Basic Treatment Planning

Optic chiasm homogeneity

• Excessive dose to optic chiasm risks optic neuropathy, potential loss of sight, blindness

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