Abstract
The Pencil Beam Scanning (PBS) technique in proton therapy uses fast magnets to scan the tumor volume rapidly. Changing the proton energy allows changing to layers in the third dimension, hence scanning the same volume several times. The PBS approach permits adapting the speed and/or current to modulate the delivered dose. We built a simple prototype that measures the dose distribution in a single step. The active detection material consists of a single layer of scintillating fibers (i.e., 1D) with an active length of 100 mm, a width of 18.25 mm, and an insignificant space (20 m) between them. A commercial CMOS-based camera detects the scintillation light. Short exposure times allow running the camera at high frame rates, thus, monitoring the beam motion. A simple image processing method extracts the dose information from each fiber of the array. The prototype would allow scaling the concept to multiple layers read out by the same camera, such that the costs do not scale with the dimensions of the fiber array. Presented here are the characteristics of the prototype, studied under two modalities: spatial resolution, linearity, and energy dependence, characterized at the Center for Proton Therapy (Paul Scherrer Institute); the dose rate response, measured at an electron accelerator (Swiss Federal Institute of Metrology).
Highlights
We intend to address this problem with the development of a stackable scintillator fiber-based dosimeter for measuring Pencil Beam Scanning (PBS) proton fields as delivered in a clinical environment of the Center of Proton Therapy (CPT)
A 150 MeV proton beam spot was scanned with a step size of
The beam current at the exit of the treatment nozzle was set to ∼600 pA, which is well-comparable to the current used for clinical treatments
Summary
The flexibility provided by this technique, especially when combined with a treatment gantry that can rotate fully around the patient such that multiple treatment fields can be delivered from different directions [2], has led to this approach being the most common proton therapy delivery modality, after the pioneering work of the Center of Proton Therapy (CPT) at the Paul Scherrer Institute [3]. The energy transferred to tissue by protons is inversely proportional to their velocity, the dose deposition varies as a function of the track length up to a maximum, which is called the Bragg. Information on clinically relevant irradiation fields, with their widely varying proton energy spectra and differing contributions from secondary particles, needs to be well-mapped on a point-to-point basis in the delivered distribution. We intend to address this problem with the development of a stackable scintillator fiber-based dosimeter for measuring PBS proton fields as delivered in a clinical environment of the CPT.
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