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Optic chiasm homogeneity
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Excessive dose to optic chiasm risks optic neuropathy,
potential loss of sight, blindness
Clinical Scenario:
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Pituitary Tumour, prescribed dose 50 Gy in 25 fractions
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Maximum dose to optic chiasm 55 Gy
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QUANTEC 55Gy <3% risk of optic neuropathy
‘safe’?
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However, 55 Gy ≈ 110% of the prescribed dose
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Each day, 2 Gy prescribed, however chiasm receives 2.2 Gy
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Biologically, higher dose per fraction increases risk of late
side effect such as blindness
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‘Double Trouble’