Pictorial reviewImaging findings of unusual anorectal and perirectal pathology: a multi-modality approach
Introduction
Primary rectal adenocarcinoma is the commonest cause of a rectal mass on imaging. Occasionally, less common lesions of the anorectum and perirectal tissues may mimic an adenocarcinoma. Imaging of the anorectum and perirectal tissues is technically challenging to perform and can be difficult to interpret. Recent advances in endorectal sonography (ERUS), magnetic resonance imaging (MRI), and multidetector computed tomography (CT) have improved imaging assessment of this region, and enable a multimodality imaging approach for the characterization of anorectal and perirectal lesions.
ERUS requires an ultrasound probe with a radial transducer of at least 7 MHz with a minimum beam width of 1.1 mm and a focal length of 3 cm. A degassed water-filled balloon covers the rotating probe and is inflated for evaluation of the rectum. ERUS permits accurate visualization of the individual layers of the rectal wall (Fig. 1) and enables precise localization of a lesion to the involved layer of the rectal wall, as well as local tumour staging by assessment of tumour size, depth of invasion, and the presence of perirectal lymph node metastases. ERUS-guided fine-needle aspiration of lesions for cytological analysis can also be performed. CT and MRI enable lesion characterization and assessment for locoregional adenopathy and distant metastases. MRI is superior to CT for lesion characterization due to superior soft-tissue contrast resolution. MRI with an endoluminal coil provides ultra-high resolution assessment of the layers of the rectal wall. The multiplanar and three-dimensional capabilities of CT and MR allow pre-operative assessment of the relationship of the lesion to the rectum, pelvic floor, and bony pelvis.
This review illustrates less common causes of rectal and perirectal disease, such as developmental cysts, non-adenomatous tumours, sacral chordoma, sacral teratoma, endometriosis, colitis cystica profunda, and extramedullary haematopoiesis, and emphasizes the radiological features that would enable a confident diagnosis and differentiation from rectal adenocarcinoma.
Section snippets
Developmental cysts
Developmental cysts are benign epithelial cysts in the retrorectal space that are thought to arise from caudal embryonic vestiges.1, 2, 3, 4 They usually affect middle-aged women and are often incidentally detected. They are classified according to their tissue of origin and histopathological features, and include epidermoid cysts, dermoid cysts, enteric cysts, and neuroenteric cysts. Epidermoid and dermoid cysts are lined with stratified squamous epithelium and are unilocular. Epidermoid cysts
Rectal carcinoid
Gastrointestinal (GI) carcinoids arise from neuroendocrine cells and have recently been reclassified as gastroenteropancreatic neuroendocrine tumours (GEP-NETs).9, 10, 11 Rectal carcinoids account for 12.6% of all carcinoids and constitute 0.7–1.3 % of all rectal tumours.11, 12 Multiple tumours occur in 2–4.5% of all cases.13 The rectum is the third most common site for GI carcinoid. Local invasion by rectal carcinoid is uncommon, but metastases are present at the time of diagnosis in 4–18% of
Chordoma
Chordoma is the most common primary malignant tumour of the sacrum and typically involves the fourth and fifth sacral vertebrae. They are thought to arise from notochord remnants, are commonest in the 30–60 year age group and show a 2:1 male predominance.38 CT often shows a large, heterogeneous, osteolytic, soft-tissue mass that extends into the presacral space and may displace the rectum and bladder.4, 39 Internal calcification is a frequent finding38 (Fig. 11a). On MRI, it is low to
Endometriosis
Endometriosis is the presence of ectopic functional endometrial glands and stroma outside the uterus.44, 45, 46 Five to 15% of women with symptomatic endometriosis have GI involvement by endometriosis,45, 46, 47 and the rectum and sigmoid are most commonly affected. The endometrial implants are usually serosal, but can infiltrate deep into the intestinal wall to produce a nodular mass from thickening and fibrosis of the muscularis propria.46, 47 Colonoscopic diagnosis is difficult due to the
Conclusion
A wide variety of less common non-carcinomatous lesions arise from the anorectum and perirectal tissues. Knowledge of their existence and differing imaging appearances using multiple modalities is required for an accurate diagnosis to facilitate appropriate management.
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