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
MRI-EMVI
scor
Predicted
Typical imaging features Schematic illustration
21 Rectal Carcinoma: Imaging for Staging
Fig. 21.7 EMVI
Grading system. With
e
permission John Wiley
and Sons [95]
0
1
2
EMVI
371
Pattern of tumour extension
through the muscle coat is not
nodular, and there are no vessels
adjacent ot areas of tumour
penetration
Minimal extramural standing/
nodular extension, but not in
NegativePositive
the vicinity of any vascular
structure
Stranding demonstrated in the
vicinity of extramural vessels, but
these vessels are of normal
calibre, and there is no definite
tumour signal within the
vessel
3
4
vascular invasion being missed even after retrosepctive review [12].
The heterogeneous relationship of perirectal
vasculature and tumour presence has led to the
formulation of a MRI-detected EMVI grading
system which consists of 4 dening criteria
(Fig.21.7): pattern of tumour margin; location of
tumour relative to major vessels; calibre of
vessels; vessel border [94]. A ve-point grading
score based on combination of the above criteria
has been proposed to improve the diagnostic
Intermediate signal intensity
apparent within vessels, although
the contour and calibre of these
vessels is only slightly expanded
Obvious irregular vessel contour
or nodular expansion of vessel by
definite tumour signal
accuracy of MRI predicted EMVI [94], such that
grade 0–2 are effectively EMVI negative, whereas
grade 3–4 are strongly suggestive of the presence
of EMVI.
Based on this grading system, MRI predicted
pathological EMVI with a high specicity ranging
between 88% and 96%, but a variable sensitivity
of 29–62% [95–97]. The low sensitivity of MRI
predicted EMVI may be due to the inability to
accurately visualise small calibre vessels
(<3 mm), even in a high resolution setting.
372
M. Dattani and G. Brown
Alternatively, it is perhaps more likely that EMVI
was underreported in histopathology evaluation
either because of vessel obliteration during
specimen handling [95], or as some authors have
alluded, sub-optimal pathological interpretation,
leading to false negative reports [97]. Indeed, it
has been suggested that MRI evaluation of EMVI
may be more accurate than histopathological
assessment [91], with an incidence of 22–39%
reported in the literature using the above MRI
grading score [95–98].
The presence of MRI detected EMVI has been
shown to predict disease relapse [95], and in a
large series of 450 rectal cancer patients, it was
found to be an independent risk factor for synchronous metastases [97]. Bugg etal. [98] subsequently showed in a cohort of 200 patients that
MRI detected EMVI was associated with an
almost fourfold risk of developing metachronous
metastases within 1year of rectal cancer diagnosis. In this context, the presence of EMVI on MRI
is of signicant relevance, as it may indicate
tumour embolisation into the systemic circulation
with possible micrometastases at the time of presentation, which remain undiagnosed because of
the limitations of current imaging modalities.
More crucially, present day multimodality
treatment in the form of neo-adjuvant chemoradiotherapy and TME surgery does not address this
issue, and may in part account for the good local
control that has been achieved with this approach,
but the failure to prevent distant relapse and
improve survival. Although not routine practice, it
may be that MRI detected EMVI patients are the
ideal candidates, amongst other high risk patients,
for up-front systematic chemotherapy in order to
secure distant control and improve survival, as is
being trialled in randomised studies [99].
MRI Evaluation ofLow Rectal Cancer
The rectum is arbitrarily divided into 3 parts
depending on the height from the anal verge, as
measured by a rigid sigmoidoscope when the
patient is in the left lateral position; low rectum is
up to 6 cm from the anal verge, mid rectum is
from 7 to 11cm, and the upper rectum is from 12
to 15 cm [100]. The importance of this
classication relates to the anatomical relationship
of the rectum to other pelvic viscera, and in
particular the safety provided by the mesorectum
in the mid rectum, or a lack thereof, in the low
rectum whereby a locally advanced tumour is
likely to compromise preservation of the anal
sphincters. Given that these distances will vary
somewhat between individual patients, a more
objective MRI based denition of low rectal
cancer has been proposed as “an adenocarcinoma
with its lower edge at, or below, the origin of the
levators on the pelvic sidewall” [101].
Approximately one third of all rectal cancers
will be ‘low’ according to this denition [102].
The mainstay of a curative surgical resection for
low rectal cancer (LRC) is either a low anterior
resection (LAR), or in up to 45% of cases, an
abdominoperineal excision (APE) that results in
a permanent stoma [103].
The merit of pre-operatively evaluating LRC
as a discrete entity, and thus individualising a
treatment plan, is related to the historically
poor outcomes associated with an APE, compared to anterior resection for upper- and mid
rectal tumours which have seen vast prognostic
improvements following the widespread adoption of TME surgery. Multiple studies have
shown that patients undergoing an APE have a
higher rate of pathological CRM involvement—
over 30% in some large series—resulting in
higher rates of local recurrence and poorer
survival [104–106]. This marked difference in
outcomes can be explained by the surgical challenges of operating within the narrow connes
of the low pelvis on a segment of the rectal
tube which is devoid of the protective mesorectum, which tapers sharply into non-existence at
the proximal insertion point of the sphincters.
These difculties culminate in the resection of
a sub-optimal surgical specimen, often with a
‘waist’ around the lowest part of the mesorectum, or in some cases an iatrogenic perforation
into the tumour or bowel [106, 107]. A greater
awareness of these pitfalls, coupled with the
renement of surgical techiniques to ensure a
more cylindrical specimen, has led to the use
of an extended or extralevator APE (ELAPE)
[108], particularly when the levators or sphincter complex have been invaded by tumour. But
21 Rectal Carcinoma: Imaging for Staging
373
the radicality of such an operation is associated
with a signicant perineal defect which is not
always suitbale forprimary closure, occasionally requiring reconstructive surgery with myocutaneous aps, or a biological mesh to achieve
wound closure and prevent herniation [109].
Despite these techniques, there remains a risk
of perineal wound morbidity which is enhanced
with the use of neo-adjuvant radiotherapy [109,
110]. Moreover, the early results of studies
comparing standard APE against ELAPE have
been mixed, and because not every LRC warrants an ELAPE, an accurate staging system to
tailor the optimal plane of surgery, or appropriately select patients for neo-adjuvant therapy is
highly valued [111].
To address these complexities in the management of LRC, an MRI based anatomical
staging system has been devised which assesses
the relationship of the tumour to both the intersphinteric space and levator muscle [102, 112]
(Table21.3).
Salerno etal. [113] retrospectively validated
this staging system and demonstrated that stage 3
and 4 low rectal tumours had an 18-fold increased
risk of a pathologically involved CRM, compared
with stage 1 and 2 tumours. When the latter group
had a standard APE without radical en-bloc
resection of the levators, the CRM was involved
in over 50% of the cases. Thus, according to the
LRC staging system, tumours categorised as
Table 21.3 MRI based staging system for low rectal
cancer
MRI stage
for LRC Description of stage assessment
Stage 1 Tumour conned to the bowel wall and
does not extend through the full
thickness; intact outer muscle coat
Stage 2 Tumour replaces the muscle coat but
does not extend into the intersphincteric
space
Stage 3 Tumour invades the intersphincteric
space or lies within 1mm of the levator
muscle
Stage 4 Tumour invades the external anal
sphincter and is within 1mm and beyond
1mm of the levator muscle, with or
without invading adjacent structures
stage 3 or 4 will require an ELAPE, or exenterative surgery if adjacent viscera are involved in
order to achieve a clear resection margin.
Conversely, stage 1 or 2 low rectal cancers can
safely achieve a clear CRM with an intersphicteric resection, with a colo-anal anastomosis
where feasible and indicated (Fig.21.8).
A composite MRI staging system for LRC has
subsequently been developed which encompasses the assessment of tumour proximity to
mesorectal fascia, as well as the involvement of
the intersphicteric space which can be used to
dene the surgical plane of excision. Thus, the
low rectal plane can be considered ‘safe’ to
achieve a complete surgical resection when
tumour does not invade both of the aforementioned structures (Fig.21.9).
The MERCURY II [103] study was the rst to
prospectively validate this LRC staging system in
279 patients recruited from 14 centres across
Europe, and achieved and overall pathological
CRM involvement of 9%, which is signicantly
lower than the historically reported rates.
Reassuringly, when the MRI predicted low rectal
cancer plane was safe (Stage 1 and 2 tumours and
MRF clear), the rate of CRM involvement was
reduced to only 4%, conrming the utility of
MRI in aiding the pre-operative decision making
process. Furthermore, the combination of a MRI
evaluated safe low rectal cancer plane with no
other adverse prognostic features (<mrT3c, mrN2
and no mrEMVI) resulted in CRM involvement
in only 1 out of 62 cases (1.6%). None of these 62
patients were deemed to require neo-adjuvant
therapy based on the MRI staging, and managed
to safely avoid the additional co-morbidity of
chemoradiation by proceeding straight to surgery, without any compromise of the short term
oncological outcomes.
On multivariate analyses, three additional
MRI risk factors were identied that predicted an
involved CRM, namely, a tumour located at a
height of less than 4cm, anterior quadrant invasion, and the presence of EMVI [103]. Based on
these ndings, the group proposed a risk stratication model according to which the presence of
all 4 adverse features predicted a risk probability
of up to 60% for an incomplete resection. These
374
ab
M. Dattani and G. Brown
Fig. 21.8 Pictorial description of low rectal cancer and
plane of resection in an oblique coronal view. (a) The
tumour on the left depicts an early stage low rectal cancer
that has not breached the muscularis propria/internal
sphincter, and would therefore be oncologically suitable
for a standard APE, or a restorative intersphicnteric resection as shown by the dashed green line in (b). The tumour
on the left in (a) has extended beyond the intersphincteric
space into the levator ani and puborectalis muscle. This
would require an extralevator APE (ELAPE) as shown by
the dashed blue lines in (b) to achieve a clear CRM
(Adapted from Battersby et al. Exp Rev Gastroenterol
Hepatol 2014;8:703–19)
multivisceral resection beyond the conventional
TME plane in order to achieve a R0 resection
[114]. Despite the risks of signicant morbidity
and mortality, and the effect on patients’ quality
of life after what is potentially mutilating surgery,
a clear pathological resection margin is the most
important determinant of survival in patient with
non-metastatic disease [115]. Accurate pre-operative staging can not only help determine the plane
of surgery required to achieve an en-bloc resection, but also mitigate the need for a default exenterative procedure and its associated consequences.
Pelvic high resolution MRI remains the choice of
imaging in most specialist centres, according to
an expert consensus statement by the ‘Beyond
Fig. 21.9 Axial MRI of low rectal cancer invading the
intersphincteric plane
TME’ collaborative [114]. More recently, MRI
staging according to pelvic surgical compartments, rather than involvement of individual
are precisely the patients who may benet from
the addition of neo-adjuvant chemoradiotherapy
to achieve downstaging from an ‘unsafe’ to a
‘safe’ low rectal cancer plane, or in the case of
poor responders, consideration of exenterative
surgery.
Staging for‘Beyond TME’ Surgery
Approximately 5–10% of patients will present
with a locally advanced rectal cancer that requires
organs, has been proposed as an oncologically
superior method of staging because it facilitates
the selection of anatomical planes of dissection.
Georgiou etal. [116] reported a high diagnostic
accuracy of compartment invasion by tumour
(ROC >87%) on such a MRI based staging system, in which there was also good inter-observer
agreement for both recurrent and primary rectal
cancer requiring curative resection beyond the
TME plane. However, prospective validation of
21 Rectal Carcinoma: Imaging for Staging
375
Fig. 21.10 Compartmental pelvic staging for ‘Beyond
TME’ surgery using high resolution MRI. Six distinct
anatomical compartments are shown: Lateral (L); central
above peritoneal reection (PR); central below peritoneal
this system in a larger study is warranted, and this
is currently underway in a multi-centre trial
whereby the pelvis is divided into six surgical
compartments as shown in Fig.21.10 [117].
MRI Staging toGuide Neo-Adjuvant
Treatment inRectal Cancer
Neo-adjuvant radiotherapy, either alone or in
combination with chemotherapy as a long course
regimen has been shown to reduce local recurrence rates [118–121], without any signicant
improvement in overall survival. But pelvic irradiation is in itself not without consequence, with
genitourinary impairment, bowel dysfunction
following restorative surgery [122], perineal
wound morbidity in patients undergoing an APE
[123], and the risks of a secondary malignancy
[124] all reported in the literature. The unstanderdised routine use of neo-adjuvant treatment
for rectal cancer, as evidenced by the wide varia-
reection (PR); posterior; infralevator. Adapted and
modied from [117], which also includes a description of
the individual organs in each compartment
tion in its worldwide use [14, 125–127], is detrimental to the patient and of little clinical benet
when optimal TME surgery is performed for
operable rectal cancer with no high risk features
[58, 128, 129]. A selective, risk stratifying policy
that spares patients from the long term toxic
effects of neo-adjuvant treatment, without necessarily compromising local control, is therefore
highly desirable [130].
In North America, the National Comprehensive
Cancer Network (NCCN) recommends
neo-adjuvant treatment for all stage II [T3/4; N0]
and stage III [T1-T4; N1/2] [131] rectal cancers,
a policy also adopted by several other international guidelines [14]. However, up to 80% of all
rectal cancers are stage II or III at presentation,
and the heterogeneity in this broad group means
that some patients will inevitably be over treated,
reecting a recent debate about multimodality
treatment in the intermediate risk cancers within
376
chemoradiotherapy
ant long concourse
‘The good’
Predicted LR risk <10%
T1 - T3a/b, N0
No EMVI
MRF clear
Rectal cancer assessed by MRI
‘The bad’
Predicted LR risk 10-20%
T3c/d - T4 or N1/2
MRF clear
M. Dattani and G. Brown
‘The ugly’
Predicted LR risk >20%
Threatened (<1mm)
or
involved MRF
or
EMVI present
Low rectal cancer:
Involved inter-sphincteric plane
or
levators
Primary TME surgery Consider neo-adjuvant short course
Fig. 21.11 MRI-based risk stratication of local recurrence in rectal cancer, with subsequent recommendation of treatment (Adapted and modied from [129–131])
this prognostically diverse group [128, 132, 133].
Consequently, one approach being adopted in the
U.K and some of the Northern European countries has been a shift from the TNM based preoperative evaluation, to MRI based risk
radiotherapy (5 x 5 Gy), or long course
advantage of eliminating the sampling biases
inherent in selecting patients from a progonostically heterogeneous stage II/III population,
which may mask the true effect of a given
treatment.
Offer neo-adjuv
chemoradiotherapy
stratication in order to guide rectal cancer management, and in particular, the use of neo-adjuvant treatment (Fig.21.11). [134, 135].
The distinction between ‘good’, ‘bad’ and
Assessment ofResponse to
Neo-Adjuvant Treatment
‘ugly’ tumours depending on MRI based risk
factors of local recurrence has started to be
incorporated into clinical guidelines for rectal
cancer management, most notably in the U.K
[136] and by the European Society of Medical
Oncology (ESMO) [137]. By selecting patients
for neo-adjuvant treatment based on these highly
discriminative MRI features, rectal cancer management can be personalised to achieve maximal
benet in clinical practice, whilst minimising the
risks of over- or under treatment. This strategy is
also increasingly being implemented in the
design of clinical trials such as the the joint
Dutch/Nordic RAPIDO trial [99], and has the
Over the last decade, the indications for
neo-adjuvant treatment in rectal cancer have
extended beyond the aim of minimising local
recurrence rates following surgery. Today, long
course chemoradiotherapy followed by delayed
surgery is recommended for locally advanced
rectal cancers that threaten the surgical CRM,
with the intention that downstaging of the tumour
will allow for a curative resection [138]. This
may facilitate a surgical procedure less radical
than was initially necessary to achieve a complete
resection, with the added benet of sphincter, or
even organ preservation through a transanal local
21 Rectal Carcinoma: Imaging for Staging
377
excision in the select few. Moreover, in a prognostically favourable subset, sufcient tumour
regression takes place such that there is no clinical evidence of a viable cancer, with the advantage that patients may be able to avoid surgery
and its associated morbidity altogether. These
decisions are challenging, contentious, and high
risk for both patients and clinicians, and are only
possible in the context of restaging the rectal cancer to evaluate its response to neo-adjuvant
treatment.
Whilst the importance of restaging is undisputed, questions remain unanswered about the
optimal time interval between completion of
neo-adjuvant treatment and surgical resection
[139], the timing of restaging the tumour in this
hiatus, the imaging modality that best assesses
response to treatment [140], and the safety of
recommending a change in the surgical strategy
[141], or in the presence of a clinical complete
response (cCR), advocating organ preservation
under a Watch-and-Wait surveillance programme
[142, 143].
Evaluating Tumour Response
Imaging the assessment of response to neo-adjuvant treatment is vital, albeit challenging, despite
attempts to standardise the reporting criteria. The
RECIST (response evaluation criteria in solid
tumours) criteria [144] have been widely adopted
and are based on changes of the tumour size in its
longest diameter, which is an objectively measurable and reproducible parameter. Response can
be categorised as complete if there has been a
total disappearance of the tumour, partial or progressive depending on percentage change from
baseline diameter, or stable disease in case of no
appreciable change in measurement. However,
pelvic irradiation results in a combination of tissue oedema, inammation, necrosis and brosis
of the tumour bed [145], all of which can be difcult to distinguish from residual tumour when
evaluating the response to neo-adjuvant treatment. For example, extensive tumour necrosis in
solid organ cancers has been shown take place
without any corresponding tumour shrinkage
[146]. Moreover, the denition of more than a
30% reduction in tumour diameter to indicate a
partial response is arbitrary, and is unvalidated
against outcome data [145]. Finally, the RECIST
criteria do not account for treatment response in
non-tumoural malignant deposits such as EMVI,
or the involvement of vital anatomical structures
such as the CRM, which remains the most important determinant of local recurrence in rectal
cancer.
The other issue in restaging rectal cancer after
neo-adjuvant treatment is the choice of imaging
modality, with MRI and ERUS being the
preferred options [140], and Computed
Tomography (CT) largely reserved for extrapelvic disease assessment. Both MRI and ERUS
are subject to the same limitations discussed
previously, with the added difculty that the
post-chemoradiation effects compromise
accurate re-staging (denoted by the prex “y”)
even further. It is therefore unsurprising that a
meta-analysis reported the average accuracy of
predicting ypT with MRI was 52%, and for
ERUS at 65% [140], with overstaging of early
tumours a particular issue for both modalities
[147]. The accuracy of predicting ypN status
was modest, with both MRI and ERUS having an
estimated average of 72% [140, 148].
But whilst the absolute prediction of the various ypTN stages is desirable, what is perhaps
more valuable from a therapeutic perspective for
a tumour that has partially responded to neoadjuvant treatment is its relationship to the MRF
or the intersphincteric plane, because of the
possibility of a more conservative curative resection than was initially possible.
Assessment ofCRM Following
Neo-Adjuvant Treatment
MRI retains a good accuracy of predicting CRM
involvement after neo-adjuvant treatment, with
reported averages of between 70% and 92% [140].
Whilst the specicity and negative predictive
values have been shown to be consistently high,
the main compromising factor is of overstaging,
such that the risk of over-treatment is increased
with a higher number false positive predictions.
This arises mainly because of the uncertainty in
differentiating brotic tissues from residual
tumour, especially if there has been a concurrent
378
M. Dattani and G. Brown
retraction of the MRF along with the tumour
[149], leading to a cautious overcall at re-staging.
In the case of low rectal cancer, the MERCURY
II study reported on 92 patients who had sphincter involvement and were deemed to require an
ELAPE to achieve a clear pCRM according to the
baseline MRI staging. Following neo-adjuvant
treatment and a re-staging MRI, 33 (36%) of the
cases were downstaged with a clear intersphincteric plane, out of which 7 (21%) avoided a stoma
by undergoing a less radical operation than the
initially planned ELAPE; all 33 patients had a
clear pCRM [103].
A change in the operative strategy based on
the re-staging MRI scan is not without contention, with concerns about the safety of dissecting
through a ‘sterile’ plane which may still harbour
cancer cells [150].
Assessment ofEMVI Following
Neo-Adjuvant Treatment
There is a paucity of data regarding assessment
of the ymrEMVI status, and its effect on oncological outcomes. There is only one study of 188
rectal cancer patients by Chand etal. [151] which
showed that patients who had persistent mrEMVI
following neo-adjuvant treatment had a worse
disease-free survival at 3years (43%) compared
to those who were mrEMVI negative on re-staging (80%). The stratication was based on a
novel tumour regression grade in which a greater
than 50% brosis in the mrEMVI was taken as a
substantial response and re-staged as ymrEMVI
negative [152].
Tumour Regression Grade
andComplete Response to
Neo-Adjuvant Treatment
A proportion of rectal cancer patients who
receive neo-adjuvant treatment will go on to
have what is termed as a pathological complete
response (pCR)—no evidence of a residual
cancer at histopathological analysis of the
surgical specimen. For the commonly used
regimens of chemoradiotherapy, the average
proportion of those who achieve a pCR is
estimated at 15–30% [142]. The benets of
being able to predict a pCR and avoid potentially
mutilating surgery are immense and obvious,
and increasing focus has therefore turned to the
concept of a clinical complete response (cCR) as
a surrogate for the former entity.
A cCR is dened when there is no clinical evidence of a tumour on imaging, endoluminal visualisation, or digital palpation where this is
possible [153]. Patients who exhibit a cCR on reassessment following neo-adjuvant treatment
may be offered the option of non-operative management under a careful ‘Watch-and-Wait’ surveillance programme to monitor their progress.
This strategy was initially met with scepticism
when rst introduced by Habr-Gama’s group
[142, 154], because of concerns about oncological compromise and the success of salvage surgery for tumour regrowths, but has since been
reported to be both feasible and safe by several
groups internationally [155–158].
In addition, the value of imaging assessment
in determining a cCR is increasingly being recognised. In a prospective cohort study with pCR
as the reference endpoint, the addition of MRI
assessment to clinical evaluation alone (digital
rectal examination and endoscopy) increased the
post-test probability of detecting a complete
response from 90% to 98% [159]. Moreover,
pCR prediction by MRI assessment of tumour
response has been shown to be tenfold higher
than clinical assessment, which has a lower sensitivity because of persistent mucosal
abnormalities [160]. In the latter study, the MRI
criteria for evaluating response to neo-adjuvant
treatment are based on the 5-grade Mandard
scoring system for assessing pathological tumour
regression, which is dependent on the relative
proportions of brosis and residual tumour in the
treated cancer [161]. Radiologically, brosis
manifests on high resolution T2-weighted images
as low signal intensity, whilst tumour is indicated
by an intermediate signal (Table21.4).
The mrTRG score has been retrospectively
validated in the MERCURY trial patients, which
found it to be an important prognostic determinant.
Good response tumours (mrTRG 1–3) had a
signicantly better disease-free survival and
overall 5-year survival than poor response tumours
(mrTRG 4–5) [162]. Moreover, mrTRG 1 and 2
ab
21 Rectal Carcinoma: Imaging for Staging
379
patients had a similar survival outcome to those
with a pCR, and may therefore be optimal candidates for a Watch-and-Wait strategy. These ndings have now been used to apply mrTRG as an
imaging biomarker to stratify rectal cancer
patients following neo-adjuvant treatment in the
multi-centre randomised controlled trial called
TRIGGER (Eudract No.: 2015-003009-40).
Rectal cancer patients receiving neo-adjuvant
therapy as part of their treatment will be
randomised to standard TME surgery which is the
current standard of care, or mrTRG directed
management where ‘good response’ patients
(mrTRG1) are offered intensive follow-up in a
Watch-and-Wait protocol, whilst the ‘poor
response’ group is recommended intensied
additional chemotherapy (Fig.21.12a, b).
There is also a growing interest in the use of
functional imaging, particularly diffusion weighted
MRI (DWI-MRI) and 18F-uorodeoxyglucose
positron emission tomography (18F-FDG PET)
with the combination of CT (PET/CT) to assess
Table 21.4 The MRI based tumour regression grade
(mrTRG)
mrTRG 1—complete radiological response (linear low
signal intensity scar only)
mrTRG 2—good response (dense brosis, no obvious
tumour signal)
mrTRG 3—moderate response (>50% brosis and
visible intermediate signal)
mrTRG 4—slight response (mostly tumour)
mrTRG 5—no response (intermediate signal
comparable to baseline MRI scan)
tumour response following neo-adjuvant treatment.
However, the evidence for these modalities is suboptimal according to a recent systematic review
[163], given that most studies comprise of small
retrospective cohorts that are not validated against
established markers of response or long-term
outcomes data. Additionally, the uptake of FDG by
metabolically active benign tissue is well a
recognised issue, and in the context of radiotherapy
induced inammation, may be a signicant confounder in accurately assessing post-treatment
tumour response. This is compounded by a lack of
consensus on the optimal cut off values for SUV
that predict for a pCR [163].
Evaluation ofExtra-Pelvic Disease
The importance of imaging beyond the primary
rectal tumour is evidenced by the high risk of
synchronous metastatic disease, currently
estimated in up to 30% of colorectal cancer
patients at presentation [164]. This has obvious
implications for the multimodality management,
prognosis, and most importantly, counselling of
the patient. The liver is the commonest site for
synchronous metastasis, with a reported incidence
of 15% [165], followed by lung and peritoneal
involvement in up to 10% of patients [166, 167].
The main focus of evaluating extra-pelvic
compartments is therefore the detection of
concurrent disease at these sites.
Fig. 21.12 (a) Axial image of a pre-treatment rectal cancer. (b) Axial MRI image of post-treatment rectal cancer
showing good response (mrTRG1)
380
M. Dattani and G. Brown
Hepatic Metastases
Computed tomography (CT) represents the most
preferred choice of primary imaging modality for
detecting distant metastatic disease, and
supersedes other imaging modality because of its
widespread availability, reproducibility and
lower cost. However, in a meta-analysis of prospective trials, the sensitivity of contrast enhanced
CT for diagnosing colorectal liver metastases
was reported to be 83.6% in patient who had no
previous treatment, compared to 88.2% and
94.1% for MRI and 18F-FDG PET, respectively
[168]; there was little variation in the specicity
of each modality (93–96%). Furthermore, in
lesions measuring less than 10mm in size, MRI
outperforms CT in being able to characterise a
metastatic deposit. The higher accuracy of MRI
is because of the enhanced soft tissue resolution,
with further gains in interpretation reported with
the use of diffusion weighted, or contrast
enhanced liver specic MRI [169]. Most hepatobiliary units that treat colorectal liver metastases
now mandate pre-operative imaging with
multiparametric MRI because of this accuracy.
Pulmonary Metastases
In most instances, a CT of the thorax is performed
as part of the extra-pelvic staging of primary rectal cancer, and is recommended in the American
College of Radiology guidelines [170]. The rationale for routine imaging of the thorax is haematogenous spread of cancer directly to the lungs
via the haemorrhoidal plexus which drain into the
vena cava, bypassing the portal circulation altogether. Indeed, isolated pulmonary metastases
arising from a primary rectal cancer are not
uncommon [171]. Moreover, rectal cancer metastases to the lung are more common than colon
cancer. Whilst the importance of detecting pulmonary metastases cannot be overstated, CT
imaging frequently identies small indeterminate
lesions because of its low specicity, with a variable incidence of between 4 and 42% reported in
the literature. Of these, only 1% are eventually
conrmed as metastases [172], and there are considerable costs of follow-up imaging, radiation
exposure, additional tests which may include
invasive diagnostics, and patient anxiety which
all have to be addressed in the decision making
process.
The addition of 18F-FDG PET to CT staging of
the thorax has also been shown to be of sub-optimal diagnostic value, with a sensitivity of 57.1%
[173] in diagnosing pulmonary metastases. This
is because of the lower resolution of 18F-FDG
PET in characterising lesions <10 mm in size
compared with thin slice CT scanning, which
remains the rst choice of investigation for pulmonary staging. Thus, current indications for the
routine use of 18F-FDG PET/CT in primary rectal
cancer staging are limited, and it may be advocated in a patient being considered for curative
hepatic or pulmonary metastectomy to rule out
an occult metastasis, where conventional CT or
MRI imaging may not have been informative.
Peritoneal Metastases
Pre-operative diagnosis of peritoneal carcinomatosis remains a major challenge, and is frequently
identied for the rst time at operation for the
primary tumour. Contrast enhanced multi-slice
CT is the principal imaging modality despite a
modest accuracy of between 60% and 88% [174,
175], with underestimation of the true extent of
peritoneal involvement. The main limitation of
all available imaging modalities is the small size
of peritoneal nodules, typically less than 1cm in
size, which frequently spread along the normal
anatomical planes in the abdomen, thus compromising the sensitivity even further. Direct visualisation of the peritoneal surfaces remains the most
accurate way of establishing the tumour burden,
and is increasingly becoming a common staging
modality in patients presenting with metachronous disease and being considered for surgery
[176]. But unlike primary staging in hepatopancreaticobiliary and gastroesophageal cancers, its
role in the detection of synchronous colorectal
peritoneal metastases is unknown at present.
Synoptic Reporting inRectal
Cancer Staging
A key component of the pre-operative evaluation
of rectal cancer is the accurate reporting and