Dosimetric properties of a proton beamline dedicated to the treatment of ocular disease

Purpose: A commercial proton eyeline has been developed to treat ocular disease. Radiotherapy of intraocular lesions (e.g., uveal melanoma, age-related macular degeneration) requires sharp dose gradients to avoid critical structures like the macula and optic disc. A high dose rate is needed to limit patient gazing times during delivery of large fractional dose. Dose delivery needs to be accurate and predictable, not in the least because current treatment planning algorithms have limited dose modeling capabilities. The purpose of this paper is to determine the dosimetric properties of a new proton eyeline. These properties are compared to those of existing systems and evaluated in the context of the specific clinical requirements of ocular treatments. Methods: The eyeline is part of a high-energy, cyclotron-based proton therapy system. The energy at the entrance of the eyeline is 105 MeV. A range modulator (RM) wheel generates the spread-out Bragg peak, while a variable range shifter system adjusts the range and spreads the beam laterally. The range can be adjusted from 0.5 up to 3.4 gcm²; the modulation width can be varied in steps of 0.3 gcm² or less. Maximum field diameter is 2.5 cm. All fields can be delivered with a dose rate of 30 Gymin or more. The eyeline is calibrated according to the IAEA TRS-398 protocol using a cylindrical ionization chamber. Depth dose distributions and doseMU are measured with a parallel-plate ionization chamber; lateral profiles with radiochromic film. The doseMU is modeled as a function of range, modulation width, and instantaneous MU rate with fit parameters determined per option (RM wheel). Results: The distal fall-off of the spread-out Bragg peak is 0.3 gcm², larger than for most existing systems. The lateral penumbra varies between 0.9 and 1.4 mm, except for fully modulated fields that have a larger penumbra at skin. The source-to-axis distance is found to be 169 cm. The doseMU shows a strong dependence on range (up to 4mm). A linear increase in doseMU as a function of instantaneous MU rate is observed. The doseMU model describes the measurements with an accuracy of ±2. Neutron dose is found to be 146 ± 102 μSvGy at the contralateral eye and 19 ± 13 μSvGy at the chest. Conclusions: Measurements show the proton eyeline meets the requirements to effectively treat ocular disease.