Comprehensive introduction to rectal cancer staging part 2.
Findings:
There is a 4 cm long low rectal tumor along the anterior, left lateral, and posterior aspects of the rectum. The distal margin is at the level of the external sphincter complex, 4 cm from the anal verge. On the left there is 1.7 cm of suspected thin, linear, intersphincteric spread without evidence of invasion into or through the external sphincter.
Along the left lateral aspect of the tumor there is 0.6 cm of extension into the meso-rectal fat in keeping with T3c disease. Tumor comes within 0.1 cm of the levator ani without evidence of levator invasion.
There are multiple small mesorectal and extra-mesorectal nodes. Only one meets criteria for pathologic involvement. This node measures 0.8mm and abuts the left anterior aspect of the meso-rectal fascia.
No evidence of macroscopic extramural vasular invasion.
Impression:
T3c, MRF+, N1, low rectal neoplasm with inferior margin at the ano-rectal junction and suspected inter-sphincteric extension over a distance of 1.7 cm anteriorly on the left without invasion of the external sphincter.
The surgeon described a low rectal cancer that he could palpate with his finger.
Step 1 - Location and size of the tumor.
The extent of the tumor is well seen on the sagittal images (1, remove annotations). Convince yourself that this is the upper (2, remove annotations) and lower (3, remove annotations) extent of the tumor on the axial T2 images.
In this case the tumor is also very nicely seen on the diffusion weighted images (DWI) (4, remove annotations). The DWI can be useful for identifying the tumor when it is hard to find on the T2 images. Because they are lower resolution than the T2 images they are not as good at determining depth of invasion. The DWI are especially valuable for assessing response to neoadjuvant chemo-radiation. Loss of diffusion restriction is sign of response to therapy. To my knowledge, however, there are no standard ADC cut-offs or changes for determining response. Most people simply look for brightness on the high b-value images.
Most sites simply obtain straight axial DWI.
I think it is critically important that the diffusion planes match the T2 planes exactly. I therefore match the slice thickness, gap, and angle of the diffusion and T2 weighted sequences.
This is the top slice with tumor on DWI (5, remove annotations). This is the bottom slice with tumor on DWI (6, remove annotations).
On the sagittal images the bottom of the tumor sits 6mm proximal to the level of the ano-rectal junction (dotted line in 7, remove annotations) as defined by the anterior aspect of the levator plate (red arrow in 7, remove annotations).
Since this is a low rectal tumor that comes close to the ano-rectal junction, images parallel to the anus are needed to evaluate the relationship of the tumor to the anal-sphincter complex. The sagittal images allow you to approximate the ano-rectal junction. The images parallel to the anus better define it.
On the T2 and post contrast images parallel to the anus the lower extent of the tumor extends all the way to the ano-rectal junction as defined by the location where the levator ani begins to run parallel to the internal sphincter (blue arrows in 8 remove annotations and 9 remove annotations).
Anteriorly on the post contrast images there is linear enhancement extending distally over a distance of 1.5 cm between the internal and external sphincters on the left (10 remove annotations). I am not convinced this is a blood vessel, so I'm calling it tumor extension into the sphincter complex even though it is not seen on the DWI or on the T2 images parallel to the anus.
Step 2: Depth of invasion.
This is best performed on the images parallel and perpendicular to the tumor. On the left lateral aspect of the tumor there is 0.6 cm of spread into the mesorectal fat (11 remove annotations, 12 remove annotations).
This is T3c disease.
Step 3: Mesorectal fascia.
On the same axial slice as above the tumor comes to within 0.1 cm of the left levator ani (red arrow in 13, remove annotations). Since the meso-rectal fascia lines the levator ani muscle, this is MRF+ disease.
Note that posteriorly the rectum contacts the levator ani, but the tumor is not transmural at this location so tumor does not abut the levator at this location. The blue line in (13 remove annotations) shows where the bowel wall remains intact.
Cross reference this to the images parallel to the anus to see that this is above the level of the ano-rectal junction.
Because the bowel makes a near 90 degree turn near the level of the tumor it is difficult to know exactly where you are without cross referencing between the different sequences.
There is not invasion of the levator ani muscle.
Step 4: Adenopathy
There is an extra-mesorectal node on the left which measures less then 5mm in short axis (14, remove annotations). Extra-mesorectal nodes must be greater than 1 cm in short axis to be considered pathologic.
There is a 0.8 mm node in the left anterior mesorectal fat which is round with irregular margins (red arrow in 15 remove annotations). This meets criteria for a pathologic node. This node contacts the anterior aspect of the meso-rectal fascia (yellow outline in 15 remove annotations). Given that there is a vessel extending from this node (16 remove annotations), it could instead represent a tumor deposit.
A left sided obturator node measures less than 5mm in short axis (17, remove annotations); therefore not considered pathologic.
There are also non enalrged external iliac chain nodes.
One node meets criteria for pathologic involvement, so this is N1 disease.
Step 5: Macroscopic extramural vascular invasion.
I do not see any evidence macroscopic extramural vascular invasion. There is a vessel extending cephalad from the tumor here (18, remove annotations), but I did not think it was sufficiently expanded to represent tumor invasion.
Step 6: Incidental findings.
No incidental findings.
This patient underwent neoadjuvant chemo-radiation with little response on followup imaging. Repeat MRI was essentially unchanged.
He then underwent a extralevator abdominoperineal excision due to proximity of the tumor to the levator. The node described above was a tumor deposit contacting the mesorectal fascia resulting in positive margins. There was no macroscopic extramural vascular invasion. Surgical staging was T3c MRF+ N1 disease.
Unfortunately, this patient developed a small bowel obstruction due to adhesions between small bowel and the mesh required to close the pelvis posteriorly. The CT scan performed to diagnose the small bowel obstruction showed innumerable liver metastases, which had not been seen on prior staging exams, including MRI liver.