Staging of Rectal Cancer: MRI Protocol, Pelvic Anatomy, and Definitions

Abstract

Rectal carcinoma is a prevalent and increasingly diagnosed cancer globally, posing significant mortality and morbidity risks, particularly among younger individuals. Prognosis hinges on several factors, such as site of involvement (upper rectum vs. lower rectum), tumor extension into surrounding tissues, involvement of critical structures like the mesorectal fascia and vessels, and the presence of distant metastases. Treatment approaches are tailored to the tumor's location and extent. Diagnostic tools including digital rectal examination and various imaging modalities like computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography-CT play crucial roles in staging, identifying high-risk patients needing neoadjuvant chemoradiotherapy (CRT), and monitoring treatment response. MRI stands out as the preferred modality for locoregional staging, detecting adverse prognostic indicators, assessing post-CRT changes, and long-term surveillance due to its superior contrast resolution. This review discusses the key concepts in the application of MRI to rectal cancer imaging.

 

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Keywords

anorectal junction - circumferential resection margin - mesorectal fascia - MRI - rectal carcinoma

Introduction

Rectal carcinoma is one of the most prevalent cancers worldwide. It has high mortality and morbidity. The incidence in the younger population is increasing.[1] [2] It is the second most common cancer among women and the third most common cancer in men.[1] [2] The prognosis depends upon various factors like site of involvement, extension beyond the muscularis into the mesorectum, mesorectal fascia (MRF) involvement, invasion of the vessels in the mesorectum, mesorectal tumor deposits, and distant metastasis. The management depends on the site and extent of the involvement. Total mesorectal excision (TME) includes removal of the rectum and peri-rectal fat limited by the thin MRF. It reduces postoperative risk of recurrence significantly.[2] The prognosis strongly depends on the ability to achieve a negative circumferential resection margin (CRM). Anterior resection and low anterior resection surgeries with TME are done for upper rectal malignancy. For lower rectal malignancy, abdominoperineal resection is done where surgical planning depends upon the involvement of the levator, sphincters, and intersphincteric space.[3] Neoadjuvant chemoradiotherapy (CRT) reduces the risk of pelvic recurrence and improves the patient's overall survival in locally advanced rectal cancers.[4] In a recent study, patients with locally advanced rectal cancer, eligible for sphincter-sparing surgery, preoperative FOLFOX with selected use of CRT was noninferior to preoperative CRT.[5] There is a gradual transition from decisions centered on specialties to generalized interdisciplinary patient management, where radiology is the foundation for risk assessment. Various diagnostic modalities, including digital rectal examination, endoscopic ultrasound, and cross-sectional imaging, including computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography-CT (PET-CT), are currently used to diagnose and follow up patients with rectal malignancies. The primary role of imaging is to precisely stage the malignancy, identify the patients (high-risk factors) requiring neoadjuvant therapy, and assessment of response after treatment.[2] [6] MRI is the most frequently used imaging test for locoregional staging, identifying poor prognostic signs, evaluating the post-CRT response, and long-term patient monitoring due to its outstanding contrast resolution. This article aims to review the radiological anatomy, MRI protocols for imaging in rectal carcinoma, the role of MRI in primary, and post-CRT staging, including high-risk markers and its pitfalls.

 

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Relevant Anatomy

The rectum and anal canal are the caudal end of the large intestine. The rectum originates at the level of sacral promontory in the pelvis and measures up to 18 cm. The adventitial taeniae bands form the outer longitudinal muscle in the rectum, distinguishing it from the colon. The valves of Houston are the submucosal folds forming curves in its lumen. It is essential to differentiate rectal cancer from sigmoid cancer due to the significant differences in the treatment approach. An international panel suggested that sigmoid take-off can be used as a landmark for distinguishing the junction of mesorectum and mesocolon on MRI.[7] On sagittal MRI, sigmoid take-off is identified where the sigmoid colon curves horizontally away from the sacrum ([Figs. 1] and [2]). Consistent use of this reference point in interdisciplinary team meetings can improve uniformity in identifying and precisely describing the tumor location. The anal canal measures 2 to 2.5 cm and is encircled by the internal and external anal sphincters. The internal anal sphincter is the continuation of the circular smooth muscles of the rectum, and the external sphincter complex is the continuation of the levator ani along with the puborectalis sling ([Fig. 3]). Anorectal junction is the transition of the rectum and anal canal. Two anatomic landmarks are used to identify the anorectal transition. The first is a sudden increase in the inner muscle layer's thickness corresponding to the internal sphincter's upper limit. The other is the superior border of the puborectalis, which is attached posteriorly to the anococcygeal raphe and anteriorly to the inferior pubic ramus on both sides of the symphysis pubis. It is the levator ani muscle complex's sling- or U-shaped component. On sagittal MRI, the anorectal junction is located at the level of a hypothetical line between the inferior margin of the pubic bone and coccyx ([Fig. 1]).

Fig. 1 Sagittal T2W images depicting sigmoid take-off (STO), anorectal junction (red line), and anal verge (yellow line). T2W, T2-weighted.

Fig. 2 Sagittal and axial T2W images showing the sigmoid take-off (dashed line, A and arrow, B). T2W, T2-weighted.

Fig. 3 (A and B) Schematic and oblique coronal T2W images showing anal sphincter complex (levator ani: long arrow; internal anal sphincter: arrow; external anal sphincter: short arrow; inter-sphincteric plane: dashed arrow), anal verge (AV), and ischial-anal fossa (*).T2W, T2-weighted.

The peritoneum covers the top two-thirds of the rectum anteriorly and the upper third laterally. There is no peritoneal covering at the lower one-third of the rectum ([Figs. 4] and [5]). The lower rectum is enveloped by an endopelvic parietal fascia called Denonvilliers fascia. Anterior peritoneal reflection (APR) is an important landmark and refers to the peritoneal reflection from the urinary bladder/seminal vesicles or uterus to the rectum. On axial T2-weighted (T2W) images, APR is identified as a hypointense line attached in a V shape along the anterior rectal wall. On sagittal images, peritoneal reflection is positioned beyond the seminal vesicle tip in men and at the uterocervical angle in females ([Fig. 4]). Rectal carcinoma is divided into upper, mid, and lower rectal cancer according to the relation with the APR ([Fig. 6]).[6] The upper rectum is defined as the segment above the APR, the mid-rectum corresponds to the segment at the level of APR, and the lower rectum is the segment below the APR. This anatomical division is clinically relevant as it influences the surgical approach, risk of local recurrence, and overall prognosis. Upper rectal tumors are typically managed like sigmoid tumors with favorable outcomes, while lower rectal tumors have higher local recurrence rates and often require more extensive surgical approaches.

Fig. 4 T2W sagittal and oblique axial images showing mesorectum outlined by thin T2W hypointense mesorectal fascia (MRF) and anterior peritoneal reflection (APR). T2W, T2-weighted.

Fig. 5 Schematic diagram showing relation of mesorectal fascia and anterior peritoneal reflection with lower, mid, and upper rectum, respectively.

Fig. 6 Sagittal T2W images depicting low, mid-, and high rectum measurements from the anal verge to defined location of anorectal tumor. T2W, T2-weighted.

Rectum is derived from the endoderm and constitutes four layers: mucosa (including lamina propria and muscularis mucosa), fat-rich submucosa, muscularis propria, and serosa. On T2W MRI, usually two-layered appearance is seen, inner hyperintense layer is mucosa and submucosa and the outer hypointense layer is muscularis propria ([Fig. 7]). These layers of the rectum may have variable thicknesses and depend upon the degree of rectal distension. The rectum is surrounded by the mesorectal fat, containing blood vessels and lymphatics. It is continuous with APR cranially and intersphincteric plane caudally.[8] MRF is the thin hypointense line on T2W images surrounding the mesorectum. Involvement of the MRF is a strong indicator of poor outcomes and early local recurrence. The CRM is a variable potential surgical margin and it depends on the type of surgery. CRM should not be used interchangeably with MRF.

Fig. 7 Schematic image and oblique axial T2W images depicting three layers of anorectum—hypointense mucosa, hyperintense submucosa, and hypointense muscularis propria. T2W, T2-weighted.

 

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Image Acquisition and MRI Protocols

Rectal MRI provides high-quality images with adequate visualization of the rectal anatomy. Using adequate imaging parameters, both 1.5 T and 3 T MRIs provide sufficient signal-to-noise ratio and spatial resolution.[9] Maas et al compared 3 T with 1.5 T for T-staging in patients with rectal malignancy.[10] The accuracy of 3 T and 1.5 T MRI for differentiating T1/T2 from early T3 lesions was comparable.

Bowel preparation is generally not required before the rectal MRI. Bowel peristalsis can cause motion artifacts that degrade the images. Spasmolytics can be administered before the study to minimize motion-related artifacts. Endorectal filling is not recommended as overdistension of the rectum causes compression of the mesorectum, leading to fallacious staging. Mesorectal nodes and small nonnodal deposits may also be displaced/compressed.[2]

The patient is positioned in the supine position. Pelvic phased array multichannel coils are routinely used for rectal imaging. Although the endorectal coils provide better resolution, they are not routinely used due to more significant patient discomfort and high cost. The European Society of Gastrointestinal and Abdominal Radiology (ESGAR) consensus on rectal imaging does not recommend endorectal coils.[9] The protocol followed in our institution for primary imaging of the rectal malignancy constitutes the following sequence:

• High-resolution fast spin echo T2W non-fat-saturated images in sagittal, oblique coronal, and oblique axial plane: the axial images are planned in the plane perpendicular to the axis of the rectal mass seen at the sagittal plane ([Fig. 8]). An incorrect plane can smudge the T2 hypointense muscularis propria, leading to inaccurate T staging. The involvement of sphincters in low rectal tumors is best assessed on oblique coronal images planned parallel to the anal canal.[2] [11]

• T1W (T1-weighted) coronal or axial image: it allows a baseline evaluation for comparison with post-contrast T1W imaging.

• Diffusion-weighted imaging (DWI): it has a limited role in evaluating the primary lesion. However, it is useful in detecting nodal involvement and tumoral deposits. Society of Abdominal Radiology (SAR) guidelines recommend the inclusion of DWI in primary imaging, whereas ESGAR guidelines state it as optional. It is mainly utilized in post-CRT response assessment. Our institution routinely includes DWI in baseline imaging and restaging protocols.

• The role of dynamic contrast-enhanced MRI (DCE-MRI) in the imaging of rectal malignancy is debatable, with no definite consensus in the literature. Patel et al concluded that DCE-MRI is valuable for nodal staging with an accuracy of 93%. Early arterial enhancement in the nodes suggested a malignant nature. However, DCE-MRI did not enhance the precision of identifying tumor stage, CRM, or recognizing complete response.[12] According to another study by Armbruster et al, the sensitivity and specificity of detecting small mesorectal lymph node metastasis (5–10 mm) is significantly enhanced by DCE-MRI, also improving specificity for suspected MRF infiltration in patients with locally advanced rectal cancer.[13] In another study by Gollub et al, the parameters derived from DCE-MRI (k-trans) showed a significant difference between tumors showing complete response versus those showing incomplete response.[14] A few similar studies also showed the considerable role of DCE-MRI in restaging and differentiating complete and incomplete responses.[15] [16] DCE-MRI is not generally recommended in the primary staging.[2] [10] [11] However, it has a role in restaging and assessing the treatment response post-CRT.

• Fat-suppressed sequences are not required in rectal cancer imaging.[2] [17]

Fig. 8 Figure representing the planning of oblique axial and oblique coronal planes during MRI acquisition. MRI, magnetic resonance imaging.

 

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Primary Staging

Tumor Morphology and Localization

Till recently rectal cancer division into upper (10–15 cm), middle (5–10 cm), and lower (<5 cm) was based on the distance of involvement from the anal verge, this leads to inaccurate assignment of therapies as there are significant interindividual variations in the length of the rectum. APR as a landmark for dividing rectum into upper (above the APR), mid (at the APR), and lower (below the APR) is more accurate and preferred.[6] It is essential to define the precise location and the extent of the tumor due to differences in the surgical approach. Longitudinal length, circumferential location, distance from the anal verge, anorectal junction, and APR involvement must be entered in the report. The site of attachment of the tumor to the rectal wall is called the invasive margin.

Rectal carcinomas can be exophytic (polypoidal or sessile) or nonexophytic (ulcerated flat lesions). Patients with exophytic lesions have better outcomes and a reduced risk of recurrence than those with nonexophytic lesions. Lymphatic and venous invasion is also more common in the nonexophytic group.[18]

Mucinous adenocarcinoma is a histological subtype with greater extracellular mucin content. It is more aggressive, affects relatively young people, and usually presents at an advanced stage with a poor prognosis and a high rate of recurrence. Mucinous adenocarcinomas do not show a significant response to neoadjuvant CRT. Thus, it is essential to differentiate it from the other subtypes. Mucinous adenocarcinomas show hyperintense signal on T2W images ([Fig. 9]).[2] [8] [18]

Fig. 9 (A and B) Axial (A) and sagittal (B) T2-weighted MRI of a patient with mucinous rectal carcinoma showing markedly hyperintense mural thickening of the mid and lower rectum (arrows) with extensive extramural hyperintense content in the mesorectum (short arrows). MRI, magnetic resonance imaging.

 

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Preoperative Staging and Risk Stratification

MRI is the modality of choice for the local staging as it provides detailed information on the extent of involvement of the rectum, mesorectal extension, and lateral pelvic wall involvement.[2] [6] [19] [20] [21] Preoperative staging requires comment on tumor extent (T), nodal involvement (N), and metastatic spread ([Fig. 10]). [Table 1] summarizes the TNM staging of rectal carcinoma according to American guidelines. The T3 sub-classification is according to the European guidelines.[6] [22] [23] Preoperative imaging helps in the treatment stratification of the malignancy into the low, intermediate, and high risk for a tailored approach.[24] [25] [26] Locally advanced disease constitutes T3–T4 lesions and node-positive nonmetastatic disease.

Table 1 TNM staging for rectal cancer

Primary tumor (T)
TX: Primary tumor cannot be assessed T0: No evidence of primary tumor Tis: Carcinoma in situ: intraepithelial or invasion of lamina propria T1: Tumor invades submucosa T2: Tumor invades muscularis propria T3: Tumor invades subserosa and perirectal tissue. (a: <1 mm, b: 1–5 mm, c: >5 mm, d: >15 mm) T4a: Tumor penetrates to the surface of the visceral peritoneum T4b: Tumor directly invades or is adherent to other organs or structures
Regional lymph nodes (N)
NX: Regional lymph nodes cannot be assessed N0: No regional lymph node metastasis N1: Metastasis in one to three regional lymph nodes N1a: Metastasis in one regional lymph node N1b: Metastasis in two to three regional lymph nodes N1c: Tumor deposit(s) in the subserosa, mesentery, or nonperitonealized pericolic or perirectal tissues without regional nodal metastasis N2: Metastasis in four or more regional lymph nodes N2a: Metastasis in four to six regional lymph nodes N2b: Metastasis in seven or more regional lymph nodes
Distant metastasis (M)
M0: No distant metastasis M1: Distant metastasis M1a: Metastasis confined to one organ or site (e.g., liver, lung, ovary, nonregional node) M1b: Metastases in more than one organ/site or in the peritoneum

Fig. 10 Schematic diagram showing T4 staging TNM classification in carcinoma rectum.

T-staging refers to the maximum depth of infiltration of the tumor into the various layers of the rectal wall, beyond muscularis into the mesorectal fat space, involvement of adjacent organs, or lateral pelvic wall. The overall T1 is invasion of the submucosa, T2 is invasion of muscularis, and T3 is invasion beyond into the mesorectum without involvement of MRF. Invasion of the peritoneum upgrades the stage to T4a. T4b is the involvement of adjacent pelvic organs. Perirectal extent can be in the direct extension of the tumor/noncontiguous deposits within the mesorectal fat space. Although high-resolution imaging with small field of view can help increase diagnostic accuracy, MRI has low sensitivity for differentiating T1 and T2 stages, overestimating the clinical stage.[27] Accurate preoperative staging is essential for planning organ-preserving surgeries. Endorectal ultrasound has high accuracy in differentiating T1 and T2 tumors and can be routinely combined with MRI to improve diagnostic accuracy.[28] [29] [30] Extramural involvement in T3 tumors appears as an interruption of the T2W hypointense muscularis mucosa. It is challenging to differentiate T1/T2 tumors from early T3 tumors in the presence of a desmoplastic reaction, which can be mistaken for extramural invasion (EMVI). Desmoplasia is usually seen as T2 hypointense spiculated strands in the mesorectum. In contrast, tumor extension is more nodular with intermediate signal intensity on T2W images and is usually seen adjacent to the site of maximum depth of the tumor. The focal disruption of the muscularis via a penetrating vessel is another pitfall leading to overestimating the clinical stage. The involvement of fat outside the mesorectum in the ischiorectal fat space and obturator space also constitutes T4b lesions ([Figs. 11] [12] [13] [14]). Mesorectum is thinned out anterior to the lower rectum and anal canal. Thus, there is a higher incidence of locally advanced disease. The prognosis and choice of surgery also depend on the involvement of the sphincter complex. Therefore, the report should mention the status of the internal and external sphincter, puborectalis, and levator ani. Involvement of the external anal sphincter, levator ani, and other pelvic floor muscles upgrades the T stage to T4b. The T2W oblique coronal plane is essential to delineate the anatomy and extent of sphincteric and extra-levator involvement ([Fig. 15]). The involvement of the internal anal sphincter/intersphincteric plane is not described in TNM staging. Sphincter-sparing surgeries like transanal local excision can be done for early-stage (T1N0) lower anorectal tumors that cover <30% of the rectal circumference. Transanal endoscopic microsurgery (TME) is a minimally invasive surgery for well-differentiated early low rectal lesions associated with reduced morbidity.[31] [32] Intersphincteric resection is done for the low rectal lesions that extend across the muscle plane with sparing of external anal sphincter. TME involves the en bloc removal of mesorectal fat, blood vessels, and lymphatics with the sparing of autonomic nerves.[33] Abdominoperineal resection, along with TME with the removal of the anal canal, is done in lesions that involve the intersphincter plane and the external anal sphincter.[34]

Fig. 11 (A and B) T2W images (sagittal and oblique axial) in a patient with rectal carcinoma with T2W intermediate signal intensity lesion involving the rectum with intact hypointense muscularis propria arrow findings suggestive of T2 stage. T2W, T2-weighted.

Fig. 12 Oblique axial T2W images showing T3 tumor stages obtained from three patients showing T2W intermediate intensity tumor. (A) Interruption of hypointense muscularis propria (T3a); (B) extending beyond muscularis propria into mesorectal fat <5 mm (T3b); (C) extending beyond muscularis propria into mesorectal fat >5 mm with MRF involvement and CRM positive (T3c). CRM, circumferential resection margin; MRF, mesorectal fascia; T2W, T2-weighted.

Fig. 13 Oblique axial (A, B) images obtained from patient with T4 stage rectal cancer showing tumor infiltration into adjacent pelvic organs. (C) Sagittal T2W image in a patient with rectal cancer depicting involvement of the anterior peritoneal reflection suggestive of T4a stage. T2W, T2-weighted.

Fig. 14 Oblique axial T2W MRI images obtained from different patients depicting (A) normal mesorectal fat, (B) linear thin T2 hypointense strands (arrow) extending into mesorectum suggesting desmoplasia, (C) T2W nodular intermediate tumor signal intensity direct tumor infiltration into the mesorectal fat. MRI, magnetic resonance imaging; T2W, T2-weighted.

Fig. 15 (A, B) T2W oblique axial and oblique coronal images of patient with lower rectal malignancy depicting circumferential asymmetrical tumor infiltrating the right internal sphincter, inter sphincteric plane, and puborectalis sling (arrow). (C) Oblique coronal image of different patient with lower rectal malignancy depicting tumor infiltrating the external sphincter and levator ani which appear altered in signal intensity. T2W, T2-weighted.

 

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N-Staging

Rectal malignancy disseminates via local extension, hematogenous route, and lymphatics. Lymph nodal extension is one of the significant prognostic factors in rectal malignancies.

The locoregional lymph node group includes mesorectal lymph nodes ([Fig. 16]) and extramesorectal lymph nodes (superior rectal, inferior mesenteric, presacral, obturator, and internal iliac lymph nodes). Distant nodes include external and common iliac and inguinal lymph nodes ([Fig. 17]). These are part of M-staging.[35] [36]

Fig. 16 Oblique axial T2W rectal images from different patients depict a mesorectal lymph node (A) and a deposit (B). T2W, T2-weighted.

Fig. 17 (A and B) Consecutive axial images illustrating the lymphatic system in rectal malignancy (superior rectal, EI: external iliac, O: obturator, II: internal Iliac and mesorectal).

The Dutch criterion uses both size and morphology for locoregional nodal metastasis. The three-point morphological criterion includes round shape, irregular, spiculated/pointed margins, and heterogeneous signal intensity (due to necrosis or variable mucin). Lymph nodes >9 mm are suspicious irrespective of the morphology; between 5 and 9 mm are considered suspicious if two morphological features are present, and <5 mm nodes require all three morphological points. Nonregional lymph nodes >10 mm are considered metastatic. TNM nodal staging depends upon the number of locoregional lymph nodes. Thus, the number should be mentioned by the radiologist in the report. N1c constitutes the tumoral deposits in the mesorectum ([Fig. 16]). They are either noncontiguous deposits from primary along the vessels or completely infiltrated lymph nodes with no residual lymphoid tissue. It is challenging to differentiate mesorectal lymph nodes from these deposits. However, the latter are more irregular in shape and heterogeneous in signal intensity. Their precise size and location must be mentioned in the report. The minimum distance between the deposit and MRF is also critical for positive CRM.

Advanced Sequences

The role of DWI and qualitative and quantitative DCE-MRI in primary nodal staging is debatable ([Fig. 18]). According to a meta-analysis by Arian et al, the sensitivity and specificity of conventional MRI for nodal staging are 74 and 77%, respectively. DCE-MRI has a sensitivity of 83% and a specificity of 86%, while DWI has 81 and 74%, respectively.[37] According to another study by Mir et al, adding DWI improved the detection of pelvic lymph nodes. However, there needs to be more standardized data and consensus on the same.[38] [39] ESGAR guidelines do not recommend the routine use of DWI for primary nodal staging. However, the SAR guidelines recommend including DWI in the routine protocol.[40] Malignant lymph nodes show peripheral rim enhancement on post-contrast images, likely due to central necrosis. On DCE-MRI, intense early arterial phase enhancement is noted in the malignant lymph nodes.[11] Although few studies have shown promising results with both quantitative and qualitative DCE-MRI, it is not recommended routinely for primary staging of rectal malignancies by both ESGAR and SAR guidelines. Lymph-node–specific agents like ultrasmall superparamagnetic iron oxide (USPIO) may improve the detection of malignant lymph nodes. Normal or inflammatory reactive lymph nodes take up USPIO; malignant lymph nodes do not show uptake and appear brighter on the T2W sequence.[8] Gadofosveset is another agent that shows selective uptake only by normal cells; thus, malignant lymph nodes show less enhancement.[41]

Fig. 18 Axial DCE-MRI sequential scans (A–D) showing no early abnormal enhancement. There is enhancement on delayed images suggesting predominant fibrosis (marked by arrows).

 

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M-Staging

The involvement of the nonregional lymph nodes is considered metastatic. CT has a role in the metastatic workup of patients for the detection of distant metastasis to the chest, upper abdomen, and bony metastasis in advanced cases. It has low sensitivity in locoregional spread of the malignancy due to poor contrast resolution. DWI of the upper abdomen allows detection of small liver metastases that may be missed at CT.[42] PET-CT has a role in preoperative metastatic workup in advanced cases. It is mainly used for postoperative follow-up for disease activity and tumor recurrence.[2] [6] [9] [12] [42]

 

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Poor Prognostic Markers

• EMVI: it gives information on the tumoral infiltration into the veins in the mesorectal fat. The vessels appear expanded, nodular, elongated projections, with signal intensity similar to the tumor on T2W images ([Fig. 19]). It is also a predictor of distant metastasis in a locally advanced rectal cancer, with a poor prognosis and a high risk of recurrence.[43] In the MERCURY II study and study by Battersby et al., EMVI was identified as an independent risk factor for poor response to treatment in preoperative staging.[44] [45]

• Sphincter complex involvement: the surgical planning of the anorectal malignancy depends on the involvement of the sphincter complex, which is well seen on MRI images. The patients with sphincteric involvement are given neoadjuvant CRT to downstage the disease for sphincter-sparing surgeries.[2] [6]

• APR involvement defines the type of surgical resection with tumor above the APR managed as sigmoid tumors.

• Extension to the adjacent organs and lateral pelvic wall must be mentioned in the report to help plan surgical resectability.

Fig. 19 (A) Oblique coronal T2W rectal image from a patient depicting a normal vessel extending from the muscularis propria into the perirectal fat (arrow). The vessel shows a thin contour. (B) Tumor involving the rectal wall extending into the mesorectal vessel (arrow). The vessel is enlarged, with altered signal and disrupted contour—EMVI. EMVI, extramural invasion. T2W, T2-weighted.

 

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Tumor Recurrence

There is a significant reduction in the recurrence rates of locally advanced rectal cancer post-advent of neoadjuvant CRT. The risk factors include poorly differentiated locally advanced cancer with higher TNM stage, CRM- and EMVI-positive lesions, lower rectal canal tumors, and mucinous and signet cell cancers. MRI is the modality of choice for locoregional recurrence. PET-CT is utilized for evaluating distant metastases.

The most common site of local recurrence is at the site of anastomosis in postoperative patients. Tumor deposits and malignant lymph nodes can be seen in the mesorectum, presacral space, ischiorectal/obturator fat space, lateral pelvic walls, adjacent pelvic organs, iliac vessels, and mesentery. They appear intermediate in signal intensity on T2W images. DWI images at a high b-value help visualize distant tumor deposits. DCE-MRI also has a role in differentiation from postoperative fibrosis. Early intense enhancement is seen in recurrent lesions as compared with fibrotic changes.

 

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Structured Reporting Pattern

The primary staging reporting should include specific information relevant to the surgical planning and treatment. The use of a structured reporting pattern improves the quality of the report, resulting in greater satisfaction among the surgeons ([Table 2]).[9] [46]

Table 2 Structured report template for primary staging

Local tumor status
1. Morphology and circumferential O'clock location   a. Solid-polypoidal growth   b. Solid-semiannular growth   c. Mucinous  2. Total length of tumor  3. T stage   T1/T2   T3 a/b/c/d   T4  4. Distance of caudal margin from anal verge and anorectal junction  5. Sphincter involvement   a. Internal sphincter   b. Intersphincteric plane   c. External sphincter
Mesorectal fascia and peritoneal involvement
1. Shortest distance between tumor and MRF   CRM—positive/negative/threatened  2. Location of shortest distance between tumor and MRF (in O'clock position)  3. Relation to anterior peritoneal reflection
Lymph node status
1. N stage  2. Total number of involved lymph nodes: yes/no  3. Location of lymph nodes (mesorectal/ extramesorectal)  4. Tumor deposits in mesorectum: yes/no (with number of deposits)
Extramural venous invasion
Yes/No

Abbreviations: CRM, circumferential resection margin; MRF, mesorectal fascia.

In conclusion, MRI is the imaging modality of choice for locoregional staging of rectal carcinoma. It allows accurate evaluation of local extent of disease which guides appropriate management. Reporting in a structured template improves the report's quality and enhances the clinicians' confidence in the report.

 

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Conflict of Interest

None declared.

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Publikationsverlauf

Artikel online veröffentlicht:
12. Juni 2025