The rapid improvement in visual field defect observed with weekly perimetry during intensity-modulated radiotherapy for optic nerve sheath meningioma Abstract During precision radiotherapy to treat optic nerve sheath meningioma, early improvement in visual function has been seen. This has been difficult to explain biologically. In the present study, we aimed to investigate this rapid improvement in visual function. To this end, we prospectively tested a single patient’s visual field (VF) using Humphrey automated perimetry at weekly intervals. The patient exhibited significant stepwise improvement in VF during an intensity-modulated radiotherapy course.