Speaker
Description
In current particle radiotherapy practice, it is necessary to evaluate and monitor the beam delivery for the purposes of quality assurance [1]. The current approach at PTC utilizes a scintillator detector (IBA Lynx PT) for determining the accuracy of beam delivery. This detector, while being heavy and large, requires individual dedicated measurements. Additionally, it has a restricted surface area of 30x30 cm, while the nozzle’s maximum field size is 30x40 cm. An alternative that could measure the maximum field size, be permanently measuring and mounted on the nozzle would represent a significant improvement. We make use of a concept exploiting scattered beam particles detected with a compact stack telescope of Timepix3 detectors. This alternative could potentially be also used for higher than conventional dose rates (ultra-high dose rate or FLASH dose rate). In this study, we evaluate the feasibility of this approach.
Protons in the delivered beam scatter along the beam path, particularly at the accelerator nozzle window and, to a lesser extent, in the air along the beam path. We measure in detail these scattered protons at a position beyond the isocenter and away from the beam axis (see Figure 1). For testing and evaluation purposes, a scattering foil (100 μm thick plastic) is placed along the beam axis between the accelerator vacuum nozzle and the isocenter. We use a miniaturized stack pixel telescope MiniPIX-Timepix3 which directionally maps energetic charged particles in a wide (60%) field of view with enhanced discrimination and high angular resolution [2].
This non-invasive approach [3, 4] is applied to track and visualize the beam from a position outside the field, several meters away. Protons reaching the pixel telescope are registered with fast timing (< 100 ns) and resolved with enhanced discrimination thanks to high-resolution spectral-sensitive particle tracking. The trajectories of individual protons are determined as 3D directional vectors with subpixel resolution (≈ μm) and high angular resolution (sub-degree) [2]. The vertex map, indicating the origin of the registered protons, can be obtained as a backplane projected image at specific positions along the beam, as shown in Figure 2. The back-projection plane corresponds to the scattering foil position at 194 cm from the detector (Figure 1). The image visualizes the beam along its path in a wide field of view.
The data shown corresponds to 7 minutes of measured time, collected with a therapeutic beam of clinical intensity. Various beam-target-detector geometries and delivered dose plans were investigated. The results and projected scattering planes produced can be used to examine treatment plans and potentially also for studies of beam quality. Information can be extracted on the intensity and positioning of the beam with this non-invasive approach.
The ultimate goal is to correlate these measurements with a dosimetric quantity, or a substitute closely correlated with it. Although this approach still requires further rigorous measurements and testing under clinical conditions, the results suggest that our objective is achievable.
| Workshop topics | Applications |
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