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Histological grading

This refers to the assessment of the cancer by the microscopist/histologist/pathologist, not so much as to the tumour type (i.e. not 'what' the diagnosis is) but rather whether it is a particularly aggressive form of the cancer or not. Aggressive features that are common to all grading systems are the number of cells in mitosis (cell division) per high power microscope field of view, the adherence of the cells to the original cells of origin of the organ from which they derived, and the homogeneity of the cellular pattern and the adoption of structures similar to that of the original organ. Thus a cancer which looks similar to the original tissue of origin (and some thyroid cancers exemplify this well), has no visible cell divisions down the microscope on high power examination and starts to make organelles that are recognisable as those typical of the organ of origin (thyroid follicles in the example just cited) will not behave as an aggressive tumour. However, a cancer that is comprised of sheets of bizarre looking cells that bear no similarity to the tissue of origin (or give no clue to the tissue of origin - if a metastasis has been biopsied) and with many of these cells in mitosis will behave as an aggressive one and tend to both grow quickly in the primary site and spread elsewhere (metastasise). Tumours that show no signs of developing characteristics of their tissue of origin are called undifferentiated or anaplastic cancers. The usual nomenclature for grading is by numbering: Grade 1 histology is usually a well differentiated tumour (the term differentiated referring to the similarity to normal tissue of origin) and a high grade (undifferentiated or anaplastic) tumour is usually grade 3-4, depending on the nomenclature. There are many different grading systems and these are often called after the histopathologists who described them: thus, the Bloom and Richardson grading system is one used very commonly in breast cancer and the Gleason system in prostate cancer, the Kernohan-Sayre system in gliomas etc. etc.

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