain. Many times, the diagnosis may not be made on physical examination, and MRI may be needed. 48 On MRI ( Fig. 14 ), endometriosis manifests as a nodular or plaquelike lesion with intermediate signal intensity on T1WI and low signal intensity on T2WI. Small T2 and T1 hyperintense foci corresponding to endometrial glands and hemorrhagic foci are almost always recognized within the endometriotic lesions. 48 A typical fistula-in-ano is mostly an idiopathic process thought to be caused by obstruction of anal glands and its diagnosis via an MRI is straightforward due to the presence of well-defined fluid contained tracts. Perianal infection in immunosuppressed patients with hematological malignancy is atypical on MRI as they have extensive anal and perianal edema and rarely have defined fistula tracts, creating confusing imaging appearance ( Fig. 14 ). 32 Common infections of the anorectum includes nonsexually transmitted infections due to Escherichia coli , Shigella spp., Campylobacter spp., and Clostridium difficile , and sexually transmitted infections due to Neisseria gonorrhoeae , Chlamydia trachomatis , Treponema pallidum , and herpes simplex virus (HSV). 49 Most of these infections do no need imaging for diagnosis and if done may show nonspecific anorectal inflammation. A fistula-in-ano is commonly complicated by abscesses and ramifications. Atypical findings of solid enhancing masslike lesions with no fluid component can represent inflammatory phlegmon in partly treated fistulas and can mimic a malignancy developing in chronic fistulas ( Fig. 14 ). 50 If differentiating them is not possible on imaging, tissue sampling needs to be done to confirm the difference.

In this article, we showed a plethora of pathological conditions in the anal and perianal region. MRI is the modality of choice for assessing the extent of anal and perianal lesions. Some tumors have characteristic imaging findings that may permit accurate diagnosis; however, tissue sampling is often needed to confirm the diagnosis. Accurate diagnosis needs good knowledge of anal and perianal anatomy as well as knowledge of the spectrum of imaging findings of common and uncommon neoplasms.

The anal canal has a mixture of different coexisting types of cells and tissues, which predispose to the development of a wide variety of neoplasms. Squamous cell carcinomas (SCCs) are the most common anal canal carcinomas, accounting for 80% of all anal canal carcinomas. 1 The less common anal canal malignancies include adenocarcinoma and melanoma, which account for 15 and 4% cases, respectively. Lymphoma, neuroendocrine tumors (NETs), and mesenchymal neoplasms are even rarer in the anal canal. Tumors originating from the adjacent structures and organs can also secondarily involve the anal canal. These include lesions arising in the ischioanal fossa, vagina, vulva, urethra, and pelvic musculoskeletal system. The perianal region refers to the skin surrounding a perimeter of 5 cm around the anal verge. 2 Tumors of the perianal region are at least five times less common than tumors of the anal canal. 1 Any cancer occurring in the skin elsewhere can occur in this region, with SCC being the most common. Less common neoplasms of perianal skin include verrucous carcinoma (VC), giant condyloma acuminatum (Buschke–Löwenstein tumor), basal cell carcinoma, Bowen's disease, and Paget's disease. Although anal and perianal neoplasms can be clinically evaluated and biopsied easily, imaging with magnetic resonance imaging (MRI) is done to define the anatomical origin and the internal characteristics of the mass, and for staging. 2 Through this pictorial review, we demonstrate the anal canal and perianal anatomy on MRI and exhibit the imaging spectrum of tumors and tumorlike lesions in these regions. We also review the radiologist's role in staging and treatment planning of common neoplasms. MR imaging performed with phased array coils provides excellent anatomical detail of the anal sphincters and the anatomical boundaries of the pelvis. High-resolution (HR) T2-weighted imaging (T2WI) in three orthogonal planes are the key sequences for knowing the extent of the lesion and proper assessment of the anal sphincter complex. 3 These are obtained with a slice thickness of 3 to 3.5 mm, with a small field of view (FOV) of 18 to 20 cm and high matrix (e.g., 320 × 256). Because of the forward tilt of the anal canal, the axial and coronal planes for HR imaging should be aligned orthogonal and parallel to the long axis of the anal canal, respectively ( Fig. 1 ). Large FOV (covering the whole of the pelvis) T1 weighted imaging (T1WI), T1 fat-suppressed imaging, short tau inversion recovery (STIR) imaging, and diffusion weighted imaging (DWI) are other useful imaging sequences for disease characterization, determination of the extent of disease, and nodal spread. Intravenous gadolinium contrast may not be needed in most cases but can be used in selected cases to characterize uncommon lesions. ( A ) Sagittal image of the anal canal demonstrating the planes to obtain axial ( yellow dashed line ) and coronal ( red dashed line ) T2 high-resolution images of the anal canal. ( B,C ) Magnetic resonance imaging (MRI) of the anatomy of the anal canal in coronal plane. The anal region includes the anal canal, anal verge (anus), and perianal skin ( Fig. 1 ). The anal canal is the terminal part of the intestine that lies between the rectum above and the anal verge below. 2 The anorectal junction is at the superior limit of the puborectalis muscle, which forms a U -shaped sling around the superior portion of the anal canal. The anal verge is a band of squamous epithelial tissue that lacks hair follicles. 2 The anal canal has the following layers from inside-out: the mucosa, submucosa, and the sphincter muscles. An important landmark of the anal canal mucosa is the dentate line, which marks the junction of the endoderm-derived upper two-thirds and ectoderm-derived lower one-third of the anal canal. 2 The lining epithelium, the lymphatic and venous drainage, and nerve supply above and below the dentate line are hence different. The lining epithelium in the lower third of the anal canal is nonkeratinized squamous epithelium, that in the upper one-third is the columnar epithelium, and the middle third is a mix of both and is hence called the transitional zone. 2 The difference in lymphatic drainage above and below the dentate line will be seen in nodal spread of anal canal tumors. Tumors above the dentate line drain into the perirectal and internal iliac nodes, whereas tumors below the dentate line and the anal margin drain into the inguinal and femoral lymph nodes. 4 Deep to the anal canal mucosa is the submucosa followed by two muscular layers, namely, the internal anal sphincter (IAS) and the external anal sphincter (EAS). The IAS is an involuntary muscle that is the thickened extension of the circular smooth muscle layer of the rectum. On MRI, IAS appears homogenous and moderately hyperintense on T2WI. 5 The EAS, which is a voluntary muscle, along with the puborectalis sling (PS), forms the outermost layer of the anal canal. The PS surrounds the upper third of the anal canal, while the EAS surrounds the lower two-thirds. According to the traditional description, EAS the is made of three muscle bundles, namely, deep, superficial, and subcutaneous bundles. 2 The deep bundle surrounds the middle two-thirds anal canal. The superficial bundle sweeps around and encircles the lower one-third of the anal canal. The subcutaneous bundle lies inferolateral to the IAS and beneath the anal verge. On MRI, the EAS complex has low signal intensity with “striated” appearance on T2WI. On HR T2WI, it may be possible to distinguish the three components of EAS. The dentate line, although not visible on MRI, is located at the level between the upper one-third and the lower two-thirds anal canal. 5 6 The perianal skin, also known as the anal margin, is the hair-bearing skin surrounding the anal verge for a radius of 5 cm 2 . The ischioanal fossa is a pyramid-shaped space situated on both sides of the anal sphincter complex with its base directed to the surface of the perineum and its apex directed anteromedially toward the pubic symphysis. 2 It is bounded superiorly by the levator ani muscle, medially by the EAS muscles, laterally by the obturator internus muscle and its fascia, and inferiorly by skin of the perineum. The World Health Organization (WHO) classifies tumors of the anal canal broadly into two categories: (1) benign epithelial tumors and precursors and (2) malignant epithelial tumors. 7 They are further divided based on the histologic type. Perianal tumors are classified just like skin tumors elsewhere. Staging of cancers arising from both the anal canal and perianal regions is based on the 8th edition of the American Joint Committee on Cancer (AJCC)/Union for International Cancer Control (UICC) TNM ( t umor size, n ode involvement, and m etastasis status) classification ( Table 1 ). 8 9 The T stage is based on size of the lesion and not based on the depth of invasion. Regional lymph nodes for N staging include inguinal, mesorectal, superior rectal, internal iliac, and external iliac nodes. Abbreviations: AJCC, American Joint Committee on Cancer; TNM, t umor size, n ode involvement, and m etastasis status; UICC, Union for International Cancer Control.