Abstract
Cross-sectional imaging techniques including magnetic resonance tomography (MRI) and computed tomography (CT) have experienced rapid development in the last decades and play a key role today in imaging benign and malignant colorectal disease, facilitating risk stratification, and procedural planning. In particular, visualization of the intestinal wall and adjacent structures enables detection of extraluminal and extraintestinal pathologies and complications.
This chapter systematically summarizes the most common benign and malignant conditions of the colon and rectum that can be diagnosed by CT and MRI.
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Keywords
- Benign and malignant diseases of the colon and rectum
- Rectal cancer
- Computed tomography
- Magnetic resonance imaging
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To provide an overview of the most common benign and malignant conditions of the colon and rectum on cross-sectional imaging (CT and MRI).
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To understand the current role of cross-sectional imaging in the detection, characterization, and differentiation of colorectal diseases.
4.1 Benign Diseases of the Colon and Rectum
4.1.1 Inflammatory Diseases of the Colon and Rectum
CT is a valuable diagnostic tool for the detection and characterization of different inflammatory conditions of the colon, including appendicitis, diverticulitis, epiploic appendagitis, chronic inflammatory bowel diseases (IBDs), as well as infectious and non-infectious colitis. CT plays an important role in detection of acute conditions including extraluminal complications and extraintestinal manifestations of inflammatory bowel disease. Despite the significant overlap in imaging findings of inflammatory bowel diseases, their findings may differ in their primary localization within the gastrointestinal tract, length of segmental involvement, degree of wall thickening, mural enhancement pattern, and extraintestinal involvement. Therefore, understanding of leading disease patterns and specific imaging features can allow accurate diagnosis.
4.1.1.1 Chronic Inflammatory Bowel Diseases
Inflammatory bowel diseases (IBD) are a group of chronic disorders that cause relapsing inflammation in the gastrointestinal tract and comprise three major subgroups of Crohn’s disease, ulcerative colitis, and unclassified. Environmental changes, genetic factors, intestinal microbiota alterations, and immune system deregulation contribute to the initiation and progression of inflammation and subsequent fibrosis [1]. Despite of the considerable overlap between the imaging findings in Crohn’s disease and ulcerative colitis, there are often certain features that can help differentiate them (Table 4.1, see also Fig. 4.1) [2].
4.1.1.2 Infectious Colitis
Infectious colitis, as its name suggests, is caused by an infection due to bacterial, viral, fungal, or parasitic agents, leading to inflammation of the colon. Although cross-sectional imaging is not the primary diagnostic tool, and imaging findings are often non-specific, standard abdominal CT may be required to assess disease extent and severity, extraluminal complications, and especially to rule out other causes of acute abdomen [3]. Typical imaging findings regardless of the infective cause are: diffuse wall thickening with homogeneous enhancement, pericolonic fat stranding, gas-fluid levels, and ascites [3].
4.1.1.2.1 Pseudomembranous Colitis
Pseudomembranous colitis is an acute, potentially life-threatening nosocomial infectious colitis caused by toxins produced by an unopposed proliferation of Clostridium difficile bacteria. In recent years, it has become a significant clinical problem, mostly due to the increased use of prophylactic and broad-spectrum antibiotics. Imaging features include marked wall thickening (which is usually more extensive compared to other infectious and non-infectious colitis), low-attenuation mural thickening corresponding to mucosal and submucosal edema, the “accordion sign” (oral contrast media trapped between the thickened colon wall folds), and the “target sign” (or “double halo sign”) (Fig. 4.2). Extracolonic features include ascites and pericolonic stranding, which may be relatively mild compared to the degree of colon wall thickening. Most commonly, the entire colon is affected. In severe cases, complications like intramural gas formation (pneumatosis coli), toxic megacolon, and perforation (pneumoperitoneum) may occur [4].
4.1.1.3 Non-infectious Colitis
Non-infectious colitis refers to the heterogeneous group of colonic inflammation caused by various causes other than infections (pathogenic organisms), for example, ischemic, drug-induced or immune-mediated.
4.1.1.3.1 Ischemic Colitis
Ischemic colitis is a condition in which inflammatory injury of the colon results from interruption and/or insufficient blood supply. It is more likely to occur in the elderly with atherosclerotic disease and/or low-flow state (e.g., due to heart disease). Low-flow state and non-occlusive vessel disease may lead to ischemic colitis in watershed areas while complete vessel occlusion produces an involvement of the dependent vascular territory (e.g., in the territory of the superior mesenteric artery). Imaging findings are mostly non-specific: uniform bowel wall thickening, “target sign” (low-density ring of submucosal edema between enhancing mucosa and serosa), bowel dilatation, pneumatosis coli (in severe cases), pericolic fluid or fat stranding, mesenteric edema, and/or asities (Fig. 4.2). Multiphase CT angiography has to be performed to identify the level of vessel occlusion and procedural planning.
4.1.1.3.2 Drug-Induced Colitis
The dramatic increase in pharmaceutical medical therapies (e.g., immune-modulating therapies with biologics, chemotherapeutics, nonsteroidal anti-inflammatory drugs) has led to an increased frequency of gastrointestinal adverse effects. Medical history and clinical presentation supported by imaging findings are the key to the diagnosis. Cross-sectional imaging may be required for the assessment of (peri-)colonic involvement, associated complications and to exclude other causes of acute abdomen (e.g., ischemic causes) [5]. Imaging findings are generally based on those seen in other infectious and non-infectious colitis (Fig. 4.2).
4.1.1.3.3 Neutropenic Colitis
Neutropenic colitis (also known as typhlitis) is a severe necrotizing inflammation occurring primarily in neutropenic patients. It mostly originates in the cecum and extends to the ascending colon, appendix, or terminal ileum [6]. As morphologic imaging findings are similar to that of other infectious and non-infectious colitis, medical history (e.g., immunodeficiency) is necessary to establish the diagnosis.
4.1.1.3.4 Radiation Colitis and Proctitis
Radiation colitis is the inflammatory injury of the colon and rectum caused by radiation therapy, which may occur between 6 months to 5 years after treatment. Depending on the onset, radiation colitis may be classified as acute or chronic. Cross-sectional imaging may be indicated for the assessment of extracolonic involvement and other complications. Imaging findings in the acute phase include non-specific wall thickening and pericolonic stranding; in the chronic phase, short or long strictures, colonic lumen narrowing, ulcerations, and/or fistulas may be present [7].
4.1.1.3.5 Graft-Versus-Host Disease
Intestinal graft-versus-host disease (GvHD) is a common, potentially life-threatening complication after hematopoietic stem cell transplantation, which may affect the entire gastrointestinal tract (large bowel involvement is present in ~25% of cases). Imaging findings are non-specific and include: moderate bowel wall thickening with mucosal enhancement, mesenteric edema, vascular engorgement, and/or pneumatosis intestinalis in severe cases (Fig. 4.2) [8].
Key Point
CT plays an essential role in the detection and characterization of inflammatory conditions, including extraluminal complications and extraintestinal manifestations.
4.1.2 Diverticular Disease and Diverticulitis
Diverticular disease is one of the most common gastroenterological disorders in the Western world. In case of acute abdomen and suspicious diverticular disease, ultrasound is routinely followed by CT, further clinical decision-making, and risk stratification. Based on the classification of diverticular disease (CDD), a differentiation can be made between uncomplicated (type 1), complicated (type 2), and chronic (type 3) diverticular disease (Fig. 4.3) [9]. In this context, CT allows the detection of associated microabscesses, macroabscesses, and free perforation as they determine the further therapeutic approach.
Key Point
CT is the method of choice to evaluate diverticulitis and allows accurate classification and guide treatment.
4.1.3 Benign Mucosal Colonic Polyp
Since the majority of colorectal cancer are believed to arise within benign adenomatous polyps that develop slowly over many years following the “adenoma to carcinoma” sequence, they are the primary target lesions for colorectal screening. Cross-sectional imaging with introduction of virtual colonoscopy (CT and MRI colonography) are promising techniques and play an increasingly important role in both symptomatic and screening patients for the selection of the appropriate therapeutic procedure (see the Abstract Book IDKD 2018).
Key Point
Cross-sectional imaging techniques with the introduction of virtual colonoscopy are promising techniques and are playing an increasing role.
4.2 Malignant Diseases of the Colon and Rectum
4.2.1 Rectal Cancer
Colorectal cancer is the third most common cancer in men and the second most common in women [10]. Nowadays, rectal MRI plays a leading role in the evaluation of rectal cancer, especially in primary local staging and assessment of response to chemotherapeutic treatment.
4.2.1.1 Elective Rectal Cancer Staging
In primary staging (pre-operative setting), MRI is important for the evaluation of tumor location and morphology, T and N category, involvement of the mesorectal fascia (MRF), extramural vascular invasion (EMVI), mucin content, and involvement of the pelvic sidewall and anal sphincter complex (Fig. 4.4). Therefore, rectal MRI is particularly performed for (1) selecting patients with locally advanced rectal cancer who are suitable for treatment with neoadjuvant chemotherapy; (2) guiding surgical planning; and (3) identifying poor prognostic factors, including EMVI, mucin content, and CRM status [10]. The prognosis of rectal cancer is directly related to mesorectal tumor infiltration and circumferential resection margins (CRMs).
Key Point
Rectal MRI plays a key role in local staging of rectal cancer and allows selection of an appropriate treatment strategy. Moreover, it allows to identify poor prognostic factors including MRF and EMVI.
4.2.2 Colon Cancer
Colon cancer is the fourth most commonly diagnosed cancer worldwide and the fifth deadliest, representing 5.8% of all cancer deaths [11]. Its incidence is 3 to 4 times higher in developed countries, making it a marker of socioeconomic development [11].
The diagnosis of colon cancer is either driven by symptoms or screening. Optical colonoscopy is the diagnostic gold standard, with detection rates of (pre)cancerous lesions >95% [12]. CT colonography may be a good alternative, particularly in patients with structural problems or comorbidities, and a good adjunct to incomplete examinations, with comparable sensitivity for lesions >10 mm [13].
Clinical staging of colorectal cancer is the most important predictor of survival and relies on the TNM system proposed by the AJCC/UICC, which is based on the pathologic analysis of the resected specimen [14]. Imaging plays an essential role, not only for surgery planning in eligible patients, but also for distant staging, detection of pre- and postoperative complications, and oncologic follow-up.
4.2.2.1 Elective Colon Cancer Staging
CT is the mainstay for colon cancer staging, but accurate T and N staging has always been a challenge and MRI has not demonstrated better results [15, 16]. Given surgery remains primary curative treatment for all TN stages, more than getting the T and N stages right, the radiologist should provide the multidisciplinary team with all the relevant information for a successful curative surgery or, alternatively, with the detailed baseline information to monitor systemic treatment. In the absence of IV contrast contraindications, a weight and concentration-adjusted acquisition in the portal venous phase of enhancement should be sufficient, oral contrast being considered unnecessary by the great majority of experts [17].
Most colon cancers present as a polyp (Fig. 4.5) or as an asymmetrical or concentrical wall thickening, the latter with lumen caliber reduction and loss of the normal layered appearance of the bowel wall (Fig. 4.6). Relative enhancement varies, most non-mucinous tumors being hyper to isoenhancing (78%) (Figs. 4.5 and 4.6) and most mucinous tumors being iso to hypoenhancing (84%) compared to adjacent bowel wall (Fig. 4.7) [18]. Enhancement pattern is usually heterogeneous, particularly in mucinous tumors (Fig. 4.7). Intratumoral calcification is unusual but relatively more frequent in mucinous tumors [18].
No reliable radiologic lymph node involvement criteria have been established so far although several have been investigated [16, 19].
The success of curative R0 resection relies on detailed imaging delineation of tumor boundaries, including any involved surrounding organs or structures (Fig. 4.8). Attention should be paid to other colon segments, particularly proximal to tumor in incomplete colonoscopies, not to miss additional lesions. Other details that matter for the selection of the best surgical approach include: specific tumor location, lesion size, and the particularities of mesenteric vascular anatomy, for which multiplanar reformations and maximum intensity projections, especially in coronal plane, may be quite illustrative [20].
4.2.2.2 Colon Cancer Presenting as Acute Abdomen
Colon cancer may present as a surgical emergency in up to 40% of cases, which occurs more frequently in the elderly population [21]. Obstruction and perforation, the most common presentations, are considered high-risk features and are linked to poorer recurrence-free survival, higher surgical morbidity and mortality, and stoma formation [21]. Other complications include acute appendicitis, ischemic colitis, and intussusception [22].
CT can localize an obstructing lesion with high sensitivity (96%) and specificity (93%) [22]. Left-sided malignancies are more likely to be obstructive. Obstructive lesions manifest with an intestinal caliber transition point at tumor level and upstream dilatation. A cecal lumen exceeding 12–15 cm, more likely to occur in patients with a competent ileo-cecal valve, should be an alert for imminent rupture, as should be the presence of any area of wall hypoenhancement [23].
Although perforation may occur proximal to an obstructing tumor, it more commonly occurs at the tumor site itself, due to necrosis and tissue friability [24]. It is the most lethal complication of colon cancer, with mortality rates as high as 50% due to secondary fecal peritonitis [21]. On CT, a focal defect in the bowel wall may be observed, accompanied by adjacent fat stranding, extraluminal air, and a variable amount of fluid. Perforation may be free or localized, the latter with eventual abscess formation and/or fistulation [21]. Oral contrast or contrast per rectum may help document a perforation but lack of extravasation of contrast does not rule it out, making its clinical utility questionable.
Key Point
Precise T and N staging with CT for colon cancer is challenging. However, it is highly valuable for M staging, planning curative surgery, diagnosing pre- and postoperative complications, assessing response to systemic treatment, and for long-term follow-up.
4.2.3 Evaluation of Response to Neoadjuvant Therapy in Rectal Cancer
Given the established advantage regarding local recurrence rates, standard treatment for locally advanced rectal cancer involves a combination of neoadjuvant radiation and chemotherapy, prior to total mesorectal excision (TME). It induces downsizing and downstaging of the disease in most patients, and in a variable proportion of them, 10–25% in most series, it leads to a complete response [11]. There are two main drives to re-stage rectal cancer after neoadjuvant therapy (NAT): to detect changes in the relation between the tumor and adjacent structures that may permit a less mutilating, yet still curative surgery; and to offer, in dedicated centers, the option of non-operative management to clinical complete responders [11].
4.2.3.1 Technique
Assessment of response to NAT prior to surgery relies on clinical evaluation, MR imaging and, in “Watch-and-Wait” dedicated centers, also on rectoscopy. The post-NAT MR imaging evaluation, just as in the staging setting, relies on high resolution T2-weighted images acquired in sagittal, parallel, and perpendicular planes relative to the tumor bed. For the identification of clinical complete responses, the use of diffusion-weighted imaging (DWI) may be of additional value, and given it is very sensitive to motion and air-induced susceptibility, patient preparation is determinant [25]. We recommend fasting for 6 h, a small enema 20 min before acquisition, and the administration of a spasmolytic agent in the absence of contraindications [25].
4.2.3.2 Re-staging to Plan Surgery
After NAT, the tumor may move away from anatomical landmarks or structures in a favorable manner. For instance, its inferior border may shift cranially making TME with a colo-anal anastomosis a possibility. Also, whenever a fat cushion becomes visible between the tumor bed and the mesorectal fascia at re-staging, or whenever the mesorectal fascia is reached only by very thin hypointense fibrotic spiculae, the specificity for a non-involved margin at pathology after TME may be 100% (Fig. 4.9a, b) [25]. On the other hand, whenever dense hypointense “fibrosis” reaches the mesorectal fascia, a resection beyond TME plane should be planned to achieve negative margins (Fig. 4.9c, d) [25].
It is also very important to evaluate the pelvic lymph nodes in the obturator and internal iliac compartments. Data from the Lateral Node Study Consortium found that internal iliac lymph nodes with a short axis >4 mm post-NAT were associated with a 52% likelihood of lateral local recurrence and that obturator lymph nodes with a short axis >6 mm post-NAT were associated with a higher 5-year rate of distant metastases, re-igniting the discussion on the need to remove lateral nodes surgically during TME in selected patients, which is not standard practice in western countries (Fig. 4.10) [26].
4.2.3.3 The Prognostic Value of Re-staging MR Imaging
Different assessment methods may be utilized to evaluate response of the primary tumor but the most important is the T2-weighted imaging-based magnetic resonance tumor regression grade (mrTRG) (Fig. 4.11), mrTRGs 4 and 5 being associated with worse patient survival [26].
Regarding low rectal cancer in particular, an mrTRG1-2 plus a tumor regression from an “unsafe” to a “safe” plane on post-NAT MR imaging is very specific for a non-involved margin at pathology (Fig. 4.12) [27].
4.2.3.4 Re-staging to Select Patients for Non-operative Management
In a variable proportion of locally advanced rectal cancer patients, there are no signs of viable tumor after NAT. In the observational studies available, the clinical criteria with the highest specificity for a pathologic complete response or a sustained clinical complete response over time are a flat white scar with or without telangiectasia at rectoscopy; and a smooth “normalized” tumor bed at digital rectal examination [28]. Regarding MR imaging, the most specific criteria depend on the analysis of T2-weighted images. They are the following:
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an mrTRG1, corresponding either to a linear/crescentic 1–2 mm hypointense scar on the endoluminal aspect of the rectal wall or to its normalization (rarely observed in our experience) [29].
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a positive split scar sign, corresponding to a specific layered reorganization of the rectal wall at tumor bed [30].
The lack or residual high signal intensity at high b value DWI images supports a complete response, but it is not specific.
An example of a patient with strict criteria for a complete response is provided in Fig. 4.13.
Key Point
Re-staging MRI after neoadjuvant therapy in rectal cancer may document sufficient tumoral regression to support less mutilating curative surgery and help identify clinical complete responders for non-operative management in dedicated centers.
4.3 Concluding Remarks
Benign and malignant diseases of the colon and rectum include a wide spectrum of neoplastic and inflammatory disorders. Cross-sectional imaging techniques (including CT and MRI) play a crucial role in imaging of benign and malignant diseases of the colon and rectum for the primary diagnosis, risk stratification, procedural planning, treatment response evaluation, and the assessment of related extraluminal and extraintestinal pathologies and complications. Despite the significant overlap in imaging findings of different bowel conditions, understanding of leading disease patterns and specific imaging features can allow accurate diagnosis and, therefore, patients’ management.
Take-Home Messages
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Abdominal CT and MRI are standard care for patients with colon and rectum diseases for primary diagnosis and assessment of complications. They allow accurate and reliable diagnosis of different colorectal diseases, including their localization, extension, in-depth mural involvement, enhancement pattern, and also associated pericolonic and extraintestinal findings.
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CT is the imaging workhorse in colon cancer. It allows adequate planning of curative resection in eligible patients, pre- and postoperative diagnose of complications, distant staging, response assessment in those selected for systemic therapy and follow-up.
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Rectal MRI plays a key role in the pre-treatment local staging of rectal cancer and enables assessment of poor prognostic factors including MRF and EMVI.
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Re-staging of rectal cancer after neoadjuvant therapy may help select patients for less mutilating surgery or for non-operative management when an mrTRG 1/split scar positive tumor bed is observed without diffusion restriction.
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Attenberger, U., Santiago, I. (2023). Benign and Malignant Diseases of the Colon and Rectum. In: Hodler, J., Kubik-Huch, R.A., Roos, J.E., von Schulthess, G.K. (eds) Diseases of the Abdomen and Pelvis 2023-2026. IDKD Springer Series. Springer, Cham. https://doi.org/10.1007/978-3-031-27355-1_4
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