Total mesorectal excision
Indication: Total mesorectal excision is used to treat cancers of the middle and lower third of the rectum.
The mesorectum is a fatty envelope around the lateral and posterior rectum, which is surrounded by the mesorectal fascia. It contains the blood and lymphatic vessels that supply and drain the rectum, as well as the regional lymph nodes. Total mesorectal excision refers to the surgical removal of this soft tissue envelope using sharp dissection under direct vision, dissecting between the mesorectum and the surrounding parietal tissues right down to the distal extremities of the pelvis in the plane of the mesorectal fascia. The excised specimen includes the whole posterior, distal and lateral mesorectum. Anteriorly it includes the intact Denonvilliers fascia and the peritoneal reflection. The characteristic bilobed encapsulated appearance of the intact mesorectum posteriorly and distally reflects the contours of the pelvic floor and the midline anococcygeal raphe. The ideal specimen has a smooth unbroken surface covered by the intact fascia.
The non-peritonealised circumferential margin in the rectum is an extensive surgically created plane of dissection produced during the removal of the rectum from its surroundings. TME reduces the likelihood of surgical transection through tumour and therefore reduces the incidence of local recurrence of tumour within the pelvis.
Transanal excision
Transanal excision is occasionally used for local treatment in selected cases. It is usually used for small tumours, less than 3cm in diameter, within 6-8 cm of the anal verge and with limited circumferential involvement. Tumours should be well/moderately differentiated and T1 on transrectal ultrasound. A full thickness rectal wall excision down to perirectal fat is performed.
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HANDLING, PREPARING AND FIXING COLORECTAL RESECTION SPECIMENS
Handling
The macroscopic assessment and correct sampling of colorectal specimens is crucial. No amount of clever microscopy can retrieve a badly prepared or poorly fixed and inadequately sampled specimen. The aim is to make a diagnosis, assess resection status and glean all other prognostic information.
Before opening a specimen have a thorough look at it. The assessment of tumour perforation is best made in the freshly received and unopened specimen. Tumour perforation is defined as a macroscopically visible defect through the tumour, such that the bowel lumen is in communication with the external surface of the intact resection specimen. Perforation of the proximal bowel as a result of a distal obstructing tumour should not be regarded as tumour perforation, although its presence should still be recorded.
Gently feel for the tumour, either from the outside or by carefully inserting a finger into the lumen. Look for areas of possible serosal involvement, look and feel for any serosal nodules and possible tumour deposits or lymph nodes. For rectal specimens it is recommended to assess and make a note of its relationship to the peritoneal reflection (above, at/astride or below) at this time. Ink the non-peritonealised circumferential margin of rectal specimens.
Opening
There are two basic methods of opening a colorectal resection specimen. The first method involves opening the specimen with scissors anteriorly up and down to the level of the tumour, which is left unopened. A wick of formalin soaked paper or gauze is then inserted into the unopened lumen to aid exposure of the tumour to the fixative. The entire specimen is then placed in formalin for complete fixation.
The second method involves opening the specimen along its length. If the tumour is not circumferential, then the specimen should be opened through an area not involved by tumour. If the tumour is circumferential then you will need to cut through the tumour at some point, but this should avoid areas of possible serosal or non-peritonealised resection margin involvement. Again, the entire specimen should then be placed in an adequate amount of formalin for complete fixation.
For rectal tumours in particular, leaving the tumour intact and bread-slicing it when fixed is recommended. This method facilitates assessment of the very important non-peritonealised resection margin. The relationship of the tumour, nodes, or extramural tumour deposits to the non-peritonealised resection margin can be assessed. It also facilitates correlation with pre-operative imaging.
Inking
The non-peritonealised resection margin of the rectum should be inked. Other cut surgical resection margins can be inked to aid in their identification microscopically if the tumour is near.
The serosal surface should not be inked. It is not a resection margin. Inking of the serosa may result in misinterpretation of serosal surface involvement as representing margin involvement. It can also mask the presence of tumour cells on the serosal surface.
Fixation
Despite the pressure by clinicians on the pathologist for rapid turnaround, adequate fixation and processing of colorectal specimens is vital for high quality pathology. Proper fixation of these large specimens facilitates assessment of those prognostic parameters on which future management will be based. Full fixation facilitates obtaining thin transverse slices through the tumour and it has also been shown to increase lymph node yield.
Fixation of at least 48 hours of the opened, cleaned specimen and long processing of blocks is recommended.
Macroscopic description and block taking
The gross description should include the specimen type, site and size of the tumour and its distance from the closest resection margin. The position of rectal tumours in relation to the peritoneal reflection should be recorded. Note the presence or absence of tumour perforation. Any background abnormalities should also be recorded.
The tumour should be sliced transversely at 3-4mm intervals and the tumour slices laid out sequentially. Block selection should target the prognostic questions that need to be answered.
Select blocks that show the greatest depth of tumour invasion. Include blocks that show tumour close to or at a serosal surface. Serosal involvement is especially prone to occur at or adjacent to peritoneal reflections, especially in the clefts adjacent to the bowel wall, and should be suspected in any areas of serosa that appear granular, dull or haemorrhagic.
Serosal involvement denotes stage T4 tumour. Local peritoneal involvement is common in colonic cancer. It predicts subsequent intraperitoneal recurrence and is a strong independent prognostic parameter. In the upper rectum, significant amounts of the bowel circumference are invested by peritoneum. In upper rectal cancer, therefore, peritoneal involvement may be just as important as margin involvement, especially for anterior tumours.
For rectal tumours, assess and record the distance of the tumour from the non-peritonealised resection margin. It is not only the continuous spread of the primary tumour that is important for this measurement, but also discontinuous spread in the form of lymph node metastases, extramural deposits, and tumour in vessels and lymphatics. Even if the main tumour appears “well clear” of this margin it is important to block the tissue between the nearest tumour edge and the non-peritonealised resection margin to ensure picking up any discontinuous areas of spread. It may be that the tissue has to be embedded in two or more sequential blocks but this margin must be well sampled.
If there is tumour perforation, then a block should be taken for histological record. This is an important prognostic factor in colorectal carcinoma. If tumour perforation is present then this indicates a T4 tumour, regardless of other factors.
Tumour at a longitudinal margin occurs only very rarely and several studies have questioned the necessity of sampling the cut end margins. If the tumour is >30mm from the cut end it is not always necessary to examine the margin microscopically, although it is often useful to have normal tissue for control purposes and uninvolved margins can provide this. The nearest cut margin should be sampled if tumour is close. It will also be necessary to examine the cut margins histologically if the initial tissue sections reveal the tumour to have an exceptionally infiltrative growth pattern, to show extensive vascular invasion or lymphatic permeation or to be a pure signet ring, small cell, or undifferentiated carcinoma.
Most of the colon has a long mesentery, so the assessment of this resection margin is rarely an issue. However, the cut margin of the mesentery is a surgical margin and if the tumour is advanced, it may potentially be involved, either by direct spread, or if there are involved nodes at its apex.
The caecum and the proximal ascending colon do not have a mesentery and posteriorly have a non-peritonealised bare area of variable size which is potentially an area of surgical margin involvement, especially in tumours arising from the posterior wall or in circumferential tumours. It is recommended that involvement of the non-peritonealised resection margin in tumours at these sites be sought and recorded when present.
Sample any background abnormalities, in particular any polyps. Their site, size and relationship to the tumour should be recorded.
How many blocks of tumour should be taken?
It is not possible to give an absolute number. Sufficient blocks should be taken to enable the pathologist to fully assess all the necessary parameters for staging and prognosis. The likelihood of identifying prognostically useful features, such as venous invasion for example, increases with the number of blocks taken.
Lymph nodes
All lymph nodes should be harvested from the specimen and examined histologically. There is no minimum number.
The finding of positive lymph nodes is a major determinant of whether a patient receives adjuvant therapy. The probability of finding a positive lymph node increases with the number of nodes found, although this probability curve flattens out after finding 12 -15 nodes. Of course, the number of nodes present depends on a number of factors, including the size of the specimen, the amount of mesenteric tissue present and whether the patient has received neo-adjuvant therapy.
Lymph nodes are easier to find in a well fixed specimen. The lymph node bearing tissue should be methodically sliced at small intervals. Gentle palpation will often release the fat revealing smaller lymph nodes. All small nodes (<5mm) should be embedded completely and at least half of larger nodes should be embedded.
Any lymph nodes lying close to the non-peritonealised resection margin should be sampled in continuity with that margin. If there is tumour in any of the lymph nodes then it is the measurement from the involved lymph node to the non-peritonealised resection margin, if it is closer, rather than from the primary tumour, that is important. This is also true for any isolated tumour deposit in the perirectal or pericolonic fat.
The N3 staging category which described cases with a positive apical lymph node has been shown not to be prognostic and so has been removed from the 6th edition of the AJCC guidelines. It is therefore not required in the colorectal minimum data set. However, it is noted that some departments will wish to continue to give a Dukes’ stage, which requires the status of the apical lymph node to be known. The apical, or highest node, is the first node identified by sectioning serially and distally from the sutured vascular margin, regardless of the actual distance between node and tie.