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Background: Several (online) adaptive radiotherapy procedures are available to maximize healthy tissue sparing in the presence of inter/intrafractional motion during stereotactic body radiotherapy (SBRT) on an MR‐linac. The increased treatment complexity and the motion‐delivery interplay during these treatments require MR‐compatible motion phantoms with time‐resolved dosimeters to validate end‐to‐end workflows. This is not possible with currently available phantoms. Purpose: Here, we demonstrate a new commercial hybrid film‐scintillator cassette, combining high spatial resolution radiochromic film with four time‐resolved plastic scintillator dosimeters (PSDs) in an MRI‐compatible motion phantom. Methods: First, the PSD's performance for consistency, dose linearity, and pulse repetition frequency (PRF) dependence was evaluated using an RW3 solid water slab phantom. We then demonstrated the MRI4D scintillator cassette's suitability for time‐resolved and motion‐included quality assurance for adapt‐to‐shape (ATS), trailing, gating, and multileaf collimator (MLC) tracking adaptations on a 1.5 T MR‐linac. To do this, the cassette was inserted into the Quasar MRI4D phantom, which we used statically or programmed with artificial and patient‐derived motion. Simultaneously with dose measurements, the beam‐gating latency was estimated from the time difference between the target entering/leaving the gating window and the beam‐on/off times derived from the time‐resolved dose measurements. Results: Experiments revealed excellent detector consistency (standard deviation ≤$\le$ 0.6%), dose linearity (R2 = 1), and only very low PRF dependence (≤$\le$0.4%). The dosimetry cassette demonstrated a near‐perfect agreement during an ATS workflow between the time‐resolved PSD and treatment planning system (TPS) dose (0%–2%). The high spatial resolution film measurements confirmed this with a 1%/1‐mm local gamma pass‐rate of 90%. When trailing patient‐derived prostate motion for a prostate SBRT delivery, the time‐resolved cassette measurements demonstrated how trailing mitigated the motion‐induced dose reductions from 1%–17% to 1%–2% compared to TPS dose. The cassette's simultaneously measured spatial dose distribution highlighted the dosimetric gain of trailing by improving the 3%/3‐mm local gamma pass‐rates from 80% to 97% compared to the static dose. Similarly, the cassette demonstrated the benefit of real‐time adaptations when compensating patient‐derived respiratory motion by showing how the TPS dose was restored from 2%–56% to 0%–12% (gating) and 1%–26% to 1%–7% (MLC tracking) differences. Larger differences are explainable by TPS‐PSD coregistration uncertainty combined with a steep dose gradient outside the PTV. The cassette also demonstrated how the spatial dose distributions were drastically improved by the real‐time adaptations with 1%/1‐mm local gamma pass‐rates that were increased from 8 to 79% (gating) and from 35 to 89% (MLC tracking). The cassette‐determined beam‐gating latency agreed within ≤$\le$12 ms with the ground truth latency measurement. Film and PSD dose agreed well for most cases (differences relative to TPS dose <$<$4%), while film‐PSD coregistration uncertainty caused relative differences of 5%–8%. Conclusions: This study demonstrates the excellent suitability of a new commercial hybrid film‐scintillator cassette for simultaneous spatial, temporal, and motion‐included dosimetry. [ABSTRACT FROM AUTHOR]

Objective. This study aimed at investigating through Monte Carlo simulations the limitations of a novel hybrid Cerenkov-scintillation detector and the associated method for irradiation angle measurements. Approach. Using Monte Carlo simulations, previous experimental irradiations of the hybrid detector with a linear accelerator were replicated to evaluate its general performances and limitations. Cerenkov angular calibration curves and irradiation angle measurements were then compared. Furthermore, the impact of the Cerenkov light energy dependency on the detector accuracy was investigated using the energy spectra of electrons travelling through the detector. Main results. Monte Carlo simulations were found to be in good agreement with experimental values. The irradiation angle absolute mean error was found to be less than what was obtained experimentally, with a maximum value of 1.12° for the 9 MeV beam. A 0.4% increase of the ratio of electrons having an energy below 1 MeV to the total electrons was found to impact the Cerenkov light intensity collected as a function of the incident angle. The effect of the Cerenkov intensity variation on the measured angle was determined to vary according to the slope of the angular calibration curve. While the contribution of scattered electrons with a lower energy affects the detector accuracy, the greatest discrepancies result from the limitations of the calculation method and the calibration curve itself. Significance. A precise knowledge of the limitations of the hybrid detector and the irradiation angle calculation method is crucial for a clinical implementation. Moreover, the simulations performed in this study also corroborate hypotheses made regarding the relations between multiple Cerenkov dependencies and observations from the experimental measurements. [ABSTRACT FROM AUTHOR]

Within the last three decades, a lot of interest focused on the development of plastic scintillators as new detectors for clinical applications for radiation therapy. During the period of 1990-2000, following the thesis work of Beddar (A.S. Beddar, Dissertation, University of Wisconsin, 1990) [1], most of the studies focused on a further and deeper characterization of these new detectors, including optical light guides and the mechanisms of the light generated in these media. Between 2010-2020, the interest shifted to refining the designs of these detectors for specific application geared toward radiotherapy treatment machine calibrations, small field dosimetry, low and high dose rate brachytherapy and small field dosimetry. During this decade most of these studies were led by two groups, one at the University of Texas MD Anderson Cancer Center (Sam Beddar) and l'Université Laval (Luc Beaulieu) collaborating with each other and carrying the work in their labs with the hope to bring these detectors to the market. During the last decade (2010-2020), the medical physics community witnessed an explosion of interest in the use of plastic scintillators in the field and many groups all over the world started studying and utilizing plastic scintillation detectors (PSDs) for additional applications which included in vivo dosimetry for brachytherapy, external beam radiation therapy, and proton therapy. In this chapter, the authors tried to cover the most important work and phase that PSDs witnessed with an emphasis on the latest advances that occurred in the field of radiation therapy. We also predict that inorganic plastic scintillators (ISDs) will gain interest in the next decade, especially in brachytherapy applications. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study was to characterize a small plastic scintillator developed for high resolution, real‐time dosimetry of therapy and imaging x‐ray beams delivered by an image‐guided small animal irradiator. Materials and Methods: A 1 mm diameter, 1 mm long polystyrene BCF‐60 scintillating fiber dosimeter was characterized with 220 kVp therapy and 40, 50, 60, 70, and 80 kVp imaging beams on the Small Animal Research Platform (SARRP). Scintillator output, sensitivity (charge per unit dose), linearity, and 0.2‐mm resolution beam profile measurements were performed. A validated in‐house Monte Carlo (MC) model of the SARRP was used to compute detailed energy spectra at locations of dosimetry, and validated scintillator measurement with MC simulations. Mass energy‐absorption coefficients from the National Institute of Standards and Technology (NIST) tables convolved with MC‐derived spectra were used in conjunction with Birks ionization quenching factors to correct scintillator output. An air kerma calibration method was employed to correct scintillator output for in‐air beam profile measurements with open, 5 × 5, and 3 × 3 mm2 square field sizes, and compared to MC simulations. Results: Scintillator dose response showed excellent linearity (R2 ≥ 0.999) for all sensitivity measurements, including output as a function of tube current. Detector sensitivity was 2.41 μC Gy−1 for the 220 kVp therapy beam, and it ranged from 1.21 to 1.32 μC Gy−1 for the 40–80 imaging beams. Percentage difference in sensitivity between the therapy and imaging beams before sensitivity correction and after using the Birks quenching factors were 52.3% and 10.2%, respectively. Percentage differences between the therapy and imaging beam sensitivities after using the air kerma calibration method for in‐air measurements was excellent and below 0.3%. In‐air beam profile measurements agreed to MC simulations within a mean difference of 2.4% for the 5 × 5 and 3 × 3 mm2 field sizes, however, the scintillator showed signs of volume averaging at the penumbra edges. Conclusions: A small plastic scintillator was characterized for therapy and imaging energies of a small animal irradiator, with output corrected for using an in‐house MC model of the irradiator. The characterization of the scintillator detector system for small fields presents steps toward implementing real‐time measurements for quality assurance and small animal treatment and imaging dose verification. [ABSTRACT FROM AUTHOR]

Purpose: The goal of this study was to prove the feasibility of using a single-fiber multipoint plastic scintillation detector (mPSD) as an in vivo verification tool during 192Ir high-dose-rate brachytherapy treatments. Methods: A three-point detector was built and inserted inside a catheter-positioning template placed in a water phantom. A hyperspectral approach was implemented to discriminate the different optical signals composing the light output at the exit of the single collection optical fiber. The mPSD was tested with different source-to-detector positions, ranging from 1 to 5 cm radially and over 10.5 cm along the longitudinal axis of the detector, and with various integration times. Several strategies for improving the accuracy of the detector were investigated. The device's accuracy in detecting source position was also tested. Results: Good agreement with the expected doses was obtained for all of the scintillating elements, with average relative differences from the expected values of 3.4 ± 2.1%, 3.0 ± 0.7%, and 4.5 ± 1.0% for scintillating elements from the distal to the proximal. A dose threshold of 3 cGy improved the general accuracy of the detector. An integration time of 3 s offered a good trade-off between precision and temporal resolution. Finally, the mPSD measured the radioactive source positioning uncertainty to be no more than 0.32 ± 0.06 mm. The accuracy and precision of the detector were improved by a dose-weighted function combining the three measurement points and known details about the geometry of the detector construction. Conclusions: The use of a mPSD for high-dose-rate brachytherapy dosimetry is feasible. This detector shows great promise for development of in vivo applications for real-time verification of treatment delivery. [ABSTRACT FROM AUTHOR]

Objective. Plastic scintillation dosimeters (PSDs) are highly suitable for real-time dosimetry on the MR-linac. For optimal performance, the primary signal (scintillation) needs to be separated from secondary optical effects (Cerenkov, fluorescence and optical fiber attenuation). This requires a spectral separation approach and careful calibration. Currently, the 'classic' calibration is a multi-step procedure using both kV and MV x-ray sources, requiring an uninterrupted optical connection between the dosimeter and read-out system, complicating efficient use of PSDs. Therefore, we present a more time-efficient and more practical novel calibration technique for PSDs suitable for MR-linac dosimetry. Approach. The novel calibration relies on prior spectral information combined with two 10 × 10 cm 2 field irradiations on the 1.5 T MR-linac. Performance of the novel calibration technique was evaluated focusing on its reproducibility, performance characteristics (repeatability, linearity, dose rate dependency, output factors, angular response and detector angle dependency) and IMRT deliveries. To investigate the calibration stability over time, prior spectral information up to 315 days old was used. To quantify the time efficiency, each step of the novel and classic calibration was timed. Main results. The novel calibration showed a high reproducibility with a maximum relative standard deviation of 0.2%. The novel method showed maximum differences of 1.2% compared to the gold-standard calibration, while reusing old classic calibrations after reconnecting fibers showed differences up to 3.0%. The novel calibration improved time efficiency from 105 to 30 min compared to the classic method. Significance. The novel calibration method showed a gain in time efficiency and practicality while preserving the dosimetric accuracy. Therefore, this method can replace the traditional method for PSDs suitable for MR-linac dosimetry, using prior spectral information of up to a year. This novel calibration facilitates reconnecting the detector to the read-out system which would lead to unacceptable dosimetric results with the classic calibration method., (Creative Commons Attribution license.)