Comparative optic and dosimetric characterization of the HYPERSCINT scintillation dosimetry research platform for multipoint applications.

This study introduces the HYPERSCINT research platform (HYPERSCINT-RP100, Medscint Inc., Quebec, Canada), 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 under the same conditions. 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 efficiency 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 doses ranging from 1 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 discrepancies 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.

Experimental and Monte-Carlo characterization of the novel compact ionization chamber PTW 31023 for reference and relative dosimetry in high energy photon beams.

INTRODUCTION: The aim of the present work is to perform dosimetric characterization of a novel vented PinPoint ionization chamber (PTW 31023, PTW-Freiburg, Germany). This chamber replaces the previous model (PTW 31014), where the diameter of the central electrode has been increased from 0.3 to 0.6mm and the guard ring has been redesigned. Correction factors for reference and non-reference measurement conditions were examined. MATERIALS AND METHODS: Measurements and calculations of the correction factors were performed according to the DIN 6800-2. The shifts of the effective point of measurement (EPOM) from the chamber's reference point were determined by comparison of the measured PDD with the reference curve obtained with a Roos chamber. Its lateral dose response functions, which act according to a mathematical convolution model as the convolution kernel transforming the dose profile D(x) to the measured signal M(x), have been approximated by Gaussian functions with standard deviation σ. Additionally, the saturation correction factors kS have been determined using different dose-per-pulse (DPP) values. The polarity effect correction factors kP were measured for field sizes from 5cm×5cm to 40cm×40cm. The influence of the diameter of the central electrode and the new guard ring on the beam quality correction factors kQ was studied by Monte-Carlo simulations. The non-reference condition correction factors kNR have been computed for 6MV photo beam by varying the field size and measurement depth. Comparisons on these aspects have been made to the previous model. RESULTS: The shifts of the EPOM from the reference point, Δz, are found to be -0.55 (6MV) and -0.56 (10MV) in the radial orientation and -0.97mm (6MV) and -0.91mm (10MV) in the axial orientation. All values of Δz have an uncertainty of 0.1mm. The σ values are 0.80mm (axial), 0.75mm (radial lateral) and 1.76mm (radial longitudinal) for 6MV photon beam and are 0.85mm (axial), 0.75mm (radial lateral) and 1.82mm (radial longitudinal) for 15MV photon beam. All σ values have an uncertainty of 0.05mm. The correction factor kS was found to be 1.0034±0.0009 for the PTW 31014 chamber and 1.0024±0.0007 for the PTW 31023 chamber at the highest DPP (0.827mGy) investigated in this study. Under reference conditions, the polarity effect correction factor kP of the PTW 31014 chamber is 1.0094 and 1.0116 for 6 and 10MV respectively, while the kP of the PTW 31023 chamber is 1.0005 and 1.0013 for 6 and 10MV respectively, all values have an uncertainty of 0.002. The kP of the new chamber also exhibits a weaker field size dependence. The kQ values of the PTW 31023 chamber are closer to unity than those of the PTW 31014 chamber due to the thicker central electrode and the new guard ring design. The kNR values of the PTW 31023 chamber for 6MV photon beam deviate by not more than 1% from unity for the conditions investigated. DISCUSSIONS: Correction factors associated with the new chamber required to perform reference and relative dose measurements have been determined according to the DIN-protocol. The correction factor kS of the new chamber is 0.1% smaller than that of the PTW 31014 at the highest DPP investigated. Under reference conditions, the correction factor kP of the PTW 31023 chamber is approximately 1% smaller than that of the PTW 31014 chamber for both energies used. The dosimetric characteristics of the new chamber investigated in this work have been demonstrated to fulfil the requirements of the TG-51 addendum for reference-class dosimeters at reference conditions.

A new reference-type ionization chamber with direction-independent response for use in small-field photon-beam dosimetry - An experimental and Monte Carlo study.

The frequently applied narrow and non-standard transverse dose profiles of intensity modulated photon-beam radiotherapy, lacking lateral secondary electron equilibrium, require the use of high-resolution dosimetry detectors, and small air-filled detectors are recommended as the reference detectors for cross-calibration of the high-resolution detectors. The present study focuses on the dosimetric properties of a novel cylindrical ionization chamber, the PTW Semiflex 3D 31021. The chamber's effective point of measurement was found to lie at (0.41±0.04) r downstream the tip of the inner surface of the spherical front wall in the axial orientation and (0.46±0.04) r upstream the chamber axis in the radial orientation. Due to its symmetrical design, the sigma values of its lateral dose response functions for all chamber's orientations are the same (2.10±0.05mm). The polarity correction factors obtained in this work do not exceed 0.1% and the saturation correction factor was below 1% up to a dose-per-pulse value of 0.956mGy. The radiation quality correction factor kQ of the chamber as a function of the tissue-phantom-ratio, TPR20,10, has been calculated by Monte Carlo simulation and has been determined experimentally at the German Metrology Institute (Physikalisch-Technische Bundesanstalt, PTB). The values of the non-reference condition correction factor kNR have been Monte-Carlo-calculated for use of the chamber at various depths and field sizes.

Ionizing radiation-induced acoustics for radiotherapy and diagnostic radiology applications.

Acoustic waves are induced via the thermoacoustic effect in objects exposed to a pulsed beam of ionizing radiation. This phenomenon has interesting potential applications in both radiotherapy dosimetry and treatment guidance as well as low-dose radiological imaging. After initial work in the field in the 1980s and early 1990s, little research was done until 2013 when interest was rejuvenated, spurred on by technological advances in ultrasound transducers and the increasing complexity of radiotherapy delivery systems. Since then, many studies have been conducted and published applying ionizing radiation-induced acoustic principles into three primary research areas: Linear accelerator photon beam dosimetry, proton therapy range verification, and radiological imaging. This review article introduces the theoretical background behind ionizing radiation-induced acoustic waves, summarizes recent advances in the field, and provides an outlook on how the detection of ionizing radiation-induced acoustic waves can be used for relative and in vivo dosimetry in photon therapy, localization of the Bragg peak in proton therapy, and as a low-dose medical imaging modality. Future prospects and challenges for the clinical implementation of these techniques are discussed.

The output factor correction as function of the photon beam field size - direct measurement and calculation from the lateral dose response functions of gas-filled and solid detectors.

The first aim of this study has been to extend the systematic experimental study of the field size dependence of the output factor correction for three micro-ionization chambers (PTW 31014, PTW 31022 and IBA Razor chamber), two silicon diodes (PTW 60017 and IBA Razor Diode) and the synthetic diamond detector microDiamond (PTW 60019) in a 6 MV photon beam down to an effective field side length of 2.6mm, and to summarize the present knowledge of this factor by treating it as a function of the dosimetric field size. In order to vary the dosimetric field size over this large range, output factors measurements were performed at source-to-surface distances of 60cm and 90cm. Since the output factors obtained with the organic scintillation detector Exradin W1 (Standard Imaging, Middleton, USA) at all field sizes closely agreed with those measured by EBT3 radiochromic films (ISP Corp, Wayne, USA), the scintillation detector served as the reference detector. The measured output correction factors reflect the influences of the volume averaging and density effects upon the uncorrected output factor values. In case of the microDiamond detector these opposing influences result in output factor correction values less than 1 for moderately small field sizes and larger than 1 for very small field sizes. Our results agree with most of the published experimental as well as Monte-Carlo simulated data within detector-specific limits of uncertainty. The dosimetric field side length has been identified as a reliable determinant of the output factor correction, and typical functional curve shapes of the field-size dependent output factor correction vs. dosimetric field side length have been associated with gas-filled, silicon diode and synthetic diamond detectors. The second aim of this study has been a novel, semi-empirical approach to calculate the field-size dependent output correction factors of small photon detectors by convolving film measured true dose profile data with measured lateral response functions of the detectors. To achieve this, the set of previously published 2D lateral dose response functions was complemented by those of the novel detectors PTW PinPoint chamber 31022 (PTW Freiburg, Freiburg, Germany), Razor chamber and Razor Diode (IBA Dosimetry, Schwarzenbruck, Germany). The output correction factors calculated from the lateral dose response functions closely fit with the directly measured output correction factors, thus supporting the latter by an independent method.