Ts Stelljes

The dose response functions of ionization chambers in photon dosimetry – Gaussian or non-Gaussian?

This study is concerned with the spatial resolution of air-filled ionization chambers in photon-beam dosimetry, i.e. with their dose response functions. These act as convolution kernels K(x,y), transforming true dose profiles D(x,y) into the measured signal profiles M(x,y). One-dimensional dose response functions have been experimentally determined for nine types of cylindrical ionization chambers both in their lateral and longitudinal directions, as well as across two plane-parallel chambers and for the single chambers of two 2D arrays. All these 1D dose response functions are closely described by Gaussian functions. The associated energy-dependent values of the standard deviations σ have been measured for 6 and 15 MV photons with an uncertainty of 0.02mm. At depths beyond secondary electron fluence build-up, there was no detectable depth dependence of the σ values. The general occurrence of Gaussian dose response functions, their extension beyond the geometrical boundaries of the chambers, and the energy dependence of their standard deviations can be understood by considering the underlying system of convolutions, which is the origin of the influences of secondary electron transport. Monte-Carlo simulations of the convolution kernels for a cylindrical, a square, and a flat ionization chamber and their Fourier analysis have been employed to show that the Gaussian convolution kernels are approximations to the true dose response functions, valid in the clinically relevant domain of the spatial frequency. This paper is conceived as the starting point for the deconvolution methods to be described in a further publication.

Performance parameters of a liquid filled ionization chamber array

PURPOSE In this work, the properties of the two-dimensional liquid filled ionization chamber array Octavius 1000SRS (PTW-Freiburg, Germany) for use in clinical photon-beam dosimetry are investigated. METHODS Measurements were carried out at an Elekta Synergy and Siemens Primus accelerator. For measurements of stability, linearity, and saturation effects of the 1000SRS array a Semiflex 31013 ionization chamber (PTW-Freiburg, Germany) was used as a reference. The effective point of measurement was determined by TPR measurements of the array in comparison with a Roos chamber (type 31004, PTW-Freiburg, Germany). The response of the array with varying field size and depth of measurement was evaluated using a Semiflex 31010 ionization chamber as a reference. Output factor measurements were carried out with a Semiflex 31010 ionization chamber, a diode (type 60012, PTW-Freiburg, Germany), and the detector array under investigation. The dose response function for a single detector of the array was determined by measuring 1 cm wide slit-beam dose profiles and comparing them against diode-measured profiles. Theoretical aspects of the low pass properties and of the sampling frequency of the detector array were evaluated. Dose profiles measured with the array and the diode detector were compared, and an intensity modulated radiation therapy (IMRT) field was verified using the Gamma-Index method and the visualization of line dose profiles. RESULTS The array showed a short and long term stability better than 0.1% and 0.2%, respectively. Fluctuations in linearity were found to be within ±0.2% for the vendor specified dose range. Saturation effects were found to be similar to those reported in other studies for liquid-filled ionization chambers. The detectors relative response varied with field size and depth of measurement, showing a small energy dependence accounting for maximum signal deviations of ±2.6% from the reference condition for the setup used. The σ-values of the Gaussian dose response function for a single detector of the array were found to be (0.72±0.25) mm at 6 MV and (0.74±0.25) mm at 15 MV and the corresponding low pass cutoff frequencies are 0.22 and 0.21 mm(-1), respectively. For the inner 5×5 cm2 region and the outer 11×11 cm2 region of the array the Nyquist theorem is fulfilled for maximum sampling frequencies of 0.2 and 0.1 mm(-1), respectively. An IMRT field verification with a Gamma-Index analysis yielded a passing rate of 95.2% for a 3 mm∕3% criterion with a TPS calculation as reference. CONCLUSIONS This study shows the applicability of the Octavius 1000SRS in modern dosimetry. Output factor and dose profile measurements illustrated the applicability of the array in small field and stereotactic dosimetry. The high spatial resolution ensures adequate measurements of dose profiles in regular and intensity modulated photon-beam fields.

High resolution ion chamber array delivery quality assurance for robotic radiosurgery: Commissioning and validation.

PURPOSE High precision radiosurgery demands comprehensive delivery-quality-assurance techniques. The use of a liquid-filled ion-chamber-array for robotic-radiosurgery delivery-quality-assurance was investigated and validated using several test scenarios and routine patient plans. METHODS AND MATERIAL Preliminary evaluation consisted of beam profile validation and analysis of source-detector-distance and beam-incidence-angle response dependence. The delivery-quality-assurance analysis is performed in four steps: (1) Array-to-plan registration, (2) Evaluation with standard Gamma-Index criteria (local-dose-difference⩽2%, distance-to-agreement⩽2mm, pass-rate⩾90%), (3) Dose profile alignment and dose distribution shift until maximum pass-rate is found, and (4) Final evaluation with 1mm distance-to-agreement criterion. Test scenarios consisted of intended phantom misalignments, dose miscalibrations, and undelivered Monitor Units. Preliminary method validation was performed on 55 clinical plans in five institutions. RESULTS The 1000SRS profile measurements showed sufficient agreement compared with a microDiamond detector for all collimator sizes. The relative response changes can be up to 2.2% per 10cm source-detector-distance change, but remains within 1% for the clinically relevant source-detector-distance range. Planned and measured dose under different beam-incidence-angles showed deviations below 1% for angles between 0° and 80°. Small-intended errors were detected by 1mm distance-to-agreement criterion while 2mm criteria failed to reveal some of these deviations. All analyzed delivery-quality-assurance clinical patient plans were within our tight tolerance criteria. CONCLUSION We demonstrated that a high-resolution liquid-filled ion-chamber-array can be suitable for robotic radiosurgery delivery-quality-assurance and that small errors can be detected with tight distance-to-agreement criterion. Further improvement may come from beam specific correction for incidence angle and source-detector-distance response.

Dosimetric characteristics of the novel 2D ionization chamber array OCTAVIUS Detector 1500.

PURPOSE The dosimetric properties of the OCTAVIUS Detector 1500 (OD1500) ionization chamber array (PTW-Freiburg, Freiburg, Germany) have been investigated. A comparative study was carried out with the OCTAVIUS Detector 729 and OCTAVIUS Detector 1000 SRS arrays. METHODS The OD1500 array is an air vented ionization chamber array with 1405 detectors in a 27 × 27 cm(2) measurement area arranged in a checkerboard pattern with a chamber-to-chamber distance of 10 mm in each row. A sampling step width of 5 mm can be achieved by merging two measurements shifted by 5 mm, thus fulfilling the Nyquist theorem for intensity modulated dose distributions. The stability, linearity, and dose per pulse dependence were investigated using a Semiflex 31013 chamber (PTW-Freiburg, Freiburg, Germany) as a reference detector. The effective depth of measurement was determined by measuring TPR curves with the array and a Roos chamber type 31004 (PTW-Freiburg, Freiburg, Germany). Comparative output factor measurements were performed with the array, the Semiflex 31010 ionization chamber and the Diode 60012 (both PTW-Freiburg, Freiburg, Germany). The energy dependence of the OD1500 was measured by comparing the arrays readings to those of a Semiflex 31010 ionization chamber for varying mean photon energies at the depth of measurement, applying to the Semiflex chamber readings the correction factor kNR for nonreference conditions. The Gaussian lateral dose response function of a single array detector was determined by searching the convolution kernel suitable to convert the slit beam profiles measured with a Diode 60012 into those measured with the arrays central chamber. An intensity modulated dose distribution measured with the array was verified by comparing a OD1500 measurement to TPS calculations and film measurements. RESULTS The stability and interchamber sensitivity variation of the OD1500 array were within ±0.2% and ±0.58%, respectively. Dose linearity was within 1% over the range from 5 to 1000 MU. The effective point of measurement of the OD1500 for dose measurements in RW3 phantoms was determined to be (8.7 ± 0.2) mm below its front surface. Output factors showed deviations below 1% for field sizes exceeding 4 × 4 cm(2). The dose per pulse dependence was smaller than 0.4% for doses per pulse from 0.2 to 1 mGy. The energy dependence of the array did not exceed ±0.9%. The parameter σ of the Gaussian lateral dose response function was determined as σ6MV = (2.07 ± 0.02) mm for 6 MV and σ15MV = (2.09 ± 0.02) mm for 15 MV. An IMRT verification showed passing rates well above 90% for a local 3 mm/3% criterion. CONCLUSIONS The OD1500 arrays dosimetric properties showed the applicability of the array for clinical dosimetry with the possibility to increase the spatial sampling frequency and the coverage of a dose distribution with the sensitive areas of ionization chambers by merging two measurements.

The “collimator monitoring fill factor” of a two-dimensional detector array, a measure of its ability to detect collimation errors

Purpose: Two‐dimensional detector arrays are routinely used for constancy checks and treatment plan verification in photon‐beam radiotherapy. In addition to the spatial resolution of the dose profiles, the “coverage” of the radiation field with respect to the detection of any beam collimation deficiency appears as the second characteristic feature of a detector array. The here proposed “collimator monitoring fill factor” (CM fill factor) has been conceived to serve as a quantitative characteristic of this “coverage”. Methods: The CM fill factor is defined as the probability of a 2D array to detect any collimator position error. Therefore, it is represented by the ratio of the “sensitive area” of a single detector, in which collimator position errors are detectable, and the geometrical “cell area” associated with this detector within the array. Numerical values of the CM fill factor have been Monte Carlo simulated for 2D detector arrays equipped with air‐vented ionization chambers, liquid‐filled ionization chambers and diode detectors and were compared with the “FWHM fill factor” defined by Gago‐Arias et al. (2012). Results: For arrays with vented ionization chambers, the differences between the CM fill factor and the FWHM fill factor are moderate, but occasionally the latter exceeds unity. For narrower detectors such as liquid‐filled ionization chambers and Si diodes and for small sampling distances, large differences between the FWHM fill factor and the CM fill factor have been observed. These differences can be explained by the shapes of the fluence response functions of these narrow detectors. Conclusions: A new parameter “collimator monitoring fill factor” (CM fill factor), applicable to quantitate the collimator position error detection probability of a 2D detector array, has been proposed. It is designed as a help in classifying the clinical performance of two‐dimensional detector arrays in photon‐beam radiotherapy.

SU‐E‐T‐131: Iterative Deconvolution of Dose Profile Measurements with Ionization Chambers Based on Lorentz‐Type Lateral Response Functions of the Detectors

Purpose: In this work it is shown that the volume effect of ionization chamber can be corrected by the application of a van‐Cittert iterative deconvolution algorithm. Methods: Due to their volume effect the reading of an ionisation chamber s(x) can be considered as a convolution of the true dose distribution d(x) with the lateral response function r(x) of the detector. A prominent effect of this convolution is the broadening of profile measurements in the penumbra region. For the analysis a Lorentz‐type response function a bell shaped function with wide tails and free parameter l is assumed. Representations of the “true” dose distribution are measured with a diode detector (detector with minimal spatial spread of r(x)). The free parameter for the response function l is found by systematical variation and subsequent application of an iterative deconvolution algorithm. The iterative procedure consists of a sequence of approximations for n(x) which quickly converges towards the desired true d(x). Each n(x) is numerically convolved with r(x) and from the comparison of the result with s(x), the next approximation n+1(x) is derived. The best estimate for r(x) is found for the 1 resulting in the best approximation of d(x). Results: For cylindrical ionisation chambers with different radii and volume effect the lateral response functions for 6 and 15 MV photonradiation and a variety of field sizes have been analyzed. It is shown that the penumbra broadening can be revoked with a sufficient accuracy. Conclusions: By an iterative deconvolution algorithm with a pre‐defined Lorentz‐shaped lateral response function can calculate an approximation of the true dose distribution from ionization chamber measurements. The resulting corrected dose profiles may act as the input for base data determination for treatment planning systems and may thus improve the accuracy of calculated dose distributions.

Determination of EPID convolution kernels for portal imaging using carbon target bremsstrahlung

Abstract Improving the accuracy and reproducibility during patient positioning is of paramount importance. Hence, the goal of this work is to characterize the aspects of image blurring occurring during carbon target bremsstrahlung portal imaging and to assess the applicability of a deconvolution algorithm. Blurring effects involved in this method of portal imaging are electron scattering inside the EPID, geometric blurring due to the photon source size and photon scattering inside the patient. These effects can all be described by convolutions using as the convolutional kernel a Lorentz function, whose FWHM is 2λ. The λ values measured for these effects range from 0.2 mm to 0.45 mm, and an iterative 2D-deconvolution of carbon target portal images was performed accordingly. A significant decrease in the image blurring of test objects has been achieved and confirmed by analyzing the RMTF. However for clinical images, the deconvolution method is presently faced with the problem of the associated increase of image noise.

SU-E-T-35: A General Fill Factor Definition Serving to Characterise the MLC Misalignment Detection Capabilities of Two-Dimensional Detector Arrays

Purpose: To present a general definition of the fill factor realistically characterizing the “field coverage”, i.e. the MLC misalignment detection capabilities of a detector array. Methods: According to Gago-Arias et al.1 the fill factor of a 2D array is defined as the ratio of the area enclosed by the FWHM of the fluence response function KM(x) of a single detector and its cell area defined by the detector spacing. More generally - accounting also for the possible overlap between FWHM’s of neighboured detectors - the fill factor is here defined as that fraction of the sum of the detector cell areas in which a defined MLC misalignment is detectable when the induced percentage signal changes exceed a detection threshold d. Ideally the generalized fill factor may reach 100 %. With user code EGS_chamber and a 2 MeV photon slit beam 0.25 mm wide, both types of the fill factor were calculated for an array with total cell area 100 cm2 for chamber widths 1–9 mm, using =1mm, d=5%. Results: For single chamber width 5 mm, fill factors were 0.49 (FWHM) and 0.61 (generalized). For chamber width 2 mm the FWHM fill factor was 0.13 whereas the generalized fill factor was 0.32. For chamber widths above 7 mm, the FWHM fill factor exceeds unity, and the general fill factor is exactly 1.00. Conclusions: An updated fill factor definition is introduced which, as a generalization of the FWHM-based definition, more closely estimates the performance of small array chambers and gives a realistic value in the case of overlapping sensitive areas of neighboured chambers. References:1A. Gago-Arias, L. Brualla-Gonzalez, D.M. Gonzalez-Castano, F. Gomez, M.S. Garcia, V.L. Vega, J.M. Sueiro, J. Pardo-Montero, “Evaluation of chamber response function influence on IMRT verification using 2D commercial detector arrays,” Phys. Med. Biol. 57, 2005–2020 (2012).

SU‐E‐T‐135: Dosimetric Properties of the OCTAVIUS Detector 1500

PURPOSE In this study the dosimetric properties of the Octavius Detector 1500 array (PTW-Freiburg-Germany) are investigated. METHODS The chambers of the array, each with an entrance window of 4.4 × 4.4 cm2 , are arranged in a checkerboard pattern in a measurement area of 27 × 27 cm2 with a sampling frequency of 0.1 mm-1 along each row which can be doubled by merging two measurements shifted by 5 mm. Linearity, stability and output factors were measured with either a Semiflex 31013 or 31010 as a reference detector. Output factors were additionally measured with a Diode 60012. The effective point of measurement was determined by comparing TPR curves of the array with Roos chamber 34001 measurements. The lateral dose response function of a single chamber was determined by comparison with a high resolution diode. An IMRT field verification was carried out with a merged OD1500 measurement. RESULTS The OD1500 was stable within ±0.15 %. Deviations in linearity did not exceed 1% from 5 to 1000 MU. The effective point of measurement was 8.2 mm below the surface. Deviations in output factors were below 0.77 % from 5 × 5 to 27 × 27 cm2 . As expected for the smallest field of 1 × 1 cm2 , the deviation from the diode was significant. The widths of the lateral dose response functions were σ6 = (2.07 ± 0.03) mm and σ1 5 = (2.09 ± 0.03) mm. Gamma Index passing rates for typical IMRT and VMAT plans were above 90 % compared to film and TPS calculations for a local 3 mm / 3 % criterion. CONCLUSION The first measurements with the OD1500 array show the excellent applicability of the array for clinical dosimetry. The response of the array to the mean photon energy and dose per pulse are under investigation.

SU‐E‐T‐177: Deconvolution of Line Dose Profiles with Ionization Chambers ‐ Experiences with the First Software Implementation in Mephisto 3.0

PURPOSE To evaluate the first implementation of a deconvolution algorithm in a commercial water phantom scanning software. METHODS Line dose profile measurements in a water phantom are an essential part of a quality assurance system and in base data measurements in radiotherapy. Usually these measurements are performed with waterproof ionization chambers of various sizes. These dose profile measurements are broadened by the Gaussian response functions of the detectors. In recent studies we showed that the undisturbed line dose profiles can be reconstruced by iterative deconvolution of the measured signal profiles with the Gaussian detector response functions. Recently, the proposed method was implemented in Mephisto 3.0. In this work we analyze the applicability and the limits of the deconvolution algorithm for several chambers by comparing the result with diode measurements. RESULTS As long as the dose gradient becomes not too steep the deconvolution algorithm is able to reconstruct the undisturbed dose profiles with sufficient accuracy. Deviations occur for smallest field sizes in which the width of the detectors lateral response functions reaches the dimensions of the field. A simple chart for those limits is derived. CONCLUSION The implemented deconvolution algorithm allows a fast and simple correction of measured dose profiles broadened by the volume effect of the ionization chambers. It offers therefore for the first time a clinical deconvolution of the profiles on a regular base and by this the implementation of undisturbed base data in the treatment planning systems as well as in the quality assurance process in modern radiotherapy.