Total mesorectal excision
In 1979 surgeon Richard John Heald introduced the total mesorectal excision (TME).
In TME the entire mesorectal compartment including the rectum, surrounding mesorectal fat, perirectal lymph nodes and its envelope, the mesorectal fascia (MRF), is completely removed by precise dissection along anatomical planes (figure).
TME is the best surgical treatment for rectal cancer provided that the resection margin is free of tumor.
It is now a standard technique and part of procedures such as low anterior resection (LAR), in which the rectum and sigmoid colon are resected or abdominoperineal resections (APR), in which the rectum and anal canal are resected.
The mesorectal fascia (MRF) plays a crucial role in the treatment plannnig.
In TME the mesorectal fascia is the resection plane and it has to be tumor-free.
A distance of the tumor to the mesorectal fascia of ⩽1 mm is regarded as not suitable for TME and is called an involved MRF.
This means that the tumor has to be downstaged before TME is possible.
On MRI the mesorectal fat has high signal intensity on both T1- and T2-weighted images.
The mesorectal fat is surrounded by the mesorectal fascia, which is seen as a fine line of low signal intensity (arrows).
High resolution T2-images are needed to clearly identify the MRF (7).
The MRF is only circumferential in the low-rectum below the anterior peritoneal reflection (see next illustration).
The MRF does not apply to the anterior peritonealized surface of the anterior mid- and high rectum.
The treatment of a patient with rectal cancer depends on the TNM-stage and whether the MRF is involved.
T1 and T2 tumors are limited to the bowel wall.
T3 tumors grow through the bowel wall and infiltrate the mesorectal fat.
They are further differentiated in:
< 1mm extension beyond muscularis propria
1-5 mm extension beyond muscularis propria
5 - 15 mm extension beyond muscularis propria
- T3d: > 15 mm
- T3 MRF+ tumor within 1mm of MRF
- MRF- no tumor within 1 mm of MRF
The N-stage is based on the number of suspicious lymph nodes:
- N0 no suspicious nodes
- N1 1-3 suspicious nodes
- N2 ⩾ 4 suspicious nodes
This figure illustrates the T-stage and mesorectal fascia involvement in the axial plane, which is usually the best imaging plane for the T-staging.
Lymph node involvement is an important factor for the treatment and the prognosis of the patient.
MR has proven to have a low diagnostic accuracy for distinguishing positive or negative lymph nodes when characterization is based on size criteria alone.
At the moment in the Netherlands we use a combination of both size and morphologic criteria as listed in the table.
Nodes larger than 9 mm are always regarded as suspicious.
Smaller lymph nodes need additional malignant characteristics to be considered suspicious.
Since staging and treatment of rectal cancer is constantly evolving, you may have to check your local oncology team for the latest developments.
The treatment is based on the clinical or cTNM.
The cTNM is based on the results of endoscopy and imaging.
- Low risk tumors
T1, T2 and borderline T3 without suspicious nodes can directly undergo surgery.
- Intermediate risk tumors
T3 with >5mm invasion or tumors with 1-3 suspicious nodes - will be treated with short term radiotherapy preoperatively.
- High risk tumors
T3 with involved MRF or T4 tumors or tumors with 4 or more suspicious nodes will receive neoadjuvant chemotherapy and long term radiation therapy and will be restaged to determine whether TME is possible
After the operation the surgical specimen is analyzed by the pathologist.
This results in a pTNM-stage (or ypTNM when neoadjuvant therapy is given).
Based on the pTNM additional therapy and follow up may be considered.
High resolution 2D T2-weighted fast spin echo sequences in the sagittal, axial and coronal plane are required for state-of-the-art staging of rectal cancer.
The slice thickness should be 3 mm.
Gadolinium-enhanced MR does not improve diagnostic accuracy and is not included in the protocol.
Start with the sagittal series.
These can be used to plan the axial images, perpendicular to the rectal wall at the level of the tumor to avoid volume averaging (yellow box).
Coronal images are planned parallel to the anal canal (green box), especially in low-rectal tumors in order to accurately evaluate the depth of tumor invasion into the anal sphincter.
The cranial border of the field of view (FOV) is vertebral body L5, the caudal border is below the anal canal.
Proper angulation is of vital importance in correctly identifying tumor borders.
In this example the axial images were originally not properly angulated (red lines not perpendicular to the tumor).
This resulted in the false impression that the MRF was involved on the anterior side (red circle).
After proper angulation it was clear that the MRF was not involved (yellow circle).
Diffusion weighted imaging can be useful for tumor and lymph node detection in primary staging.
The figure shows a semicircular T3 tumor with perirectal invasion extending from 3-9 o'clock of the circumference.
Corresponding diffusion restriction on the ADC map and calculated DWI (b = 1000 s/mm2)
DWI in restaging
DWI is very useful in determining the response to chemoradiation.
In this case there is persistent high signal on images with high B-values. which indicates incomplete response.
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 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 mesorectal fascia, pelvic wall and surrounding organs.
It will be more difficult for the surgeon to get a tumor free resection (see figure).
The report should describe the relationship of the tumor to the anal sphincter complex in case of low rectal cancer. The internal sphincter is the distal continuation of rectal circular fibers.
Consequently, if a tumor extends caudally into the internal sphincter, it is considered a T3 tumor.
Involvement of the intersphincteric plane, external sphincter and levator musculature should be assessed, as this may influence treatment planning (see section surgery).
Involvement of the intersphincteric plane is best observed on coronal planes (figure)(7).
The table shows an overview of the T-staging.
T1 and T2
T1 and T2 tumors are limited to the bowel wall and have a good prognosis.
MR imaging is unable to distinguish between tumor extension into the mucosa, submucosa and muscularis propria and therefore can not differentiate between Tis (in situ), T1 and T2 tumors.
Although T1 tumors could be treated with local excision, the treatment of choice in both T1 and T2 tumors is TME.
Only if there is a preference for local excision through transanal endoscopic microsurgery (TEM-procedure), endorectal US can be helpful, because it sometimes can differentiate between T1 and T2 tumors.
Key finding in T1 and T2 rectal tumors is an intact external muscularis layer, which is identified as a hypointense thin line surrounding the rectum (figure).
T3-tumors grow through the external muscularis into the surrounding mesorectum.
As the rectum does not contain a serosal layer, tumor invades directly into the mesorectal fat and can spread to lymph nodes and beyond.
Spread into the mesorectum can be depicted as spicules of low signal intensity in the hyperintense mesorectal fat or distortion of the hypointense muscularis propria.
T3-tumors are further differentiated in:
- T3a: tumour extends <1 mm beyond muscularis propria
- T3b: tumour extends 1-5 mm beyond muscularis propria
- T3c: tumour extends 5-15 mm beyond muscularis propria
- T3d: tumour extends > 15 mm beyond muscularis propria
- MRF- no tumor within 1mm of MRF
- MRF+ tumor within 1mm of MRF
Difficulty in distinguising true mesorectal tumor invasion from desmoplastic reaction, is the main cause of overstaging.
However, to prevent understaging, it is recommended to stage a tumor as T3 when stranding is present.
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.
T3 with MRF involvement
In the description of T3-tumors, the report should include the shortest distance between the tumor margin and the MRF.
Involvement of the MRF results in an increased risk for local recurrence.
The MRF is considered involved when the distance between the tumor margin and MRF is less than 1mm.
Although a positive margin due to a suspicious lymphnode should be assessed and reported, this is not regarded as a determination factor in defining MRF involvement.
The image shows a tumor that infiltrates the mesorectal fat with involvement of the resection margin on the posterior side (arrow).
This tumor is classified as T3 MRF+.
This patient will be treated with chemotherapy and a long course of radiotherapy.
If the treatment is successful, as demonstrated by a restaging MRI, a TME will be performed.
T4a - Invasion peritoneal reflection
The low rectum is totally covered by the mesorectal fascia.
In the mid-rectum it is covered by the mesorectal fascia on the posterior and lateral side, but on the anterior side it is covered by the visceral peritoneum.
Growth into the visceral peritoneum means spread to the peritoneal cavity.
On a sagittal image the anterior peritoneal reflection is the transition between the non-peritonealized and peritonealized portion of the rectum.
It is important to notice if tumor spread on the anterior side is below or above the peritoneal reflection.
On sagittal T2-weighted images the peritoneal reflection can be depicted as a hypointense thin line connecting the bladder with the anterosuperior aspect of the rectum.
On this axial T2-weighted image there is tumor ingrowth along the visceral peritoneum (arrow).
The peritoneal reflection is marked by the arrow - you may have to enlarge the image.
The peritoneal reflection can be difficult to recognize.
It is the border between the intraperitoneal mesocolic fat and the mesorectal fat.
Continue with next image.
On this sagittal image of the same patient peritoneal metastases are seen (arrow).
Notice that there are also suspicious lymph nodes in the mesorectum.
T4b - Invasion surrounding organs
A T4b-tumor invades the surrounding structures such as pelvic wall, vagina, prostate, bladder or seminal vesicles.
Tumor invasion is defined as loss of the intervening fat plane and corresponding T2 signal abnormality within the involved surrounding structure.
On the sagittal T2W-image there is loss of fat plane between the rectum and the posterior wall of the vagina.
On axial images the relatively low signal intensity of the tumor is seen to extend into the posterior wall of the vagina (arrow).
Scroll through the axial images and see how the low signal intensity of the tumor is seen to extend into the posterior wall of the vagina (arrows).
These images demonstrate a tumor extending into the posterior wall of the uterus.
Extramural vascular invasion (EMVI)
Vascular invasion is a risk factor for recurrent disease and is to be included in standardized MR reporting.
EMVI is associated with T3- and T4 tumors (10,11).
EMVI is suspected if a vascular structure in close proximity to the tumor is expanded, irregular or infiltrated by tumor signal intensity (see figure).
The N-stage is an important risk factor for local recurrence.
MR has a low accuracy for distinguishing positive or negative lymph nodes if characterization is based on size alone.
Prediction of nodal involvement is improved by using the border contour and signal intensity characteristics of lymph nodes (Table) (12).
- Lymph nodes < 5mm are considered suspicious if three malignant characteristics are present (see table).
- Lymph nodes 5 - 9mm are considered malignant if two out of three malignant characteristics are present.
- Lymph nodes with a short axis ⩾ 9mm are always included in the number of suspicious nodes.
When in doubt, a borderline suspicious lymph node should not be considered as suspicious.
Consequently, a lesser N-stage should be assigned.
Diffusion weighted images can be helpful in detecting lymph nodes (figure).
However diffusion images are not suitable for characterization.
High resolution T2W-images are used to determine size and morphologic characteristics.
Notice that the diffusion image is inverted.
On this sagittal T2W-image a low rectal cancer with multiple nodes in the mesorectal fat on the posterior side.
Some of the nodes on this image are heterogenous and have irregular borders.
There were more than 4 suspicious nodes in this patient (N2-stage).
This patient will receive neoadjuvant chemoradiation and a TME depending on the findings of the follow-up MRI.
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, because in a standard TME procedure these extramesorectal lymph nodes will not be resected.
Suspicious extramesorectal lymph nodes have to be included in the standard reporting, so the radiation and surgical planning can be adapted.
The image shows a circular T3 tumor with extramural vascular invasion (EMVI), bridging to the right extramesorectal space (yellow arrow).
In addition there is a suspicious extramesorectal lymph node (green circle).
This axial T2W-image is of a patient with extramesorectal nodal recurrence after TME (arrow).
In a standard TME procedure these extramesorectal lymph nodes are not 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.
Regional Lymph nodes
Regional lymph nodes are located along the providing vessels of the rectum.
Note that lymph nodes are potentially suspicious at the level - or proximally of the primary tumor, following the normal lymph drainage (figure).
The AJCC confines locoregional lymphnode involvement to the perirectal, sigmoid mesenteric, inferior mesenteric, lateral sacral, presacral, internal iliac, sacral promontory (Gerota's), internal iliac, superior rectal (hemorrhoidal), middle rectal (hemorrhoidal), and inferior rectal (hemorrhoidal) lymphnodes.
Lymph nodes outside of these areas are considered metastatic disease (M1).
For example suspicious inguinal lymph nodes if the distal anal sphincter complex is involved.
Low Anterior Resection (LAR)
TME is the standard surgical procedure of rectal carcinoma.
LAR or Low Anterior Resection is an extension of the TME procedure in high rectal cancer.
The following structures are removed:
- Rectum - mesorectum - mesorectal fascia (TME)
- Sigmoid colon (or part of the sigmoid colon)
The standard abdomino-perineal-resection or excision (APR or APE) is performed to remove low rectal tumours that invade the anal canal or levator ani.
Two incisions are made:
- Abdominal - to remove the rectum and mesorectum (TME)
- Perineal - to remove the anal canal
Intersphincteric APR and ELAPE
The Intersphincteric abdomino-perineal and the extralevator abdominoperineal excision (ELAPE) are variations to the standard APR.
When the intersphincteric plane is tumor-free, the external sphincter can be spared with an intersphincteric abdomino-perineal resection.
The ELAPE is a more extensive procedure which includes the levator muscles.
Chemo- and Radiotherapy
There are differences in rectal cancer treatment between countries and between institutions.
Check with your local oncology-group what the treatment protocols are in your institution.
It is widely accepted that TME is the gold standardf or 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 observed 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.
Locally advanced tumors such as T3 with involved mesorectal fascia or suspicious lymph nodes or T4-tumors first receive high dose chemoradiation.
Further action depends on the response to this treatment.
Structured MR report
The radiological report should describe the tumor localization and characteristics as listed in the table.
The circumferential location of the tumor within the wall can be described as lateral, anterior or posterior or in a clockwise manner, i.e. 6-12 o' clock.
In T3-tumors describe the location and the depth of the perirectal fat infiltration and the exact distance to the mesorectal fascia.
Description of suspicious nodes should be subdivided into mesorectal - and extramesorectal nodes, as location of the nodes may influence radiation planning and surgical excision (see section N-stage).
The report should conclude with a cTNM.
Here an example of a standardized report (8).
After neoadjuvant treatment the patient is restaged to evaluate the preoperative locoregional status.
Restaging is done 6 - 8 weeks after completing neoadjuvant therapy.
Similar to the initial MRI, the level of the residual tumor, length, circumferential location, involvement of the MRF and number of mesorectal and extramesorectal lymphnodes should be reported.
After neoadjuvant therapy, the stage is reported as yTNM.
In addition to the conventional T2 weighted assessment, comparing diffusion weighted imaging before and after therapy is crucial to assess residual tumor.
Diffusion restriction suggests residual tumor.
DWI and ADC sequences are also used to interpret residual tumor location and morphology.