Silica-based scintillators: basic properties of radioluminescence kinetics.

Radioluminescent silica-based fiber dosimeters offer great advantages for designing miniaturized realtime sensors for high dose-rate dosimetry. Rise and fall kinetics of their response must be properly understood to better assess their performances in terms of measurement speed and repeatability. A standard model of radioluminescence (RL) has already been quantitatively validated for doped silica glasses, but beyond conclusive comparisons with specific experiments, a comprehensive understanding of the processes and parameters determining transient and equilibrium kinetics of RL is still lacking. We analyze in detail the kinetics inherent in the standard RL model. Several asymptotical regimes in the RL growth are demonstrated in the case of a pristine sample (succesive quadratic, linear and power-law time dependencies before the plateau is reached). We show how this situation is modified when a pre-irradiation partly fills traps beforehand. RL growth is then greatly accelerated because of the pre-formation of recombination centers (RCs) from dopant ions, but not due to pre-filling of trapping levels. In all cases, the RL intensity eventually tends to a constant level equal to the pair generation rate, long before all carrier densities themselves reach equilibrium. This occurs late under irradiation, when deep traps get to saturation. The fraction of dopants converted into RCs is then 'frozen' at a lower level the smaller the density of deep traps. Controlling RL kinetics through the engineering of material traps is not an option. Pre-irradiation appears to be the simplest way to obtain accelerated and repeatable kinetics.

A generalized model for monitor units determination in ocular proton therapy using machine learning: A proof-of-concept study.

Objective.Determining and verifying the number of monitor units is crucial to achieving the desired dose distribution in radiotherapy and maintaining treatment efficacy. However, current commercial treatment planning system(s) dedicated to ocular passive eyelines in proton therapy do not provide the number of monitor units for patient-specific plan delivery. Performing specific pre-treatment field measurements, which is time and resource consuming, is usually gold-standard practice. This proof-of-concept study reports on the development of a multi-institutional-based generalized model for monitor units determination in proton therapy for eye melanoma treatments.Approach.To cope with the small number of patients being treated in proton centers, three European institutes participated in this study. Measurements data were collected to address output factor differences across the institutes, especially as function of field size, spread-out Bragg peak modulation width, residual range, and air gap. A generic model for monitor units prediction using a large number of 3748 patients and broad diversity in tumor patterns, was evaluated using six popular machine learning algorithms: (i) decision tree; (ii) random forest, (iii) extra trees, (iv) K-nearest neighbors, (v) gradient boosting, and (vi) the support vector regression. Features used as inputs into each machine learning pipeline were: Spread-out Bragg peak width, range, air gap, fraction and calibration doses. Performance measure was scored using the mean absolute error, which was the difference between predicted and real monitor units, as collected from institutional gold-standard methods.Main results.Predictions across algorithms were accurate within 3% uncertainty for up to 85.2% of the plans and within 10% uncertainty for up to 98.6% of the plans with the extra trees algorithm.Significance.A proof-of-concept of using machine learning-based generic monitor units determination in ocular proton therapy has been demonstrated. This could trigger the development of an independent monitor units calculation tool for clinical use.

Characterization of the HollandPTC proton therapy beamline dedicated to uveal melanoma treatment and an interinstitutional comparison.

PURPOSE: Eye-dedicated proton therapy (PT) facilities are used to treat malignant intraocular lesions, especially uveal melanoma (UM). The first commercial ocular PT beamline from Varian was installed in the Netherlands. In this work, the conceptual design of the new eyeline is presented. In addition, a comprehensive comparison against five PT centers with dedicated ocular beamlines is performed, and the clinical impact of the identified differences is analyzed. MATERIAL/METHODS: The HollandPTC eyeline was characterized. Four centers in Europe and one in the United States joined the study. All centers use a cyclotron for proton beam generation and an eye-dedicated nozzle. Differences among the chosen ocular beamlines were in the design of the nozzle, nominal energy, and energy spectrum. The following parameters were collected for all centers: technical characteristics and a set of distal, proximal, and lateral region measurements. The measurements were performed with detectors available in-house at each institution. The institutions followed the International Atomic Energy Agency (IAEA) Technical Report Series (TRS)-398 Code of Practice for absolute dose measurement, and the IAEA TRS-398 Code of Practice, its modified version or International Commission on Radiation Units and Measurements Report No. 78 for spread-out Bragg peak normalization. Energy spreads of the pristine Bragg peaks were obtained with Monte Carlo simulations using Geant4. Seven tumor-specific case scenarios were simulated to evaluate the clinical impact among centers: small, medium, and large UM, located either anteriorly, at the equator, or posteriorly within the eye. Differences in the depth dose distributions were calculated. RESULTS: A pristine Bragg peak of HollandPTC eyeline corresponded to the constant energy of 75 MeV (maximal range 3.97 g/cm2 in water) with an energy spread of 1.10 MeV. The pristine Bragg peaks for the five participating centers varied from 62.50 to 104.50 MeV with an energy spread variation between 0.10 and 0.70 MeV. Differences in the average distal fall-offs and lateral penumbrae (LPs) (over the complete set of clinically available beam modulations) among all centers were up to 0.25 g/cm2 , and 0.80 mm, respectively. Average distal fall-offs of the HollandPTC eyeline were 0.20 g/cm2 , and LPs were between 1.50 and 2.15 mm from proximal to distal regions, respectively. Treatment time, around 60 s, was comparable among all centers. The virtual source-to-axis distance of 120 cm at HollandPTC was shorter than for the five participating centers (range: 165-350 cm). Simulated depth dose distributions demonstrated the impact of the different beamline characteristics among institutions. The largest difference was observed for a small UM located at the posterior pole, where a proximal dose between two extreme centers was up to 20%. CONCLUSIONS: HollandPTC eyeline specifications are in accordance with five other ocular PT beamlines. Similar clinical concepts can be applied to expect the same high local tumor control. Dosimetrical properties among the six institutions induce most likely differences in ocular radiation-related toxicities. This interinstitutional comparison could support further research on ocular post-PT complications. Finally, the findings reported in this study could be used to define dosimetrical guidelines for ocular PT to unify the concepts among institutions.

Study of the responses and calibration procedures of neutron and gamma area and environmental detectors for use in proton therapy.

Ambient dose equivalent measurements with radiation protection instruments are associated to large uncertainties, mostly due to the energy dependence of the instrument response and to the dissimilarity between the spectra of the standard calibration source and the workplace field. The purpose of this work is to evaluate its impact on the performance of area and environmental detectors in the proton therapy environment, and to provide practical solutions whenever needed and possible. The study was carried out at the Centre Antoine Lacassagne (CAL) proton therapy site, and included a number of commercially available area detectors and a home-made environmental thermoluminescent dosimeter based on a polyethylene moderator loaded with TLD600H/TLD700H pairs. Monte Carlo simulations were performed with MCNP to calculate, first, missing or partially lacking instrument responses, covering the range of energies involved in proton therapy. Second, neutron and gamma spectra were computed at selected positions in and outside the CAL proton therapy bunkers. Appropriate correction factors were then derived for each detector, workplace location and calibration radionuclide source, which amounts to up to 1.9 and 1.5 for neutron and photon area detectors, respectively, and suggest that common ambient dose equivalent instruments might not meet IEC requirements. The TLD environmental system was calibrated in situ and appropriate correction factors were applied to account for the cosmic spectra. Measurements performed with this system from 2014 to 2017 around the installation were consistent with reference natural background dose data and with pre-operational levels registered at the site before the construction of the building in 1988, showing thus no contribution from the site clinical activities. An in situ verification procedure for the radiation protection instruments was also implemented in 2016 at the low energy treatment room using the QA beam reference conditions. The method presents main methodological, practical and economic advantages over external verifications.

Occurrence of Phosphenes in Patients Undergoing Proton Beam Therapy for Ocular Tumor.

PURPOSE: Phosphenes are frequently reported by patients irradiated in the head and neck area. The aim of the present study was to characterize and investigate potential mechanisms of proton beam therapy (PBT)-induced phosphenes in a large population of patients undergoing PBT for ocular tumors. DESIGN: Prospective cohort study. METHODS: Consecutive patients who underwent PBT in a single center were included. Immediately after the first session, all patients completed a questionnaire collecting information about the presence of phosphenes as well as their color, shape, and duration. Patient, tumor and treatment characteristics (dose volume histograms) were also collected. RESULTS: Among the 474 patients included, 62.8% reported phosphenes during the first session of PBT. Reported colors were mainly blue-violet (70.5%) and white (14.1%). The prevalence of phosphenes was higher in younger patients (P = .003); other patient or ocular characteristics were not associated with the occurrence of phosphenes. Irradiation of the macula (P < .001) and/or optic disc (P < .001) were significantly associated with the presence of phosphenes, whereas blue-violet color was only associated with young age and irradiation of macular area (P = .04). Pupillary constriction was reported for 57.1% of patients with phosphenes vs 18.5% of patients without (P < .001). Blue-violet phosphenes (P < .001) and irradiation of macula (P = .001) were statistically associated with pupillary constriction. CONCLUSIONS: The present study reported a high rate of phosphenes in patients irradiated by PBT for ocular tumor. Their blue-violet color and their association with a pupillary constriction probably indicates the stimulation of S-cones and retinal ganglion cells that reflects the activation of the afferent visual pathway.