SU-E-T-297: Proton-Therapy System for Treatment of Macular Degeneration and Ocular Malignancies.

Abstract

To commission a proton-therapy system for the treatment of uveal melanoma and age-related macular degeneration. Proton therapy system is the proto-type of a commercial product developed by Ion Beam Applications. Proton beam is brought into the treatment room at 105 MeV through a fixed beam line. A single-scattering system with absorber/scattering foils spreads the beam into a Gaussian profile. A library of 10 range-modulator wheels and 16 range-modulator blocks generate spread-out Bragg peaks of various range and modulation width. Source-to-axis distance of the system is 169 cm. Two orthogonal digital x-ray panels are used for alignment. EyePlan software is used both for both treatment planning and in-room alignment. Range can be varied continuously between 0.5 and 3.4 g/cm2 . Range accuracy is measured to be better than 0.05 g/cm2 . Modulation width can be varied in steps of =0.3 g/cm2 with an accuracy of 0.05 g/cm2 or 2%. Maximum aperture diameter is 2.5 cm and maximum dose rate >32 Gy/min. Strong dependence of output on range (7%/mm) and dose rate (0.2%/(Gy/min)) is found. Distal and lateral fall-off (80%-20%) are =0.23 and =0.18 g/cm2 and do not depend much on range or depth. When reducing the aperture diameter to 6 mm no significant change is observed in shape of depth-dose curve or absolute dose (<2.5%). Measurements show a significant portion of the dose at shallow depth (=0.7 g/cm2 ) is delivered by protons scattering off of snout elements. Simple collimation could reduce this effect. The dosimetric and positioning properties of the IBA ocular proton system are adequate to treat ocular lesions with acceptable clinical margins. Suggested improvements include limiting the output-dependence on range and reducing snout scatter.