What is hiding in the hindgut sac? Looking beyond rectal carcinoma

Vivek Virmani Subramaniyan Ramanathan Vineeta Sethi Virmani John Ryan Najla Fasih Objectives Although rectal cancer is by far and large the most common pathology involving the rectum that needs imaging, there are many other important but less common pathological conditions affecting anorectal region. The objective of this pictorial review is to discuss the cross-sectional imaging features of less common anorectal and perirectal diseases. Results Although a specific histological diagnosis cannot usually be made due to considerable overlap in the imaging appearances of anorectal diseases, this review illustrates the cross-sectional imaging findings with emphasis on magnetic resonance imaging (MRI) that can help in narrowing down the differentials to a reasonable extent. Teaching points Variety of pathology exists in the anorectum apart from common rectal carcinoma Anorectal diseases present as non-specific wall thickening indistinguishable from rectal carcinoma Computed tomography (CT) and MRI can help in narrowing down the differentials, although often biopsy is warranted. - Primary rectal adenocarcinoma is a common malignancy with a high mortality rate in the western world. Initially computed tomography (CT) and endoluminal ultrasound (EUS) have been the mainstay of diagnosis and staging. In the past decade magnetic resonance imaging (MRI) has become the imaging modality of choice for loco-regional staging of rectal cancer. The complex anatomy of the anorectal region makes imaging and interpretation challenging. The advent of MRI, with its high soft tissue contrast, multiplanar capability and no radiation risk, has simplified imaging of the anorectal region. The role of MRI in preoperative staging of rectal cancer has been well established now. Though rectal cancer is the commonest cause of rectal mass, there are many other common and uncommon diseases which affect rectum and perirectal region, many of which can mimic rectal carcinoma and their distinction is essential as the management strategy changes significantly [1]. The objective of this review is to describe and illustrate the specific imaging findings of uncommon and atypical diseases affecting the anorectal region. Although a specific diagnosis cannot usually be made due to considerable overlap in the imaging appearances of anorectal diseases, this review illustrates the cross-sectional imaging findings with emphasis on MRI that can help to narrow down the differential diagnoses to a reasonable extent. Uncommon anorectal diseases include congenital cysts, benign and malignant neoplasms excluding adenocarcinoma, atypical infections, inflammatory conditions (excluding common inflammatory bowel disease and post radiation changes) and a few other rare miscellaneous conditions. MR imaging technique MRI of the rectum may be performed with either an endorectal coil or a phased-array surface coil. The decision depends on the availability of coils, technical expertise of radiologists, surgeons preference and practical issues like time constraints. In our institution we use phased array surface coils routinely for all rectal diseases. The advantage of an endorectal coil lies in its high-resolution images that fully depict the wall layers of the bowel but has the disadvantage in evaluating rectal strictures, high rectal carcinomas and assessment of the perirectal structures due to its smaller field of view (FOV). Phased-array surface coil yields high-spatialresolution images, albeit with less distinction of bowel wall layers but with additional advantage of a large field of view, patient comfort and ease of use in structuring cancers and high rectal, rectosigmoid tumours [2, 3]. In our institution we do no routine bowel preparation, rectal contrast or antispasmodic agents. Opacification rectal lumen with contrast is still controversial with use of various agents like super paramagnetic iron oxide solutions, methylcellulose, barium suspensions and aqueous gel [4]. A sagittal T2-weighted turbo spin-echo (TSE) sequence is obtained first to locate the rectal lesion. Based on the sagittal sequence, axial and coronal T2-weighted TSE sequences are planned, and they are angled to the plane exactly perpendicular and parallel to the lesion. We have a standard rectal cancer protocol which is applied for all other rectal lesions as well [2]. Our MRI protocol consists of sagittal T2-weighted single-shot images and T2-weighted TSE images in the axial and coronal planes. High-resolution images are obtained in the axial and coronal planes with a slice thickness of 3 mm and small FOV. Unenhanced and contrast-enhanced axial and coronal highresolution T1-weighted fat-saturated images of the rectum are also obtained. The use of gadolinium in rectal cancers is now becoming optional, as all the staging parameters are adequately depicted in T2-weighted sequences. However, rectal lesions other than usual cancers still benefit from gadolinium. As of now, we routinely do post gadolinium multiplanar T1 VIBE (volume interpolated breath-hold examination) sequences. Diffusion-weighted imaging (DWI) is a functional imaging tool that yields information about water mobility and tissue cellularity. It also allows calculation of the apparent diffusion coefficient (ADC) from images with different b values. Although diffusion sequences are not part of our routine rectal MRI protocol, there is increasing literature on their usefulness, especially for malignant lesions. The reader is requested to refer to various articles available on principles, imaging parameters and pitfalls of diffusion imaging, as these are beyond the scope of our article [5, 6]. Malignant tumours are generally depicted as foci of increased intensity on DWI and decreased signal intensity on ADC images, because water diffusion is restricted in highly cellular tissues in malignant tumours [5, 7, 8]. However, blood, fat, abscesses, lymph nodes, and melanin can show restricted diffusion and can be resolved by referring to standard T1- and T2-weighted images [9]. Examples include dermoid cyst, rectal lipoma, melanoma and endometriosis. DWI has been proven useful in diagnosis, assessing treatment response and recurrence of rectal carcinoma [1012]. Other malignant lesions that can show diffusion restriction include lymphoma, stromal tumours and mucinous carcinoma [10, 1315]. Data on other rectal tumours are not well established and mostly are extrapolated from similar tumours occurring at other sites. Cystic lesions (duplication cyst, tail gut cyst, pseudomyxoma) show T2 shine-through effect, which is high signal intensity on low- and high-b-value images and on ADC images [9]. Developmental retro-rectal cysts Developmental cysts are benign epithelial cysts in the retro-rectal space, arising from caudal embryonic vestiges. Often these are incidentally detected in middle-aged women. Developmentally these are sub-typed in to epidermoid cysts, dermoid cysts, enteric cysts and neuroente (...truncated)