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As small radiation fields play an ever-increasing role in radiation therapy, accurate dosimetry of these fields becomes critical to ensure high quality dose calculation and treatment optimization. The PRB-0002, a new plastic scintillation detector (PSD) part of the Hyperscint RP-200 dosimetric platform from Medscint, can potentially facilitate small field measurements. In this work our objective is twofold: first we perform a detailed characterization of the field output correction factors of the PRB-0002 over a wide range of field sizes; second we demonstrate how this PSD can be used to determine the field output correction factors for other small field detectors. Uncertainties were carefully considered and a detailed uncertainty budget is provided. EGSnrc Monte Carlo simulations were conducted to determine the impact of detector composition, surrounding materials, dose averaging within the sensitive volume as well as ionization quenching. From these simulations, the field output correction factors of this PSD were determined. Then, by experimental comparisons, field output correction factors for 2 solid state detectors and 3 small volume ion chambers were. With PRB-0002, the material composition was found to be balanced with the ionization quenching making the field output correction factor near unity. For fields between 0.6 $\times$ 0.6 and 30 $\times$ 30~cm$^2$, the field output correction factors of the PRB-0002 were between 1.002 and 0.999 with a total uncertainty of 0.5 %. Analysis of the uncertainty budget showed that an overall uncertainty of 0.59% can be achieved for a 1 $\times$ 1~cm$^2$ using PRB-0002. We conclude that PRB-0002 is a near-ideal detector for small field dosimetry. Furthermore, it can be used to experimentally determine the field output correction factors of other dosimeters with great accuracy.

Hyperspectral unmixing aims at decomposing a given signal into its spectral signatures and its associated fractional abundances. To improve the accuracy of this decomposition, algorithms have included different assumptions depending on the application. The goal of this study is to develop a new unmixing algorithm that can be applied for the calibration of multi-point scintillation dosimeters used in the field of radiation therapy. This new algorithm is based on a non-negative matrix factorization. It incorporates a partial prior knowledge on both the abundances and the endmembers of a given signal. It is shown herein that, following a precise calibration routine, it is possible to use partial prior information about the fractional abundances, as well as on the endmembers, in order to perform a simplified yet precise calibration of these dosimeters. Validation and characterization of this algorithm is made using both simulations and experiments. The experimental validation shows an improvement in accuracy compared to previous algorithms with a mean spectral angle distance (SAD) on the estimated endmembers of 0.0766, leading to an average error of $(0.25 \pm 0.73)$ % on dose measurements., Comment: 12 pages, 6 figures

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 lesser than what was obtained experimentally, with a maximum value of 1.12 degree 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, 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 corroborates hypotheses made regarding the relations between multiple Cerenkov dependencies and observations from the experimental measurements.

Due to its angular dependency, Cerenkov light has long been considered a contamination signal in plastic scintillating dosimeters. In this study, we propose a novel approach designed to take advantage of this angular dependency to perform a direct measurement of an external beam radiation angle of incidence. A Cerenkov probe composed of a 10-mm long filtered sensitive volume of clear PMMA optical fibre was built. Both filtered and raw Cerenkov signals from the transport fibre were collected through a single 1-mm diameter transport fibre. An independent PSD composed of 10-mm BCF12 scintillating fibre was also used for simultaneous dose measurements. A first series of measurements aimed at validating the ability to account for the Cerenkov electron energy spectrum dependency by simultaneously measuring the deposited dose. A cylindrical phantom was then used to obtain an angular calibration curve for fixed dose irradiations and perform incident angle measurements using electron and photon beams. The beam nominal energy was found to have a significant impact on the shapes of the angular calibration curves obtained for irradiation angle measurements. This can be linked to the electron energy spectrum dependency of the Cerenkov emission cone. Irradiation angle measurements exhibit an absolute mean error of 1.86{\deg} and 1.02{\deg} at 6 and 18 MV, respectively. Similar results were obtained with electron beams and the absolute mean error reaches 1.97, 1.66, 1.45 and 0.95 degrees at 9, 12, 16 and 20 MeV, respectively. Reducing the numerical aperture of the Cerenkov probe leads to an increased angular dependency for the lowest energy while no major changes were observed at higher energy. This allowed irradiation angle measurements at 6 MeV with a mean absolute error of 4.82{\deg}. The detector offers promising perspectives for external beam radiotherapy and brachytherapy applications.

This study introduces a novel hybrid Cerenkov-scintillation dosimeter which is intended to be used for irradiation angle measurements based on the Cerenkov angular dependency. First measurements aimed at validating the ability to account for the Cerenkov electron energy spectrum dependency by simultaneously measuring the deposited dose, thus isolating signal variations resulting from the angular dependency. The Cerenkov probe is composed of a 10-mm long sensitive volume of clear PMMA optical fiber separated by an absorptive filter from a 1-mm diameter transport fiber. Filtered and raw Cerenkov signals from the sensitive volume and transport fiber, respectively, were collected using the Hyperscint RP-200 scintillation dosimetry platform. The total signal was unmixed using a hyperspectral approach. Dose calibration of the detector signal was accomplished with photon and electron beams. Using a solid-water phantom, measurements at fixed incident angles covering a wide range of doses and output factors were realized. For fixed incident angle, signal characterization of the Cerenkov detector displays a linear dose-light relationship. As expected, the sensitive volume signal was found to be energy dependent. Output factors were accurately measured within 0.8 % for field size up to 25 cm x 25 cm with both photons and electrons. First validation of the Cerenkov angular dependency shows a linear dose-light relationship for the whole range of angles tested. As expected, the Cerenkov signal intensity per dose unit varies based on the irradiation angle due to the angular dependency. Results showed that using calibration conditions where the electron energy spectrum is similar to the measurement conditions allows to rely on deposited dose to account for this dependency. These preliminary results constitute a first step toward experimental irradiation angle measurements., Comment: The two-part study was merged in a single paper as requested by the editor of the journal. Findings of both papers can now be found in the updated version of the second part "External beam irradiation angle measurement using Cerenkov emission"

This study introduces the HYPERSCINT research platform, the first commercially available scintillation dosimetry platform capable of multi-point dosimetry through the hyperspectral approach. Optic and dosimetric performances of the system were investigated through comparison with another commercially available solution, the Ocean Optics QE65Pro spectrometer. The optical characterization was accomplished by measuring the linearity of the signal as a function of integration time, photon detection efficiency and spectral resolution for both systems. Dosimetric performances were then evaluated with a 3-point plastic scintillator detector (mPSD) in terms of signal to noise ratio (SNR) and signal to background ratio (SBR) associated with each scintillator. The latter were subsequently compared with those found in the literature for the Exradin W1, a single-point plastic scintillator detector. Finally, various beam measurements were realized with the HYPERSCINT platform to evaluate its ability to perform clinical photon beam dosimetry. Both systems were found to be comparable in terms of linearity of the signal as a function of the intensity. Although the QE65Pro possesses a higher spectral resolution, the detection efficency of the HYPERSCINT is up to 1000 time greater. Dosimetric measurements shows that the latter also offers a better SNR and SBR, surpassing even the SNR of the Exradin W1 single-point PSD. While the doses ranging from 1 cGy to 600 cGy were accurately measured within 2.1% of the predicted dose using the HYPERSCINT platform coupled to the mPSD, the Ocean optics spectrometer shows discrepencies up to 86% under 50cGy. Similarly, depth dose, full width at half maximum region of the beam profile and output factors were all accurately measured within 2.3% of the predicted dose using the HYPERSCINT platform and exhibit an average difference of 0.5%, 1.6% and 0.6%, respectively.