- Pictorial Review
- Open Access
MRI and CT of anal carcinoma: a pictorial review
© The Author(s) 2012
- Received: 27 September 2012
- Accepted: 31 October 2012
- Published: 4 December 2012
The Erratum to this article has been published in Insights into Imaging 2013 4:221
Squamocellular anal carcinoma is increasingly diagnosed in patients with risk factors.
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.
Cross-sectional imaging techniques allow the identification of coexisting complications, and differentiation from other perineal abnormalities.
Cross-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.
• 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.
- Anal carcinoma
- Human immunodeficiency virus
- Chronic inflammatory bowel diseases
- Computed Tomography (CT)
- Magnetic Resonance Imaging (MRI)
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 (40–60 years) HIV-positive individuals, most often men who have sex with men (MSM) [3–5].
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].
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 [6–8].
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 [11–13].
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. No special patient preparation is needed. 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, 13–15].
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 [16–18].
MRI provides a detailed visualisation of the anal canal and nearby anatomical structures. 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 90–100 %, with high concordance regarding tumour size, although on a limited number of patients, and more precise results with ultrasound for smaller, superficial tumours .
Tumour-node-metastasis (TNM) staging of anal carcinoma according to lesion site of origin
Primary tumour (T)
Primary tumour cannot be assessed
No evidence of primary tumour
Tumour ≤2 cm in greatest dimension
Tumour 2–5 cm in greatest dimension
Tumour >5 cm in greatest dimension
Tumour of any size invading adjacent organ(s) e.g. vagina, urethra, bladder
Tumour invading deeper structures (skeletal muscle, cartilage)
Regional lymph nodes (N)
Regional lymph nodes cannot be assessed
No regional lymph node metastasis
Metastasis in perirectal lymph node(s)
Regional lymph node metastasis
Metastasis in unilateral internal iliac and/or inguinal lymph node(s)
Metastasis in internal iliac and perirectal lymph nodes and/or bilateral internal iliac and/or bilateral inguinal lymph nodes
Distant metastasis (M)
No distant metastasis
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) [7, 9, 10, 13].
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 18F-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].
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 40–68 % 3-year disease-free survival. New neoadjuvant and adjuvant drugs are being investigated to treat advanced disease. Salvage surgery with abdomino-perineal resection is reserved for persistent or recurrent tumours [3, 6, 23–25].
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/μl, 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, 26–29].
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].
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. 15, 16) [33–35].
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].
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