*5.1.1 Challenges for T-staging*

*Colorectal Cancer*

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**Figure 8.**

**Figure 7.**

*Radiology Kaufbeuren.*

**Figures 5, 6** and **7.**

Several histopathologic studies have shown that T3 tumors with more than 5 mm mesorectal invasion have a cancer-specific 5-year survival rate of approximately 54% [11]. On the other hand, for tumors of 5 mm or less in diameter, the cancer-specific survival exceeds 85% [12, 13]. Therefore, it is crucial to report the depth of extramural spread in detail, with the precise substage **T3a, b, c** or **d**. The overall reported accuracy for T staging using a pelvic phased-array coil ranges from 59% to 95% [12, 13]. Differences in T2 signal intensity between the tumor, submucosa, muscular layer, and mesorectum play the main role while detecting and staging rectal cancers using MRI. T stage must be assessed on planes strictly perpendicular to the tumor. Incorrect prescription of the acquisition plane leads to blurring of the muscularis propria and may lead to overstaging. When the tumor is not visible on sagittal T2 WI: obtaining high-resolution images of the entire length of the rectum and adding DWI may help localize the mass. The depth of extramural spread must be measured in millimeters beyond the outer edge of the longitudinal muscular layer [13], as depicted in

*A. Stage T4a tumor involves visceral peritoneum or anterior peritoneal reflection (green arrow). B. Stage T4b tumor involves an adjacent organ, uterus (yellow arrow), and the mesorectum (yellow star). Source: F. Bauer,* 

*Axial T2wi shows a rectal tumor (yellow star) staged: T3d (>15 mm), CRM+, EMVI+, N1). The extramural spread is measured from the level of the supposed muscularis propria (red dashed line) to the maximal point of mesorectal involvement (red arrow). Notice also the invasion of the venous structures (EMVI, blue arrow) and the extramesorectal metastatic node (yellow arrow). This node group will not be removed in a regular TME!* 

*CRM = FMR (green arrow). Source: F. Bauer, Radiology Kaufbeuren.*

Differentiation between T2 and borderline T3 lesions is still challenging today. The main issue is to distinguish true mesorectal tumor invasion from desmoplastic reactions [14]. In this case, the inflammatory accompanying reaction in the adjacent mesorectal fat masks the actual tumor spread. In particular, fine spicular extensions in the mesorectum should be evaluated carefully - if these are mistakenly interpreted as a tumor (T3 instead of T2), overstaging and thus overtherapy may occur.

One often error source is the use of thicker sections and lower resolution techniques. Therefore, using fine sections in T2WI should help clarifying such cases. Indeed, desmoplasia associated with ulcerating tumors at the invasive border is typically seen as fine low-signal-intensity spicules on T2WI. These spicules do not show restricted diffusion. Tumor extension into the mesorectum, on the other hand, forms thicker, intermediate signal- intensity nodular bands with restricted diffusion and disruption of muscularis propria [15].

From the therapeutic point of view, the differentiation between T2 and T3a, b stages is not important since the treatment of these lesions is identical: TME alone or short term RCT followed by TME.

## *5.1.2 Specific issues related to low-lying tumors*

Low-rectal tumors are associated with higher rates of positive resection margins, higher local recurrence rates, and poorer survival [16]. This is largely due to anatomic considerations and the fact that the mesorectal envelope tapers and narrows at this level. These rates can be improved by using CRT in locally advanced lowrectal tumors. The results show a good response with higher sphincter preservation rates and disease-free survival [15]. In consequence, a tumor that would have previously required an abdominoperineal excision may instead be treated with ultralow resection and coloanal anastomosis.

Our experience has shown that, particularly in the case of low-lying tumors, the primary surgical concept changed relatively often after CRT and restaging. Consequently, tumors that had required abdominoperineal excision before CRT only needed ultralow resection and coloanal anastomosis after CRT.

All these require a very good quality MRI beforehand, to define the location of the tumor relative to the sphincter complex precisely, so that we select correctly the patients who will profit from preoperative CRT.

For the assessment of the anal canal, T1w lipid-saturated T1FS sequences with contrast medium are superior to T2w-sequences, since m. levator ani and m. sphincter ani externus are reliably separated from m. sphincter ani internus due to their signal and contrast medium behavior.

Rectal carcinoma usually shows low signal intensity compared to the normal intestinal wall and sphincters. **Stage T3** implies the infiltration of the external sphincter. At **stage T4**, the tumor infiltrates also of the m. levator ani. As a matter of fact, as soon as a rectal carcinoma crosses the mesorectal fascia and infiltrates the visceral peritoneum, the diagnosis is T4. Here, it must be differentiated whether adjacent organs (vagina, uterus, ovaries, prostate, seminal vesicles, bladder and ureter) are reached by the tumor (T4b) or whether only the visceral peritoneum (T4a) is infiltrated **(Figure 8A** and **B).** The contact of the tumor with surrounding

organs (without a preserved fat layer adjacent to the organ) automatically requires classification as T4 in the findings report, even if the adhesion later turns out histopathologically to be a peritumorous inflammation.

Tips for T-staging of low-lying tumors with MRI

	- a.If tumor is located above the puborectalis sling: sphincter involvement can be easily excluded.
	- b.If the tumor extends below the puborectalis sling, 3 areas have to be evaluated and reported on, **Figure 1B**, and **C** (see Appendix on structured reporting).
		- 1.The internal sphincter (IAS)
		- 2.The intersphincteric plane (ISP)
		- 3.The external sphincter (EAS)

## **5.2 Mesorectal fascia (MRF) = Circumferential resection margin (CRM)**

A central component of preoperative local staging is the assessment of the distance of the tumor from the mesorectal fascia (MRF) and thus from to the circumferential resection margin (CRM). CRM infestation is an important prognostic indicator for the occurrence of local recurrences [5].

In the case of MRI-based surgery of the rectum, we deliberately equated fascia mesorectalis (MRF) with Circumferential Resection Margin (CRM) in the MDT conference, which naturally led to a need for clarification at the beginning of the discussions. In the meantime, this discussion has been clarified, if one considers the following anatomical and surgical conditions.

The CRM is the non-peritoneal surgical resection plane that is prepared during surgery and has no direct anatomical correlate in the MRI, as it is de facto only determined by the surgeon during the procedure. In practice, however, the surgeon orients himself or herself on the MRF, so that the MRF serves as the most important anatomical landmark in preoperative staging and is practically equated with the surgical resection plane. Accordingly, the visceral peritoneum or peritoneal flap are not part of the CRM, as they cannot be influenced by the surgeon. Consequently, the CRM is only "circumferential" in the lower third of the rectum and thus strongly dependent on the height of the respective rectal section, since in the middle third, the rectum is already covered anteriorly by peritoneum, and the CRM accordingly only exists laterally and posteriorly. In the upper third of the rectum, the CRM is only present on the dorsal side, since the rectum is predominantly peritoneal at this height. The distance between the rectal carcinoma and the circumferential resection

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*Imaging and Diagnosis for Planning the Surgical Procedure*

therefore special importance must be attached to the CRM.

margin (CRM) is the most important risk factor for a local tumor recurrence,

The CRM is considered positive (MRI predicted "cut edge positivity") if the distance between the rectal carcinoma and the mesorectal fascia is 1 mm or less (=

Therefore, we need to document the minimum distance to the MRF in millimeters in the findings. There is no general consensus regarding the evaluation of the lymphatic of extramural vascular infiltration if these are closer to the MRF than the primary tumor. In our clinic, we consider clear lymph node metastases and clear extramural vascular infiltration a CRM positive criteria, when the shortest distance

The lower third of the rectum poses a particular challenge for the assessment of CRM due to its anatomical situation. Therefore, the best possible image quality is essential here, including the exact angulation of the layers with respect to the anal canal. The mesorectal fascia fuses in the lower third of the rectum on the levator ani and ends at the upper edge of the sphincter complex. CRM positivity here depends in particular on the surgical procedure. In this context, the intersphincterian fat lamella is an important anatomical guiding structure in addition to the m. levator ani. If the m. sphincter ani internus is infiltrated, but there is a distance between the tumor and the intersphincterian fat lamella or m. levator ani of more than 1 mm, the CRM for an intersphincterian resection is negative. If, on the other hand, the intersphincterian fat lamella or the m. levator ani is infiltrated, an extended resec-

tion must be performed, otherwise CRM positivity would be present.

EMVI is defined as tumoral invasion of large vessels, typically veins, in close proximity to the muscularis propria. It represents an important criterion for the individual prognosis, as positive EMVI leads significantly more often to local tumor recurrence and metastases (both local and distant). The probability of metastasis increases with the caliber of the infiltrated vessel, whereas larger vessels with a caliber of ≥3 mm greatly increase the probability of metastasis. On the other hand, smaller vessels are difficult to differentiate from lymph vessels, which have a somewhat better prognosis. This distinction is difficult even for histopathology, where it may be achieved using special staining. EMVI indicates at least stage T3, since EMVI expands per continuitatem and represents a tumor infiltration through

MRI has shown an increasing sensitivity for the detection of EMVI with the increasing use of 3 T systems. The infiltration can be detected much easier using a higher resolution, where it is shown as an intravascular substrate having an identical T2w signal intensity as the primary tumor. At the same time, no flow signal can

The MRI-EMVI point score system recommended by Smith and Brown in 2008 was not practical for us, and with the increasing use of 3 T equipment, we are now

All radiological imaging procedures, including MRI, have limited sensitivity and specificity in assessing lymph node metastasis, but we can significantly improve this

result by consistently applying the DLC system, as depicted in **Figure 9** [2]: **D** – Detection using axial DWI (number of lymph nodes), see also **Table 1**;

increasingly successful in directly detecting vascular infiltration.

*DOI: http://dx.doi.org/10.5772/intechopen.93873*

to the MRF is lower than or equals 1 mm.

**5.3 Extramural venous invasion (EMVI)**

be detected inside the vessel (**Figure 7)**.

the muscularis propria.

**5.4 Lymph node staging**

CRM positivity), see **Figure 7**.

### *Imaging and Diagnosis for Planning the Surgical Procedure DOI: http://dx.doi.org/10.5772/intechopen.93873*

*Colorectal Cancer*

puborectalis sling:

involved.

be easily excluded.

organs (without a preserved fat layer adjacent to the organ) automatically requires classification as T4 in the findings report, even if the adhesion later turns out

• Protocol of choice: High-spatial-resolution T2W and T1 FS coronal imaging after i.v. administration of Gd, because it depicts optimally the tumor relationship with the levator and puborectal muscles, sphincter complex, and inter-

• First focus on the location of the lower edge of the tumor in relation to the

a.If tumor is located above the puborectalis sling: sphincter involvement can

b.If the tumor extends below the puborectalis sling, 3 areas have to be evaluated and reported on, **Figure 1B**, and **C** (see Appendix on structured reporting).

• In case of stage T4: Levator and puborectalis muscles or external sphincter are

**5.2 Mesorectal fascia (MRF) = Circumferential resection margin (CRM)**

A central component of preoperative local staging is the assessment of the distance of the tumor from the mesorectal fascia (MRF) and thus from to the circumferential resection margin (CRM). CRM infestation is an important prognostic

In the case of MRI-based surgery of the rectum, we deliberately equated fascia mesorectalis (MRF) with Circumferential Resection Margin (CRM) in the MDT conference, which naturally led to a need for clarification at the beginning of the discussions. In the meantime, this discussion has been clarified, if one considers the

The CRM is the non-peritoneal surgical resection plane that is prepared during surgery and has no direct anatomical correlate in the MRI, as it is de facto only determined by the surgeon during the procedure. In practice, however, the surgeon orients himself or herself on the MRF, so that the MRF serves as the most important anatomical landmark in preoperative staging and is practically equated with the surgical resection plane. Accordingly, the visceral peritoneum or peritoneal flap are not part of the CRM, as they cannot be influenced by the surgeon. Consequently, the CRM is only "circumferential" in the lower third of the rectum and thus strongly dependent on the height of the respective rectal section, since in the middle third, the rectum is already covered anteriorly by peritoneum, and the CRM accordingly only exists laterally and posteriorly. In the upper third of the rectum, the CRM is only present on the dorsal side, since the rectum is predominantly peritoneal at this height. The distance between the rectal carcinoma and the circumferential resection

histopathologically to be a peritumorous inflammation. Tips for T-staging of low-lying tumors with MRI

sphincteric plane, as depicted in **Figure 1A**, and **B**.

1.The internal sphincter (IAS)

2.The intersphincteric plane (ISP)

3.The external sphincter (EAS)

indicator for the occurrence of local recurrences [5].

following anatomical and surgical conditions.

**88**

margin (CRM) is the most important risk factor for a local tumor recurrence, therefore special importance must be attached to the CRM.

The CRM is considered positive (MRI predicted "cut edge positivity") if the distance between the rectal carcinoma and the mesorectal fascia is 1 mm or less (= CRM positivity), see **Figure 7**.

Therefore, we need to document the minimum distance to the MRF in millimeters in the findings. There is no general consensus regarding the evaluation of the lymphatic of extramural vascular infiltration if these are closer to the MRF than the primary tumor. In our clinic, we consider clear lymph node metastases and clear extramural vascular infiltration a CRM positive criteria, when the shortest distance to the MRF is lower than or equals 1 mm.

The lower third of the rectum poses a particular challenge for the assessment of CRM due to its anatomical situation. Therefore, the best possible image quality is essential here, including the exact angulation of the layers with respect to the anal canal. The mesorectal fascia fuses in the lower third of the rectum on the levator ani and ends at the upper edge of the sphincter complex. CRM positivity here depends in particular on the surgical procedure. In this context, the intersphincterian fat lamella is an important anatomical guiding structure in addition to the m. levator ani. If the m. sphincter ani internus is infiltrated, but there is a distance between the tumor and the intersphincterian fat lamella or m. levator ani of more than 1 mm, the CRM for an intersphincterian resection is negative. If, on the other hand, the intersphincterian fat lamella or the m. levator ani is infiltrated, an extended resection must be performed, otherwise CRM positivity would be present.

### **5.3 Extramural venous invasion (EMVI)**

EMVI is defined as tumoral invasion of large vessels, typically veins, in close proximity to the muscularis propria. It represents an important criterion for the individual prognosis, as positive EMVI leads significantly more often to local tumor recurrence and metastases (both local and distant). The probability of metastasis increases with the caliber of the infiltrated vessel, whereas larger vessels with a caliber of ≥3 mm greatly increase the probability of metastasis. On the other hand, smaller vessels are difficult to differentiate from lymph vessels, which have a somewhat better prognosis. This distinction is difficult even for histopathology, where it may be achieved using special staining. EMVI indicates at least stage T3, since EMVI expands per continuitatem and represents a tumor infiltration through the muscularis propria.

MRI has shown an increasing sensitivity for the detection of EMVI with the increasing use of 3 T systems. The infiltration can be detected much easier using a higher resolution, where it is shown as an intravascular substrate having an identical T2w signal intensity as the primary tumor. At the same time, no flow signal can be detected inside the vessel (**Figure 7)**.

The MRI-EMVI point score system recommended by Smith and Brown in 2008 was not practical for us, and with the increasing use of 3 T equipment, we are now increasingly successful in directly detecting vascular infiltration.

### **5.4 Lymph node staging**

All radiological imaging procedures, including MRI, have limited sensitivity and specificity in assessing lymph node metastasis, but we can significantly improve this result by consistently applying the DLC system, as depicted in **Figure 9** [2]:

**D** – Detection using axial DWI (number of lymph nodes), see also **Table 1**;

### **Figure 9.**

*Nodal staging using the DLC system. D = Detection using DWI, L = Localization using T2w, and C = Characterization using high resolution systems with 3 T. Red arrow: intramesorectal nodes. Yellow arrow: extramesorectal nodes. Green arrow: fascia mesorectalis (CRM). Blue dashed arrow: characterization (inhomogeneity, round-oval with spiculae). Red star: tumor. Source: F. Bauer, Radiology Kaufbeuren.*


### **Table 1.**

*Extended N-classification for rectal cancer.*

**L** – Localization of lymph nodes (no. of intra and extra mesorectal) using T2w high resolution multiplanar imaging using a 3 T system (axial, coronal, and sagittal planes);

**C** – Characterization using T2w high resolution imaging using a 3 T system: tumor size in mm and morphological criteria like inhomogeneity, round-oval with spiculae, etc.

We can answer all therapeutically relevant questions using this scheme. In addition, the increase use of 3 T devices has significantly improved the resolution. Our experience shows that many lymph nodes previously considered round and smooth show distinct spiculae in high resolution images, which is a clear criterion for malignancy. We have also previously seen this correlation between focal findings

**91**

data sets.

*Imaging and Diagnosis for Planning the Surgical Procedure*

and resolution in mammography. A good resolution is the key to a correct morphological assessment of the lymph nodes. Currently, the morphology of lymph nodes

*Lymph nodes of same size (4 mm) but with totally different morphology in MRI. A. Lymph node metastasis in a patient with rectal cancer. Note the typical aspect of malign lymph nodes: inhomogeneous signal; irregular border with spikes (red arrow). B. Benign (reactive) nodes (arrows), characterized by homogeneous signal and well-defined borders on the background of anal fistula (no cancer!). Source: F. Bauer, Radiology Kaufbeuren.*

The mesorectal fatty tissue offers a unique and excellent opportunity for a very clear demarcation of lymph nodes. In signal-rich fatty tissue (light), the signal-poor lymph nodes (dark) can be excellently demarcated and characterized (see **Figure 10**). Unfortunately, we do not have this unique situation everywhere in the abdomen! In general, we have no problems with the assessment of the larger lymph nodes over 5 mm near the tumor or proximal to the primarius, which are usually always positive. We only have problems with smaller lymph nodes below 4 mm, which as we know can contain micrometastases. Here, morphology with good resolution and

In our tumor conference, we focus on the localization of lymph nodes, because it is crucial to assess the presence of potentially malignant extramesorectal lymph nodes. While intramesorectal lymph nodes are standardly removed in TME, extramesorectal/obturator lymph nodes are usually left out. If the latter ones present malignancy aspects in MRI, the surgical procedure may change to a D3 lymphadenectomy remov-

We recommend the consistent use of structured reporting (see template in Appendix) for primary MRI staging of rectal cancer. This report includes all thera-

Nodal metastases must be detected and characterized preoperatively, as they are critical for surgical planning, prognosis, and the decision to administer adjuvant/

As in all areas of life, knowledge and experience are also the key to success in dealing with technology. One of these new technologies that deserves application and experience is "high resolution 3D imaging". Perhaps, it will even change the way

3D imaging does not mean, as the term might suggest, image representation in spatial form, but rather the generation of images by means of 3-dimensional

3D imaging provides numerous benefits for experienced surgeons, from the facilitated planning of complex operations to the use of realistic models. The latter

powerful 3 T devices provides a valuable help, as shown in **Figure 10**.

ing extramesorectal lymph nodes (depending on the surgical strategy).

*DOI: http://dx.doi.org/10.5772/intechopen.93873*

is becoming more important than their size!

**Figure 10.**

peutically and diagnostically important points.

**6. MRI and the newer 3D technology**

neoadjuvant chemoradiation.

we scan in MRI in the future.

**Figure 10.**

*Colorectal Cancer*

**90**

planes);

N1

**Figure 9.**

N2

**Table 1.**

spiculae, etc.

**Class Interpretation**

Nx Regional lymph nodes cannot be assessed N0 No involved regional lymph nodes

a 1 involved regional lymph node b 2-3 regional lymph nodes involved

a 4-6 regional lymph nodes involved b > = 7 lymph nodes involved

*Extended N-classification for rectal cancer.*

**L** – Localization of lymph nodes (no. of intra and extra mesorectal) using T2w high resolution multiplanar imaging using a 3 T system (axial, coronal, and sagittal

c No involved regional lymph nodes, but tumor deposits in subserosa, mesentery or non-

*Nodal staging using the DLC system. D = Detection using DWI, L = Localization using T2w, and* 

*C = Characterization using high resolution systems with 3 T. Red arrow: intramesorectal nodes. Yellow arrow: extramesorectal nodes. Green arrow: fascia mesorectalis (CRM). Blue dashed arrow: characterization (inhomogeneity, round-oval with spiculae). Red star: tumor. Source: F. Bauer, Radiology Kaufbeuren.*

peritonealized pericolic or perirectal/mesorectal tissues

**C** – Characterization using T2w high resolution imaging using a 3 T system: tumor size in mm and morphological criteria like inhomogeneity, round-oval with

We can answer all therapeutically relevant questions using this scheme. In addition, the increase use of 3 T devices has significantly improved the resolution. Our experience shows that many lymph nodes previously considered round and smooth show distinct spiculae in high resolution images, which is a clear criterion for malignancy. We have also previously seen this correlation between focal findings

*Lymph nodes of same size (4 mm) but with totally different morphology in MRI. A. Lymph node metastasis in a patient with rectal cancer. Note the typical aspect of malign lymph nodes: inhomogeneous signal; irregular border with spikes (red arrow). B. Benign (reactive) nodes (arrows), characterized by homogeneous signal and well-defined borders on the background of anal fistula (no cancer!). Source: F. Bauer, Radiology Kaufbeuren.*

and resolution in mammography. A good resolution is the key to a correct morphological assessment of the lymph nodes. Currently, the morphology of lymph nodes is becoming more important than their size!

The mesorectal fatty tissue offers a unique and excellent opportunity for a very clear demarcation of lymph nodes. In signal-rich fatty tissue (light), the signal-poor lymph nodes (dark) can be excellently demarcated and characterized (see **Figure 10**). Unfortunately, we do not have this unique situation everywhere in the abdomen!

In general, we have no problems with the assessment of the larger lymph nodes over 5 mm near the tumor or proximal to the primarius, which are usually always positive. We only have problems with smaller lymph nodes below 4 mm, which as we know can contain micrometastases. Here, morphology with good resolution and powerful 3 T devices provides a valuable help, as shown in **Figure 10**.

In our tumor conference, we focus on the localization of lymph nodes, because it is crucial to assess the presence of potentially malignant extramesorectal lymph nodes. While intramesorectal lymph nodes are standardly removed in TME, extramesorectal/obturator lymph nodes are usually left out. If the latter ones present malignancy aspects in MRI, the surgical procedure may change to a D3 lymphadenectomy removing extramesorectal lymph nodes (depending on the surgical strategy).

We recommend the consistent use of structured reporting (see template in Appendix) for primary MRI staging of rectal cancer. This report includes all therapeutically and diagnostically important points.

Nodal metastases must be detected and characterized preoperatively, as they are critical for surgical planning, prognosis, and the decision to administer adjuvant/ neoadjuvant chemoradiation.
