anal carcinoma is increasingly diagnosed in patients with human immunodeficiency virus ( hiv ) , anoreceptive intercourse , chronic inflammatory bowel disease . an established association exists with human papillomavirus ( hpv ) infection and premalignant intra - epithelial dysplasia . phased - array mri is recommended as the preferred imaging modality for regional staging . an uncommon malignancy in the general population , squamocellular anal carcinoma ( scac ) accounts for approximately 1 % of all gastrointestinal neoplasms and less than 5 % of anorectal tumours . in past decades , scac was usually diagnosed at a relatively advanced age with a significant female predominance , and believed to be an indolent disease secondary to chronic irritation . in recent years , similarly to uterine cervix dysplastic changes , oncogenic human papillomavirus ( hpv ) has been detected in the vast majority ( up to 90 % ) of invasive scacs , and linked to the development of low- and high - grade premalignant anal intra - epithelial neoplasms ( ain ) , particularly with high - risk or multiple hpv serotypes infection [ 1 , 2 ] . furthermore , the incidence of scac is steadily increasing , particularly in patients with risk factors such as human immunodeficiency virus ( hiv ) infection , history of anoreceptive intercourse , coexistent cervical dysplasia or cancer , immunosuppression , inflammatory bowel diseases ( ibd ) and cigarette smoking . currently , at least half of scacs occur in relatively young ( 4060  years ) hiv - positive individuals , most often men who have sex with men ( msm ) [ 35 ] . the surgical anus is about 4  cm long from the anorectal junction to the perianal skin on the external anal margin ( verge ) . the internal anal sphincter consisting of smooth muscle is separated from the external , striated muscle sphincter by the fatty intersphincteric space . along with the puborectalis and levator ani muscles , the external anal sphincter forms the sphincter complex . located approximately halfway along the anus , the dentate line marks the transition from the squamous epithelium to the intestinal mucosa . thus , histologically scacs can be either keratinising or non - keratinising according to their origin below or above the dentate line , although with similar biological behaviour [ 6 , 7 ] . anal margin and anal canal scac originating distal to the dentate line drain to the inguinal and femoral lymph nodes . when the primary tumour arises above the dentate line , regional lymph nodes include the inguinal , internal iliac and perirectal nodes , whereas the external , common iliac and para - aortic nodes are considered non - regional [ 68 ] . because of its anatomical location , in most cases scac is diagnosed clinically in patients with rectal bleeding , pain , discharge or palpable masses . alternatively , lesions may be detected during follow - up of high - risk individuals . following physical examination including digital rectal and vaginal examination , ano - proctoscopy and biopsy , imaging is required to evaluate the local extent of the lesion , lymph node involvement , possible invasion of adjacent organs and distant metastases [ 6 , 9 ] . imaging the anal canal and perianal structures may prove technically challenging to perform and interpret . in past years , trans - anal ultrasound ( trus ) and magnetic resonance imaging ( mri ) techniques allowed an accurate assessment of tumour size and depth of mural invasion [ 7 , 10 , 11 ] . unfortunately , in patients with anal lesions , positioning of endoanal sonography probes and mri coils is hampered by pain and stricture . trans - anal imaging combines an excellent spatial detail with a limited field - of - view that prevents panoramic assessment of entire ischiorectal spaces and of regional lymph nodes . furthermore , trus has limited specificity for differentiation of residual tumour versus post - treatment fibrosis [ 1113 ] . currently , mri performed using external phased - array coils on high - magnetic - field scanners is the imaging modality of choice to investigate the anal region . significant advantages of mri include its native multiplanar capability , superior soft - tissue differentiation , biological non - invasiveness and optimal safety profile of gadolinium - based contrast agents . acquisition protocols heavily rely on high - resolution t2-weighted sequences along three planes , with coronal and axial scans planned slightly oblique , respectively parallel and perpendicular to the long axis of the anal canal . despite the increased tumour conspicuity provided by background fat suppression , short - tau inversion recovery ( stir ) sequences are less useful because of limited spatial detail and difficulty to delineate anatomic landmarks . at our centre , t1-weighted sequences including fat suppression in at least one plane are routinely acquired following standard - dose intravenous gadolinium contrast , to allow detection of lesion enhancement . conversely , other authors discourage post - contrast mri acquisitions by stating that enhanced images do not offer additional information to the high soft tissue contrast intrinsic to t2-weighted imaging [ 7 , 10 , 1315 ] . although with limited contrast resolution compared with mri , volumetric multidetector computed tomography ( mdct ) acquisitions including image reformations along arbitrary planes allow visualisation of anorectal abnormalities in their cranio - caudal extent with relationship to key anatomical landmarks such as the sphincter complex [ 1618 ] . although the dentate line is not directly recognisable , its position can be inferred as it corresponds approximately to the upper portion of external sphincter muscles . sensitivity of trus and mri for the identification of scac has been reported to approach 90100 % , with high concordance regarding tumour size , although on a limited number of patients , and more precise results with ultrasound for smaller , superficial tumours . neoplastic tissue in the anal canal has low - to - intermediate t1 signal intensity and positive enhancement after intravenous gadolinium contrast . on t2-weighted and stir sequences , untreated neoplasms display intermediate signal intensity , lower to that of normal ischioanal fat and almost always superior to the internal reference standard represented by uninvolved anal sphincters and gluteal muscles ( figs .  1 , 2 ) . although with limited sensitivity compared with mri , on ct images scac may be detected as solid , enhancing nodules or masses within the anus ( figs .  2 , 3 ) . axial ( a ) and sagittal ( b ) t2-weighted images show 2-cm hyperintense nodule contained within the internal sphincter muscle , intensely enhancing as seen on post - contrast fat - suppressed coronal t1-weighted image ( c ) , consistent with t1 tumour ( arrowheads)fig axial t2-weighted ( a ) , post - contrast fat - suppressed axial ( b ) and coronal ( c ) t1-weighted images , and corresponding enhanced image from body ct ( d ) show a 5.5-cm long ( t3 ) enhancing tumour with infiltration of the left ischioanal fatty space ( arrowheads)fig . post - contrast axial ( a ) and coronal reformatted ( b ) detailed images of the anorectal region identify an unexpected 2-cm right - sided enhancing anal nodule . subsequent clinical and bioptic assessment confirmed poorly symptomatic ulcerated scacfig . 4an elderly , 92-year - old man with previous prostatectomy and kidney failure has unenhanced mri . sagittal t2- ( a ) and axial t1-weighted ( b ) images show 5-cm long solid , inhomogeneous neoplastic tissue ( * ) extending from the anus to encase the proximal urethra ( note catheter in place ) a 40-year - old msm with bioptic diagnosis of scac . axial ( a ) and sagittal ( b ) t2-weighted images show 2-cm hyperintense nodule contained within the internal sphincter muscle , intensely enhancing as seen on post - contrast fat - suppressed coronal t1-weighted image ( c ) , consistent with t1 tumour ( arrowheads ) a 62-year - old female with biopsy - proven scac . axial t2-weighted ( a ) , post - contrast fat - suppressed axial ( b ) and coronal ( c ) t1-weighted images , and corresponding enhanced image from body ct ( d ) show a 5.5-cm long ( t3 ) enhancing tumour with infiltration of the left ischioanal fatty space ( arrowheads ) a 57-year - old woman undergoing abdomino - pelvic mdct for unrelated reasons . post - contrast axial ( a ) and coronal reformatted ( b ) detailed images of the anorectal region identify an unexpected 2-cm right - sided enhancing anal nodule . subsequent clinical and bioptic assessment confirmed poorly symptomatic ulcerated scac an elderly , 92-year - old man with previous prostatectomy and kidney failure has unenhanced mri . sagittal t2- ( a ) and axial t1-weighted ( b ) images show 5-cm long solid , inhomogeneous neoplastic tissue ( * ) extending from the anus to encase the proximal urethra ( note catheter in place ) staging is essential for both prognostic information and correct therapeutic planning , and is performed according to the uicc / ajcc tumour - node - metastasis ( tnm ) system ( table  1 ) , including local lesion extent , lymph node status and distant metastatic spread . in 2010 the european society for medical oncology ( esmo ) recommended mri as the primary imaging modality to accurately stage scac , taking into account the maximum tumour diameter , possible invasion of adjacent organs and nodal involvement [ 6 , 13].table 1tumour - node - metastasis ( tnm ) staging of anal carcinoma according to lesion site of originanal canalanal marginprimary tumour ( t)txprimary tumour can not be assessedt0no evidence of primary tumourt1tumour 2  cm in greatest dimensiont2tumour 25  cm in greatest dimensiont3tumour > 5  cm in greatest dimensiont4tumour of any size invading adjacent organ(s ) e.g. vagina , urethra , bladdertumour invading deeper structures ( skeletal muscle , cartilage)regional lymph nodes ( n)nxregional lymph nodes can not be assessedn0no regional lymph node metastasisn1metastasis in perirectal lymph node(s)regional lymph node metastasisn2metastasis in unilateral internal iliac and/or inguinal lymph node(s)n3metastasis in internal iliac and perirectal lymph nodes and/or bilateral internal iliac and/or bilateral inguinal lymph nodesdistant metastasis ( m)m0no distant metastasism1distant metastasis tumour - node - metastasis ( tnm ) staging of anal carcinoma according to lesion site of origin particular care should be applied to choosing the longest lesion diameter on t2-weighted images , since correct t parameter staging relies on this measure being below 2  cm , over 5  cm or intermediate ( fig .  1 ) mri demonstrates good correlation with physical findings concerning t stage , whereas infiltration of adjacent organs and sometimes tumour size are clinically underestimated . extramural neoplastic spread may involve the sphincter complex muscles ( external sphincter , levator ani and puborectalis ) and most commonly occurs towards the anterior urogenital triangle with possible vaginal , urethral or bladder involvement . sometimes , the tumour may also extend laterally with invasion of the ischioanal fossa ( fig . 2 ) , superiorly to the rectum and mesorectal compartment , or inferiorly to the skin and subcutaneous planes of the perianal region . in such instances , t2-hyperintense solid tissue is seen infiltrating the even more hyperintense fat in the ischiorectal ( fig .  2 ) and subcutaneous spaces , encasing the lower signal intensity skeletal muscles , or isointense structures such as the vagina ( fig .  5 ) , prostate and urethra ( fig . notably , radiologists should remember that anal canal carcinoma directly invading the rectal wall , perianal skin , subcutaneous or the sphincter muscle does not imply assessing the tumour stage as t4 . for anal margin scac , a t4 lesion is defined by invasion of deeper structures such as the skeletal muscle or cartilage [ 9 , 10 , 13].fig . axial t2-weighted ( a ) and post - contrast fat - suppressed axial t1-weighted ( b ) images show inhomogeneous anal tissue invading the left aspect of the vagina ( arrowheads ) , with internal non - enhancing necrosis and peripheral enhancement . biopsy diagnosed scac with superimposed infection a 32-year - old hiv - positive woman with clinical diagnosis of anovaginal fistula . axial t2-weighted ( a ) and post - contrast fat - suppressed axial t1-weighted ( b ) images show inhomogeneous anal tissue invading the left aspect of the vagina ( arrowheads ) , with internal non - enhancing necrosis and peripheral enhancement . biopsy diagnosed scac with superimposed infection the incidence of regional nodal involvement increases with primary tumour size . lymph node metastases may be present ( in 25 % of cases ) even with superficial ( up to t2 ) scacs , and are unreliably assessed clinically . nodal staging evaluation relies on the distance from the primary tumour rather than on the number of involved nodes . mri is highly helpful to assess lymph node metastatic involvement , although the mere size criterion is far from accurate and associated with both false - positive and false - negative results . short - axis threshold values of 8  mm , 5  mm and 10  mm have been suggested for pelvic , perirectal and inguinal lymph nodes , respectively . additional helpful features to increase specificity include loss of the normal bean - shaped morphology and fatty hilum , internal t1 and t2 signal heterogeneity with central necrosis , and inhomogeneous enhancement ( figs . 6a 65-year - old woman with history of previously treated small scac 3  years earlier . axial t2- ( a ) and post - contrast t1-weighted ( b ) images show roundish 1-cm left inguinal node ( arrowheads ) with internal fluid - like necrosis and inhomogeneous enhancement , confirmed by ultrasound ( c ) as hypoechoic with loss of normal nodal structure . surgical exeresis ( postoperative status as seen in d , follow - up mri ) confirmed metastatic node from scacfig . axial t2-weighted ( a , b ) and post - contrast fat - suppressed t1-weighted ( c ) images show inguinal nodal metastases , larger on left side ( arrowheads ) plus bilateral enhancing perirectal adenopathies ( arrows ) a 65-year - old woman with history of previously treated small scac 3  years earlier . axial t2- ( a ) and post - contrast t1-weighted ( b ) images show roundish 1-cm left inguinal node ( arrowheads ) with internal fluid - like necrosis and inhomogeneous enhancement , confirmed by ultrasound ( c ) as hypoechoic with loss of normal nodal structure . surgical exeresis ( postoperative status as seen in d , follow - up mri ) confirmed metastatic node from scac the same patient as in fig .  2 . axial t2-weighted ( a , b ) and post - contrast fat - suppressed t1-weighted ( c ) images show inguinal nodal metastases , larger on left side ( arrowheads ) plus bilateral enhancing perirectal adenopathies ( arrows ) as suggested by ecco guidelines , search for distant spread is usually performed by means of contrast - enhanced body mdct , with conventional imaging appearances of liver and lung metastases . dissemination is very uncommon ( less than 5 % of patients at initial diagnosis , and is usually encountered in association with post - treatment recurrence . alternatively , in immunocompetent patients f - fluorodeoxygluocose positron emission tomography ( fdg - pet / ct ) has high specificity for nodal and visceral dissemination . at diagnosis , fdg - pet / ct may alter staging of anal scac in 20 % of patients , leading to inclusion of involved pelvic or inguinal lymph nodes in the radiation field [ 6 , 7 ] . in our experience , not infrequently anal tumours coexist with inflammatory conditions such as proctitis and abscesses . in such instances , mdct and mri provide confident detection of perirectal inflammatory changes and purulent collections that are differentiated from solid neoplastic tissue , thus allowing a correct therapeutic choice including surgical drainage as necessary ( figs .  5 , 8 , 9 , 10 ) . resolution of associated inflammatory changes during treatment is easily monitored by cross - sectional imaging ( figs . 8a 60-year - old man with aids and clinical finding of ulcero - fungating anal mass . axial ( a ) and coronal reformatted ( b ) ct images show moderately heterogeneous tissue ( * ) in its entire longitudinal extent from the anorectal junction to below the anal verge , associated with large necrotic iliac adenopathies ( * in c ) . axial images ( b , c ) from urgent contrast - enhanced mdct show solid circumferential thickening of the anal canal ( arrowheads ) associated with abscess collections with mixed gas - fluid content ( * ) and fistulas crossing the ischioanal space . surgical examination under anaesthesia including biopsies revealed ulcerated scac with superinfection . after surgical drainage and subsequent chemo - radiotherapy , mri ( c ) shows complete resolution of both inflammatory and neoplastic changes with residual t2-hypointense fibrotic tracksfig . staging mri confirms left - sided anal thickening ( arrowheads ) with abnormal t2 signal intensity ( a ) and strong contrast enhancement ( b ) . incidentally , two abscess collections with necrotic content and peripheral enhancement are seen ventrally , connected to the anal canal by a fistulous track ( arrow in c ) . subtotal regression of changes after treatment is seen on follow - up mri ( d ) a 60-year - old man with aids and clinical finding of ulcero - fungating anal mass . axial ( a ) and coronal reformatted ( b ) ct images show moderately heterogeneous tissue ( * ) in its entire longitudinal extent from the anorectal junction to below the anal verge , associated with large necrotic iliac adenopathies ( * in c ) . biopsy confirmed superinfected scac a 53-year - old man with purulent drainage and clinical diagnosis of perianal inflammation . axial images ( b , c ) from urgent contrast - enhanced mdct show solid circumferential thickening of the anal canal ( arrowheads ) associated with abscess collections with mixed gas - fluid content ( * ) and fistulas crossing the ischioanal space . surgical examination under anaesthesia including biopsies revealed ulcerated scac with superinfection . after surgical drainage and subsequent chemo - radiotherapy , mri ( c ) shows complete resolution of both inflammatory and neoplastic changes with residual t2-hypointense fibrotic tracks a 39-year - old hiv - infected man with biopsy - proven scac . staging mri confirms left - sided anal thickening ( arrowheads ) with abnormal t2 signal intensity ( a ) and strong contrast enhancement ( b ) . incidentally , two abscess collections with necrotic content and peripheral enhancement are seen ventrally , connected to the anal canal by a fistulous track ( arrow in c ) . subtotal regression of changes after treatment is seen on follow - up mri ( d ) furthermore , cross - sectional imaging particularly with mri also proves useful to differentiate anal carcinoma from other causes of local pain and perineal masses , such as pilonidal sinus diseases , gartner duct or bartolini gland cysts , tailgut cysts , uncommon soft - tissue neoplasms , urethral cancer , lymphoma or metastases [ 14 , 22 ] . in the past , scac was treated with abdomino - perineal resection and permanent colostomy . currently , anal margin and small canal tumours without evidence of nodal spread may be successfully excised . in all other cases , the standard treatment includes radiation combined with mitomycin - c plus infusional 5-fu chemotherapy , yielding an 80 % 5-year survival rate with preservation of sphincter function . advanced t3/t4 tumours have worse outcomes with a 4068 % 3-year disease - free survival . salvage surgery with abdomino - perineal resection is reserved for persistent or recurrent tumours [ 3 , 6 , 2325 ] . after radio - chemotherapy , imaging follow - up with mri represents a useful complement to clinical evaluation in the assessment of therapeutic response . shortly after treatment completion , interpretation of mri is usually challenging due to the superimposition of inflammatory changes resulting from radiotherapy . performed at least 68  weeks after treatment completion , mri provides confident , reproducible assessment of post - treatment modifications . findings indicative of a positive response include size reduction and diminished t2 signal intensity of the treated tumour and associated adenopathies ( figs . the appearance of t2-hypointense signal is consistent with fibrosis , although it does not allow excluding minor residual neoplastic foci for sure ( fig . stability in size and signal intensity of any residual abnormality visible at mri in the site of the treated lesion 1 year after therapy has been reported to be strongly associated with a favourable outcome . locoregional and/or distant recurrence occurs in up to 35 % of treated patients , and is strongly associated with advanced ( t3t4 ) stage and nodal involvement at presentation [ 7 , 20 ] . persistent and locally recurrent tumours often display an aggressive behaviour , with possible extensive invasion of the adjacent organs and pelvic bony structures , and a tendency for lymphatic dissemination ( figs . initial mri shows moderate circumferential thickening of the anus with t2-hyperintense signal ( a ) and contrast enhancement ( b ) ( arrowheads ) . complete disappearance of the lesion is observed on axial t2 ( c ) and post - contrast fat - suppressed t1-weighted ( d ) images following chemo - radiotherapyfig . mri staging including axial ( a ) and coronal ( b ) t2-weighted images detect a solid , 3-cm eccentric anal mass ( arrowhead ) consistent with t2 tumour , associated with right inguinal adenopathies with analogous signal features . after chemo - radiotherapy , mri follow - up ( c , d ) shows complete disappearance of both anal neoplastic solid tissue and lymphadenopathiesfig . initially , mdct ( a ) shows left - sided thickening with involvement of the external sphincter ( arrowheads ) . after chemo - radiotherapy , lesion regression is observed on mri with appearance of t2-hypointense fibrosis ( * ) and hyperintense nodule ( arrow ) consistent with residual tumourfig . 14an elderly , 85-year - old lady with biopsy - proven scac and multiple comorbidities . initial mri ( a , b ) shows showed circumferential anal wall thickening with abnormal solid signal ( arrowheads ) measuring 6  cm in length , plus a suspicious centimetric left inguinal lymph node ( arrow in a ) , findings consistent with t3n2 lesion . after reduced chemo - radiotherapy , follow - up mri ( c , d ) 4 months later disclosed progression and partial necrosis of both primary tumour ( arrowhead ) and inguinal adenopathy ( arrow ) , plus appearance of an exophytic tissue mass protruding from the external anal orifice ( * in d ) an 83-year - old man with known scac . initial mri shows moderate circumferential thickening of the anus with t2-hyperintense signal ( a ) and contrast enhancement ( b ) ( arrowheads ) . complete disappearance of the lesion is observed on axial t2 ( c ) and post - contrast fat - suppressed t1-weighted ( d ) images following chemo - radiotherapy a 46-year - old female patient with biopsy - proven scac . mri staging including axial ( a ) and coronal ( b ) t2-weighted images detect a solid , 3-cm eccentric anal mass ( arrowhead ) consistent with t2 tumour , associated with right inguinal adenopathies with analogous signal features . after chemo - radiotherapy , mri follow - up ( c , d ) shows complete disappearance of both anal neoplastic solid tissue and lymphadenopathies a 46-year - old hiv - infected man with biopsy - proven scac . initially , mdct ( a ) shows left - sided thickening with involvement of the external sphincter ( arrowheads ) . after chemo - radiotherapy , lesion regression is observed on mri with appearance of t2-hypointense fibrosis ( * ) and hyperintense nodule ( arrow ) consistent with residual tumour an elderly , 85-year - old lady with biopsy - proven scac and multiple comorbidities . initial mri ( a , b ) shows showed circumferential anal wall thickening with abnormal solid signal ( arrowheads ) measuring 6  cm in length , plus a suspicious centimetric left inguinal lymph node ( arrow in a ) , findings consistent with t3n2 lesion . after reduced chemo - radiotherapy , follow - up mri ( c , d ) 4 months later disclosed progression and partial necrosis of both primary tumour ( arrowhead ) and inguinal adenopathy ( arrow ) , plus appearance of an exophytic tissue mass protruding from the external anal orifice ( * in d ) after the introduction of highly active anti - retroviral treatment ( haart ) , people with hiv infection or acquired immunodeficiency syndrome ( aids ) gained a greatly improved life expectancy with better immune conditions , at the price of an increased tendency to develop tumours . although it is considered a non - aids - defining malignancy , currently scac ranks third ( 8.2 % ) among neoplasms observed in hiv / aids populations , with a substantially higher incidence in msms and in long - standing infected people [ 4 , 26 ] . dysplastic intraepithelial lesions are highly prevalent in hiv - infected people , and hiv represents a marker for coinfection with other sexually transmitted diseases such as hpv . since the risk of developing scac increases with the total time elapsed with cd4 + count below 200  cells/l , it has been hypothesised that hiv - related immune suppression acts as a cofactor to hpv in the development of anal dysplasia and progression to overt carcinoma [ 1 , 3 , 4 ] . screening procedures including high - resolution anoscopy and cytology smears are increasingly adopted at hiv care centres , to allow detection of scac precursors and early - stage tumours amenable to limited excision and topical therapies . prevention should limit the occurrence of advanced stages at diagnosis in the future [ 4 , 2629 ] . a high prevalence ( at least 30 % ) of anorectal complaints is characteristic of hiv - positive patients , particularly those practicing anoreceptive intercourse . differential diagnosis encompasses a wide spectrum of abnormalities , including non - specific anal diseases such as haemorrhoids , fissures , fistulas and abscesses , along with venereal infections , viral ulcers , and a non - negligible ( 7 % ) rate of neoplasms such as scac , lymphoma and kaposi s sarcoma . therefore , when performing cross - sectional imaging procedures on hiv - infected patients , even for unrelated complaints , special attention should be paid to the anal region , with a focus on the possible identification of solid , enhancing tissue consistent with tumour that indicates need for biopsy ( figs .  5 , 8 , 10 , 13 ) [ 21 , 30 , 31 ] . despite initial discouraging reports , in the haart era hiv - positive patients are likewise treated with standard chemo - radiation regimens , reaching satisfactory results in terms of local control and survival rates , although with increased toxicity and frequent local recurrences [ 4 , 5 , 26 , 27 , 32 ] . some literature reports have highlighted the increased risk of anus and lower rectum carcinomas associated with long - standing , severe perianal fistulising crohn s disease ( cd ) ( fig . , scac reaches a 14 % proportion among all colorectal cancers , which is ten - times higher than the usual figure . furthermore , cd patients develop anorectal carcinomas at a younger age ( 20  years earlier ) than the general population . according to the hypothesised pathogenesis , fistulas probably allow hpv an easier access to the epithelial layers , and chronic mucosal regeneration may ultimately lead to neoplastic changes . however , anal tumours may occasionally develop in patients with ulcerative colitis ( uc)-related ( fig . 15a 40-year - old male patient with long - standing perianal crohn s disease , being treated with seton . contrast - enhanced mdct images ( a , b , c in cranio - caudal order ) show right - sided levator ani abscess ( * ) , extensive perianal fistulisation occupying the ischioanal space ( arrow ) , and marked solid - appearing circumferential anorectal thickening ( arrowheads ) . initial multiplanar mdct ( a , b ) identified enhancing anal thickening ( arrowheads ) with right - sided vaginal infiltration and solid tissue ( * ) extending to reach the internal obturator muscle . after biopsy confirmation of scac and surgical debulking with colostomy , repeat mdct ( c , d ) detected enlarging neoplastic residue ( * ) . shortly after chemo - radiotherapy , mri ( e ) detected the formation of a thick hypointense fibrotic band in the site of the regressed tumour ( thin arrows ) . mri follow - up ( e ) identified appearance of a contralateral enhancing tissue band interpreted as suspicious for local recurrence ( arrow ) . after negative clinical reassessment and pet findings , this post - treatment finding remained stable on further mri studies ( not shown ) a 40-year - old male patient with long - standing perianal crohn s disease , being treated with seton . contrast - enhanced mdct images ( a , b , c in cranio - caudal order ) show right - sided levator ani abscess ( * ) , extensive perianal fistulisation occupying the ischioanal space ( arrow ) , and marked solid - appearing circumferential anorectal thickening ( arrowheads ) . abdomino - perineal resection for scac was performed a 39-year - old woman with history of ulcerative colitis and perianal inflammation . initial multiplanar mdct ( a , b ) identified enhancing anal thickening ( arrowheads ) with right - sided vaginal infiltration and solid tissue ( * ) extending to reach the internal obturator muscle . after biopsy confirmation of scac and surgical debulking with colostomy , repeat mdct ( c , d ) detected enlarging neoplastic residue ( * ) . shortly after chemo - radiotherapy , mri ( e ) detected the formation of a thick hypointense fibrotic band in the site of the regressed tumour ( thin arrows ) . mri follow - up ( e ) identified appearance of a contralateral enhancing tissue band interpreted as suspicious for local recurrence ( arrow ) . after negative clinical reassessment and pet findings , this post - treatment finding remained stable on further mri studies ( not shown ) diagnosis is often unsuspected or delayed because of pre - existent , unspecific complaints and clinical assessment is hampered by complex inflammation with stricture and local pain . as a result , ibd - associated anal cancers are often advanced at presentation , may require extensive surgery plus chemotherapy and radiotherapy , and are associated with a severe prognosis . therefore , patients with early - onset or long - standing perianal cd should undergo clinical and imaging surveillance , particularly when new or changed symptoms develop . radiologists should be aware of the increased risk for anorectal cancer in middle - aged ibd patients , and clearly report any solid tissue as suspicious for neoplasm and suggest biopsy ( figs . the established association with hpv infection and premalignant intra - epithelial dysplastic changes provides insight into the pathogenesis of hiv- and ibd - related anal cancers , and the possibility of prevention or early diagnosis through screening of high - risk individuals [ 1 , 2 ] . state - of - the - art cross - sectional imaging with high - resolution mri using external phased - array coils and multiplanar mdct allow detailed , comprehensive visualisation of abnormalities involving the anus and perineal region . currently , mri represents the modality of choice for primary regional staging of scac , assessment of complications , of therapeutic response following chemo - radiotherapy , and of possible recurrences [ 9 , 10 , 13 ] .
backgroundsquamocellular anal carcinoma is increasingly diagnosed in patients with risk factors.methodsstate-of-the-art imaging with magnetic resonance imaging ( mri ) using phased - array coils and volumetric multidetector computed tomography ( ct ) provides detailed visualisation of anal disorders , identification and extent assessment of neoplastic tissue , detection and characterisation of nodal and visceral metastases . 
 mri has been recommended by the european society for medical oncology ( esmo ) as the preferred modality of choice to stage anal cancer , taking into account the maximum tumour diameter , invasion of adjacent structures and regional lymph node involvement.resultscross-sectional imaging techniques allow the identification of coexisting complications , and differentiation from other perineal abnormalities.conclusioncross-sectional imaging is useful for planning radiotherapy , surgical drainage or salvage abdomino - perineal resection . 
 after chemo - radiotherapy , mri follow - up provides confident reassessment of therapeutic response , persistent or recurrent disease.teaching points anal carcinoma is increasingly diagnosed in patients with human immunodeficiency virus ( hiv ) , anoreceptive intercourse , chronic inflammatory bowel disease. an established association exists with human papillomavirus ( hpv ) infection and premalignant intra - epithelial dysplasia. phased - array mri is recommended as the preferred imaging modality for regional staging. imaging allows detection of infectious complications , planning of radiotherapy or salvage surgery. follow - up mri allows reliable assessment of therapeutic response after chemo - radiotherapy .