Abstract-In vivo verification of dose delivery in proton therapy by means of positron emission tomography (PET) or prompt gamma imaging is mostly based on fast scintillation detectors. The digital silicon photomultiplier (dSiPM) allows excellent scintillation detector timing properties and is thus being considered for such verification methods. We present here the results of the first investigation of radiation damage to dSiPM sensors in a proton therapy radiation environment. Radiation hardness experiments were performed at the AGOR cyclotron facility at the KVI-Center for Advanced Radiation Technology, University of Groningen. A 150-MeV proton beam was fully stopped in a water target. In the first experiment, bare dSiPM sensors were placed at 25 cm from the Bragg peak, perpendicular to the beam direction, a geometry typical for an in situ implementation of a PET or prompt gamma imaging device. In the second experiment, dSiPM-based PET detectors containing lutetium yttrium orthosilicate scintillator crystal arrays were placed at 2 and 4 m from the Bragg peak, perpendicular to the beam direction; resembling an in-room PET implementation. Furthermore, the experimental setup was simulated with a Geant4-based Monte Carlo code in order to determine the angular and energy distributions of the neutrons and to determine the 1-MeV equivalent neutron fluences delivered to the dSiPM sensors. A noticeable increase in dark count rate (DCR) after an irradiation with about 10 8 1-MeV equivalent neutrons/cm 2 agrees with observations by others for analog SiPMs, indicating that the radiation damage occurs in the single photon avalanche diodes and not in the electronics integrated on the sensor chip. It was found that in the in situ location, the DCR becomes too large for successful operation after the equivalent of a few weeks of use in a proton therapy treatment room (about 5×10 13 protons). For PET detectors in an in-room setup, detector performance was unchanged even after an irradiation equivalent to three years of use in a treatment room (3 × 10 15 protons). Manuscript
Introduction Cancer is one of the leading causes of death in The Netherlands. In 2017, all types of cancer combined caused 47,000 of 150,000 recorded causes of death (Centraal Bureau voor de Statistiek, 2017). There are several ways to treat cancer. The most common treatments include radiotherapy, surgery, chemotherapy, targeted therapy, hormonal therapy and immunotherapy. Often, different treatment modalities are combined to maximize their efficacy. For example, patients might receive radiotherapy after surgery to remove any traces of cancer cells that were left. Radiotherapy uses ionizing radiation to kill tumor cells by damaging their DNA. This radiation can be applied internally (brachytherapy) or externally. For brachytherapy, radioactive sources are implanted in and around the tumor, which deliver dose directly at the right location. However, for this method, the tumor needs to be in a relatively easily accessible location. For some patients, radioactive substances that accumulate in the tumor are injected. This radiation then delivers most dose at the site where it accumulates. More often, the radiation is applied using a source outside of the body. In the past, radioactive sources such as 60 Co were used to supply MeV gamma rays. Nowadays, a linear accelerator is used in most radiotherapy facilities to produce MeV electron beams. These electrons are stopped in a tungsten absorber to generate MeV X-rays, which penetrate deeply into the body. Other particles can also be used, such as protons or even heavier nuclides. Accelerating these particles to clinically useful energies requires large particle accelerators. Already in 1946, Robert R. Wilson wrote about how protons with an energy in the order of 100 MeV are very interesting for radio-3. Beam-on imaging of short-lived positron emitters during proton therapy proton bunches, such as prompt gamma rays, were removed from the data via an anti-coincidence filter with the cyclotron RF. The resulting energy spectrum allowed good identification of the 511 keV PET counts during beam-on. A method was developed to subtract the long-lived background from the 12 N image by introducing a beam-off period into the cyclotron beam time structure. We measured 2D images and 1D profiles of the 12 N distribution. A range shift of 5 mm was measured as 6 ± 3 mm using the 12 N profile. A larger, more efficient, PET system with a higher data throughput capability will allow beam-on 12 N PET imaging of single spots in the distal layer of an irradiation with an increased signal-to-background ratio and thus better accuracy. A simulation shows that a large dual panel scanner, which images a single spot directly after it is delivered, can measure a 5 mm range shift with millimeter accuracy: 5.5 ± 1.1 mm for 1.64 × 10 8 protons and 5.2 ± 0.5 mm for 8.2 × 10 8 protons. This makes fast and accurate feedback on the dose delivery during treatment possible.
Dose delivery verification in proton beam radiotherapy is used to ensure the delivery of the dose to the correct location. A positron emission tomography (PET) scanner can be used to detect the secondary radiation during the treatment, so-called in-beam PET. This is a challenging application for PET due to the low counts and limited angular coverage. We propose a maximum a posteriori (MAP) reconstruction with median root prior (MRP) for the reconstruction of in-beam PET data. The proposed method was compared against MAP with total variation (TV) prior and maximum likelihood expectation maximization (MLEM), which have previously been used for this application. The effects of different ring configurations and time-of-flight information were tested with simulations of a geometrical phantom and a realistic patient treatment plan. The results indicate that both MAP methods produced sharper edges than MLEM, allowing more accurate edge localization in the reconstructed images. Even for the partial ring configurations, no elongation was observed with MAP methods. MAP-MRP successfully reduced the noise, whereas MAP-TV resulted in checkerboard artifacts. MAP-MRP was also more stable against the selection of the reconstruction parameters. In conclusion, MAP-MRP offers a simple and robust alternative for the reconstruction of in-beam PET data. Index Terms-List-mode (LM) positron emission tomography (PET), partial ring scanner, particle beam radiotherapy, proton beam radiotherapy, time-of-flight (TOF). I. INTRODUCTIONT HE BEAMS used in particle beam radiotherapy have a well-defined, finite penetration depth with high dose deposition close to the end of the beam's trajectory, the so-called Bragg peak. This enables treatments in which less healthy tissue is irradiated as compared to irradiation with photons, leading to a reduction in irradiation-induced complications. However, as a result of the Bragg peak, large dose deposition errors can occur if the actual treatment situation is different from the situation assumed during Manuscript
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