REVIEW ARTICLE
Local Magnetic Resonance Imaging Staging of Rectal Adenocarcinoma Tina Sprouse, MD, Corey T. Jensen, MD, Rafael Vicens, MD, Randy Ernst, MD, and Priya Bhosale, MD DISCUSSION
Abstract: Successful multidisciplinary evaluation of potentially resectable rectal adenocarcinoma depends on high-resolution preoperative magnetic resonance imaging (MRI). Magnetic resonance imaging accurately identifies important risk factors of local recurrence and distant metastasis, thus facilitating enhanced preoperative prognostic stratification and treatment. When combined with appropriate neoadjuvant chemotherapy and total mesorectal excision, the treatment of rectal cancer has dramatically improved. Accurate local staging by MRI requires a robust combination of imaging sequences. Herein, we review MRI imaging and rectal anatomy related to the staging of rectal adenocarcinoma. Key Words: rectal carcinoma imaging, rectal MRI staging, rectal anatomy, total mesorectal excision (J Comput Assist Tomogr 2014;38: 885–889)
A
56-year-old man with a medical history significant only for recent suspected diverticulitis treated with antibiotics presented to the emergency center with severe lower abdominal pain and a 7-month history of bright red blood per rectum. Computed tomography of the abdomen and the pelvis demonstrated circumferential rectal wall thickening and perforation (Fig. 1). Differential considerations for circumferential rectal wall thickening with perforation include inflammatory and infectious etiologies such as diverticulitis, infectious or inflammatory colitis, radiation enteritis, and ischemic colitis.1,2 Although spontaneous perforation related to colorectal carcinoma is uncommon, authors have reported it to occur in approximately 1.6% of colorectal carcinoma cases.3 Malignancy was a strong consideration in this patient owing to the patient's age and chronic abdominal pain associated with hematochezia. Colonoscopy with biopsy of the rectum confirmed the presence of invasive moderately differentiated rectal adenocarcinoma (Fig. 2). As part of the multidisciplinary evaluation, magnetic resonance imaging (MRI) staging was performed (Fig. 3A), revealing tumor extension through the mesorectal fascia and abutting the adjacent prostate and seminal vesicles. The patient underwent total mesorectal excision (TME) after 4 months of neoadjuvant chemoradiation (Fig. 3B). The surgery included en bloc resection of the prostate and seminal vesicles because of the tumor abutment noted on MRI. The final pathology report indicated a ypT3 tumor and 11 negative regional lymph nodes. Given the initial presence of significant extramural tumor extension and perforation (cT4a), the patient is currently undergoing adjuvant chemotherapy.
From the Department of Diagnostic Radiology, The University of Texas MD Anderson Cancer Center, Houston, TX. Received for publication July 29, 2014; accepted September 17, 2014. Reprints: Corey T. Jensen, MD, Department of Diagnostic Radiology, Unit 1473, The University of Texas MD Anderson Cancer Center, 1515 Holcombe Blvd, Houston, TX 77030 (e‐mail: [email protected]). An expanded discussion of this “Case of the Quarter” can be found at the SCBTMR Web site: scbtmr.dnnstaging.com. The authors declare no conflict of interest. Copyright © 2014 by Lippincott Williams & Wilkins
Adenocarcinoma of the colon is one of the most common malignancies in the United States, with 30% to 40% of cases originating in the rectum.4 Historically, radical resections were performed for rectal adenocarcinoma, often with blunt dissections, which resulted in dismal outcomes. During recent decades, TME has become the standard surgical treatment for most rectal cancers, consisting of en bloc resection of the rectum and the surrounding mesorectum using sharp dissection to the levator musculature. This surgical approach and neoadjuvant chemoradiation therapy have led to dramatically improved outcomes.5–7 The rectum is traditionally divided into three 5-cm sections: the lower, middle, and upper thirds. Tumor position is classified according to the location of the inferior-most portion of the tumor as measured from the anal verge. For example, because the measurement from the anal verge includes the anus, the inferior rectum is typically the first 7- to 10-cm segment from the anal verge. The middle rectum is the next 5-cm segment and the upper rectum is the following 5-cm portion.7 These divisions of the rectum have unique characteristics, which require special considerations given their associated clinical implications. For instance, the anterior portion of the upper rectum typically abuts the peritoneal reflection (Fig. 4); thus, superior rectal tumors have a higher rate of intraperitoneal extension resulting in classification as stage T4 disease.8 The middle rectum has less common but variable exposure to the peritoneal cavity. The lower third of the rectum is extraperitoneal, but tumors in this region require special consideration because they often approach the anal sphincter complex and pelvic organs. Consequently, inferior rectal tumors are associated with the highest rate of local recurrence and worse overall outcomes.9 Each rectal level is surrounded by the mesorectal fascia, which is a connective tissue sheath (Fig. 4). The mesorectum contains the rectum, mesorectal fat, vasculature, and lymph nodes.10,11 Resection of the mesorectum as an intact sleeve to the levator ani level constitutes TME. Adequate distal margins of superior rectal tumors can often be obtained without complete TME; thus, tumor-specific TME is often preferred, which does not require dissection to the levator ani level.12 Determination of the primary rectal tumor level is important for selecting the appropriate operative approach and is a predictor of outcomes. For instance, a 2-cm distal margin of resection is standard for TME. Thus, detailed evaluation of tumors near this distance from the anal sphincter complex is of extreme importance; although the exact treatment will depend on clinician experience and other clinical factors, the decision of whether low anterior resection or abdominoperineal resection is appropriate will be largely affected by evaluation of this potential circumferential resection margin (CRM). This preoperative stratification is critical to maintaining the lowest possible recurrence rates.13,14 Preoperative MRI assessment of rectal cancer has greatly improved patient stratification and has become an integral component of multidisciplinary assessment. Magnetic resonance imaging facilitates accurate staging of the primary tumor, which is derived
J Comput Assist Tomogr • Volume 38, Number 6, November/December 2014
www.jcat.org
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
885
J Comput Assist Tomogr • Volume 38, Number 6, November/December 2014
Sprouse et al
FIGURE 1. Axial image from the patient's pelvis CT obtained with IV contrast demonstrates circumferential rectal wall thickening, rectal wall disruption, and an adjacent left anterolateral fluid collection consistent with perforation (arrow).
FIGURE 4. Coronal oblique T2-weighted MR image of the pelvis depicts the mesorectal fascia (white arrow) and peritoneal reflection (clear arrow).
FIGURE 2. Photograph taken during colonoscopy shows a portion of the patient's rectal mass, which extended from 2 to 12 cm from the anal verge. Figure 2 can be viewed online in color at www.jcat.org.
from a robust assessment of the rectal wall layers. Specifically, the submucosa, muscularis propria, and mesorectal fat are reliably depicted as distinct MRI layers, allowing for clinically relevant T staging (Table 1; Figs. 5A, B).15 In addition to providing the distance of tumor extension beyond the muscularis propria, MRI details the distance of the tumor to the mesorectal fascia, which provides accurate assessment of the CRM, thus assisting presurgical and neoadjuvant therapy planning (Fig. 4).7 Technical parameters and consistency are critical to ensuring a high level of MRI staging accuracy for rectal carcinoma. Highresolution, triplane T2-weighted fast spin echo sequences without fat saturation make up the foundation of proper MRI assessment.8 A sagittal T2 sequence is typically performed first to provide a planning sequence for subsequent oblique axial sequences that are angled orthogonal to the tumor plane. When tumors are large or tortuous, often 2, and sometimes 3, oblique series are obtained.
FIGURE 3. A, Sagittal T2-weighted MR image depicts the patient's invasive middle rectal tumor extending anteriorly to abut the prostate and seminal vesicles (dashed arrow). B, Response of the tumor to neoadjuvant therapy is noted based on significant reduction in the size of the mass and increased fibrosis.
886
www.jcat.org
© 2014 Lippincott Williams & Wilkins
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
J Comput Assist Tomogr • Volume 38, Number 6, November/December 2014
TABLE 1. MRI T Staging of Rectal Adenocarcinoma T Stage Tx T0 T1 T2 T3a T3b T3c T3d T4
MRI Findings Primary tumor cannot be assessed No evidence of primary tumor Tumor invades the submucosal layer Tumor invades the muscularis propria Tumor invades 15 mm beyond the muscularis propria Tumor invades adjacent organs or peritoneal reflection
The coronal T2 sequence is performed without angulation, which typically provides the best assessment of the anal sphincter complex (Fig. 6). Diffusion-weighted imaging (DWI) is increasingly used in the evaluation of rectal cancer. Initial experience with DWI demonstrated that some otherwise occult primary rectal tumors could be detected with the use of DWI. Diffusion-weighted imaging also provides high contrast of lymph nodes, thus assisting in the general localization of nodes, although DWI does not provide improved specificity for detecting nodal disease. Although DWI can assist in the initial assessment of rectal cancer, evolving evidence suggests that it can be used as a functional technique in predicting and restaging tumor response after chemoradiation.16 However, the evidence regarding this is conflicting, requiring further investigation.7,17 Other sequences and techniques can be used for rectal carcinoma assessment but are often used only in certain clinical scenarios because current evidence does not support their standard use. For example, researchers have suggested intravenous (IV) administration of contrast for restaging of rectal cancer in patients who have undergone chemoradiation; however,
Local MRI Staging of Rectal Adenocarcinoma
accuracy for the initial staging of rectal carcinoma does not improve with the use of IV contrast.16,18 T1-weighted sequences are not typically performed for rectal carcinoma assessment but can be added when an incidental or otherwise confounding finding is present such as an ovarian mass. A 3-dimensional T2-weighted sequence is used in some centers because this can potentially improve signal-noise ratio and, if thinsectioned, can allow for postprocessing 3-dimensional reformations.8 Rectal distension with gel can be useful to delineate small, low–T-stage, and polypoid tumors, but this is not standard and it often provides redundant information because patients will undergo endoscopy. Lastly, use of endorectal coils is not recommended because it causes rectal distension, which may lead to incorrect T staging.16 The T stage is determined according to the depth of tumor invasion (Table 1). For T3 tumors, determining the closest distance of the primary tumor or tumor deposits to the mesorectal fascia (Fig. 5) is critical because it predicts the involvement of the CRM. On MRI, a distance of 1 mm from the primary tumor or tumor deposits to the mesorectal fascia indicate involvement of the CRM, with an accuracy rate of 92%. If the distance from the tumor to the mesorectal fascia is greater than 6 mm on MRI, a tumor-free margin of at least 2 mm is predicted with 97% accuracy.19,20 Special consideration must be given to low rectal tumors, for which assessment of the intersphincteric plane is key (Table 2; Fig. 6). If this plane is clear, clinicians with appropriate experience can often perform adequate sphincter-preserving resection.17 In addition to T staging, MRI can also provide valuable assessment of pelvic lymph nodes. Although mesorectal lymph nodes are removed during TME, it is important to identify concerning nodes and tumor deposits that approach the mesorectal fascia, thus threatening the CRM.7 Lymph node size alone is a poor predictor of nodal tumor involvement, but using the presence of irregular nodal morphology and irregular internal signal to indicate involvement does improve accuracy, with sensitivity and specificity reported to be 85% and 97%, respectively.9,20 Because removal of nodes external to the mesorectal fascia is not standard,
FIGURE 5. A, Illustration of rectal cancer T stages. B, Oblique T2-weighted MR image of a companion rectal cancer case demonstrates T3 extramural extension of the tumor (dashed arrow) and EMVI (clear arrow). Separate, uninvolved hypointense muscularis propria and intermediate-intensity submucosa are well delineated as separate layers (white arrow). Figure 5 can be viewed online in color at www.jcat.org. © 2014 Lippincott Williams & Wilkins
www.jcat.org
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
887
J Comput Assist Tomogr • Volume 38, Number 6, November/December 2014
Sprouse et al
FIGURE 6. A, Coronal T2-weighted MR image from another patient shows a normal low–signal-intensity external sphincter (clear arrow) as a continuation of the levator ani. The internal sphincter continues from the muscularis propria (white arrow). The T2 hyperintense intersphinteric fat plane (dashed arrow) separates the internal and external sphincters. B, Axial T2-weighted MR image reveals the external sphincter (clear arrow), intersphincteric plane (dashed arrow), and internal sphincter (white arrow).
reporting of concerning nodes in these regions can assist in modification of the treatment approach to prevent untreated adenopathy; possible treatment modifications can include both an altered radiation field and/or extended lymphadenectomy.8 In a majority of cases, nodal disease occurs in the mesorectal, superior hemorrhoidal, and inferior mesenteric chains, but drainage toward the pelvic sidewall can occur, often via the middle rectal lymphatic drainage. This lateral pathway of drainage is more often seen with mid to lower rectal tumors. In general, lymphatic drainage occurs in a cephalad manner, but adenopathy has been reported up to 4 cm distal to primary rectal tumors.22 An increasingly recognized and important feature in MRI assessment of rectal carcinoma is extramural venous invasion (EMVI). This can be identified on MRI by loss of normal vascular signal voids, venous expansion, and/or discrete intravascular soft tissue. Importantly, EMVI can threaten the CRM; thus, the distance from EMVI to the mesorectal fascia should be reported.7 Furthermore, the detection of EMVI is an independent predictor of increased risk for synchronous and metachronous metastases. Currently, whereas the presence of EMVI does not directly influence initial treatment planning, it may become a factor in treatment stratification for rectal cancer patients in the future.23 Magnetic resonance imaging is increasingly used in the evaluation of neoadjuvant therapeutic response of rectal cancer and can provide useful prognostic information (Table 3). Magnetic resonance imaging assessment of details such as the degree of therapy-related fibrotic change has evolved from pathology studies demonstrating prediction of local recurrence, disease-free survival,
TABLE 2. Proposed MRI Staging of Low Rectal Adenocarcinoma21 Stage 1
Tumor confined to bowel wall (partial thickness) Tumor confined to bowel wall (full thickness) but does not extend into the intersphincteric fat plane Tumor invades the intersphincteric plane or lies within 1 mm of the levator muscle Tumor invades the external anal sphincter
Stage 2
Stage 3
Stage 4
888
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and overall survival using posttreatment specimens. Importantly, MRI assessment of treatment response requires strict control of technique to ensure that pretreatment and posttreatment scans are as similar as possible, including use of the same obliquities for T2-weighted sequences. Magnetic resonance imaging evidence of treatment response includes increased fibrosis and reduced tumor size. Interestingly, although mucinous tumors have a worse prognosis than nonmucinous rectal carcinoma, the internal development of mucin related to neoadjuvant therapy often denotes a favorable response.17 This postchemoradiation MRI evaluation can aid in determining the timing of surgery and need for additional neoadjuvant treatment as well as surgical planning.7,17 In summary, MRI is a critical tool in the multidisciplinary staging of primary rectal tumors. Identification of tumor location, local involvement, and nodal disease provides key information for treatment stratification and presurgical planning. Future technical advances in MRI will likely lead to shorter examination times, improved anatomic detail, and further improvement of staging accuracy. In patients with low–T-stage tumors, improved diagnostic accuracy may aid the trend toward more conservative treatment strategies. Combined with clinical advances, improved MRI
TABLE 3. Prognostic MRI Features for Rectal Carcinoma7 Predictors of Recurrence Extramural tumor extension > 5 mm Tumor within 1 mm of mesorectal fascia Inferior rectal tumor involving the intersphincteric plane Presence of EMVI
Good Prognostic Features CRM uninvolved on preoperative MRI (tumor > 1 mm from the mesorectal fascia) No evidence of EMVI Early MRI T stage (
Local Magnetic Resonance Imaging Staging of Rectal Adenocarcinoma Tina Sprouse, MD, Corey T. Jensen, MD, Rafael Vicens, MD, Randy Ernst, MD, and Priya Bhosale, MD DISCUSSION
Abstract: Successful multidisciplinary evaluation of potentially resectable rectal adenocarcinoma depends on high-resolution preoperative magnetic resonance imaging (MRI). Magnetic resonance imaging accurately identifies important risk factors of local recurrence and distant metastasis, thus facilitating enhanced preoperative prognostic stratification and treatment. When combined with appropriate neoadjuvant chemotherapy and total mesorectal excision, the treatment of rectal cancer has dramatically improved. Accurate local staging by MRI requires a robust combination of imaging sequences. Herein, we review MRI imaging and rectal anatomy related to the staging of rectal adenocarcinoma. Key Words: rectal carcinoma imaging, rectal MRI staging, rectal anatomy, total mesorectal excision (J Comput Assist Tomogr 2014;38: 885–889)
A
56-year-old man with a medical history significant only for recent suspected diverticulitis treated with antibiotics presented to the emergency center with severe lower abdominal pain and a 7-month history of bright red blood per rectum. Computed tomography of the abdomen and the pelvis demonstrated circumferential rectal wall thickening and perforation (Fig. 1). Differential considerations for circumferential rectal wall thickening with perforation include inflammatory and infectious etiologies such as diverticulitis, infectious or inflammatory colitis, radiation enteritis, and ischemic colitis.1,2 Although spontaneous perforation related to colorectal carcinoma is uncommon, authors have reported it to occur in approximately 1.6% of colorectal carcinoma cases.3 Malignancy was a strong consideration in this patient owing to the patient's age and chronic abdominal pain associated with hematochezia. Colonoscopy with biopsy of the rectum confirmed the presence of invasive moderately differentiated rectal adenocarcinoma (Fig. 2). As part of the multidisciplinary evaluation, magnetic resonance imaging (MRI) staging was performed (Fig. 3A), revealing tumor extension through the mesorectal fascia and abutting the adjacent prostate and seminal vesicles. The patient underwent total mesorectal excision (TME) after 4 months of neoadjuvant chemoradiation (Fig. 3B). The surgery included en bloc resection of the prostate and seminal vesicles because of the tumor abutment noted on MRI. The final pathology report indicated a ypT3 tumor and 11 negative regional lymph nodes. Given the initial presence of significant extramural tumor extension and perforation (cT4a), the patient is currently undergoing adjuvant chemotherapy.
From the Department of Diagnostic Radiology, The University of Texas MD Anderson Cancer Center, Houston, TX. Received for publication July 29, 2014; accepted September 17, 2014. Reprints: Corey T. Jensen, MD, Department of Diagnostic Radiology, Unit 1473, The University of Texas MD Anderson Cancer Center, 1515 Holcombe Blvd, Houston, TX 77030 (e‐mail: [email protected]). An expanded discussion of this “Case of the Quarter” can be found at the SCBTMR Web site: scbtmr.dnnstaging.com. The authors declare no conflict of interest. Copyright © 2014 by Lippincott Williams & Wilkins
Adenocarcinoma of the colon is one of the most common malignancies in the United States, with 30% to 40% of cases originating in the rectum.4 Historically, radical resections were performed for rectal adenocarcinoma, often with blunt dissections, which resulted in dismal outcomes. During recent decades, TME has become the standard surgical treatment for most rectal cancers, consisting of en bloc resection of the rectum and the surrounding mesorectum using sharp dissection to the levator musculature. This surgical approach and neoadjuvant chemoradiation therapy have led to dramatically improved outcomes.5–7 The rectum is traditionally divided into three 5-cm sections: the lower, middle, and upper thirds. Tumor position is classified according to the location of the inferior-most portion of the tumor as measured from the anal verge. For example, because the measurement from the anal verge includes the anus, the inferior rectum is typically the first 7- to 10-cm segment from the anal verge. The middle rectum is the next 5-cm segment and the upper rectum is the following 5-cm portion.7 These divisions of the rectum have unique characteristics, which require special considerations given their associated clinical implications. For instance, the anterior portion of the upper rectum typically abuts the peritoneal reflection (Fig. 4); thus, superior rectal tumors have a higher rate of intraperitoneal extension resulting in classification as stage T4 disease.8 The middle rectum has less common but variable exposure to the peritoneal cavity. The lower third of the rectum is extraperitoneal, but tumors in this region require special consideration because they often approach the anal sphincter complex and pelvic organs. Consequently, inferior rectal tumors are associated with the highest rate of local recurrence and worse overall outcomes.9 Each rectal level is surrounded by the mesorectal fascia, which is a connective tissue sheath (Fig. 4). The mesorectum contains the rectum, mesorectal fat, vasculature, and lymph nodes.10,11 Resection of the mesorectum as an intact sleeve to the levator ani level constitutes TME. Adequate distal margins of superior rectal tumors can often be obtained without complete TME; thus, tumor-specific TME is often preferred, which does not require dissection to the levator ani level.12 Determination of the primary rectal tumor level is important for selecting the appropriate operative approach and is a predictor of outcomes. For instance, a 2-cm distal margin of resection is standard for TME. Thus, detailed evaluation of tumors near this distance from the anal sphincter complex is of extreme importance; although the exact treatment will depend on clinician experience and other clinical factors, the decision of whether low anterior resection or abdominoperineal resection is appropriate will be largely affected by evaluation of this potential circumferential resection margin (CRM). This preoperative stratification is critical to maintaining the lowest possible recurrence rates.13,14 Preoperative MRI assessment of rectal cancer has greatly improved patient stratification and has become an integral component of multidisciplinary assessment. Magnetic resonance imaging facilitates accurate staging of the primary tumor, which is derived
J Comput Assist Tomogr • Volume 38, Number 6, November/December 2014
www.jcat.org
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
885
J Comput Assist Tomogr • Volume 38, Number 6, November/December 2014
Sprouse et al
FIGURE 1. Axial image from the patient's pelvis CT obtained with IV contrast demonstrates circumferential rectal wall thickening, rectal wall disruption, and an adjacent left anterolateral fluid collection consistent with perforation (arrow).
FIGURE 4. Coronal oblique T2-weighted MR image of the pelvis depicts the mesorectal fascia (white arrow) and peritoneal reflection (clear arrow).
FIGURE 2. Photograph taken during colonoscopy shows a portion of the patient's rectal mass, which extended from 2 to 12 cm from the anal verge. Figure 2 can be viewed online in color at www.jcat.org.
from a robust assessment of the rectal wall layers. Specifically, the submucosa, muscularis propria, and mesorectal fat are reliably depicted as distinct MRI layers, allowing for clinically relevant T staging (Table 1; Figs. 5A, B).15 In addition to providing the distance of tumor extension beyond the muscularis propria, MRI details the distance of the tumor to the mesorectal fascia, which provides accurate assessment of the CRM, thus assisting presurgical and neoadjuvant therapy planning (Fig. 4).7 Technical parameters and consistency are critical to ensuring a high level of MRI staging accuracy for rectal carcinoma. Highresolution, triplane T2-weighted fast spin echo sequences without fat saturation make up the foundation of proper MRI assessment.8 A sagittal T2 sequence is typically performed first to provide a planning sequence for subsequent oblique axial sequences that are angled orthogonal to the tumor plane. When tumors are large or tortuous, often 2, and sometimes 3, oblique series are obtained.
FIGURE 3. A, Sagittal T2-weighted MR image depicts the patient's invasive middle rectal tumor extending anteriorly to abut the prostate and seminal vesicles (dashed arrow). B, Response of the tumor to neoadjuvant therapy is noted based on significant reduction in the size of the mass and increased fibrosis.
886
www.jcat.org
© 2014 Lippincott Williams & Wilkins
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
J Comput Assist Tomogr • Volume 38, Number 6, November/December 2014
TABLE 1. MRI T Staging of Rectal Adenocarcinoma T Stage Tx T0 T1 T2 T3a T3b T3c T3d T4
MRI Findings Primary tumor cannot be assessed No evidence of primary tumor Tumor invades the submucosal layer Tumor invades the muscularis propria Tumor invades 15 mm beyond the muscularis propria Tumor invades adjacent organs or peritoneal reflection
The coronal T2 sequence is performed without angulation, which typically provides the best assessment of the anal sphincter complex (Fig. 6). Diffusion-weighted imaging (DWI) is increasingly used in the evaluation of rectal cancer. Initial experience with DWI demonstrated that some otherwise occult primary rectal tumors could be detected with the use of DWI. Diffusion-weighted imaging also provides high contrast of lymph nodes, thus assisting in the general localization of nodes, although DWI does not provide improved specificity for detecting nodal disease. Although DWI can assist in the initial assessment of rectal cancer, evolving evidence suggests that it can be used as a functional technique in predicting and restaging tumor response after chemoradiation.16 However, the evidence regarding this is conflicting, requiring further investigation.7,17 Other sequences and techniques can be used for rectal carcinoma assessment but are often used only in certain clinical scenarios because current evidence does not support their standard use. For example, researchers have suggested intravenous (IV) administration of contrast for restaging of rectal cancer in patients who have undergone chemoradiation; however,
Local MRI Staging of Rectal Adenocarcinoma
accuracy for the initial staging of rectal carcinoma does not improve with the use of IV contrast.16,18 T1-weighted sequences are not typically performed for rectal carcinoma assessment but can be added when an incidental or otherwise confounding finding is present such as an ovarian mass. A 3-dimensional T2-weighted sequence is used in some centers because this can potentially improve signal-noise ratio and, if thinsectioned, can allow for postprocessing 3-dimensional reformations.8 Rectal distension with gel can be useful to delineate small, low–T-stage, and polypoid tumors, but this is not standard and it often provides redundant information because patients will undergo endoscopy. Lastly, use of endorectal coils is not recommended because it causes rectal distension, which may lead to incorrect T staging.16 The T stage is determined according to the depth of tumor invasion (Table 1). For T3 tumors, determining the closest distance of the primary tumor or tumor deposits to the mesorectal fascia (Fig. 5) is critical because it predicts the involvement of the CRM. On MRI, a distance of 1 mm from the primary tumor or tumor deposits to the mesorectal fascia indicate involvement of the CRM, with an accuracy rate of 92%. If the distance from the tumor to the mesorectal fascia is greater than 6 mm on MRI, a tumor-free margin of at least 2 mm is predicted with 97% accuracy.19,20 Special consideration must be given to low rectal tumors, for which assessment of the intersphincteric plane is key (Table 2; Fig. 6). If this plane is clear, clinicians with appropriate experience can often perform adequate sphincter-preserving resection.17 In addition to T staging, MRI can also provide valuable assessment of pelvic lymph nodes. Although mesorectal lymph nodes are removed during TME, it is important to identify concerning nodes and tumor deposits that approach the mesorectal fascia, thus threatening the CRM.7 Lymph node size alone is a poor predictor of nodal tumor involvement, but using the presence of irregular nodal morphology and irregular internal signal to indicate involvement does improve accuracy, with sensitivity and specificity reported to be 85% and 97%, respectively.9,20 Because removal of nodes external to the mesorectal fascia is not standard,
FIGURE 5. A, Illustration of rectal cancer T stages. B, Oblique T2-weighted MR image of a companion rectal cancer case demonstrates T3 extramural extension of the tumor (dashed arrow) and EMVI (clear arrow). Separate, uninvolved hypointense muscularis propria and intermediate-intensity submucosa are well delineated as separate layers (white arrow). Figure 5 can be viewed online in color at www.jcat.org. © 2014 Lippincott Williams & Wilkins
www.jcat.org
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
887
J Comput Assist Tomogr • Volume 38, Number 6, November/December 2014
Sprouse et al
FIGURE 6. A, Coronal T2-weighted MR image from another patient shows a normal low–signal-intensity external sphincter (clear arrow) as a continuation of the levator ani. The internal sphincter continues from the muscularis propria (white arrow). The T2 hyperintense intersphinteric fat plane (dashed arrow) separates the internal and external sphincters. B, Axial T2-weighted MR image reveals the external sphincter (clear arrow), intersphincteric plane (dashed arrow), and internal sphincter (white arrow).
reporting of concerning nodes in these regions can assist in modification of the treatment approach to prevent untreated adenopathy; possible treatment modifications can include both an altered radiation field and/or extended lymphadenectomy.8 In a majority of cases, nodal disease occurs in the mesorectal, superior hemorrhoidal, and inferior mesenteric chains, but drainage toward the pelvic sidewall can occur, often via the middle rectal lymphatic drainage. This lateral pathway of drainage is more often seen with mid to lower rectal tumors. In general, lymphatic drainage occurs in a cephalad manner, but adenopathy has been reported up to 4 cm distal to primary rectal tumors.22 An increasingly recognized and important feature in MRI assessment of rectal carcinoma is extramural venous invasion (EMVI). This can be identified on MRI by loss of normal vascular signal voids, venous expansion, and/or discrete intravascular soft tissue. Importantly, EMVI can threaten the CRM; thus, the distance from EMVI to the mesorectal fascia should be reported.7 Furthermore, the detection of EMVI is an independent predictor of increased risk for synchronous and metachronous metastases. Currently, whereas the presence of EMVI does not directly influence initial treatment planning, it may become a factor in treatment stratification for rectal cancer patients in the future.23 Magnetic resonance imaging is increasingly used in the evaluation of neoadjuvant therapeutic response of rectal cancer and can provide useful prognostic information (Table 3). Magnetic resonance imaging assessment of details such as the degree of therapy-related fibrotic change has evolved from pathology studies demonstrating prediction of local recurrence, disease-free survival,
TABLE 2. Proposed MRI Staging of Low Rectal Adenocarcinoma21 Stage 1
Tumor confined to bowel wall (partial thickness) Tumor confined to bowel wall (full thickness) but does not extend into the intersphincteric fat plane Tumor invades the intersphincteric plane or lies within 1 mm of the levator muscle Tumor invades the external anal sphincter
Stage 2
Stage 3
Stage 4
888
www.jcat.org
and overall survival using posttreatment specimens. Importantly, MRI assessment of treatment response requires strict control of technique to ensure that pretreatment and posttreatment scans are as similar as possible, including use of the same obliquities for T2-weighted sequences. Magnetic resonance imaging evidence of treatment response includes increased fibrosis and reduced tumor size. Interestingly, although mucinous tumors have a worse prognosis than nonmucinous rectal carcinoma, the internal development of mucin related to neoadjuvant therapy often denotes a favorable response.17 This postchemoradiation MRI evaluation can aid in determining the timing of surgery and need for additional neoadjuvant treatment as well as surgical planning.7,17 In summary, MRI is a critical tool in the multidisciplinary staging of primary rectal tumors. Identification of tumor location, local involvement, and nodal disease provides key information for treatment stratification and presurgical planning. Future technical advances in MRI will likely lead to shorter examination times, improved anatomic detail, and further improvement of staging accuracy. In patients with low–T-stage tumors, improved diagnostic accuracy may aid the trend toward more conservative treatment strategies. Combined with clinical advances, improved MRI
TABLE 3. Prognostic MRI Features for Rectal Carcinoma7 Predictors of Recurrence Extramural tumor extension > 5 mm Tumor within 1 mm of mesorectal fascia Inferior rectal tumor involving the intersphincteric plane Presence of EMVI
Good Prognostic Features CRM uninvolved on preoperative MRI (tumor > 1 mm from the mesorectal fascia) No evidence of EMVI Early MRI T stage (
