Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_894_Библиотеки_им_академика_М_И_Перельмана.pdf
ab
21 Rectal Carcinoma: Imaging for Staging
c
uT0 - tumor confined to the mucosa
uT1 - tumor confined to the submucosa
1
2
uT2 - tumor invades the muscularis
propria but does not penetrate through it
3
uT3 - tumor invades the perirectal tissue
4
uT4 - tumor invades an adjacent organ
5
361
Fig. 21.1 (a) uT1 lesion. The middle white hyperechoic
layer between the white arrows has been disrupted by the
bulging tumour, indicating invasion into the submucosa.
However, the outer black hypoechoic line indicating
the muscularis propria is preserved. (b) uT1 lesion.
Submucosal invasion by the tumour (white arrow) is indi-
cated by the loss of the hypoechoic muscularis propria
Conversely, a locally advanced and unresectable
tumour may benet from neo-adjuvant treatment,
rendering it operable and improving the chances
of a curative resection following sufcient downsizing. These decisions depend on the accuracy
of ERUS, or other imaging modalities, at determining the depth of rectal wall invasion, or ‘T’
stage. As discussed previously, ERUS evaluation
of T-stage is reported according to a modied
TNM classication system (see Table 19.2) and
denoted by the prex ‘u’ to reect the imaging
modality.
layer. However, the outermost white layer which is indicative of the perirectal fat is still preserved, demonstrating that
the tumour is still conned to the bowel wall. (c) The velayer rectal wall model of an ERUS and the corresponding
depth of invasion (uT stage) as determined by degree of
preservation of the different anatomical layers. Images
courtesy of Dr. A Corr, St. Mark’s Hospital, London. U.K.
There is a wide variation in the accuracy of
ERUS predicted T-stage, with estimates ranging
between 63% and 96% [5], and a reported mean of
approximately 85% [6]. A meta-analysis of 42
studies with over 5000 patients who underwent
ERUS for rectal cancer staging between 1984 and
2006 found a pooled sensitivity of 81–96% and
specicity of 91–98% for evaluating the T stage
when compared to histopathology [7]. Whilst these
results are certainly impressive, and comparably
better than the reported accuracy of CT and MRI in
assessing the depth of mural invasion [8], caution is
362
M. Dattani and G. Brown
warranted in the absolute interpretation of pooled
analyses of heterogeneous studies. Several studies
included in the meta-analysis were single centre
assessments, often with a small sample size, which
was less than 30 patients in some cohorts, or a
selection bias tending towards the inclusion of early
and late stage disease in which the risks of inaccurate staging are low. Indeed, a common nding of
ERUS is the low accuracy in T2 rectal cancer staging, with frequent overstaging cited in several series
[9]. The main reason postulated is the difculty in
accurately distinguishing between a T2 tumour that
invades deep into the muscularis propria, from an
early T3 with microscopic inltration of the perirectal fat [5]. This is a consequence of the inability
to differentiate peritumoural desmoplastic reaction
from genuine neoplastic invasion in the perirectal
tissues with ERUS [10]. It is argued that the preoperative misclassication of a T2 rectal cancer bears
little clinical consequence [11, 12], and whilst this
is true for outcomes of T2 and ‘early’ T3 rectal cancers that undergo primary resectional surgery [13],
the risks of overstaging could expose patients to
contend with the toxicity of neo-adjuvant treatment, particularly in countries where this is standard practice for a T3N0 staged rectal cancer with
no other adverse features [14]. Conversely, an
understaged T2 cancer may be offered a local excision treatment in selective cases, with the prospect
of having to undergo a potentially difcult salvage
procedure subsequent to histological analysis.
Harewood [6] assessed the accuracy of ERUS
staging for rectal cancer in his review of 41 publications in the English literature, and made two
important observations worthy of discussion.
Firstly, that the accuracy of ERUS for evaluating
the depth of mural invasion in rectal cancer was
inversely proportional to the sample size of the
study. He suggested that publication bias of previously reported smaller studies may have over
inated the accuracy of ERUS as the imaging
modality of choice. In this context, his second
observation was the decline in ERUS accuracy
for rectal cancer T-staging in the more recently
published data, which is shown in Table 21.1
[15–24] for studies with over 100 patients.
Table 21.1 Accuracy of ERUS in determining the depth of rectal wall invasion
Study Year Patient recruitment Setting No. of patients
Akasu etal. [15] 2000 1991–1996 Single-centre, Japan 309 80
Garcia-Aguilar
etal. [16]
Marusch etal. [17]b2002 1999 Multi-centre, Germany
Mackay etal. [11] 2003 1991–2001 Multi-centre, Australia
Manger etal. [18] 2004 1994–2002 Single-centre, Germany 357 77
Kauer etal. [19] 2004 1990–2000 Single-centre, Germany 458 69
Zammit etal. [20] 2005 1998– Single-centre, U.K 117 76
Ptok etal. [21] 2006 2000–2003 Multi-centre, Germany
Badger etal. [22]
Goertz etal. [23]
Morris etal. [24]
Marusch etal. [25] 2011 2000–2008 Multi-centre, Germany
Ashraf etal. [26]
Restivo etal. [27] 2015 1997–2012 Single-centre (Italy) 220 65
a
uT0-T4 tumours; uT1-T4in all other studies
b
No information on neo-adjuvant treatment provided. In all other studies, patient who had neo-adjuvant treatment
were excluded
c
Some patient had short course neo-adjuvant radiotherapy with immediate surgery
d
uT0-T3 tumours only
2002 Data not given Single-centre, U.S.A 545 69
(n=49)
(n=2)
c
2007 1999–2004 Single-centre, U.K 131 72
c
2008 1990–2003 Single-centre, Germany 333 71
c
2011 1999–2007 Single-centre, Australia 233 82
b
2012 1992–2008 Multi-centre, U.K (n=21) 165 55
(n=331)
(n=384)
422 63
356 77
3501 66
7096 65
T staging
accuracy (%)
a
d
21 Rectal Carcinoma: Imaging for Staging
363
A notable pattern in Table 21.1 is the relatively lower accuracy of ERUS in multi-centre
studies, apart from Mackay etal. [11], in which
the staging was carried out by two surgeons
across two hospital sites. The inference is that
widespread adoption of ERUS outside expert
centres can compromise its reliability as an
accurate staging modality. Marusch et al. [25]
found that the accuracy of T-staging dropped
from 73% at centres performing more than 30
ERUS per year to 63% at centres with less than
10 cases per year, thus advocating for a centralised
service provided by experienced operators.
Indeed, in a large study from Minnesota [16], the
pathological T-stage of the tumour and ERUS
operator were the only two independent factors
affecting the accuracy of staging. Moreover,
ERUS has been plagued with high inter-observer
variability, which although in some cases may be
secondary to procedural aspects, it is more
commonly a consequence of operator experience
[19]. In the context of a protracted learning curve
associated with ERUS [5], the initial operator
variability plateaus with increasing the case load,
as well as standardising the technique and
interpretation of staging criteria [26–28].
Lymph Node Involvement
Beyond the rectal wall is the moderately echogenic fatty tissue which contains the lymphovascular drainage system of the rectum. Assessment
of potentially metastatic lymph nodes within the
mesorectum continues to present a problem for
all imaging modalities, including ERUS. The
reported accuracy for ERUS based nodal staging
is even less than that for evaluating the depth of
invasion. A recent meta-analysis of over 5000
patients found a 57% sensitivity and 80% specicity for ERUS predicted involved lymph nodes
[29], with a mean accuracy of 73% [5]. The main
limitation in assessing lymph nodes is the lack of
valid ultrasonographic criteria to discriminate
between malignant and inammatory nodal tissue. The commonly applied size criterion of 5mm
or more to dene a malignant lymph node is
arbitrary and of poor predictive value when
compared to gold standard histopathology. Kim
etal. [30] found that up to 18% of lymph nodes
less than 5mm contained metastases, and whilst
reducing the size threshold to 3mm improved the
overall accuracy of predicting metastatic nodes,
this signicantly compromised the specicity
with the inherent risks of over-treatment in this
group [15].
The addition of secondary characteristics,
such as echogenicity, border features and contour, to dimensional assessment of lymph nodes
may increase the discriminatory power of
ERUS in evaluating metastatic nodes. Thus, a
hypoechoic round node with a smooth delineated
border is more likely to be metastatic lymph node
than an inammatory one, although even this
combination of additive features has been shown
to give conicting results [22, 31, 32]. One way
of ameliorating these issues, particularly in
equivocal cases, has been the suggestion of ne
needle aspiration (FNA) for cytology sampling of
mesorectal lymph nodes [31]. Whilst technically
feasible and safe to perform, ERUS guided FNA
has not been shown to improve the accuracy of
lymph node staging in rectal cancer [33], and is
certainly not routinely practised.
Limitations ofERUS
Technical limitations of ERUS are best considered in relation to the rectal anatomy, or the
tumour itself. In the latter case, large polypoid or
stenotic tumours cannot be adequately assessed
because of the inability of the probe to traverse
the narrow lumen [5, 23]. Moreover, lesions in
the upper rectum may be inaccessible for evaluation, and those in the lower rectum have been
subject to difcult staging owing to loss of the
ve layer sonographic appearance in the rectal
ampulla wall [34]. Further anatomical distortion
is seen with a wide bore and rigid ERUS probe,
or the water lled balloon, which both stretch the
rectal wall and impair accurate visualisation.
Finally, the angulation of the probe at the tumour
interface, faecal residue in the rectum, prior
biopsy of the lesion, and the valves of Houston
are all potential confounders that have resulted in
over-staging of the rectal lesion [9, 23].
The major limitation of ERUS, however, is its
inability to accurately dene important perirectal
anatomical structures, most notably the mesorectal
364
M. Dattani and G. Brown
fascia. The relationship of a rectal cancer to this
crucial oncological landmark is predictive of
circumferential resection margin (CRM)
involvement, which is an independent risk factor
for local recurrence following TME surgery [35].
Current multimodality treatment of rectal cancer
therefore mandates the precise evaluation of the
mesorectal fascia, as it determines the need for
neo-adjuvant treatment, or the plane and type of
surgery where curative resection is achievable,
both of which have signicant implications for
the patient.
Future Perspectives ofERUS
Advancements in ERUS technology have overcome some of the limitations described above,
including the advent of endoscopic ultrasound
miniprobes, which enables access to the more
proximal rectal lesions, as well as assessment of
large stricturing tumours [36]. Furthermore,
three-dimensional (3D) ERUS has recently been
show to improve diagnostic performance because
of higher resolution and multi-planar image
acquisition of the rectal wall and surrounding
anatomy, including the mesorectal fascia in few
cases [9, 18, 30]. Strain elastography is another
novel and complementary technique to ERUS,
which relies on the differential resistance of tissues to strain, depending on whether there is any
malignant inltration [37]. Early indications
from these evolving technologies hold promise
for the future of accurate preoperative staging,
which will be dictated by robust evidence of their
effectiveness in clinical practice.
Magnetic Resonance Imaging
High-resolution magnetic resonance imaging
(MRI) plays a fundamental role in modern day
multidisciplinary management of rectal cancer.
The ability of MRI to precisely depict and characterise a rectal cancer, and its relationship to
important pelvic anatomical structures such as
the sphincter complex and mesorectal fascia, is
pivotal in its effectiveness as a preoperative staging modality. Thus, MRI evaluated local staging
of rectal cancer has important implications not
only in prognostication of the disease, but also in
tailoring treatment that whilst maximising the
chances of a successful cure, spares the patient
from the effects of unwarranted multimodality
treatment. The introduction of phased-array surface coils has enabled the acquisition of high spatial resolution images with equally good soft
tissue contrast for interpretation, which makes
MRI based stratication an ideal staging
modality.
MRI Technique
Whilst institutional protocols will vary somewhat, the main principles of MRI technique for
local rectal cancer staging are universal, and
require thin slice, high spatial resolution
T2-weighted images (1mm 3 voxel size) with a
small eld of view encompassing the rectal
tumour and its surrounding perirectal tissue.
Bowel preparation is unnecessary, and an intramuscular injection of 20mg hyoscine butylbromide is helpful to minimise peristaltic artefact
from the small bowel. Endorectal coil use is not
recommended because of the several limitations
that apply to ERUS as well, and not least because
of the superior quality imaging acquired with
modern day pelvic surface coils. Moreover, the
routine use of endorectal contrast is also not
advocated, and may in fact impair accurate interpretation of mural invasion by stretching of a full
rectum. Intravenous gadolinium contrast does not
improve diagnostic accuracy, and is also not recommended as it needlessly prolongs the examination [38]. Similarly diffusion weighted MRI
has not added to the diagnostic accuracy in rectal
cancer staging and results in unacceptable prolongation of the scans which patients may nd
distressing.
The patient lies supine on the MRI table and a
phased-array surface coil is placed around the
pelvis to secure the patient in position, thus minimising movement artefact. MRI scanning is then
initiated in a cranio-caudal fashion as the patient
advances into the magnet core, which should ideally be of 1.5T eld strength, or higher. The rst
sequences are to locate the rectal tumour being
investigated, and are acquired as T2-weighted
fast spin-echo (T2W-FSE) images in the sagittal
21 Rectal Carcinoma: Imaging for Staging
365
and coronal plane. Based on the longitudinal axis
of the tumour from these initial images, thin section orthogonal sequences of the tumour and
adjacent perirectal tissues are then obtained in
the axial plane. The meticulous planning of this
angulation, prior to imaging, is critical to enable
accurate assessment of depth of invasion by the
tumour. These images should be acquired using a
16 cm eld of view and a matrix resolution of
256×256 pixels with a 3mm thickness to achieve
a 0.6×0.6mm in plane resolution and 1.1mm3
voxel resolution. To achieve adequate signal to
noise and interpretability a minimum of 4–6 signal averages is required (approximately 7 min
scan duration). A further scan should be performed to cover at least 5cm above the top of
tumour to evaluate the draining nodes and vessels
for tumour spread. The rest of the pelvis is then
imaged from the iliac crests to the pubic symphysis with a large eld of view.
Appropriate consideration must also be given
to MRI planning for distal third rectal cancers,
owing to anatomical features of the anorectum in
the low pelvis. At this level, the mesorectum and
its fascia taper sharply into non-existence, being
replaced by the sphincter complex. The abrupt
change in calibre and angulation of the rectum
risks a tumour at this level being overstaged,
especially if perpendicularity of the imaging
planes is not maintained. Optimal angulation to
achieve high-resolution coronal sequences will
clearly delineate the sphincter complex and its
relationship to the tumour, the benets of which
cannot be overstated for the planning of a primary surgical resection. The awareness of common pitfalls and artefacts related to MRI of the
pelvis, and how to minimise their impact on diagnostic interpretation have been reviewed by Zand
etal. [39].
Primary Rectal Cancer Staging:
TheRole ofMRI
Depth ofInvasion
Brown etal. [40] rst reported MRI as an accurate
method of staging mural spread in rectal cancer,
by prospectively comparing section-for-section
in vivo MRI images to in vitro rectal cancer
specimen images, and nally correlating them to
gold standard histopathology. They argued that
the poor results in previous studies could be
accounted for by the use whole body surface coils
and thick sections that afforded poor quality
imaging for interpretation. Since then, high spatial
resolution thin slice MRI has become the imaging
choice for preoperative evaluation of rectal cancer,
with a sensitivity of 87% and a specicity of 75%
for T-stage assessment as reported in a metaanalysis of just under 2000 patients [41]. The
depth of invasion is reported according to the
signal intensity criteria, with the tumour having
an intermediate signal intensity that is higher than
the muscularis propria, but lower than the
submucosa. The latter is normally seen as a thick
layer just deep to a very ne line of low signal
intensity mucosa. An overall mrT-stage is
accorded, as derived from TNM staging of rectal
cancers (Table21.2) [12].
As in the case of ERUS staging, some of the
reported variability in the accuracy of mrT-stage
evaluation is because of an apparent difculty in
differentiating a T2 from an ‘early’ T3 rectal
cancer. The risk of overstaging at this interface is
well recognised, but can be avoided by recognis-
Table 21.2 MRI staging criteria for depth of invasion
(mrT-stage)
MRI signal characteristics
Low signal in the submucosal layer,
replacement of the submucosal layer by
abnormal signal not extending into circular
muscle layer
Intermediate signal intensity within
muscularis propria. Outer muscle coat
replaced by tumour of intermediate signal
intensity that does not extend beyond the
outer rectal muscle into the perirectal fat
Broad-based bulge or nodular projection (not
ne spiculation) of intermediate signal
intensity projecting beyond outer muscle coat
Extension of abnormal signal into adjacent
organ, extension of tumour signal through the
peritoneal reection
Adapted from [12]
*T3 is further subdivided depending on the depth of extramural invasion from the muscularis propria to the outer
edge of the tumour; T3a: <1 mm, T3b: 1–5 mm, T3c:
5–15mm; T3d: >15mm
MRI
T-stage
T1
T2
T3*
T4
366
M. Dattani and G. Brown
a
b
Fig. 21.2 (a) Axial T2-weighted MR sections of a mrT2
rectal cancer conned to the bowel wall. The long, low
intensity spikes of brosis in the perirectal fat (arrows in
a and b) demonstrate extramural desmoplasia, and not T3
extension (b) Corresponding histopathology slice
(haematoxlin-eosin stain) of the same tumour showing the
tumour margin (arrowheads) which is still intramural
(Adapted from Ref. [40])
ing the obvious differences between the peritumoural desmoplastic reaction in the extramural
fat and the characteristic nodular broader front of
inltration that is observed in early T3 tumours
(Fig.21.2a, b).
More recently, the depth of extramural invasion within the mesorectum has been found to be
a risk factor for local recurrence, and is therefore
of prognostic relevance in the preoperative setting, particularly because the largest proportion
of rectal cancers seen at presentation are of T3
stage. Merkel etal. [13] subdivided a large series
of over 800 pT3 rectal cancers into whether
they had extramural invasion of less than 5mm
(pT3a), or more than 5mm (pT3b), and found a
signicant difference in the 5year cancer-specic
survival of 85% and 54%, respectively, irrespective of the lymph node status. They also showed
that survival for a pT2 cancer was comparable to
a pT3a cancer, and on this basis, it can be argued
that neo-adjuvant treatment for this cohort of
patients is unlikely to be of any signicant benet. The precise evaluation of extramural spread,
and thus the sub-staging of T3 rectal cancers
(Table21.2), is of major importance in the era of
multimodality treatment. The sub-classication
of this heterogeneous group of rectal cancers has
been reected in the pathological TNM staging
of rectal cancers [42]. Based on this classication, the MERCURY [43] study found that the
MRI assessment of extramural invasion in rectal
cancer correlated to within 0.5mm of histological
measurement in 295 cases who had primary surgery. In this context, the group later reported that
for MRI staged ‘good’ prognosis rectal cancers
(mrT1-T3b), the 5-year local recurrence rate was
3% with primary surgery alone, regardless of the
MRI reported lymph node status [44]. Thecombination of MRI staging followed by good quality
TME surgery avoided the need forneo-adjuvant
treatment in 30% of rectal cancer patients without any major oncological compromise [44].
Lymph Node Involvement
The soft tissue contrast afforded by MRI makes
it the optimal imaging modality to identify local
mesorectal lymph nodes. Despite this, the lack
of a universally accepted set of diagnostic criteria in discriminating between a benign and a
malignant lymph node presents the main challenge in accurate prediction of nodal involvement. Whilst the presence of any visible
mesorectal lymph nodes is largely obsolete for
dening metastatic involvement, size criterion
remains an important, if controversial, determinant. In a morphometric analysis of nearly
13,000 lymph node retrieved from rectal cancer specimens, Dworak etal. [45] demonstrated
considerable overlap in the size of reactive and
metastatic lymph nodes. Despite this, a range of
21 Rectal Carcinoma: Imaging for Staging
Fig. 21.3 A 8mm mesorectal lymph node (arrow) close
to the mesorectal fascia on a T2-weighted axial image,
showing a homogenous signal and regular border contour.
This was subsequently conrmed on histology as a benign
node
cut-offs have been used over the years to predict
a metastatic lymph node, with size thresholds of
3–10mm used as a diagnostic criteria [46–48].
Unsurprisingly, these arbitrary measurements
have yielded an accuracy of between 43-85%
when compared with standard histological
assessment, conrming the view that lymph
node size has a poor predictive value for nodal
staging [49], as shown in Fig.21.3.
Focus has subsequently shifted to the morphological appearances of lymph nodes on MRI,
which has been shown to be a better discriminator of nodal involvement. This is because tumour
inltration into a lymph node disrupts the capsular integrity, as well as the signal intensity owing
to necrosis within the node. Brown etal. [50]
rst demonstrated this phenomenon in their
meticulous study of comparing lymph nodes in
the dissected pathology specimen to their corresponding MRI images in a section-by-section
analysis. They reported that an irregular border
and mixed signal intensity in a MRI detected
lymph node had a sensitivity of 85%, and a specicity of 97% in correctly predicting histological
nodal involvement. Conversely, normal or reactive nodes are likely to have a homogeneous signal intensity with a well-dened, smooth border.
367
The combination of these features as a diagnostic criteria had a predictive accuracy which was
superior to various size cut-offs applied to the
same cohort [50].
A recent meta-analysis of MRI studies for rectal cancer staging conrmed the poor accuracy
that has plagued preoperative lymph node evaluation with all types of imaging modalities.
Al-Sukhni and colleagues [41] reported a sensitivity of 77% and specicity of only 71% for
MRI predicted lymph node involvement, which
is probably a reection of the heterogeneity in the
diagnostic criteria employed by the individual
studies. Indeed, most studies continue to use size
criteria of various cut-offs [51, 52], and it is plau-
sible that pooling these results into a meta-analysis leads to the reported sub-optimal accuracy for
MRI nodal evaluation.
A novel technique to improve the accuracy of
MRI nodal assessment has been the development
of lymph node specic contrast agents, most
notably ultra-small superparamagnetic iron oxide
particles (USPIO) [53]. These particles are preferentially taken up by normal lymphoid tissue
and then ingested by macrophages, resulting in a
‘dark’ signal intensity after delayed imaging on a
T2*-weighted sequence. Thus, inammatory
nodes with a high concentration of macrophages
should be discernible from metastatic lymph
nodes, which demonstrate a high ‘white’ signal
intensity because of no USPIO uptake. Although
early results of its application were promising
[54, 55], the lack of validating evidence from
larger population studies has limited its current
utilisation in clinical practice.
The overarching debate in recent times has
been the signicance of mesorectal lymph nodes
[56, 57], especially with the popularisation of
anatomically precise rectal cancer surgery based
on the principles of TME.Quirke etal. [58] found
that in good quality mesorectal plane TME specimens, where the rectum and its surrounding lymphovascular mesorectum were excised as an
intact package, the risk of local recurrence at
3years was 4% without neo-adjuvant radiotherapy. Thus, some have argued that as optimal
TME surgery achieves good locoregional control, lymph node status should not mandate routine adjunctive treatment [56], suggesting instead
368
M. Dattani and G. Brown
a ‘hierarchical’ set of adverse prognostic features
that include extramural venous invasion and the
circumferential resection margin. At present
however, assessment of lymph nodes in the preoperative setting has an important role, for example where local excision is being considered to
avoid the risks of major resectional surgery.
Prediction ofCircumferential Resection
Margin Involvement
The relevance of the circumferential resection
margin (CRM) was rst postulated in a seminal
paper by Quirke etal. [59] in 1986, in which they
reported the association of pelvic recurrence with
tumour presence at what they termed the ‘lateral’
margin of rectal cancer specimens. An involved
CRM has since become one of the most important
and well-established prognostic factor in rectal
cancer, and is associated with an increased risk of
local recurrence, distant metastases and reduced
survival [60–63]. The preoperative prediction of
an involved or threatened CRM, therefore, has
signicant implications on the need for neo-adjuvant treatment, planning the type and plane of surgery in order to maximise the chances of a curative
resection, and most crucially, counselling the
patient about the potentially adverse outcomes.
The anatomical equivalent of the surgical
CRM is the mesorectal fascia [64]; a thin embryological broareolar sheath that envelopes the
rectum and its surrounding fatty lymphovascular
mesorectum, thus forming a natural oncological
barrier. It is beyond this fascia that they ‘holy
plane’ of TME is pursued to achieve the surgical
excellence of resecting an oncologically intact
specimen. On axial MRI sections, the mesorectal
fascia appears as a low signal intensity circumferential structure that encases the mesorectum
(Fig. 21.4a, b), which itself is of high signal
intensity similar to fat.
Brown and colleagues [40] rst demonstrated
the feasibility of MRI in visualising the mesorectal
fascia in a small, single centre study comprising 28
patients. Subsequently, they reported a 92% accuracy in predicting involvement of the pathological
CRM if tumour invasion was within 1mm of the
mesorectal fascia on MRI [12]. Whilst there was
initial uncertainty about the optimal distance
a
b
Fig. 21.4 (a) Axial T2-weighted MRI section showing the
circumferential mesorectal fascia (white arrows) encasing
the mesorectum. In this case, the mesorectal fascia has
not been invaded by the tumour (not shown). (b) AxialOblique T2-weighted image of a mid rectal tumour (white
arrow) that has perforated through the mesorectal fascia on
the right, with surrounding inammatory changes
between tumour and mesorectal fascia to predict a
threatened margin, a <1 mm threshold is now
universally accepted as a standard, and has the
benet of conforming to the histopathological definition of an involved CRM [63]. Distance criteria
of <2 mm [65], or even <5mm [66] have been
used and advocated, and although this unsurprisingly increases the sensitivity of predicting CRM
involvement, the overall accuracy of a <1mm cutoff is far superior as reported in a recent metaanalysis [67]. Moreover, Taylor etal. [68] showed
in a large multi-centre study that with a cut-off
criteria of tumour <5mm from the mesorectal fascia to predict pathological CRM involvement, up
to 13 additional patients would need neo-adjuvant
21 Rectal Carcinoma: Imaging for Staging
369
treatment in order to prevent one local recurrence,
representing overtreatment in this group.
The MERCURY study [69], a European
multi-centre prospective observational study,
reported a MRI accuracy of 88% in predicting
CRM involvement, and an even higher negative
predictive value of 94% when validated against
the gold standard pathological assessment of
CRM involvement. On this basis, when the MRI
predicted mesorectal fascia is clear of tumour,
primary surgery based on the principles of TME
can be recommended to achieve a R0 resection
with a high degree of certainty. The MERCURY
group subsequently found that MRI assessment
of the CRM status was an independent prognostic
marker, predictive of the 5-year local recurrence
and survival rate [35]. This series forms the largest contributor to a recent meta-analysis of 1600
patients, which found a 74% sensitivity and a
93% specicity in the use of MRI to assess
involvement of the mesorectal fascia in rectal
cancer staging, prior to any treatment [67].
Pelvic Side Wall Lymph Nodes
As alluded to previously, MRI staging enables
evaluation of the whole pelvis, including assessment of the lateral wall compartment in which
prominent lymph nodes are occasionally seen.
The clinical signicance and management of
these pelvic side wall lymph nodes (PSWLN) in
rectal cancer remains contentious, but it is purported to be one of the reasons for locoregional
failure despite optimal TME surgery, which does
not address extra-mesorectal disease [70]. This is
particularly pertinent in the distal third rectal cancers, owing to the preferential lateral lymphatic
spread of low lying tumours along the internal
iliac artery, and then to various lymph node stations on the pelvic side wall [71, 72]. Sugihara
et al. [73] reported a histologically conrmed
15% incidence of metastatic PSWLN in cancers
below the peritoneal reection, compared to a signicantly lower 8% incidence in upper rectal cancers from a large multi-centre cohort. In the East,
and particularly Japan, PSWLN dissection is
therefore widely pursued and advocated, accepting the risks associated with what is an otherwise
major and high morbidity procedure [72, 74]. A
meta-analysis of 20 studies, which were mostly
retrospective non-randomised cohorts, comparing
curative TME resection and PSWLN dissection
versus TME alone found no difference in the local
recurrence rates between the two strategies [75].
In this context, clinical practice in North America
and Europe has been to either ignore suspicious
PSWLN seen on pre-operative imaging, or
more commonly, to offer chemoradiotherapy.
Currently, there is no evidence based consensus
on which approach offers the best oncological
outcome, largely limited by the lack of an accurate predictor of PSWLN involvement that can
guide treatment choice.
Debate continues about what constitutes a
‘suspicious’ PSWLN on MRI, much like in the
optimal diagnostic criteria for predicting a metastatic mesorectal lymph node, as discussed previously. Although size is considered a poor
discriminant, it is still employed with various
cut-offs to base decisions about neo-adjuvant
treatment, or the need to pursue PSWLN dissection [76, 77]. The MERCURY group have instead
adopted morphological MRI criteria of mixed
signal intensity and irregular contour to dene an
involved PSWLN (Fig.21.5a, b) [78]. Judged by
these criteria, they reported a 12% incidence of
MRI suspicious PSWLN in 325 patients with
rectal cancer, limited of course by the absence of
histological validation. These patients had a signicantly poorer disease free survival compared
to those without MRI suspicious PSWLN, however, when neo-adjuvant radiotherapy was factored into the analysis, the difference was no
longer apparent between the two groups [78].
Extramural Vascular Invasion
Extramural vascular invasion (EMVI) is dened
as the presence of tumour cells in the microvasculature beyond the muscularis propria, and is a
well-established adverse prognostic factor
[79–81]. Whilst EMVI is more prevalent inlocally
advanced T3/4 rectal cancers, it can present as
discontinuous foci with early stage intramural
tumours [82], and must be sought for to improve
the accuracy of prognostication. Post-operatively,
histologically conrmed EMVI has been shown
to be associated with a higher risk of local and
370
M. Dattani and G. Brown
a
b
a
b
Fig. 21.5 Axial (a) and Saggital (b) T2-weighted MRI
section showing a 14mm metastatic left pelvic side wall
lymph node (white arrow). Note the heterogeneous signal
intensity and irregular border which is suggestive of
tumour inltration. (Images courtesy of Mr. B Moran,
Basingstoke and North Hampshire Hospital, U.K.)
distant failure [80, 81, 83, 84], and poorer overall
survival [85, 86], regardless of the nodal status or
depth of mural invasion [87, 88]. The reported
incidence of histopathological EMVI in rectal
cancer varies from 9 to 61% [89], and to a large
extent reects the variability in reporting amongst
pathologist. The diagnostic yield is inuenced by
Fig. 21.6 (a) Axial T2-weighted MRI section showing
EMVI with the corresponding H&E stained histological
whole mount section. (b) A tongue of tumour is seen
extending into the perirectal fat (arrow) associated with
a vessel, which appears as a signal void on MRI
(arrowheads)
several factors, including the lack of standardised
pathological reporting criteria [90], the processing and number of tissue blocks examined [86,
91], expertise of the reporting pathologist [92],
and the use, or not, of ancillary specialised stains
to improve detection [91, 93].
Recent interest has been generated by the
ability of MRI to detect EMVI pre-operatively,
as part of standard rectal cancer staging.
Brownand colleagues [12] characterise EMVI
as intermediate signal intensity serpiginous
structures in the mesorectal fat, similar to
tumour signal, that are seen extending into a signal void tubular structure which represents a
blood vessel (Fig.21.6a, b).
In this seminal study, MRI correctly predicted
histological EMVI in 15 out of 18 cases where
large veins were involved, with small calibre