Mid rectal cancer with important discussion about the mesorectal fascia in Step 3.
Findings:
There is an 8 x 5 x 6 cm , near circumferential, mid rectal cancer with distal end 5.5 cm from the anorectal junction.
The neoplasm is transmural through the rectal wall over most of the rectal circumference. Laterally on the right tumour invades through the circumferential resection margin and into the adjacent pelvic sidewall fat. There is probably invasion of the lateral aspect of the right seminal vesicle. Posteriorly the tumor extends over a distance of greater than 2 cm into the presacral fat distal to the S2/3 level. Therefore, there is no definite peritoneal spread of disease at this site.
There is no evidence of involvement of the anal sphincters.
There are enlarged lymph nodes within the distal aspect of the sigmoid mesocolon, along both pelvic sidewalls, and within the mesorectal fat.
There is evidence of direct vascular tumour invasion.
There is diffusely decreased marrow signal within the ischium, iliac wings, sacrum, and imaged lower vertebral bodies. No focal lytic or blastic lesion is seen on the CT scan from 4 days previously. Given that this appears a diffuse process, I suspect this represents marrow conversion as opposed to diffuse metastatic disease.
Impression:
T4b MRF + N2 M0 (no distant metastases on the CT from 4 days previously) mid rectal cancer.
Step 1: Size and location of the tumor.
This is a large tumor that can be seen on the T2 (1 remove annotations), diffusion weighted (2 remove annotations), and post contrast (3 remove annotations) images.
The distal end of the tumor is 5.5 cm from the ano-rectal junction (dotted blue line in 4, remove annotations) defined on the sagittal images as the anterior most aspect of the levator (blue arrow in 4, remove annotations).
Since this is well proximal to the anal sphincter I do not investigate further using images parallel to the anus.
This is a mid rectal cancer (distal end 5-10 cm from the ano-rectal junction).
Step 2: Depth of tumor spread, i.e. the T staging.
Posteriorly there is greater than 1.5 cm of direct spread into the mesorectal fat (5 remove annotations) This is therefore at least T3d disease.
Step 3: Involvent of the mesorectal fascia.
On this coronal image there is extension of low signal tumor (red arrow in 6, remove annotations) through the mesorectal fascia (yellow line in 6, remove annotations). Tumor extending to within 1mm of the mesorectal fascia or resulting in thickening of the mesorectal fascia is MRF + disease. Tumor passing through the fascia is T4b disease. This is T4d disease.
View the same area in the axial plane (7 remove annotations). The mesorectal fascia (yellow line) is thickened and there is low signal tumor in the pelvic side wall fat (red arrow).
Some of the tumor touches the right seminal vesicle (8 remove annotations).
What about posteriorly? For low rectal cancers the rectum is completely surrounded by the mesorectal fascia. However, mid and upper rectal cancers are located in places where part of the rectum is surrounded by mesorectal fascia and part is surrounded by a peritoneal reflection. Distinguishing the two is important because it is relatively easy for a surgeon to achieve a negative surgical margin at a peritoneal reflection. It is harder to get a negative margin along the mesorectal fascia.
The sigmoid colon is entirely intraperitoneal and sits on a mesocolon that the surgeon can encircle with his or her hands. At approximately the level of S2/3 the sigmoid becomes the rectum and fat posteriorly becomes retroperitoneal. A surgeon following the bowel distally would be able to wrap hands around the anterior and lateral parts of the rectum but not be able to pass fingers posterior the upper rectum (i.e. the posterior rectum is on a pedicle extending from the pre-sacral fat). To fully mobilize the recum at this point would involve a posterior dissection and the plane of the dissection is the posterior mesorectal fascia.
More distally, the sides of the rectum become retroperitoneal and to mobilize the rectum involves a dissection both posteriorly and laterally.
Even more distally the anterior aspect of the rectum becomes retroperitoneal. The location where the anterior aspect of the rectum becomes retroperitoneal is called the anterior peritoneal reflection. This is sometimes seen as a thin line extending from the bladder or uterus to the anterior aspect of the rectum and we will see it in subsequent cases.
The red line in 9 (remove annotations) is the posterior mesorectal fascia and I call it this because it is distal to the S2/3 level which is the average level of the posterior peritoneal reflection but you should always check on the axial images.
Image 10 (remove annotations) is just proximal to the upper red arrow in 9. The thicker circumferential line overlaid in red (10 remove annotations) is the peritoneal reflection anteriorly and laterally. The thinner line overlaid in yellow (10 remove annotations) is the mesorectal fascia. The low signal tissue on the other side of the yellow line is tumor in the presacral fat (which is also T4b disease).
Many radiologists use the thickness of the reflection to distinguish peritoneal reflection from mesorectal fascia. I find the distinction can be very difficult and we will discuss this in subsequent cases.
Step 4: Adenopathy
There were multiple positive nodes.
This node (11 remove annotations) measures 6mm, round, and heterogeneous. Nodes 5- 9mm need two positive features.
This node (12 remove annotations) is extra-mesorectal and measures > 9 mm and is therefore meets criteria for involvement.
This 6mm (13 remove annotation) extramesorectal node does not meet criteria for involvement.
This 6mm (14 remove annotation) node is mesorectal and is round and heterogeneous.
This 7mm (15 remove annotation) extramesorectal node does not meet criteria.
This 8mm (16 remove annotation) mesorectal node is round and heterogeneous.
There are at least 4 nodes with criteria for pathologic involvement, which is N2 disease.
Step 5: Macroscopic extramural vascular invasion.
This vessel (17 remove annotation) is too thick and too high signal.
Step 6: Incidental findings.
None.
T4b disease means there is no chance for a negative resection margin. This patient underwent neoadjuvant chemotherapy and long course of radiation. Followup MRI showed moderate reduction in disease with persistent thickening of the mesorectal fascia and reduced but persistent low signal soft tissue in the pelvic sidewall and presacral fat.
He is set to undergo a lower anterior resection and we will learn if the residual tissue is tumor or fibrosis.