Non-Interceptive Beam Current and Position Monitors for a Cyclotron Based Proton Therapy Facility

In PSI’s dedicated proton therapy facility PROSCAN a pulsed 250 MeV proton beam is delivered by a superconducting cyclotron. During the proton-irradiation treatments, there is a need to accurately measure beam current, in the range of 0.1-10 nA, and beam position (required accuracy 0.5 mm). The beam current is directly associated with the dose-rate in the treatment and the beam position with the quality of the dose distribution in the patient. However, the presently used measurements compromise the beam quality. Nevertheless, it is a necessity to perform these measurements online and with minimal beam disturbance. This thesis reports on the development of two types of cavity resonators to perform non-interceptive measurements of these beam parameters, within the required accuracy. For beam current measurements, a single cavity resonator has been built. For the beam position measurements, a cavity resonator consisting of four separate segments has been built. Both cavity resonators have been tuned to the second harmonic of the beam pulse rate, i.e., 145.7 MHz. In test bench experiments and with proton beams, a good agreement between the expected and measured sensitivity of these resonators has been found. The cavity used to measure beam current can measure currents down to 0.15 nA with a resolution of 0.05 nA. The cavity for measuring beam position delivers position information with the required accuracy and resolution demands of 0.5 mm. The design, tests and performance in the beam as well as special applications, future improvements and limitations are discussed.

Monte Carlo Simulations of Neutron Ambient Dose Equivalent in a Novel Proton Therapy Facility Design

Purpose The neutron shielding properties of the concrete structures of a proposed proton therapy facility were evaluated with help of the Monte Carlo technique. The planned facility's design omits the typical maze-structured entrances to the treatment rooms to facilitate more efficient access and, instead, proposes the use of massive concrete/steel doors. Furthermore, straight conduits in the treatment room walls were used in the design of the facility, necessitating a detailed investigation of the neutron radiation outside the rooms to determine if the design can be applied without violating existing radiation protection regulations. This study was performed to investigate whether the operation of a proton therapy unit using such a facility design will be in compliance with radiation protection requirements. Methods A detailed model of the planned proton therapy expansion project of the University of Texas, M. D. Anderson Cancer Center in Houston, Texas, was produced to simulate secondary neutron production from clinical proton beams using the MCNPX Monte Carlo radiation transport code. Neutron spectral fluences were collected at locations of interest and converted to ambient dose equivalents using an in-house code based on fluence to dose-conversion factors provided by the International Commission on Radiological Protection. Results and Conclusions At all investigated locations of interest, the ambient dose equivalent values were below the occupational dose limits and the dose limits for individual members of the public. The impact of straight conduits was negligible because their location and orientation were such that no line of sight to the neutron sources (ie, the isocenter locations) was established. Finally, the treatment room doors were specially designed to provide spatial efficiency and, compared with traditional maze designs, showed that while it would be possible to achieve a lower neutron ambient dose equivalent with a maze, the increased spatial (and financial) requirements may offset this advantage.