Keywords:
Rectal neoplasms/surgery - Colorectal surgery - Colorectal neoplasms - Magnetic resonance
imaging
Descritores:
Neoplasias retais/cirurgia - Cirurgia colorretal - Neoplasias colorretal - Imagem
por ressonância magnética
INTRODUCTION
The rectum is a 15-cm section of the gastrointestinal tract that extends from the
anal canal to the rectosigmoid junction, and is divided into upper, middle, and lower
segments. The upper, middle, and lower segments are approximately 10-15 cm, 5-10 cm,
and 0-5 cm from the anal verge, respectively.[1] Lower rectal tumors appear to be more aggressive and are associated with high rates
of positive radial margins, local recurrence, and mortality, which are related to
the complex anatomy of the rectum. For example, these tumors are located near the
pelvic organs, the sphincter complex, the narrowing of the bony pelvis, and the progressive
tapering of the mesorectum caudal to its encounter with the levator muscle of the
anus.[2]-[4] ([Figure 1])
Figure 1 T2-weighted coronal sections demonstrating the anatomy of the inferior rectus and
sphincter complex.
Previous studies have revealed that abdominoperineal resection for lower rectal tumors
is associated with a poorer prognosis, compared to lower anterior resection, based
on the 30% higher risk of tumor perforation and high rates of positive radial margins
(0-10%).[5]-[9] Furthermore, patients with lower rectal cancer who receive neoadjuvant chemoradiotherapy
have shorter disease-free survival, even after achieving a complete pathological response,
compared to patients with upper and middle rectal tumors.[10]
Thus, it is important to adopt new strategies that aim to optimize the outcomes of
treatment for lower rectal cancer. Magnetic resonance imaging (MRI) has become a crucial
tool for successfully selecting and completing surgery, based on its high anatomical
resolution and ability to define the tumor’s relationship with the sphincter complex.
Developments
This article is a narrative review that evaluates the modern approach to surgically
treating lower rectal cancer based on MRI findings. The PubMed database was searched
using the terms “lower rectal cancer”, “colorectal surgery”, and “magnetic resonance
imaging”. Milestone reports from 2001 to 2017 were selected to be addressed in this
review, choosing the most recent articles to carry out a current revision.
This study was submitted to and approved by the INCA Research Ethics Committee. Images
taken from articles included in the literature review were authorized for the use
in this article.
Staging based in MRI
Shihab et al. have proposed specific criteria for staging lower rectal tumors based
on MRI findings, which helps the surgeon select the resection type and degree of sphincter
preservation.[2],[8],[11] Lower rectal tumors are stratified into three levels based on coronal T2-weighted
images:
-
Level 1 or supra-levator: the tumor is above the insertion of the levator ani. At
this level, the mesorectal fat is clearly visible. Total mesorectal excision with
lower colorectal anastomosis is indicated for tumors with ≤1 mm of extension into
the mesorectal fascia. Broader resection is indicated for cases with involvement of
the mesorectal fascia or adjacent structures.
-
Level 2 or intra-levator: the tumor is between the origin of the levator ani and the
upper margin of the puborectal muscle. At this level, there is progressive tapering
of the mesorectal fat and the evaluation of radial tumor extension is crucial. Intersphincteric
resection is indicated for tumors that are limited to the muscularis propria or tumors
with extension to the interphincteric plane up to 1 mm from the levator ani. Extra-elevator
abdominoperineal resection is indicated for tumors with a potentially positive radial
margin (i.e., invasion or <1 mm from the levator ani).
-
Level 3 or infra-levator: the tumor is at or below the puborectal muscle. This region
(the anal canal) is devoid of mesorectum, with an inner sphincter that is formed by
the lower segment of the circular muscular layer of the distal rectum and an external
sphincter that is formed of the puborectal fibers as they join the lower portion of
the levator ani.[12] Inter-sphincter resection can be performed when there is extension of ≤1 mm from
the outer margin of the internal sphincter, while extra-elevator abdominoperineal
resection is indicated for cases with greater involvement of the intersphincteric
plane or the external sphincter.[2],[8],[11]
There is also a proposal for staging lower rectal cancer according to the involvement
of the sphincter complex structures ([Figures 2] and [3]).[3],[11]
-
Stage 1: tumors confined to the rectal wall without involvement of the entire muscle’s
thickness.
-
Stage 2: tumors that involve the entire muscular layer of the internal sphincter but
without extension to the intersphincteric plane. When the tumor is above the sphincter,
it is confined to the mesorectum.
-
Stage 3: tumors that invade the intersphincteric plane but maintain a distance of
≥1 mm from the levator ani.
-
Stage 4: tumors that invade the external sphincter or are <1 mm from the levator ani.
The tumor may or may not have invaded the adjacent organs.
Figure 2 Coronal MRI cut-off scheme of staging of lower rectum tumors by Shihab et al.[10] Figure published by Nougaret et al.[4] IS: internal sphincter; ES: external sphincter; EA: anus lift.
Figure 3 Tumor invading the levator ani and puborectal muscle on the right. Coronal and axial
T2-weighted cuts (Stage 4, according to Nougaret et al. Staging proposal).[4]
Therapeutic strategy
Shihab et al. have also proposed a therapeutic strategy that addresses this staging
proposal. Neoadjuvant treatment is indicated for more advanced tumors (Stages 3 and
4), in an attempt to achieve tumor regression to Stages 1 or 2, which would allow
for intersphincteric resection with anal preservation.[3],[4]
[Figure 4] shows the modified scheme of different surgical techniques that can be used to treat
lower rectal cancer.
Figure 4 Modified scheme of the different surgical techniques for lower rectum. Figure published
by Nougaret et al.[4] IS: internal sphincter; ES: external sphincter; L: levator muscle of the anus, *:
interphincteric plane. Black line: low anterior resection (rectum and mesorectum in
block). Green line: Low anterior resection with inter-sphincter resection. Dotted
line: Conventional abdominoperineal resection. Gray line: Extra-elevated abdominoperineal
resection.
Comments
The previous articles have clearly highlighted the value of MRI for guiding rectal
cancer treatment, especially in cases of lower rectal cancer. In addition, it is important
to request MRI results from before and after neoadjuvant treatment, in order to ensure
that an accurate comparison can be used to evaluate the clinical response, disease
progression, and possibility of sphincter preservation. Furthermore, post-treatment
imaging is commonly performed because it can alter the treatment choice in up to 32%
of cases.[13]-[18] Moreover, preoperative MRI should ideally be performed 8 weeks after the end of
the neoadjuvant treatment (chemoradiotherapy), as the response rate appears to be
related to time.[19]-[21] At the Brazilian National Cancer Institute, our conduct matches the literature with
MRI being performed pretreatment and again 8 weeks after neoadjuvant therapy. Surgery
is performed 10-12 weeks after neoadjuvant therapy.
The National Cancer Institute uses a similar classification for assessing involvement
of the sphincter complex ([Table 1]), and this classification was useful for therapeutic decision-making. A preliminary
analysis of our study (INCAGI004) revealed that sphincter involvement before and after
neoadjuvant treatment was associated with the type of surgery that was performed for
49 patients. In that series, 42% of patients with intersphincteric plane involvement
(Grade 2) underwent sphincter-preserving surgery after neoadjuvant treatment, whereas
only 20% of patients with external sphincter involvement (Grade 3) in the baseline
MRI underwent sphincter-preserving surgery. The final analysis is currently being
performed and we hope to publish the results. These MRI-based classifications appear
to help guide the selection of surgical treatment for patients with lower rectal cancer,
which can help provide better outcomes with fewer sequelae.
Table 1
Degree of sphincter impairment and type of operation indicated
|
Involvement of the sphincter complex
|
Therapeutic decision
|
|
Grade 0: No involvement
|
Low anterior resection
|
|
Grade 1: Internal sphincter involvement
|
Inter-sphincter resection
|
|
Grade 2: Intersphincteric plane
|
Conventional abdominoperineal resection
|
|
Grade 3: External sphincter involvement
|
Conventional abdominoperineal resection
|
In summary, the current treatment of lower rectal cancer should be decided based on
the anatomical features evaluated by the high resolution resonance. The new strategies
of sphincter preservation can be applied as long as they are guided by good quality
images, allowing to reduce the rate of mutilating surgeries (abdominoperineal resection)
without prejudice to oncological results.
Bibliographical Record
Marcus Valadão, Eduardo R.Z. Câmara, Rodrigo Araújo, José Paulo Jesus, Eduardo Linhares,
Claudia Carrada. Low rectal cancer: current approach based on magnetic resonance imaging.
Brazilian Journal of Oncology 2017; 13: e-BJO20171345A133.
DOI: 10.26790/BJO20171345A133
