Dissection

Cancer resections: for optimal demonstration of the deepest point of tumour spread, its relationship to the CRM and correlation with ELUS/CT cross sectional imaging multiple, serial, 3-4 mm thick slices of the cancer in the transverse axis are recommended. The slices can then be laid out in sequence and a photographic or digital record provided. Generally, four or five blocks of the tumour and wall are selected to adequately define the pT stage. Some pathologists leave the tumour segment unopened during fixation and transverse slicing to keep the CRM intact -others open it carefully avoiding suspect areas of the CRM to ensure adequate tumour fixation and ascertain tumour measurements. Either approach is justifiable as long as it is done with care and consistency. Sometimes the local anatomy or proximity of the tumour to a longitudinal margin necessitate dissection in the longitudinal plane. Such a block can be useful in a poorly differentiated carcinoma when the adjacent mucosa may show a point of origin or clue as to its histological type. Mucosal blocks away from the tumour may also demonstrate its histogenesis, e.g., metaplasia/dysplasia/cancer sequence in the stomach, or multifocality. Multiple colonic cancers are blocked and reported individually.

All regional lymph nodes should be sampled as size alone is not a reliable indicator of metastatic involvement and pN staging relates to total and involved numbers of nodes. Small nodes seen histologically in the tumour blocks are also counted and may only measure > 1 mm diameter but are recognisable by their subcapsular sinus. A limit node is identified adjacent to a mesen-teric pedicle suture tie - some specimens, e.g., transverse colon, may have more than one. Dukes staging for colorectal cancer varies according to whether the limit node is involved (C2) or not (C1). Techniques such as xylene clearance have been advocated to increase nodal yields but in general there is no substitute for experienced, careful dissection. A target number can be useful, e.g., a harvest of eight nodes will identify the vast majority of Dukes C colorectal carcinomas. Preoperative radio-/chemotherapy can lead to marked tumour degeneration and fibrotic reaction compromising nodal yields and identification of residual primary tumour or nodal deposits. Most general laboratories submit small nodes (< 5 mm) intact or bisected, and a mid-slice of larger ones. It is important that the same node is not counted twice.

Non-neoplastic resections: an important descriptive feature in differential diagnosis is disease distribution, e.g., diffuse, segmental, mucosal or transmural. Overt lesions may show only endstage non-specific florid ulceration and reactive changes - the disease distribution and changes in the intervening mucosa give important diagnostic clues. For example, ulcerative colitis is mucosal and diffuse, Crohn's disease segmental and transmural with intervening aphthous ulcers, chronic ischaemic stricture is preferentially located at the splenic flexure and clostridium difficle infection shows mucosal pseudomembranes. Non-neoplastic colonic specimens therefore require sequential labelled blocks of abnormal and normal (e.g., every 10 cm) areas. As the mucosa is arranged in transverse folds, long-axis blocks are taken. Longitudinal limits are transverse sectioned to look for disease involvement and although mesenteric nodes are usually reactive only, they may show helpful diagnostic pointers such as granulomas in Crohn's disease. In ischaemic conditions, mesenteric vessels are also sampled for signs of vasculitis or embolic thrombi. Some vascular anomalies, e.g., angiodysplasia of the colon, may require close liaison with the surgical and radiological teams necessitating perioperative injection of radio-opaque contrast medium. In some cases, e.g., gastric resections, it is not possible to tell macroscopically if the ulcer, adjacent mucosa or regional nodes are benign or malignant or to gauge the extent of mural spread - dissection and block selection must be sufficiently comprehensive to allow for this.

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