Comprehensive Introduction to Rectal Cancer Staging
Findings:
There is a 4 cm long, low rectal tumor with distal margin at the level of the ano-rectal junction abutting the proximal-most portion of the external sphincter complex 4 cm from the anal verge. The tumor does not extend into the anal canal nor does it undermine the anal mucosa. There is not extension of the tumor to the internal or external sphincter. More proximally the tumor does not contact the levator ani.
Laterally on the right there is 5mm of extramural extent of tumor into the meso-rectal fat. At the level of the anorectal junction the rectum contacts the levator ani at the 6 o'clock position. I believe that the tumor is nontransmural at this location and therefore this would not represent involvement of the mesorectal fascia.
There are at least four pathologic loco-regional nodes measuring up to 0.8 cm.
One is extra-mesorectal, being located high in the sigmoid mesocolon. One node contacts the mesorectal fascia on the left.
There is no evidence of macroscopic vascular extension.
The endometrium measures 1.1 cm which is too thick for a post-menopausal woman.
No focal endometiral mass.
Impression:
T3b, MRF-, N2 low rectal cancer extends to the level of the ano-rectal junction.
Homogeneous thickening of the endometrium up to 1.1 cm. This could represent hyperplasia, polyp, or neoplasm.
There are five things you must define do to locally stage rectal cancer
In this comprehensive introduction we will go through these steps in detail.
Step 1: Size and location of the tumor.
Find the tumor on the sagittal images and identify the proximal and distal most margins of the neoplasm. This can be difficult if there is stool or incomplete distension of the bowel lumen. For this reason some people like to insert gel to distend the rectum. However, often instead of distending the rectum it instead just fills the more proximal bowel without distending it. Patients don't like it an technologists don't like doing it. I generally do not insert gel into the rectum.
Identifying the tumor requires practice. Hopefully, working through the cases in this series will improve your confidence. I obtain images perpendicular ("axial") and parallel ("coronal") to the lumen of the bowel at the location of the tumor. Cross reference the T2 series for this case to see these planes, which are prescribed off the true sagittal images. For low rectal cancers, you will also need images parallel to the anus.
Why? The bowel makes a near 90 degree turn at the ano-rectal junction. The "coronal" images cannot be used to define the relationship of the tumor to the anal sphincter complex because it is oblique to the tumor. Likewise, if the distal edge of the tumor is not adjacent to the ano-rectal junction the images parallel to the anus cannot be used to measure the distance from the tumor to the ano-rectal junction. This is because these images are oblique to the tumor. For very low rectal tumors you need images parallel to both the tumor and the anus to make these measurements. If the tumor is far from the ano-rectal junction, these measurements can be made on the sagittal images, which are parallel to both the tumor and the anterior aspect of the levator plate. The elevator plate is a good estimate of the level of the and-rectal junction.
It is helpful if your surgeon can tell you approximately where the tumor is because this can help you find the tumor on the sagittal images. In this case he told me the tumor was low and that he could palpate it with his finger.
The length of the tumor is delineated on this sagittal image (1, remove annotation). This is the axial image at the proximal end of the tumor (2, remove annotation) and the arrow points to what appears to me to be the superior most soft tissue growing from the wall of the bowel. This is the same soft tissue in the coronal plane (3, remove annotation). This is the axial image at the distal most end of the tumor (4, remove annotation) with the arrow pointing at what appears to me to be the inferior most region where soft tissue protrudes from the wall of the bowel. This is the same soft tissue in the coronal plane (5, remove annotation).
Scroll through the tumor on the T2 and subtraction images and cross reference them to the the coronal and sagittal images to convince yourself that, to within one or two slices, you agree with these margins. You should do this for all tumors.
What about this soft tissue here (6 remove annotation, 7 remove annotation) ? To me this looks like smooth thickening of the bowel wall.
Compare to the middle of the tumor (8 remove annotation, 9 remove annotation). The neoplasm is slightly higher signal than the bowel wall, and has overhanging edges.
Now that we have identified the tumor we need to describe its location. The distance from the ano-rectal junction to the distal edge of the tumor is very important. As mentioned in the mastery course, there are several ways to determine the ano-rectal junction. For the purposes of initial measurement I use the anterior aspect of the levator plate which is seen on the sagittal images as the anterior most extent of the levator ani muscle (arrow in 10, remove annotation), and the ano-rectal junction is approximately the line drawn at this level perpendicular to the lumen (dashed line in 10. remove annotation).
I measure the anus to be 4 cm long (11, remove annotation). Verify by scrolling through the sagittal images that I used the distal part of the external sphincter as the anal verge. The distance from the ano-rectal junction to the distal tumor is 0 mm. The distal margin is at the ano-rectal junction.
Because the tumor is centered between 0-5 cm from the ano-rectal junction this is a low rectal tumor. 6-10 cm is a mid rectal tumor. 11-15 cm is a high rectal tumor.
For tumors that approach the ano-rectal junction, you need imagers parallel to the anus. How far into the anus does the tumor extend and does it invade through the internal sphincter?
If you look at the images parallel to the anus you can also see the inferior extent of the tumor (red outline on 17, remove annotation and 18 remove annotation) is just at the level where the levator ani begins to run parallel to the anus as the external sphincter (blue arrows on 17 remove annotation and 18, remove annotation).
Remember, the internal sphincter is the outer muscular wall of the anus. The external sphincter is the inferior continuation of the levator-ani which surrounds the anus laterally and posteriorly. Note that the blue arrow on the right (17 remove annotation ) is not at the point where the levator contacts the anal wall; rather it is at the point where the levator begins to run parallel to the anus.
Therefore, this tumor extends just to the level of the external sphincter.
Tumors can involve the anus in several ways.
The tumor can prolapse into the anal canal, i.e. be in the anus. It is difficult to determine if the tumor is invading the anal mucosa on imaging. Sometimes the surgeon can use their finger to determine how mobile the tumor is, i.e. is it prolapsed into the anal canal or growing along the mucosa. If the tumor is merely prolapsing the surgeon may be able to perform a sphincter sparing surgery.
The tumor can undermine the anal mucosa, meaning it is growing between the mucosa and the internal sphincter. We will see some cases of this. These patients normally require an APR.
The tumor can invade the sphincter- either internal or external. The surgeon will want to know this information.
In this case, the tumor does not extend into the anal canal nor does it undermine the anal mucosa. There is not extension of the tumor to the internal or external sphincter.
Both the coronal images and the images reformatted parallel to the anus show that there is no tumor in the fat between the internal and external sphincters. There is more fat on the right than on the left; I am uncertain this is significant.
Step 2: Depth of tumor spread, i.e. the T staging.
MRI is only useful for differentiating T2 from T3 from T4 disease and is not used to define the presence or depth of invasion into the muscular wall. The images obtained perpendicular to the lumen of the bowel ("axial") are most useful for measuring the depth of invasion into the meso-rectal fat.
On these two axial images (12 remove annotation and 13 remove annotation) there is between 1 and 5 mm of extension of tumor into the adjacent fat. On every other image I think there is an intact, low signal, bowel wall between the tumor and the fat.
Less than 5mm but greater than 1mm of extension of tumor into the meso-rectal fat is T3b disease.
Step 3: Involvent of the mesorectal fascia.
Is there is tumor within 1mm of the mesorectal fascia or peritoneal cavity? This is important because tumors with involvement of the mesolectal fascia will usually get a long course of radiotherapy in addition to neoadjuvant chemotherapy.
This is a low tumor and the MRF is outlined in yellow on this image (14 remove annotation).
This far inferiorly, the MRF covers the levator ani. There are no areas where tumor comes to within 1 mm of these structures so this is a MRF- tumor.
There are places where the bowel wall comes to within 1 mm of the left levator ani (15 remove annotation), but the tumor is NOT transmural at this location, so the tumor remains > 1mm away.
We will see that there are also areas where nodes abut the MRF. This needs to be mentioned, but does not constitute MRF+ disease.
Note that if a low tumor invades the levator-ani this would be T4 disease (the levator is on the other side of the mesorectal fascia).
Step 4: Adenopathy
Starting from top to bottom.
This node measures 0.8 cm in short axis (19). It is round and heterogeneous therefore meeting criteria for pathologic involement. The node is located in the sigmoid mesocolon so that it contacts the peritoneal reflection is not critical; it is already in the peritoneal cavity. However, it is high in the mesocolon and this should be noted. It might not be removed in a lower anterior resection; which is why it is important to mention.
This node is 0.4 cm in short axis (20). It is round, heterogeneous, and has irregular margins; therefore meetin criteria for pathologic involvement. It contacts the levator ani. A node contacting the MRF does not make for an MRF+ tumor, but this should be mentioned.
This node measures 0.2 cm in short axis (21).
It is heterogeneous and round but does not have irregular margins. It therefore does not meet criteria for being a pathologic node.
This node measures 0.4 cm in short axis (22).
It is heterogeneous, round, and has irregular margins. It is a pathologic node.
This node measures 0.6 cm (23). It is round and heterogeneous, therefore meets criteria for a pathologic node.
So there are four nodes with features of a malignant node. One is high in the sigmoid meso-colon and thus extra-mesorectal. One abuts the left levator ani. This is N2 disease.
Step 5: Macroscopic extramural vascular invasion.
I do not see any evidence macroscopic extramural vascular invasion.
Step 6: Incidental findings.
The endometrium measures 1.1 cm which is too thick for an 80 year old woman. The appearance is nonspecific. This could represent hyperplasia, polyp, or neoplasm. No focal mass is seen in the endometrium.
The patient underwent an abdominoperineal resection (APR) that extended proximally for 25 cm to remove the node in the sigmoid meso-colon.
I asked my surgeon how much space he needs between the bottom of the tumor and the ano-rectal junction to perform a lower anterior resection (LAR) and it turns out he only needs a few millimeters. However, the tumor felt rigid and immobile to his finger on physical exam which suggeted to him that he would not be able to get a clean margin with an LAR.
Post-operative staging was T3, MRF-, N2 which agreed with the imaging findings.
The tumor extended into the anus over a distance of 1.5 cm but did not extend through the internal sphincter.