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during the preoperative assessment is key, as it may change
the surgical strategy, often by performing a subtotal colectomy instead of a segmental colectomy. This is usually performed by a complete colonoscopy which also allows
confirmation of the diagnosis (using biopsy), tattoo marking
of the tumor site(s) and removal of pre-cancerous lesions. If
a complete colonoscopy is not feasible, combining flexible
sigmoidoscopy and CT colonography may constitute an
alternative (Argiles etal. 2020). In the event a synchronous
lesion cannot be excluded before the removal of the primary
lesion, in situations such as emergency presentation, colonoscopy (preferentially) or CT-colonography can be performed when the clinical situation allows, but ideally within
three months of emergency resection (Argiles etal. 2020).
CT colonography, also known as virtual colonoscopy, allows
assessment of the colonic lumen using low-dose CT combined
with oral contrast and CO
5). CT colonography is usually indicated for colorectal cancer screening in patients refusing a more invasive procedure,
for assessment of strictures and to assess the colon proximal
to a partially obstructing colorectal cancer or for excluding
synchronous lesion(s) in patients refusing complete colonoscopy or in whom a complete colonoscopy is unsuccessful. In
patients with positive fecal occult blood testing, CT colonography has been reported to have a sensitivity and specificity of
88.8% and 75.4% respectively for the detection of colorectal
cancer and adenomas ≥6
tivity was demonstrated to be similar to the sensitivity of colonoscopy for the detection of polyps >5
but was dependent on the expertise of the reporting radiologist
(Plumb etal. 2014b).
2
insufflation of the colon (Figure
mm (Plumb etal. 2014a). Its sensi-
mm (Graser etal. 2009),
Rectal Cancer
Recommended Imaging
The introduction of total mesorectal excision (TME) as a surgical technique and preoperative downstaging (chemo)radiotherapy have markedly improved both local recurrence rates
and survival for patients with rectal cancer. The role of imaging
is to provide detailed staging, including an assessment of the
tumor in relation to the circumferential resection margin
(CRM) and the presence or absence of poor prognostic factors,
such as extramural venous invasion (EMVI). This will then
allow discussion on the suitability and planning of optimal surgery and the need for neoadjuvant treatment.
The minimum required imaging for rectal cancer consists of
CT imaging of the abdomen and pelvis to assess for enlarged
lymph nodes (cN stage) and distant metastases (cM stage), in
addition to CT imaging of the chest to evaluate for lung metastases. The primary lesion is optimally evaluated using pelvic
MRI, which allows accurate locoregional staging by determining the cT stage and assessment of the circumferential
resection margin (CRM). In the case of early rectal cancer,
endoscopic ultrasound may be used as an adjunct to more precisely define the depth of invasion of T1 cancer, given the limitations of MRI in this specific area (Glynne-Jones etal. 2017).
Moreover, the tumor’s distance to the anal verge and from the
sphincters has to be assessed by a combination of digital rectal
examination, rigid rectoscopy and pelvic MRI (Glynne-Jones
etal. 2017), as it will help in surgical planning and whether a
sphincter-preserving surgical approach may be an option.
The presence of a synchronous cancer in the colon is ruled
out using a complete colonoscopy in an adequately prepared
patient, or CT colonography. In the case of an obstructing
rectal cancer, emergency resection of the primary rectal lesion
is usually not performed and a surgical diversion by a loop
stoma is the preferred option prior to further pre-operative
staging and consideration of neo-adjuvant treatment.
Classication System
The classification of the primary cancer and its eventual
secondary lesions relies on the latest version of the Union for
International Cancer Control (UICC) TNM system, which is
similar to the one used for colon cancer (Table 1). As this
classification is – in the context of preoperative imaging – provisional, it is usually preceded by a “c” (meaning “clinical”).
Alternatively, classification of the primary lesion can be preceded by the prefix “mr,” meaning that the stage was provided
by MRI, which is usual in rectal cancer. If neo-adjuvant
treatment was given before imaging, existing prefix and/or
stage indications are preceded by “y.” This can be the case in the
context of restaging after (chemo)radiotherapy.
Assessing the Primary Lesion (T)
MRI is the imaging modality of choice when performing the
local staging of rectal cancer and usually helps to inform the
therapeutic management. Therefore, quality of MRI reporting
is key to optimal care, and the use of a structured template for
MRI reporting is recommended (Glynne-Jones et al. 2017;
Gupta etal. 2020), as depicted in Table 2. Alternatively, the
Korean Society of Abdominal Radiology (KSAR) template can
be used (Cancer KSGfR 2017).
Depth of Invasion
At MRI the mucosal layer of the bowel wall appears as a fine,
low signal intensity line with the thicker, higher signal intensity
submucosal layer beneath. The muscularis propria is seen as a
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Example of standardized case-report form for pelvic MRI to
Table 2
assess rectal cancer.
Patient identifiants:
Clinical indication:
Date:
Contrast product:
Tumor location (lower <5 cm, middle 5-10 cm,
upper 10-15 cm from anal verge)
Distal tumor to anal verge (cm)
Distal tumor to top of puborectalis muscle (cm)
Length of tumor (cm)
Tumor at or below the top of puborectalis (yes/no)
Relationship to peritoneal reflection (above/below)
Clock face of tumor
Morphology of tumor
Mucinous (yes/no)
Extramural depth of invasion (mm)
Tumor/EMVI/node closest to MRF (mm, clock face)
Structure closest to MRF (tumor/EMVI/node)
Clock face of closest MRF
MRF involved? (yes, if ≤ 1
T stage
T4b involvement (detail)
EMVI (and location)
Mesorectal lymph nodes
Pelvic side wall nodes (number, side, location)
N stage
Other findings
Conclusion (mrTNM stage, MRF)
mm)
low-intensity outer layer that may occasionally be depicted as
two distinct layers – the inner circular layer and the outer
longitudinal layer. The outer layer has an irregular corrugated
appearance and numerous surface interruptions caused by vessels entering the rectal wall. The perirectal fat displays high
signal intensity with vessels depicted as signal voids within,
while the mesorectal fascia is visible as a fine, low signal layer
enveloping the perirectal fat and rectum (Brown etal. 1999).
Tumors typically appear as intermediate signal lesions on
T2-weighted MRI, being of higher signal than muscle but lower
than fat or the rectal submucosa.
Given its higher resolution, endorectal ultrasound allows
better definition of the layers of the rectal wall than MRI.
Images demonstrate a hyperechogenic line (interface with the
probe), the mucosa and muscularis mucosae (hypoechogenic
line), the submucosa (hyperechogenic line), the muscularis
propria (hypoechogenic line) and the perirectal tissue (peritoneum or mesorectum, hyperechogenic line) (Nuernberg etal.
2019). Endorectal ultrasound is not routinely performed for
staging of rectal cancer, but can be used for pre-operative
assessment of early tumors and in patients in whom MRI is
contra-indicated such as those with incompatible pacemakers
or cerebral aneurysm clips. It has a pooled sensitivity and specificity to determine T1 stage of 87.8% and 98.3%, respectively
(Puli etal. 2009) and allows early and locally advanced rectal
cancer to be differentiated with 96% sensitivity and 85% specificity (Zorcolo et al. 2009) (Figure 6). Precise pre-operative
substaging of T1 tumors into the sm1/sm2/sm3 Kikuchi stages
Figure 6 (A) Endorectal ultrasonographic image of a T1 tumor (outlined in (B)) invading the submucosa (bright, hyperechoic band illustrated with solid
arrow) but not muscularis propria (hypoechoic/dark line, open arrow). Image courtesy of Dr Ed Godfrey, Cambridge University Hospitals.
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(Kikuchi et al. 1995) is not possible. Moreover, endorectal
ultrasound is operator-dependent with reduced accuracy for
assessing more advanced tumors and nodal stage (GarciaAguilar etal. 2002). Endorectal ultrasound constitutes a useful
adjunct for surgical planning and identifying rectal cancer eligible for transanal endoscopic microsurgery (TEMS) or transanal minimally invasive surgery (TAMIS). Its optimal accuracy
seems to be when cancer is located 3-6 cm from the anal verge
(Ren etal. 2018).
In rectal cancer, T3 stage is synonymous with tumor extension
into the mesorectum and/or into the intersphincteric plane (for
low rectal cancer) (Tapan etal. 2014). This translates on MRI
to tumor signal extending into the perirectal fat with a rounded
or nodular advancing margin and in continuity with the intramural portion of the tumor. The muscularis propria often has
an irregular corrugated appearance in the absence of tumor
and interruptions of this outer layer may occur normally due
to penetrating blood vessels. Therefore, irregularity or disruption of the low signal muscularis propria itself is not sufficient
to diagnose T3 disease. Spiculations within the mesorectal fat
adjacent to the tumor are not necessarily a manifestation of
extramural tumor spread, given that tumors may elicit a desmoplastic response or cause perivascular cuffing of the penetrating vessels. Nor can MRI reliably differentiate T2 from early
T3 tumors, however this distinction is not clinically important
as it will not generally affect the choice of treatment modality.
T3 stage can be further divided into substage depending on
the depth of invasion beyond the muscularis propria. An invasion into the mesorectum <5 mm is classified as mrT3a, and an
invasion >5
the subclassification can be as follows: mrT3a (<1
(1-5
mm as mrT3b (Zinicola etal. 2017). Alternatively,
mm), mrT3b
mm), mrT3c (6-15 mm) and mrT3d (>15 mm) (GlynneJones etal. 2017) (Figures 7 and 8). This subclassification has
prognostic value, as it also indirectly indicates the distance of
Figure 7 Axial oblique T2-weighted MR image of a T3a mid rectal tumor
extending to 5–9 o’clock (solid white arrow). Muscularis propria is visible
as a hypointense (dark) band (black arrow). Early soft tissue extension
beyond muscularis propria by 1 mm is in keeping with T3a disease (open
white arrow).
Figure 8 Sagittal (A) and axial oblique (B) T2-weighted MR image of a cT3b mid rectal tumor extending from 3–9 o’clock in the axial plane (asterisk).
Muscularis propria is visible as a hypointense (dark) band (black arrow). Irregular tissue extending into the mesorectal fat by 6 mm is consistent with T3c
disease (solid white arrow). More marked hyperintensity within the mesorectal fat represents edema (open arrow).
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Figure 9 Locally advanced, T4b tumor. Axial T2-weighted MR image
demonstrates a semi-annular tumor extending from 12–7 o’clock (asterisk)
with extension anteriorly through the prostatic capsule (white arrow).
the tumor from the CRM (Merkel etal. 2001), and therefore the
surgical margin in case of planned surgery with TME.
Finally, tumor extension into adjacent organs, pelvic sidewall
or peritoneal reflection represents T4 disease (Figure 9). Tumor
invasion beyond the intersphincteric plane into the external
anal sphincter also represents T4 stage (Tapan etal. 2014). This
corresponds to locally advanced rectal cancer, which is defined
as cancer that is predicted by MRI to require an extended surgical resection beyond the TME plane to achieve a resection
specimen with a pathologically clear resection margin (R0)
(Beyond 2013).
Distance from the Anal Verge
Rectal cancer is usually classified as high rectal cancer (10–15
cm from the anal verge), mid rectal cancer (5–10 cm from the
anal verge) and low rectal cancer (<5 cm from the anal verge)
(Glynne-Jones etal. 2017). More recently, low rectal cancer was
defined as a cancer with its lower edge at or below the origin of
the levator muscles at the pelvic side-wall based on MRI
(Salerno etal. 2006). The tumor’s distance to the anal verge is
assessed by a combination of digital rectal examination, rigid
rectoscopy and pelvic MRI.
Estimating the precise location of rectal cancer is of crucial
importance for therapeutic management. For instance, high
rectal cancer (defined as >12cm from the anal verge and/or
above the peritoneal reflection) may not require neo-adjuvant
treatment in some countries and patients can benefit from
anterior resection with partial mesorectal excision, likely
without the need for a stoma. Mid and low rectal cancers, however, should be appropriately staged and consideration given
for neoadjuvant (chemo)radiotherapy and followed by surgery
with low anterior resection including TME if not entered into a
“watch and wait” program.
A key question is whether to preserve the sphincters or not.
If a stapling device can be safely applied at a distance below the
lower edge of the tumor and an anastomosis performed above
the sphincters (even if it involves a disconnection from the perineal approach and intersphincteric dissection), the sphincters
(sometimes only the external sphincter) can be preserved. If
not, and for example if the sphincter(s) are involved, an abdomino-perineal excision of the rectum (APER) should be performed. In addition to the anatomical findings, a patient may
choose a stoma rather than a restorative procedure, but this is
outside of the scope of this chapter.
Circumferential Resection Margin (CRM)
The circumferential resection margin (CRM) at the level of the
mesorectum is represented by the mesorectal fascia (MRF),
which surrounds the mesorectum. At MRI the MRF is identified
as a thin low signal intensity linear structure encompassing the
mesorectum. The tumor may directly spread to the MRF or may
be present within lymph nodes, veins, lymphatics or as tumor
deposits (Langman etal. 2017). Once identified, the presence of
tumor close to the MRF may imply a threatened margin, which
is usually defined as being within 1 mm (Figure 10). The prevalence of threatened MRF during staging MRI was estimated to
be 13.2% (Group MS 2006). Involvement of the CRM constitutes an important risk factor for increased five-year incidence
of local recurrence and distant metastases, and decreased fiveyear disease-free survival and overall survival (Detering et al.
2021). Therefore, if the CRM is threatened based on preoperative staging, the tumor is considered as locally advanced and
neoadjuvant chemoradiotherapy should be proposed (Beyond
2013; Glynne-Jones etal., 2017). Of note, the nature of CRM
involvement may influence oncological outcomes. For instance,
some evidence indicates that patients with a MRF threatened by a lymph node have better oncological outcomes than
patients with a MRF threatened by the primary tumor, which
may be similar to the outcomes of patients with non-threatened
MRF, notably in terms of local recurrence (Birbeck etal. 2002;
Nagtegaal etal. 2002; Patel etal. 2019; Sao Juliao and Perez 2019;
Smith etal. 2022; Suarez etal. 2018).
In the upper rectum, the anterior wall is covered by peritoneum, rather than mesorectum. Identification of the peritoneal reflection is important as peritoneal involvement by
tumor represents T4a disease. Although the peritoneal reflection is a variable structure, it may be identified on the sagittal
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Figure 10 (A) Mesorectal fascia threatened at 2 o’clock (white arrow) by extension of a T3c semi-annular tumor (asterisk). (B) The fascia is also
threatened at 6 o’clock by an enlarged and heterogeneous signal intensity node with malignant features (arrow).
images as a low signal intensity structure passing over the
urinary bladder, which passes posteriorly to its point of attachment to the rectum.
In the lower rectum, the mesorectal plane is replaced by the
intersphincteric plane. Invasion into the intersphincteric plane
is considered as T3 disease, whereas invasion of the external
anal sphincter or within 1 mm of the levator ani is considered
as T4 disease (Santiago etal. 2020; Tapan etal. 2014) and may
require extralevator abdomino-perineal excision (ELAPE) or a
more extended procedure.
Vascular Invasion
In addition to direct extension, tumor may invade along blood
vessel and lymphatics. Extramural venous invasion (EMVI)
is present in up to one third of patients with rectal cancer
(Siddiqui etal., 2017) and was identified as a strong predictor
of disease-free survival (Gu etal. 2019) and both synchronous
and metachronous metastases (Siddiqui et al. 2017). EMVI
can be detected by CT but is better assessed by MRI. On thinsection MRI, extramural venous invasion (EMVI) by tumor
can be readily identified and is demonstrated as serpiginous or
tubular extension of tumor beyond the muscle coat (Figure 11).
Sensitivity of MRI for detection of EMVI ranges between 50%
to 83% (Brown etal. 2003; Zhang etal. 2018). A score to allow
prediction of the presence of EMVI on histology based on MRI
criteria has been developed (Smith etal. 2008)
Figure 11 EMVI. Sagittal T2-weighted MR image of a bulky locally
advanced mucinous tumor of the mid rectum (solid white arrow) with
invasion into the prostate anteriorly (open arrow). Extensive mucinous tumor
invasion (EMVI) of the superior rectal vein is clearly visible (black arrows).
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Assessing Lymph Nodes (N)
As with colonic cancer, CT can only use size as a criterion for
nodal involvement by tumor and as such suffers the same limitations. However, given the improved ability of MRI to resolve
nodal morphology, further criteria have been developed. A wide
variety of suggested cut-off values have been suggested for lymph
node size ranging from 3 to 10 mm, with variable sensitivities
and specificities. Other criteria based on the morphology of the
node have therefore been developed. An irregular or indistinct
border has consistently been shown to be a specific marker of
likely lymph node positivity. Similarly, an irregular or heterogeneous signal from within the node also correlates with likely
nodal positivity (Kim etal. 2004). Such criteria are independent
of nodal size but when combined can increase the sensitivity
and specificity of nodal staging to 90% (Brown 2005). However,
imaging is unable to determine the presence of micrometastases
(tumor deposits around 1–2
Local nodes are located within the mesorectum. As with
colonic cancer, the second group of nodes is related to the
arterial blood supply. However, the rectum receives supply both
from the inferior mesenteric artery and the internal iliac artery,
via branches of the pudendal artery. Therefore, it is possible for
nodal metastases to occur outside the mesorectum along the
internal iliac chain with lateral spread to the pelvic sidewall or
retroperitoneum (Figures 10 and 12).
mm) within individual nodes.
Colorectal Cancer Metastases
Recommended Imaging
Tumor cells can spread to distant organs to cause metastatic
disease. This can occur through the hematogenous route, the
lymphatic route or the transcoelomic route. At primary presentation of colorectal cancer, 17.6% of patients have metastasis,
which defines stage IV disease (Moghadamyeghaneh et al.
2016). The most common site of metastasis are the liver (70%),
the lungs (24%), distant lymph nodes (16%) and the peritoneum (15%) (Holch etal. 2017).
Recommended imaging looking for potential metastases of
colorectal cancer includes CT of the chest, abdomen and pelvis,
which provides a first assessment of central lymph nodes, liver
and chest (Van Cutsem etal. 2016). CT has a sensitivity and
specificity of 85% and 98% respectively for M-staging in colorectal cancer (Leufkens etal. 2011). Optimal detection of liver
metastases requires intravenous contrast and scanning in the
portal venous phase of enhancement. When metastatic disease
is shown or suspected, the diagnostic management is then tailored to the patient and potential therapeutic strategy in a stepwise approach with imaging guided according to the potentially
affected organs (Van Cutsem etal. 2016).
PET-CT is not routinely performed and should be reserved for
decision-making in patients with stage IV disease (to look for
additional sites of metastasis) and also in patients with normal
staging CT but at high risk of metastatic disease, for example in
case of extensive EMVI or high CEA (Glynne-Jones etal. 2017).
Figure 12 Same patient as in Figure 11. Large, heterogeneous signal
intensity lymph node with an irregular border within the presacral space,
consistent with metastasis. The node is in continuity with EMVI as
demonstrated in Figure 11.
Liver Metastases
The liver is the most common site of metastatic disease in
patients suffering from colorectal cancer. Optimal imaging is
key to evaluate the potential resectability of metastases, which
can be performed either by surgery or ablative treatments, such
as thermal devices (radiofrequency ablation, cryoablation,
microwave ablation), non-thermal devices (brachytherapy electroporation, external body radiotherapy with high-precision
radiation therapy), embolic devices (selective internal radiation
therapy (SIRT), transarterial chemoembolization (TACE)) or
local chemotherapy (Van Cutsem etal. 2016). Usual indications
for resection include the possibility to completely remove the
liver disease (R0) while preserving an adequate future liver remnant (FLR) without concomitant unresectable extrahepatic disease (Adam etal. 2012; Van Cutsem etal. 2016).
At CT, liver metastases usually appear as lesions of decreased
attenuation compared to the liver parenchyma. In the event of
mucinous primary tumors, differentiation of cystic metastases
from simple cysts may be difficult and is a recognized pitfall.
Contrast-enhanced MRI is superior to contrast-enhanced CT
for the detection of lesions above and below 1 cm in diameter
(Figure 13). However, for colorectal cancer, it is not sufficient to
simply diagnose the presence of metastatic disease within the
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Figure 13 Enlarging liver metastasis. (A) Portal venous phase axial CT image demonstrates a hypoenhancing lesion within segment 7 (arrow), which
subsequently enlarges in the interval prior to MR imaging. (B) Diffusion weighted MR image demonstrating the metastasis as a high signal, diffusionrestricting lesion which is hyperintense on T2-weighted imaging (C). Hepatobiliary contrast medium is taken up by normal liver parenchyma but not the
metastasis, which appears as a hypointense (dark) lesion in (D).
liver. If the potential treatment is surgical resection, then it is
necessary to detect all lesions and also to characterize their location. As a result both MRI and PET-CT are performed, the latter
also enabling the identification of unsuspected extrahepatic metastatic disease (Adam etal. 2012; Bipat etal. 2005) although with
reduced sensitivity for the detection of metabolically active
deposits less than 1 cm in size or the detection of mucinous
metastases. As a result, liver MRI is also indicated, while PET-CT
may detect extrahepatic metastatic disease in up to 29% of
patients considered for surgery, hence altering decision-making
(Khan etal. 2006). For instance, metastasectomy was shown to
improve cancer-specific survival in patients with liver metastases, but not in patients with lung metastases or synchronous
lung and liver metastases (Siebenhuner etal. 2020).
Peritoneal Metastases
Synchronous and metachronous peritoneal metastases are present in 8.3% of patients with colorectal cancer (Segelman etal.
2012) (Figure 14). Of these patients, only 3% of patients are
potentially eligible for surgery, which consists of cytoreductive
surgery (CR) with hyperthermic intra-peritoneal chemotherapy
(HIPEC) or pressurized intraperitoneal aerosol chemotherapy
(PIPAC) in patients with a Sugarbaker’s peritoneal carcinomatosis index (PCI) <20 and no extra-peritoneal metastases (as
verified by PET-CT). Therefore, diagnosing the presence of peritoneal metastases and characterizing their number and size is
key for optimal surgical management and avoiding potentially
unnecessary surgery. Unfortunately, pre-operative imaging techniques have poor sensitivity for diagnosing peritoneal disease.
For instance, CT has a sensitivity ranging between 60 to 76%
(de Bree etal. 2004; Marin etal. 2010) which varies according to
the location of the metastases and their diameter. Its sensitivity
further drops to 43% for lesions of a diameter smaller than 0.5
cm (Marin etal. 2010). Sensitivity of MRI has previously been
similar to CT. However improved techniques – in particular
diffusion-weighted imaging – have allowed its sensitivity to
reach 84% with the better detection of smaller metastases (Low
etal. 1997). Adding PET sequences does not seem to increase
the diagnostic accuracy of pre-operative imaging techniques
(Elekonawo etal. 2020). In the United Kingdom, these patients
are still discussed routinely with the nominated national referral
centers for peritoneal malignancy. In order to compensate for the
low sensitivity of imaging techniques, patients with T4 colorectal
cancer may benefit from laparoscopy for early detection of metachronous peritoneal carcinomatosis (Bastiaenen etal. 2019).
Lateral Pelvic Sidewall Lymph Nodes
Mid and low rectal cancers below the peritoneal reflection may
metastasize to lateral pelvic sidewall lymph nodes, such as the
internal iliac nodes, common iliac nodes, obturator nodes, and
external iliac nodes (Bell etal. 2009). The prevalence of lateral
pelvic sidewall lymph node metastases in patients with mid/low
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Figure 14 (A) Axial CT image demonstrating a large hepatic metastasis (asterisk) with peritoneal deposit adjacent to the jejunum and splenic flexure
(white arrow). (B) Coronal MPR image demonstrating the cecal primary tumor.
Figure 15 Metastatic lateral pelvic sidewall node. (A) Axial T2-weighted MR image demonstrates a high signal enlarged sidewall node adjacent to
internal iliac vessels on the right, with signal characteristics consistent with a known mucinous primary (not shown). (B) 18F-fluorodeoxyglucose (18F-FDG)
PET-CT fused axial image confirms avidity within the node, in keeping with metastatic disease.
rectal cancer is estimated to be around 17% (Christou etal. 2021)
and is inversely proportional to tumor height (Ueno etal. 2005).
Involvement of lateral pelvic sidewall lymph nodes constitutes a
cause of recurrence in patients treated with preoperative chemoradiotherapy and rectal resection (Kim etal. 2008). Therefore,
identifying these lateral pelvic sidewall lymph nodes at risk of
lymph node metastases in the pre-operative setting is key for
optimal multidisciplinary treatment. These nodes can be identified using CT, MRI, PET-CT or PET-MRI (Figure 15).
When using definitive histopathological analysis as a gold-
standard, MRI has a sensitivity and specificity of 68.6% and
79.7% respectively for detecting lateral pelvic sidewall lymph
nodes (with a cutoff axis of 5 mm) (Ogawa etal. 2016). If uncertainty remains the addition of PET, combined with either CT or
MRI, may increase sensitivity. For instance, the metabolic
tumor volume of the primary tumor and the maximum standardized uptake value (SUV) of suspected lymph nodes were
demonstrated to be independent predictive factors for nodal
metastases (Kim etal. 2019).
In Western countries, suspected involvement of these nodes
will constitute an indication for chemoradiotherapy with or
without a radiotherapy boost (Glynne-Jones etal. 2017). In case
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of absence or partial response after neoadjuvant treatment, the
therapeutic strategy varies between radiotherapy boost to surgical excision of the rectum with lateral lymph node dissection
(Hazen etal. 2021). Restaging MRI should be performed as it
allows identification of lymph nodes at higher risk of recurrence
(those with short axes of ≥0.5 cm) (Malakorn etal. 2019; Ogura
etal. 2019). In Asia, routine lateral lymph node dissection is the
preferred approach and can be performed unilaterally or bilaterally (Kanemitsu et al. 2017). Ongoing controversy exists
regarding treatments for suspicious lateral pelvic side wall lymph
nodes and indications for lateral lymph node dissection. Some
authors recommend a selective approach (Kim etal. 2020).
Key Take Home Messages
Pre-operative imaging of colorectal cancer is key to assess
the local and distant extents of the disease and to exclude
synchronous cancer, with the objective of determining surgical
resectability and eventual need for neo-adjuvant therapy. The
three components of the clinical TNM stage should be assessed:
the T(umor) stage, the N(odal) stage and the M(etastasis) stage.
Imaging of colon cancer relies on CT of the chest, abdomen
and pelvis, associated with a complete colonoscopy. Imaging
of rectal cancer relies on the same modalities, but optimal
assessment of local extension requires the addition of pelvic
MRI. Pelvic MRI has to be reported according to a standardized
form which provides the surgeon with the required minimal
information for surgical planning. In case of suspected metastasis, optimal imaging has to be tailored to the patient.
Areas for Further Research
Although imaging and staging of colorectal cancer has seen
major improvements over the last decade, the field deserves
further improvements. Of note, more sensitive and specific
methods for detecting peritoneal carcinomatosis and metastatic lateral lymph nodes are awaited. Moreover, assessment of
tumor response to neo-adjuvant treatment could be improved,
notably by the use of functional imaging methods. And finally,
molecular imaging using radio-labeled antibodies could potentially assess the bio-distribution and bio-availability of potential targets for immunotherapy (Van Cutsem etal. 2016).
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