Precise On-line Position Measurement for Particle Therapy

An on-line beam position monitoring and regular beam stability tests are of utmost importance for the Quality Assurance (QA) of the patient treatment at any particle therapy facility. The Gantry$\hspace{0.5 mm}2$ at the Paul Scherrer Institute uses a strip ionization chamber for the on-line beam position verification. The design of the strip chamber placed in the beam in front of the patient allows for a small beam penumbra in order to achieve a high-quality lateral beam delivery. The position error of 1 mm in a lateral plane (plane perpendicular to the beam direction) can result in a dose inhomogeneity of more than $5 \%$. Therefore the goal of Gantry 2 commissioning was to reach a sub-millimeter level of the reconstruction accuracy in order to bring a dose uncertainty to a level of $1 \%$. In fact, we observed that for beams offered by Gantry 2 signal profiles in a lateral plane can be reconstructed with a precision of 0.1 mm. This is a necessary criterion to perform a reliable patient treatment. The front end electronics and the whole data processing sequence have been optimized for minimizing the dead time in between two consecutive spots to about 2 ms: the charge collection is performed in about 1 ms, read-out takes place in about 100 $\mu$s while data verification and logging are completed in less than 1 ms.