• proton therapy
• proton beam
• Ocular melanoma
• dosimetric characterization
• small target treatment

Particle radiotherapy is a rapidly growing technique becoming an important treatment modality within the field of radiation therapy. Protons and charged ions show an increasing energy deposition with the penetration depth, leading to a maximum, the Bragg-peak, near the end of the particle range. This makes possible the creation of a highly conformal dose region, the spread-out Bragg-peak, with the possibility of covering even small tumor volumes with high accuracy delivering lower doses to healthy tissues. Like other highly conformal therapy techniques, proton therapy is of particular interest for those tumors located close to serially organized tissues, where a small local overdose may cause fatal complications, such as most of the eye tumors, which are close to the optic nerve.
The international standard radiation therapy for eye treatment is based on dedicated passive beam line with protons produced by cyclotron. The National Center for Oncological Hadrontherapy (CNAO) of Pavia is provided with a synchrotron, a circular accelerator of about 25 meters in diameter, designed for a fully active 3D dose distribution system. The facility is not equipped with a dedicated eyeline developed to treat ocular disease and, for these reason, the treatment room 3 has been adapted to this purpose. This work presents the dosimetric measurements needed to characterize the proton beam in the new treatment setup, which involves the use of a range shifter, a patient-specific collimator and a dedicated synchrotron protocol to reduce the treatment time.
First of all, the new setup requires an ad-hoc re-calibration of the beam monitors in terms of delivered dose per Monitor Units. To check the distal fall-off of the Bragg curve the integrated depth-dose-distributions were measured with the Peakfinder (a variable depth water column equipped with two plane-parallel ionization chambers), while the lateral penumbra measurements were performed using EBT3 films irradiated with square monoenergetic uniform fields. At last, different Spread Out Bragg Peaks were also measured to check that the superposition of the Bragg peaks and the resulting dose distribution were both correct.
With the new setup, specifically commissioned for protontherapy of the eye, a study was performed to evaluate the main dosimetric properties of the configuration adopted. In the energy range of clinical interest, the depth dose distribution and the lateral penumbra are comparable with specifications of other eye-dedicated facilities, making possible the treatment of ocular disease.