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33rd Annual Scientific Meeting proceedings

Stream: SA   |   Session: Pathology Session
Date/Time: 07-07-2023 (11:00 - 11:40)   |   Location: Conference Hall Complex A
Tumour margins & Special stains
Priestnall SL, Suarez-Bonnet A
The Royal Veterinary College, Hatfield, United Kingdom.

The first half of this session will focus on surgical margins of tumours, specifically the histologic tumour free margin (HTFM). The definition of this term varies considerably amongst pathologists, oncologists and surgeons alike, not helped by the lack of a standardised way of reporting.

Factors that can be managed by the surgeon such as clear identification (e.g. use of surgical inks) of important surgical margins, whether a sample is incisional or excisional, and a concise clinical history and gross intra-operative description can all greatly influence the usefulness of the histopathological interpretation and comment. Other factors affecting the surgical margins such as tissue shrinkage, both naturally post-excision e.g. contraction of smooth muscle or elastin, or following fixation in formalin, are largely impossible to avoid, but should be considered by surgeon and pathologist.

Tissue trimming in the histology laboratory aims to, as best as possible, represent the entirety of the sample but must recognise that the histological tissue section is a two-dimensional, 4µm slice of a much larger three-dimensional tissue. Various methods for this will be discussed but often this is a practical consideration in terms of cost, time and individual laboratory procedures.

The Residual (R) Tumour Classification Scheme, widely used in human oncology, will be discussed. Essentially this system designates R0 as no residual tumour (or HTFM  >0mm) and R1 as ‘tumour on ink’. Whilst this may seem a simple system which could cut through the varied terminology used in histopathology reports it perhaps oversimplifies a more nuanced situation which may vary by tissue, tumour type and grade, anatomical location etc.

The second part of the session will look at the use of ‘special’ stains in histopathology. These histochemical stains are only ‘special’ as they differ from the standard haematoxylin and eosin (H&E) stain which is the staple of diagnostic histopathology worldwide. Each pathologist and laboratory will have their own preferred set of go-to ‘specials’ for identification of pathogens (e.g. Gram, Ziehl-Neelsen and Giemsa), specific cellular components (e.g. Masson’s trichrome for collagen/fibrosis or Perl’s Prussian Blue for iron-containing pigment such as haemosiderin) or extracellular material (e.g. Congo red for amyloid or Martius scarlet blue for fibrin). Toluidine blue is classically used to identify the granules within mast cells and can be particularly useful with poorly granulated (less well-differentiated) tumours to distinguish them from other round cell tumours.

Whilst special stains have somewhat fallen out of fashion in many laboratories – they are costly, time-consuming and many are technically challenging to perform correctly – they are still a valuable part of the pathologist’s toolbox and used appropriately can transform a diagnosis.

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