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Optic chiasm
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Probably most crucial organ at risk
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Lies below hypothalamus, third ventricle and its optic recess
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Anterior cerebral arteries lie ventrally, and ICA’s laterally
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Approx 14mm transverse width, 8mm AP, 2-5mm height
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Slopes slightly upwards and backwards
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Must contour on multiple slices – gaps in contour will lead to
missing essential volume for computing dose-volume histogram
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Check contour on sagittal or coronal images