Abstract
A typical proton CT (pCT) detector comprises a tracking system, used to measure the proton position before and after the imaged object, and an energy/range detector to measure the residual proton range after crossing the object. The Bergen pCT collaboration was established to design and build a prototype pCT scanner with a high granularity digital tracking calorimeter (DTC) used as both tracking and energy/range detector. In this work the conceptual design and the layout of the mechanical and electronics implementation, along with a Monte Carlo (MC) simulation of the new pCT system are reported. The DTC is a multilayer structure with a lateral aperture of 27 cm × 15 cm, made of 41 detector/absorber sandwich layers (calorimeter), with aluminum (3.5 mm) used both as absorber and carrier, and 2 additional layers used as tracking system (rear trackers) positioned downstream of the imaged object; no tracking upstream the object is included. The rear tracker’s structure only differs from the calorimeter layers for the carrier made of ~200 μm carbon fleece and carbon paper (carbon-Epoxy sandwich), to minimize scattering. Each sensitive layer consists of 108 ALPIDE chip sensors (developed for ALICE, CERN) bonded on a polyimide flex and subsequently bonded to a larger flexible printed circuit board. Beam tests tailored to the pCT operation have been performed using high-energetic (50–220 MeV/u) proton and ion beams at the Heidelberg Ion-Beam Therapy Center (HIT) in Germany. These tests proved the ALPIDE response independent of occupancy and proportional to the particle energy deposition, making the distinction of different ion tracks possible. The read-out electronics is able to handle enough data to acquire a single 2D image in few seconds making the system fast enough to be used in a clinical environment. For the reconstructed images in the modeled MC simulation, the water equivalent path length error is lower than 2 mm, and the relative stopping power accuracy is better than 0.4%. Thanks to its ability to detect different types of radiation and its specific design, the pCT scanner can be employed for additional online applications during the treatment, such as in-situ proton range verification.
Highlights
Particle therapy, especially with proton beams, has been used and become widely accepted in the last 20 years
Most Proton CT (pCT) scanners currently available utilize a tracking system consisting of two layers of tracking detectors upstream and two more layers downstream of the object to be imaged
The signal produced by the 241Am source has a time over threshold 4–6 μs, so for each pulse, a data frame window matched up with the signal from the analog frontend and the hit was read out
Summary
Especially with proton beams, has been used and become widely accepted in the last 20 years. Due to the stochastic nature of the particle energy loss, precise calculation of proton range is inevitably uncertain even for simple geometries and materials For this reason, range uncertainty has become a crucial and still debated topic in proton therapy. Due to multiple Coulomb scattering the proton track across the target is not a straight line, affecting the spatial resolution of proton imaging. To address this issue, several trajectory estimation methods [7,8,9] are employed to reconstruct each single proton trajectory using the most likely path (MLP) formalism. The expected effect of an improved spatial resolution was observed, once the ion fragmentation was taken into account [25,26,27,28,29,30,31]
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