MRI and CT of anal carcinoma: a pictorial review

Massimo Tonolini Roberto Bianco Background Squamocellular anal carcinoma is increasingly diagnosed in patients with risk factors. Methods State-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. Results Cross-sectional imaging techniques allow the identification of coexisting complications, and differentiation from other perineal abnormalities. Conclusion Cross-sectional imaging is useful for planning radiotherapy, surgical drainage or salvage abdominoperineal resection. After chemo-radiotherapy, MRI followup 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. - 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]. Fig. 1 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 Regional anatomy and imaging techniques 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 seen on post-contrast fat-suppressed coronal T1-weighted image (c), consistent with T1 tumour (arrowheads) above the dentate line, although with similar biological behaviour [6, 7]. Lymphatic drainage of anal neoplasms varies according to the primary lesion site. 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 Fig. 2 A 62-year-old female with biopsy-proven SCAC. Axial T2weighted (a), post-contrast fat-suppressed axial (b) and coronal (c) T1weighted 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. 3 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 Fig. 4 An elderly, 92-year-old man with previous prostatectomy and kidney failure has unenhanced MRI. Sagittal T2- (a) and axial T1weighted (b) images show 5-cm long solid, inhomogeneous neoplastic tissue (*) extending from the anus to encase the proximal urethra (note catheter in place) 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 noninvasiveness and optimal safety profile of gadoliniumFig. 5 A 32-year-old HIV-positive woman with clinical diagnosis of anovaginal fistula. Axial T2-weighted (a) and post-contrast fatsuppressed 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 sup (...truncated)