Design and verification of an external radiobiological beam port on a 16.5 MeV GE PETtrace proton cyclotron.

PURPOSE: Protons and heavy ions are considered to be ideal particles for use in external beam radiotherapy due to the superior properties of the dose distribution. While a photon (x-ray) beam delivers considerable dose to healthy tissues around the tumor, a proton beam that is delivered with sufficient energies has: a low entrance dose (the dose in front of the tumor); a high-dose region within the tumor, known as the Bragg peak; and, no exit dose beyond the tumor. Proton therapy is the next major step in advancing radiotherapy treatment. The purpose of this project was to adapt an existing radioisotope production cyclotron, a General Electric (GE) PETtrace, to enable radiobiological studies using proton beams. During routine use the PETtrace delivers 16.5 MeV protons to target with beam currents in the range of 10-100 µA resulting in dose rates in the order of kGy/s. To achieve the aim of the project the dose rate had to be reduced to the Gy/min range, without attenuating the proton energy below 5 MeV. This paper covers the design, construction and validation of the beam port. METHODS: Monte Carlo simulations were performed, using GEANT4, SRIM and PACE4 to design the beam port and optimize its components. Once the beam port was fabricated, validation experiments were performed using EBT3 and HD-V2 Gafchromic™ films, and a Keithley 6485 picoampere meter. RESULTS AND CONCLUSION: The external beam port was successfully modeled, designed and fabricated. By using a 0.25 mm thick gold foil and a brass pin-hole collimator the beam was spread from a narrow full beam diameter of 10 mm to a wide beam with a 5% flatness area in the center of the beam that had a diameter of ~20 mm. In using this system the dose rate was reduced from kGy/s to ~30 Gy/min.

Evaluation of silica and PMMA optical fibre response when irradiated with 16.5 MeV protons.

An investigation into the response of optical fibres to 16.5 MeV protons is presented here. A silica and a poly(methyl methacrylate) (PMMA) optical fibre was exposed to 16.5 MeV protons from a GE PETtrace cyclotron. The optical fibres were exposed to beam currents of 30nA - 270nA and the emission spectrum analysed. The silica fibre was the most sensitive and had two main peaks at 460 nm and 650 nm. The ratio between the peaks was observed to increase as irradiation of the fibres continued, where the 460 nm peak increased at a rate >4 times the 650 nm peak. The rate of increase of the ratio between the peaks was observed to be constant at a constant target current and linear with target current. In the case of the PMMA fibre, significant spectral changes were observed during the exposure to 16.5 MeV protons. A simple method for estimating the effect of photodarkening and activation is presented here and indicated that the changes in the spectrum for the PMMA fibres may be due to photodarkening and activation.