Introduction

Colorectal carcinoma (CRC) remains one of the leading causes of cancer-related deaths in developed countries. Dietary habits and hereditary and environmental factors remain risk factors for development of CRC. Most colon cancers are thought to directly develop from adenomatous polyps. The malignant potential of polyps are determined by their size. Polyps greater than 2 cm are at greater than 40% risk of being cancerous, while polyps less than 0.5 cm have essentially no malignant potential.1

Depending upon the stage of disease, patients may present with a range of symptoms starting from rectal bleeding, abdominal pain, and change in bowel habits to fatigue, weight loss, and anemia in advanced stages.2

As anatomy, lymphatic drainage and treatment options of colon cancer and rectal cancer differ significantly. The present review will summarize the application of computed tomography (CT), CT colonography (CTC), and magnetic resonance imaging (MRI), which play an important role in the diagnosis, staging, response evaluation, and follow-up of CRC.

Initial Diagnosis

Colon cancer: As most of the cancers are thought to directly develop from adenomatous polyps, early detection of polyps play an important role in the prevention or early diagnosis of colon cancer. This will also lead to more patients being potentially eligible for curative treatment. Any patient suspected of having a colon cancer should undergo detailed clinical evaluation with proper history taking and measurement of carcinoembryonic antigen (CEA).3 The sensitivity to detect CRC range around 95% for both colonoscopy and CTC.4 However, colonoscopy remains the first line of examination for localization of tumor with an added advantage of taking subsequent biopsy from the lesion. CTC remains the second-line option in patients in whom the lesion could not be reached by colonoscopy due to a non-negotiable stricture or in patients in whom colonoscopy is contraindicated.5 Other techniques like MRI and abdominal CT have inferior performance in early detection of polyps or colon cancers and, therefore, should not be considered primary diagnostic tools.6

Rectal cancer: Primary diagnosis of rectal cancer is more straightforward as compared with colon cancer by using digital rectal examination or proctoscopy. Locoregional staging plays a very important role in the management of rectal cancers. MRI and endoscopic ultrasound (EUS) remain the modalities of choice for locoregional staging.3

Techniques of Computed Tomography

Bowel preparation is done for all planned cases a day prior to the scan. For imaging of colon cancers, oral contrast opacification of colon is of utmost importance when abdominal CT is performed. Water soluble oral contrast agents can be administered 60 to 90 minutes prior to the study to ensure the contrast has reached the colon at the time of scan, which can be confirmed by taking a CT topogram image. Alternatively, positive rectal contrast can also be administered through a rectal tube. Neutral contrast in the form of water7 or negative contrast material in the form of air8 could also be administered via rectal tube for colonic imaging.

Negative contrast in the form of air or carbon dioxide used to distend the colon for CTC can particularly be used for detection of polyps and small masses. While using negative contrast, both prone and supine images should be taken to differentiate residual fecal matter from polyps and help displace fluid in dependent segments that may obscure an underlying lesion. It also helps in distension of the segment of the colon that may remain collapsed in the supine position. Recent advances with virtual colonoscopy with fly-through images can also be performed after 3D reconstruction of dataset for detection of polyps and early colon cancers. Due to high contrast between air and bowel wall/lesion, CTC can be performed with a lower radiation dose than CT scan.

The abdomen is routinely imaged from the diaphragm to the pubic symphysis. Intravenous contrast administration with arterial and venous phase CT images is important in complete staging of a known case of colon cancer and for evaluation of residual/recurrent disease. Routinely, 100 to 120 mL of iohexol contrast is administered intravenously at a rate of 2 to 3 mL/s.

Techniques of MRI

MRI should be performed with at least 1.5-T field strength with an external phased-array coil. The use of endorectal coil is no longer recommended. No dedicated consensus is available on whether spasmolytics or bowel preparation should be used prior to examination.9 In most centers, bowel preparation is not used and spasmolytics are recommended. High-resolution T2-weighted images in axial, coronal, and sagittal planes should be obtained with a slice thickness of 1 to 3 mm. Diffusion-weighted sequences are increasingly used mainly in restaging of rectal cancers after chemoradiotherapy (CRT). Use of intravenous contrast in MRI does not give any additional prognostic information and is not recommended.

Staging

Once the diagnosis is made, staging should be performed using the latest version (8th edition) of the American joint Committee on Cancer (AJCC) tumor, node, and metastasis (TNM) classification10 (Table 1).

T staging of colon cancer: Contrast-enhanced CT scan of the abdomen with arterial and venous phase images are most widely used and are the recommended modality for local staging of colon cancer, and MR is the modality of choice for rectosigmoid cancer staging.3 CT scan helps differentiate tumors localized to the bowel wall from those exceeding the bowel wall with or without infiltration of adjacent structures (Fig. 1 and Fig. 2). However, differentiating T2 and T3 lesions on CT scan can prove challenging in some cases. Apart from T4 tumors, subclassification of T stage does not change the surgical management.11 Application of CT scan in the detection of T1 tumor is limited due its low soft tissue contrast.

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Fig. 1: Oral and intravenous (IV) contrast-enhanced axial computed tomography images showing a well-defined polypoidal mass lesion (arrow) arising from the posterior wall of the descending colon with a mass lesion involving the muscularis propria and no evidence of involvement of pericolonic tissue, suggestive of a T2 lesion.

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Fig. 2: Contrast-enhanced axial computed tomography images showing a large circumferential well-defined heterogeneously enhancing centrally necrotic infiltrating mass lesion causing involvement of pericolonic tissue and infiltration into adjacent small bowel loops (arrow), suggestive of a T4 lesion.

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T staging of rectal cancer: MRI is the modality of choice for locoregional staging of rectal cancers; however, MRI does not differentiate T1 lesions from T2 lesions.1213 Therefore, for patients suspected with T1 lesion, EUS would be the modality of choice. MRI is very useful for evaluating the tumor extension into the mesorectal fat and involvement of the mesorectal fascia (MRF; Fig. 3). The distance of the tumor from the MRF (or circumferential resection margin) plays an important role in therapeutic decision-making, surgical planning, and subclassification of T3 tumors. The distance of the tumor from the MRF also exhibits an important prognostic value. Tumors (or involved nodes) lying ≤1 mm from the MRF or the infiltrating MRF are associated with a high risk of local recurrence.14 MRI also plays an important role in measuring the distance between the anorectal junction and the distal part of the tumor, determining the length of the tumor, sphincteric involvement, and extramural invasion of the tumor.

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Fig. 3: Portal phase axial contrast-enhanced computed tomography scan image through the liver demonstrates an ill-defined poorly enhancing hypodense lesion in the right lobe of the liver (arrow) in a known case of carcinoma of the sigmoid colon, suggesting liver metastasis.

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N staging of CRC: All routinely available radiological modalities exhibit low sensitivity and specificity for identifying nodal metastasis. Unlike other gastrointestinal malignancies, the size criteria are unreliable as up to 50% of metastatic nodes are ≤5 mm in short axis diameter.14 More reliable methods for identifying nodal metastasis are based on morphological factors like irregularity of lymph node borders, presence of round shape, and signal heterogenicity within the nodes.3 Risk assessment based on nodal metastasis should be done with caution with ≤T3b tumors, which have a favorable prognosis irrespective of the presence or absence of nodal metastasis.15 Special attention should be given to the pelvic side wall nodes and nodes in the obturator fossa, as these nodes fall out of standard resection plane and radiation field. If these nodes are not mentioned in the report, there are high chances of these nodes being left untreated and a high local recurrence rate. Recently, a novel MRI technique, diffusion-weighted imaging with background body signal suppression (DWIBS), that gives functional data about tumor cellularity and helps in the detection of suspicious lymph nodes has been introduced.16

M staging: Preoperative M staging is important as, at the time of diagnosis, distant metastases are present in approximately 25% of colon cancers (19% liver and 3% lung metastasis) and 18% of rectal cancers (15% liver and 4% lung metastasis).16 According to the European Registration of Cancer Care (EURECCA) consensus, contrast-enhanced abdominal CT and chest CT should be performed for primary M staging of CRC (Fig. 4).3 Ultrasonography and MRI remain the second-line tools in cases of doubtful liver lesions on CT scan. For detection of liver metastasis ≥1 cm, contrast-enhanced CT scan is considered equal to MRI; however, the sensitivity drops for small lesions, which become important if liver resection is planned.17 To detect smaller lesions, MRI should be done using hepatocyte-specific contrast agents and diffusion-weighted images.18

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Fig. 4: Axial T2-weighted magnetic resonance image showing a normal anatomy of the rectum, mesorectal fat (*), and mesorectal fascia (arrow).

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Response Evaluation After Neoadjuvant Treatment

Restaging after neoadjuvant CRT is usually performed using MRI after 6 to 8 weeks of treatment or before surgery as the surgical approach may differ if the cancer is downstaged following CRT. Restating following CRT is also important as the likelihood for complete pathological response following CRT is around 25%, and these patients might be candidates for a wait-and-watch strategy, which is currently investigated.19 As compared with conventional imaging techniques, use of DWI has increased in sensitivity in detecting residual tumor from 50% to nearly 80%.20 In 2000, the Response Evaluation Criteria in Solid Tumors (RECIST 1.0) were proposed for evaluating treatment response, followed by a revision in 2009 (RECIST 1.1).21 More recently, the MR tumor regression grade (mrTRG) is commonly used for grading tumors.2223

Magnetic Resonance Tumor Regression Grade (mrTRG)

The following MR tumor regression grade (mrTRG) categories are used to assess tumor regression (Fig. 5):

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Fig. 5: Sagittal T2-weighted (T2W) magnetic resonance imaging showing low rectal cancer with intermediate signal before treatment. Following radiation therapy treatment, there is now dense low T2W signal in keeping with fibrosis and a tumor regression grade 2 response.

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• TRG 1: Complete radiological response (linear scar only); no evidence of treated tumor.

• TRG 2: Good response (dense hypointense fibrosis, no obvious tumor signal); no tumor or minimal residual disease.

• TRG 3: Moderate response (∼50% fibrosis/mucin and visible intermediate signal) signifying residual tumor.

• TRG 4: Slight response (mostly tumor, minimal fibrosis/mucinous degeneration).

• TRG 5: No response/regrowth of tumor (same as baseline or progression.

Follow-up: There is poor evidence for a routine use of radiological imaging for follow-up in CRC patients. Chest X-ray is recommended for annual follow-up of patients with rectal cancers in stages II and III within the first 5 years after treatment; however, there is no evidence for routine use of chest X-ray in patients with colon cancer for detecting lung metastasis.14 For detecting liver metastasis, several guidelines recommend an annual abdominal CT for the first 3 years after treatment.24 In case of suspected recurrence or unclear findings on CT/MRI, further evaluation with positron emission tomography (PET) CT might be helpful.14

Conclusion

Radiological imaging plays a very important role in the initial diagnosis, staging, response evaluation, and follow-up of patients with CRC.