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True extent of tumour
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Gliomas:
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Grade II: Everything that is T2 hyperintense is tumour!
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Grade III and IV:
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GTV: Contrast-enhancing area
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T2-hyperintensities: Grade II components plus edema -
> CTV
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Tumour to be expected even in seemingly normal brain
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Metastases:
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Tumor confined to contrast-enhancing area
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T2 hyperintensities are oedema!
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Lymphoma:
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Is a generalised CNS disease