Rectal Cancer - MR imaging

MR staging

Max Lahaye, Regina Beets-Tan and Robin Smithuis

Radiology Departement of the Maastricht University Hospital and the Rijnland Hospital in Leiderdorp, , the Netherlands

Publicationdate

The major advancement in the treatment of rectal cancer is total mesorectal excision (TME), which involves complete removal of the tumor along with the mesorectal tissue which contains the lymphatics.
The other advancement is the shift from adjuvant to neoadjuvant radiochemotherapy.
Both have dramatically changed the local recurrence rates and survival.
A complete remission rate as high of 30% can be achieved with neoadjuvant treatment.

The issue is whether a patient with rectal cancer is candidate for TME only or preoperative chemoradiotherapy followed by TME.
MRI can answer that question, since it is the most accurate tool for the local staging of rectal cancer.

Introduction

Images can be enlarged by clicking on them.

Traditionally rectal cancer surgery consisted of excision of the tumor with a margin of surrounding perirectal fat.
This however resulted in high local recurrence rates up to 40%.
In 1982 the surgeon Richard John Heald introduced the total mesorectal excision.
After many years TME was widely accepted, which caused a drop in local recurrence rates from 40% to 11% (1,2).

The role of MRI is to determine whether TME-surgery is possible or whether there is an advanced tumor that should be treated with chemoradiation and followed by TME in a later stage.

Total mesorectal excision

Total mesorectal excision (TME) is the best surgical treatment for rectal cancer provided that the resection margin is free of tumor.

In TME the entire mesorectal compartment including the rectum, surrounding mesorectal fat, perirectal lymph nodes and its envelope, i.e. the mesorectal fascia is completely removed (figure).
This minimizes the chance of tumor remnants in the surgical bed.

On the left a coronal illustration of the rectum with a tumor extending through the rectal wall into the mesorectal fat and with some lymph nodes.
The resection margin along the mesorectal fascia is free of tumor and a TME can be performed.
Notice the anal verge (blue arrow).

Rectum is surrounded by mesorectal fat within the mesorectal fascia (red arrows). P: prostate and V: seminal vesicles.

On MRI the mesorectal fat has a high signal intensity on T1- and T2-weighted images.
The mesorectal fat is bounded by the mesorectal fascia, which is seen as a fine line of low signal intensity (red arrows).
In a TME the mesorectal fascia is the resection plane.
The shortest distance from the tumor or lymph nodes to the mesorectal fascia is called the cirumferential resection margin (CRM).

The shortest distance between tumor to the mesorectal fascia is the most powerful predictor for local recurrence.

The mesorectal fascia is involved if the distance between the tumor and mesorectal fascia is ⩽1 mm.
The mesorectal fascia is threatened if the distance between the tumor and mesorectal fascia is between 1-2mm.
If the distance is > 2mm the mesorectal fascia is not threatened.

Circumferential resection margin

On the left an illustration with:

  • T2-tumor limited to the bowel wall
  • T3-tumor with a wide circumferential resection margin or T3 CRM-
  • T3-tumor with an involved circumferential resection margin or T3 CRM+ (red arrow)
  • T4-tumor with ingrowth into seminal vesicles and prostate

Whenever there are lymph nodes or tumor deposits within 2 mm of the mesorectal fascia we need to report this, because they can be malignant and the CRM may be involved (blue arrow).

MRI has to determine the following:

  1. Location of the tumor
    Is it a low or high rectal tumor, what is the size, circumferential growth?
  2. T-stage: T1, T2, T3 or T4
  3. Distance of the tumor to the mesorectal fascia. Is it threatened or involved?
  4. Tumor growth or lymph nodes within 1 mm of the resection margin?
  5. N-stage: Are there any lymphnodes within the mesorectum or beyond the mesorectum?

Location of the tumor

The rectum extends from the anorectal junction to the sigmoid.
The rectosigmoid junction is arbitrarily defined as 15 cm above the anal verge.
A tumor more than 15 cm above the anal verge is regarded and treated as a sigmoid tumor.
Since we cannot detect the anal verge on MR, it is best to measure from the anorectal angle.
A tumor more than 15 cm above the anorectal angle is regarded and treated as a sigmoid tumor.

Rectal cancer can be divided into:

  • Low rectal cancer:
    distal border is 0- 5 cm from the anorectal angle
  • Mid rectal cancer:
    distal border is 5-10 cm from the anorectal angle
  • High rectal cancer:
    distal border is 10-15 cm from the anorectal angle

Low rectal cancer

Low rectal cancer has a higher local recurrence rate.
The distal tapering of the mesorectal fat implies that low rectal cancer more easily invades the surrounding structures and it will be more difficult for the surgeon to get a tumor free resection (see figure).

T-stage and mesorectal fascia (MRF)

When we know the exact location of the tumor, the next step is to determine the T-stage.
MR cannot distinguish tumor growth limited to the submucosa or invasion to the muscularis externa, so it cannot differentiate between T1 and T2 tumors.
Often a T1 tumor will be treated with local excision.
In these cases endorectal US is more accurate in differentiating between T1 and T2 tumors.

T1 and T2

T1 and T2 tumors are limited to the bowel wall and have a good prognosis.
They can be accurately identified with MR, because the rectal wall will have an intact black line, i.e. musclaris externa, surrounding the tumor (3).

The figure shows a rectal tumor that is completely surrounded by the black layer of the musculari externa.
This is a T2 tumor.

T3 MRF- rectal cancer

T3 MRF-

A T3 tumor grows through all wall layers and extends into the perirectal fat tissue.
In these tumors it is important to determine whether the mesorectal fascia is involved.

On the left a tumor that probably infiltrates the mesorectal fat, i.e. T3 (arrow).
There is a wide resection margin around the tumor and there are no lymphnodes adjacent to the mesorectal fascia.
This tumor is classified as T3 MRF-.
In the Netherlands, like in most European countries, this patient will be treated with a short preoperative course of radiotherapy followed by TME.

Perirectal stranding
MRI has a sensitivity of 82% to detect perirectal tissue invasion.
The pitfall is when perirectal stranding is seen.
This can be the result of tumor ingrowth or a desmoplastic reaction.
To be on the safe side and to avoid understaging, it is advised to stage tumors with perirectal stranding as T3 tumors. The extent of extramural growth is a prognostic factor.

Here we see two tumors with a similar MR-appearance.
In A there was perirectal tumor invasion.
In B the tumor was limited to the bowel wall, i.e. a T2-tumor.
The perirectal stranding in the latter case was the result of a desmoplastic reaction.

For therapeutic purposes it does not have any consequences to differentiate accurately between a T2 MRF- and a T3 MRF- tumor.
Both tumors will be treated either with TME only or with preoperative low dose radiotherapy of 5x5 Gy followed by TME.

T3 MRF+ rectal cancer

T3 MRF+

On the left a tumor that infiltrates the mesorectal fat with infiltration of the resection margin on the anterior side (arrow).
This tumor is classified as T3 MRF+.
This patient will be treated with a long course of radiotherapy and chemotherapy preoperatively.

If the treatment is successful, as demonstrated by a restaging MRI, it will be followed by TME.

Rectal cancer with invasion of the prostate

T4

A T4 tumor is an advanced tumor that invades surrounding structures like pelvic wall, vagina, prostate, bladder or seminal vesicles.
These patients require a long course of chemoradiation and extensive surgery.
For adjacent organ invasion all imaging modalities show similar sensitivity: 70% for transrectal US, 72% for CT and 74% for MR imaging.

On the left a T4-tumor with invasion of the prostate.

N-stage

The N-stage is an important risk factor for local recurrence.
Unfortunately MR, like most other imaging modalities, has a low sensitivity and specificity for the detection of lymph node metastases.
When lymph nodes have a short axis of > 5 mm or a spiculated and indistinct border or a mottled heterogeneous appearance, than you can be sure that these nodes contain metastases.
However not all positive lymph nodes meet these criteria.

Most use the N-stage of the TNM5.
The N-stage in the TNM7 is just like in TNM5, but the N1- and N2-stage is more differentiated.

Even in T1 and T2 tumors there is a considerable risk for lymph node metastases (Table).

The low sensitivity using only size criteria can be explained by the fact that in rectal cancer small lymph nodes still have a high prevalence of malignancy, 9% in 1-2 mm nodes and 17% in 2-5 mm nodes respectively (11).
As is demonstrated in the table on the left the majority of positive nodes are 1-5 mm in size.
In order not to understage patients, all lymph nodes are regarded as possible malignant.

On the left a low rectal cancer with multiple nodes in the perirectal fat on the posterior side.
This has a big influence on the prognosis of the patient and based on the advanced stage of the tumor with MRF+ and N+ status, the patient will therefore receive a more aggressive treatment with neoadjuvant chemoradiation.

Local recurrence of rectal cancer after TME due to positive extramesorectal lymph nodes

It is important to look beyond the mesorectum for lymph nodes (arrow).
These extramesorectal nodes are important, because they can be a cause of local recurrence.
When they are detected by MR, the radiation and surgical planning has to be adapted.

On the left a patient with extramesorectal nodal recurrence after TME (arrow).
During a standard TME procedure these extramesorectal lymph nodes will not be resected.
This means that after TME surgery not all tumor deposits will have been removed.
The finding of malignant extramesorectal lymph nodes entails that at least a more extensive surgical approach is necessary to remove all the cancer deposits or a boost of radiotherapy to the areas of risk.

On the left axial T2-weighted images of two different rectal cancer patients.
These cases illustrate the problems for the radiologist to accurately stage the nodal status.
On the far left there is a small extra mesorectal lymph node depicted.
On the right there are numerous large mesorectal lymph nodes and also a right extramesorectal lymph node with indistinct borders (red arrow).

Although the nodes of these two patients have very different characteristics in size, border and heterogenous appearance, they all proved to be malignant.
Rectal cancer is notorious for small metastatic lymphnodes

MR protocol

Only FSE T2 and no Gadolinium

The only sequence that is required is a T2-weighted fast spin echo sequence.
Gadolinium-enhanced MR does not improve diagnostic accuracy and is therefore not included in the protocol.
Images are made in the sagittal, coronal and axial plane.
First start with the sagittal series and plan the axial images perpendicular to the rectal wall at the level of the tumor (blue lines).

Coronal images are planned perpendicular to the axial images (yellow line)and in distal tumors parallel to the anal canal.
In this way we avoid partial volume problems and we will be able to accurately evaluate the depth of tumor invasion and ingrowth in the anal canal.

It helps if the level of the tumor position is indicated by the referring surgeon for proper planning of the MR-sequences.
The cranial border of the field of view (FOV) is L5, the caudal border is below the anal canal.

The use of an external coil is recommended and an endorectal coil is therefore not needed. There is no consensus on whether 1.5T or 3.0T MRI should be used.

Diffusion weighted images can be used in restaging MRI.
High signal on B1000 images indicates incomplete response.

Angulation

Axial images have to be angulated perpendicular to the axis of the tumor to avoid volume averaging.
At first the axial images were not properly angulated. This resulted in the false impression, that the MRF was involved on the anterior side (red circle).
After proper angulation it was noted, that the MRF was not involved (yellow circle).

No fat suppression and no bowelpreparation

Fat suppression is not helpfull in delineating the tumor.
Patients do not need bowel or any other preparation.

The use of rectal contrast is not advised, because stretching of the bowel wall may lead to overestimation of an involved MRF.
Furthermore the mesorectal nodes in the distal mesorectum are not well appreciated.

Radiological report

A. Primary staging

The radiological report must consist of the following tumor variables:

  • Location of the tumor in low, mid or high rectum.
  • Distance from the anorectal junction to the lower pole of the tumor.
  • Length of the tumor.
  • Circular or semi-circular. The circumferential location of the tumor within the wall, i.e. lateral, anterior and posterior.
  • T-stage:
    • T1 / T2: tumor limited to the bowel wall
    • T3: perirectal fat invasion and extent of extramural growth in mm.
    • T4: invasion of surrounding structures. Mention which organs are involved.
  • The smallest distance (in mm) between the tumor and the mesorectal fascia and its location
  • N-stage: nodes inside of the mesorectum and mention distance to resection margin and look for nodes outside mesorectum. Also mention the number of suspicious lymph nodes.

B. Restaging after neoadjuvant treatment

Restaging is done in the same way as the primary staging, except that the prefix y is added to the stage.
For instance YT3 means, that the patient neoadjuvant therapy and there is remaining tumor with depositis in the mesorectum.

Treatment

There are differences in rectal cancer treatment between countries and between institutions.
Everyone agrees that TME is the best radical treatment for all tumors with free resection margins.
In the Netherlands, like in most European countries, a short course of 5x5Gy radiotherapy is given to the majority of patients prior to TME, because additional benefit was seen in the large TME trial.
In some institutions this preoperative radiotherapy is not given to tumors that already have a good prognosis, like high-rectal T1N0 and T2N0.
Preoperative short course of radiotherapy immediately followed by TME does not result in down-staging and is therefore not suitable for locally advanced tumors.
That is why all T4-tumors or tumors with involved resection margins and tumors with suspicious malignant lymph nodes near or beyond the resection margin first receive high dose chemoradiation.
Further action depends on the response to the preoperative treatment.
In cases of tumor regression from the mesorectal fascia this will be a less extensive resection.
In cases of tumor downstaging to only a small tumor remnant and sterilized nodes (yN0) the next step will be TME.

If there is complete disappearance of tumor and involved nodes a wait-and-see policy might be considered instead of surgery. In cases of complete tumor regression and yN0, a wait-and-see policy is possible with omission of surgery.
This latter option however is still controversial and not standard practise.

Cases

case 1
Radiology report:

Tumor

  • At 0.5 cm above the anorectal junction there is tumor dorsally in the rectal wall.
  • The tumor length is 3.5 cm.
  • The tumor invades the perirectal fat and has a close relation with the muscles of the pelvic floor especially on the right side.
  • At 7 o' clock the MRF is involved.
  • T-stage: T3 tumor

Lymph nodes

  • Number of suspicious mesorectal lymph nodes (>5mm): 0
  • Number of suspicious extramesorectal lymph nodes: 0
  • N-stage: N0

Conclusion

  • Low-rectal T3N0 tumor with an involved MRF at the pelvic floor muscles on the right side.

case 2
Radiology report:

Tumor

  • There is a solid circular rectal tumor at 6 cm above the anorectal junction.
  • The tumor length is 5.5 cm.
  • Invasion of the perirectal fat.
  • Close relation with the peritoneal fold at 12 o' clock.
  • No extramural vascular invasion.
  • T-stage: T3 tumor

Lymph nodes

  • Number of suspicious mesorectal lymph nodes: 0
    There are only a few small mesorectal lymph nodes (not all depicted): not suspicious.
  • Number of suspicious extramesorectal lymph nodes: 0
  • N-stage: N0

Conclusion

  • Midrectal T3N0 tumor close to the peritoneal fold.

case 3
Radiology report:

Tumor

  • There is a solid circular rectal tumor at 10 cm above the anorectal junction.
  • The tumor length is 7 cm.
  • The tumor invades the perirectal fat and is adjacent to the peritoneal fold at 9 to 11 o' clock.
  • No extramural vascular invasion.
  • T-stage: T3 tumor

Lymph nodes

  • Number of suspicious mesorectal lymph nodes: 4 (not all depicted)
    There are lymph nodes, that are larger than 8 mm, which makes them very suspicious for malignancy. At various levels 4 nodes were detected.
  • Number of suspicious extramesorectal lymph nodes: 0
  • N-stage: N2

Conclusion

  • High-rectal T3N2 tumor with an involved peritoneal fold on the right ventral side.