D Poppinga

A new correction method serving to eliminate the parabola effect of flatbed scanners used in radiochromic film dosimetry

PURPOSE The purpose of this study is the correction of the lateral scanner artifact, i.e., the effect that, on a large homogeneously exposed EBT3 film, a flatbed scanner measures different optical densities at different positions along the x axis, the axis parallel to the elongated light source. At constant dose, the measured optical density profiles along this axis have a parabolic shape with significant dose dependent curvature. Therefore, the effect is shortly called the parabola effect. The objective of the algorithm developed in this study is to correct for the parabola effect. Any optical density measured at given position x is transformed into the equivalent optical density c at the apex of the parabola and then converted into the corresponding dose via the calibration of c versus dose. METHODS For the present study EBT3 films and an Epson 10000XL scanner including transparency unit were used for the analysis of the parabola effect. The films were irradiated with 6 MV photons from an Elekta Synergy accelerator in a RW3 slab phantom. In order to quantify the effect, ten film pieces with doses graded from 0 to 20.9 Gy were sequentially scanned at eight positions along the x axis and at six positions along the z axis (the movement direction of the light source) both for the portrait and landscape film orientations. In order to test the effectiveness of the new correction algorithm, the dose profiles of an open square field and an IMRT plan were measured by EBT3 films and compared with ionization chamber and ionization chamber array measurement. RESULTS The parabola effect has been numerically studied over the whole measuring field of the Epson 10000XL scanner for doses up to 20.9 Gy and for both film orientations. The presented algorithm transforms any optical density at position x into the equivalent optical density that would be measured at the same dose at the apex of the parabola. This correction method has been validated up to doses of 5.2 Gy all over the scanner bed with 2D dose distributions of an open square photon field and an IMRT distribution. CONCLUSIONS The algorithm presented in this study quantifies and corrects the parabola effect of EBT3 films scanned in commonly used commercial flatbed scanners at doses up to 5.2 Gy. It is easy to implement, and no additional work steps are necessary in daily routine film dosimetry.

Experimental determination of the lateral dose response functions of detectors to be applied in the measurement of narrow photon-beam dose profiles.

This study aims at the experimental determination of the detector-specific 1D lateral dose response function K(x) and of its associated rotational symmetric counterpart K(r) for a set of high-resolution detectors presently used in narrow-beam photon dosimetry. A combination of slit-beam, radiochromic film, and deconvolution techniques served to accomplish this task for four detectors with diameters of their sensitive volumes ranging from 1 to 2.2 mm. The particular aim of the experiment was to examine the existence of significant negative portions of some of these response functions predicted by a recent Monte-Carlo-simulation (Looe et al 2015 Phys. Med. Biol. 60 6585-607). In a 6 MV photon slit beam formed by the Siemens Artiste collimation system and a 0.5 mm wide slit between 10 cm thick lead blocks serving as the tertiary collimator, the true cross-beam dose profile D(x) at 3 cm depth in a large water phantom was measured with radiochromic film EBT3, and the detector-affected cross-beam signal profiles M(x) were recorded with a silicon diode, a synthetic diamond detector, a miniaturized scintillation detector, and a small ionization chamber. For each detector, the deconvolution of the convolution integral M(x)  =  K(x)  ∗  D(x) served to obtain its specific 1D lateral dose response function K(x), and K(r) was calculated from it. Fourier transformations and back transformations were performed using function approximations by weighted sums of Gaussian functions and their analytical transformation. The 1D lateral dose response functions K(x) of the four types of detectors and their associated rotational symmetric counterparts K(r) were obtained. Significant negative curve portions of K(x) and K(r) were observed in the case of the silicon diode and the diamond detector, confirming the Monte-Carlo-based prediction (Looe et al 2015 Phys. Med. Biol. 60 6585-607). They are typical for the perturbation of the secondary electron field by a detector with enhanced electron density compared with the surrounding water. In the cases of the scintillation detector and the small ionization chamber, the negative curve portions of K(x) practically vanish. It is planned to use the measured functions K(x) and K(r) to deconvolve clinical narrow-beam signal profiles and to correct the output factor values obtained with various high-resolution detectors.

Magnetic field influences on the lateral dose response functions of photon-beam detectors: MC study of wall-less water-filled detectors with various densities

The distortion of detector reading profiles across photon beams in the presence of magnetic fields is a developing subject of clinical photon-beam dosimetry. The underlying modification by the Lorentz force of a detectors lateral dose response function-the convolution kernel transforming the true cross-beam dose profile in water into the detector reading profile-is here studied for the first time. The three basic convolution kernels, the photon fluence response function, the dose deposition kernel, and the lateral dose response function, of wall-less cylindrical detectors filled with water of low, normal and enhanced density are shown by Monte Carlo simulation to be distorted in the prevailing direction of the Lorentz force. The asymmetric shape changes of these convolution kernels in a water medium and in magnetic fields of up to 1.5 T are confined to the lower millimetre range, and they depend on the photon beam quality, the magnetic flux density and the detectors density. The impact of this distortion on detector reading profiles is demonstrated using a narrow photon beam profile. For clinical applications it appears as favourable that the magnetic flux density dependent distortion of the lateral dose response function, as far as secondary electron transport is concerned, vanishes in the case of water-equivalent detectors of normal water density. By means of secondary electron history backtracing, the spatial distribution of the photon interactions giving rise either directly to secondary electrons or to scattered photons further downstream producing secondary electrons which contribute to the detectors signal, and their lateral shift due to the Lorentz force is elucidated. Electron history backtracing also serves to illustrate the correct treatment of the influences of the Lorentz force in the EGSnrc Monte Carlo code applied in this study.

SU‐E‐I‐87: Experimental Study of Anisotropic Light Scattering and Polarization Effects of EBT3‐Films

PURPOSE Further experiments are needed to understand the underlying optical properties of flat-bed scanned EBT3-films. METHODS EBT3-films, arranged in landscape orientation and irradiated with different doses, were illuminated with a homogeneous spot of unpolarized white light. Polarizer foils could be added in front of and behind the EBT3-film. The light scattered by the film was collected by a plano-convex lens and focused onto a diffusing glass plate placed at focal length distance. Thereby, the scattering angle was transformed into an offset from the optical axis, forming a characteristic corona. This image was digitized with a DSLR-camera and the red color channel used for analysis. The effect of incident light polarization was investigated by stepwise rotating the electrical vector of a polarizer in front of the film, with 0° parallel to the preferred direction of the polymer fibres. The polarization of the scattered light was investigated by a second polarizer behind the lens. RESULTS With an unpolarized light source, anisotropic scattering is preferently propagated orthogonal to the direction of the active polymers, for both landscape and portrait orientation, and the amount of scattered light increases with dose. Scattering by the EBT3-film varies from almost zero to a maximum when the electrical vector of the light source varies from parallel to orthogonal with the direction of the active polymers. The electrical vector of the scattered light is rotated by 90° compared with the incident light. CONCLUSION By experimental separation between scattered and unscattered transmitted light it was proved that incident light polarization only affects the scattered component. This effect hints upon a role of the magnetic component in the excitation of anisotropic scattering. The effect may be utilized to enhance or suppress scattering influences in EBT-3 film dosimetry.

Determination of the active volumes of solid‐state photon‐beam dosimetry detectors using the PTB proton microbeam

Purpose This study aims at the experimental determination of the diameters and thicknesses of the active volumes of solid‐state photon‐beam detectors for clinical dosimetry. The 10 MeV proton microbeam of the PTB (Physikalisch‐Technische Bundesanstalt, Braunschweig) was used to examine two synthetic diamond detectors, type microDiamond (PTW Freiburg, Germany), and the silicon detectors Diode E (PTW Freiburg, Germany) and Razor Diode (Iba Dosimetry, Germany). The knowledge of the dimensions of their active volumes is essential for their Monte Carlo simulation and their applications in small‐field photon‐beam dosimetry. Methods The diameter of the active detector volume was determined from the detector current profile recorded by radially scanning the proton microbeam across the detector. The thickness of the active detector volume was determined from the detectors electrical current, the number of protons incident per time interval and their mean stopping power in the active volume. The mean energy of the protons entering this volume was assessed by comparing the measured and the simulated influence of the thickness of a stack of aluminum preabsorber foils on the detector signal. Results For all detector types investigated, the diameters measured for the active volume closely agreed with the manufacturers’ data. For the silicon Diode E detector, the thickness determined for the active volume agreed with the manufacturers data, while for the microDiamond detectors and the Razor Diode, the thicknesses measured slightly exceeded those stated by the manufacturers. Discussion The PTB microbeam facility was used to analyze the diameters and thicknesses of the active volumes of photon dosimetry detectors for the first time. A new method of determining the thickness values with an uncertainty of ±10% was applied. The results appear useful for further consolidating detailed geometrical knowledge of the solid‐state detectors investigated, which are used in clinical small‐field photon‐beam dosimetry.

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.

Test study of boron nitride as a new detector material for dosimetry in high-energy photon beams

The aim of this test study is to check whether boron nitride (BN) might be applied as a detector material in high-energy photon-beam dosimetry. Boron nitride exists in various crystalline forms. Hexagonal boron nitride (h-BN) possesses high mobility of the electrons and holes as well as a high volume resistivity, so that ionizing radiation in the clinical range of the dose rate can be expected to produce a measurable electrical current at low background current. Due to the low atomic numbers of its constituents, its density (2.0 g cm-3) similar to silicon and its commercial availability, h-BN appears as possibly suitable for the dosimetry of ionizing radiation. Five h-BN plates were contacted to triaxial cables, and the detector current was measured in a solid-state ionization chamber circuit at an applied voltage of 50 V. Basic dosimetric properties such as formation by pre-irradiation, sensitivity, reproducibility, linearity and temporal resolution were measured with 6 MV photon irradiation. Depth dose curves at quadratic field sizes of 10 cm and 40 cm were measured and compared to ionization chamber measurements. After a pre-irradiation with 6 Gy, the devices show a stable current signal at a given dose rate. The current-voltage characteristic up to 400 V shows an increase in the collection efficiency with the voltage. The time-resolved detector current behavior during beam interrupts is comparable to diamond material, and the background current is negligible. The measured percentage depth dose curves at 10 cm  ×  10 cm field size agreed with the results of ionization chamber measurements within  ±2%. This is a first study of boron nitride as a detector material for high-energy photon radiation. By current measurements on solid ionization chambers made from boron nitride chips we could demonstrate that boron nitride is in principle suitable as a detector material for high-energy photon-beam dosimetry.

SU-E-T-176: Examination of Surface Dose Enhancement Using Radiochromic EBT3-Films in a Cylindrical Setup

PURPOSE This study was undertaken to optimize the measurement techniques with radiochromic EBT3 films to offer accurate surface dose measurements and at the same time high resolution depth dose curves of the backscattering from high Z materials. METHODS Radiochromic EBT3 films (Ashland ISP, Wayne, USA) were wrapped around a PET hollow cylinder with a diameter of 41.5 mm and fixed upon the surface of a lead block. The setup was immersed in water and exposed to a dose of 2 Gy at 6MV acceleration voltage using a Siemens Primus linear accelerator. Water reference measurements were undertaken under equal conditions. An Epson Expression 10000 XL flatbed scanner (Epson, Suwa, Japan) with a preset resolution of 72 dpi was used for digitization. RESULTS The dose enhancement could be measured with a high resolution of measurement points along the axis normal to the lead surface. A dose enhancement of 70 % was measured at a distance of 134 μm from the lead surface. The data has been compared with results presented by Das et al (Med. Phys. 16(3) (1989)) and is consistent within the uncertainty of the measurements. The results are in consistence with the results from time-tested EBT3 setups, i.e. normal-to-beam EBT3 film stacks and parallel to beam EBT3-films. CONCLUSION The cylindrical film setup offers a powerful tool for surface measurements. The advantages of a stacked film setup and a parallel to beam setup could be combined.

SU-E-T-44: Angular Dependence of Surface Dose Enhancement Measured On Several Inhomogeneities Using Radiochromic EBT3 Films

PURPOSE The quantification of the relative surface dose enhancement in dependence on the angle of incidence and the atomic number Z of the surface material. METHODS Experiments were performed with slabs made of aluminum, titanium, copper, silver, dental gold and lead. The metal slabs with equal sizes of 1.0×8.0×8.8mm3 were embedded in an Octavius 4D phantom (PTW Freiburg, Germany). Radiochromic EBT3 films were used to measure the surface dose for angles of incidence ranging from 0° to 90°. The setup with the metals slabs at the isocenter was irradiated with acceleration voltages of 6MV and 10MV. Water reference measurements were taken under equal conditions. RESULTS The surface dose enhancement is highest for angles of incidence below 30° and drops significantly for higher. The surface dose enhancement produced by lead and dental gold at 6MV showed a peak of 65%. At 90°, the surface dose enhancement dropped to 15% for both materials. The surface dose enhancements for silver, copper, titanium and aluminum were 45%, 32%, 22% and 12% at 0°, respectively. At an angle of incidence of 80°, the values dropped to 22%, 18%, 12% und 6%. The values for 10MV were very similar. Lead and dental gold showed peaks of 65% und 60%. Their values dropped to 18% at an angle of 90°. The surface dose enhancements for silver, copper, titanium and aluminum were 45%, 30%, 20% and 8% at 0°. At 80° the values dropped to 30%, 20%, 12% and 5%. A dependence of the magnitude of the surface dose enhancement on the atomic number of the surface material can be seen, which is in consistence with literature. CONCLUSION The results show that the surface dose enhancements near implant materials with high Z-values should be taken into consideration in radio therapy, even when the angle of incidence is flat.

SU-E-T-228: Liquid Ionisation Chamber Array and MicroDiamond Measurements with the CyberKnife System

PURPOSE The aim of this study was to measure the dose profile and output factors with a CyberKnife accelerator using a TM60019 microDiamond detector and a 1000SRS liquid chamber array (both PTW Freiburg, Germany). METHODS An MP3 water phantom (PTW, Freiburg) was positioned along the robotic world coordinate system. The TM60019 detector was adjusted to the center of the according fields and the semiconductor axis was aligned with the beam direction. Profiles at 5cm water depth and SSD = 80 cm were measured along the robotic x axis and y axis for the cylindrical collimators of the CyberKnife (diameter 60, 50, 40, 30, 20, 15, 12.5, 10, 7.5 and 5mm). To determine the output factors the dose profile was measured at 0.1 mm steps around the field center to find the maximum dose value. The liquid chamber array (1000SRS) measurement was performed with the same setup, but with RW3 buildup. RESULTS The 1000SRS measurements closely conform with the TM60019 profile measurement in all profile regions and for all collimator sizes. The profile measurement is influenced by the almost equal spatial resolution of the TM60019 detector (radius of the sensitive area 1.1mm) and of the 1000SRS liquid chamber array (single chamber width 2.3mm). The measured dose profiles have not been corrected for this limited spatial resolution. Rather we purpose to consider that spatial dose averaging over 2 mm wide regions might be justified in view of patient positioning inaccuracies and of the spaces in tissue participating in the biological radiation responses. CONCLUSION The 1000SRS data points conform with the TM60019 profile measurements at all profile regions showing the applicability of liquid ion chamber arrays with the CyberKnife system.