ON-LINE LUMINESCENT DOSIMETRY OF PRODUCT PROCESSING MODE AT AN ELECTRON LINAC

Radiation-technological processes on the basis of electron accelerators, in particular, sterilization of medical de-vices, are regulated by international standards. The distribution of the electron flux density and absorbed dose are the main parameters of such processes, that require constant monitoring. In this work, we investigated the possibil-ity to determine in real time the absorbed dose profile on the surface of an object processed using the effect of ca-thodoluminescence, that occurs when the electron flux acts on technical materials (mainly amorphous dielectrics). The results of calibration of the cathodoluminescent radiators of different composition against the absorbed dose measured by a calorimetric method are presented. The study of the novel technique at an LU-10 electron Linac of NSC KIPT made it possible to identify the sources of uncertainty and evaluate their contribution to the dose meas-urement.

Dosimetric characteristics of a thin bolus made with real time variable shape tungsten rubber for photon radiotherapy

In this study, we aim to clarify the dosimetric characteristics of a real time variable shape rubber containing tungsten (STR) as a thin bolus in 6-MV photon radiotherapy. The percentage depth doses (PDDs) and lateral dose profiles (irradiation field = 10 × 10 cm2) in the water-equivalent phantom were measured and compared between no bolus, a conventional 5-mm gel bolus, and 1-, 2-, and 3-mm STR boluses. The characteristics of the PDDs were evaluated according to relative doses at 1 mm depth (D1mm) and depth of maximum dose (dmax). The water-equivalent thicknesses of the STR boluses were determined by shifting the distance of the PDD’s build-up curve until it overlaid that with no bolus. The penumbra size and width of the 50% dose were evaluated using lateral dose profiles. The D1mm with no bolus, 5-mm gel bolus, and 1-, 2-, and 3-mm STR boluses were 47.6%, 91.5%, 86.6%, 89.3%, and 89.4%, respectively, and the respective dmax values were 15, 10, 12, 11, and 10 mm. The water-equivalent thicknesses of the 1-, 2-, and 3-mm STR boluses were 4.4, 4.9, and 5.1 mm, respectively. There were no differences for those in lateral dose profiles. The 1-mm-thick STR thin bolus shifted the depth dose profile by 4.4 mm and could be used as a customized bolus for photon radiotherapy.

Development of a time-resolved mirrorless scintillation detector

Purpose We developed a compact and lightweight time-resolved mirrorless scintillation detector (TRMLSD) employing image processing techniques and a convolutional neural network (CNN) for high-resolution two-dimensional (2D) dosimetry. Methods The TRMLSD comprises a camera and an inorganic scintillator plate without a mirror. The camera was installed at a certain angle from the horizontal plane to collect scintillation from the scintillator plate. The geometric distortion due to the absence of a mirror and camera lens was corrected using a projective transform. Variations in brightness due to the distance between the image sensor and each point on the scintillator plate and the inhomogeneity of the material constituting the scintillator were corrected using a 20.0 × 20.0 cm2 radiation field. Hot pixels were removed using a frame-based noise-reduction technique. Finally, a CNN-based 2D dose distribution deconvolution model was applied to compensate for the dose error in the penumbra region and a lack of backscatter. The linearity, reproducibility, dose rate dependency, and dose profile were tested for a 6 MV X-ray beam to verify dosimeter characteristics. Gamma analysis was performed for two simple and 10 clinical intensity-modulated radiation therapy (IMRT) plans. Results The dose linearity with brightness ranging from 0.0 cGy to 200.0 cGy was 0.9998 (R-squared value), and the root-mean-square error value was 1.010. For five consecutive measurements, the reproducibility was within 3% error, and the dose rate dependency was within 1%. The depth dose distribution and lateral dose profile coincided with the ionization chamber data with a 1% mean error. In 2D dosimetry for IMRT plans, the mean gamma passing rates with a 3%/3 mm gamma criterion for the two simple and ten clinical IMRT plans were 96.77% and 95.75%, respectively. Conclusion The verified accuracy and time-resolved characteristics of the dosimeter may be useful for the quality assurance of machines and patient-specific quality assurance for clinical step-and-shoot IMRT plans.

Simultaneous Measurements of Dose and Microdosimetric Spectra in a Clinical Proton Beam Using a scCVD Diamond Membrane Microdosimeter

A single crystal chemical vapor deposition (scCVD) diamond membrane-based microdosimetric system was used to perform simultaneous measurements of dose profile and microdosimetric spectra with the Y1 proton passive scattering beamline of the Center of Proton Therapy, Institute Curie in Orsay, France. To qualify the performance of the set-up in clinical conditions of hadrontherapy, the dose, dose rate and energy loss pulse-height spectra in a diamond microdosimeter were recorded at multiple points along depth of a water-equivalent plastic phantom. The dose-mean lineal energy (y¯D) values were computed from experimental data and compared to silicon on insulator (SOI) microdosimeter literature results. In addition, the measured dose profile, pulse height spectra and y¯D values were benchmarked with a numerical simulation using TOPAS and Geant4 toolkits. These first clinical tests of a novel system confirm that diamond is a promising candidate for a tissue equivalent, radiation hard, high spatial resolution microdosimeter in beam quality assurance of proton therapy.