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ECR 2018 / C-3146
Magnetic resonance imaging assessment of low rectal tumors
Congress: ECR 2018
Poster No.: C-3146
Type: Educational Exhibit
Keywords: Cancer, Diagnostic procedure, MR, Pelvis, Oncology, Gastrointestinal tract
Authors: R. M. Antunes, P. G. M. G. Ferreira; Lisbon/PT
DOI:10.1594/ecr2018/C-3146

Findings and procedure details

Special considerations need to be taken into account when staging LRT, namely when assessing the tumor relationship to the adjacent structures, so to define the tumor free margin. The staging is based on MRI and allows to assess the need for chemoradiation prior to surgery and the choosing of the most suitable surgical excision plane. MRI staging relies primarily on T2-weighted images that better defines the tumor relationship to the rectal wall and to the adjacent structures. The protocol starts with imaging the pelvis in the sagittal and axial planes in order to assess the tumor location and plan the next sequences. High-resolution T2-weighted images in the axial and coronal planes, oriented according to the long axis of the tumor, are then acquired [3]. For LRT tumors, an additional high-resolution coronal sequence is advisable to depict the levator ani muscle, anal sphincter complex and intersphincteric space to a better extent [4]. Straight coronal images are an alternative [5]

 

LRT staging is challenging due to the complex anatomy of this area. The distal end of the rectum is seen on MRI as the level where the levator ani muscle inserts on the rectal wall. Bellow this plane, the anal canal is located. The mesorectum surrounds the rectum and is composed of fat, fibrous tissue, lymph nodes and neurovascular structures enclosed by the mesorectal fascia (seen on MRI as a thin hypointense line). It acts as a primary barrier to tumor spread, but it tapers distally and in the lower rectum only a thin layer of fat can be seen separating the rectal wall from the mesorectal fascia. Bellow the puborectalis sling plane lies the anal canal, a cylindrical structure composed by internal and external anal sphincters and the intersphincteric space. The internal sphincter, composed of smooth muscle, is a continuation of the rectal circular muscular layer. The external sphincter, on the other hand is composed of skeletal muscle with fibers composed with fibers from the levator ani muscle, puborectalis sling and by the external sphincter muscle. Between the two sphincters the intersphincteric plane is locater [2, 5].

 

When staging LRT, the first question to answer is where the lower edge of the tumor is located in relation to the puborectalis sling. This distinction determines the feasibility of sphincter-sparing surgery [5]. In tumors located above the puborectalis sling, sphincter involvement can be safely excluded. These tumors are mainly managed based on the depth of muscularis propria invasion, the same way as tumors located in the rectal upper two thirds [1] Fig. 1 Fig. 2 .

 

Fig. 1: Sagittal (A), oblique coronal (B) and axial (C) T2-weighted images, showing a large polypoid tumor in the lower rectum, located above the plane of the puborectalis sling. In the axial plane (C), there is tumor extension beyond the mesorectal fascia with invasion of the right seminal vesicle.
References: Radiology, Centro Hospitalar de Lisboa Ocidental - Lisbon/PT
Fig. 2: Sagittal (A) and oblique coronal (B) T2-weighted images shows a large tumor in the distal rectum. The tumor is located above the puborectalis sling and anteriorly it invades the Denonvillier fascia and the prostate gland.
References: Radiology, Centro Hospitalar de Lisboa Ocidental - Lisbon/PT

 

With tumors extending bellow the puborectalis sling, special attention should be given to the anal sphincter complex, so to a suitable surgical technique is used. A specific set of criteria for LRT staging was proposed [4]. Based on this four stages were defined. Stage1 corresponds to tumor that invade the muscular layer of the rectal wall but not extend through full thickness. With Stage 2, there’s invasion of the rectal wall in its full thickness and of the internal sphincter, but not of the intersphincteric plane. In stage 3 tumors the intersphincteric plane is invaded or the tumor is located at least 1mm from the levator ani muscle Fig. 4 Fig. 5 . With the more advanced Stage 4 tumors, there is also invasion of the external sphincter and/or the tumor lies less than 1mm from the levator ani muscle Fig. 3 Fig. 6.

Fig. 4: Sagittal (A), oblique coronal (B) and axial (C) T2-weighted images shows a low rectal tumor that extends to the anal canal. There is invasion of the internal sphincter and eventually invasion of the intersphincteric space.
References: Radiology, Centro Hospitalar de Lisboa Ocidental - Lisbon/PT
Fig. 5: Sagittal (A) and oblique coronal (B) T2-weighted images shows a mucinous tumor in the distal rectum. Anteriorly it invades the vaginal wall and the cervix. The is also extension to the anal canal with invasion of the intersphincteric space on the left.
References: Radiology, Centro Hospitalar de Lisboa Ocidental - Lisbon/PT
Fig. 3: Sagittal (A), oblique coronal (B) and axial (C) T2-weighted images, showing tumor in the lower rectum, extending bellow the puborectalis sling. On the left, the tumor extends beyond the mesorectal fascia and contacts the levator ani muscle (B).
References: Radiology, Centro Hospitalar de Lisboa Ocidental - Lisbon/PT
Fig. 6: Sagittal (A), oblique coronal (B) and axial (C) T2-weighted images shows a rectal tumor that extends bellow the level of the puborectalis sling to the anal canal. The tumor invades the internal sphincter, intersphincteric space and external sphincter. There is also invasion of the posterior wall of the vagina.
References: Radiology, Centro Hospitalar de Lisboa Ocidental - Lisbon/PT

 

 

With all the LRT, the relationship of the tumor to structures mentioned before should be carefully evaluated and clearly stated. LRT usually are managed with low anterior resection, intersphincteric resection or abdominoperineal excision, depending on the invasion of the anal sphincter complex. In tumors with adjacent organ invasion, pelvic exanteration could be indicated [3].

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