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Oldenburg, Germany

Zink K.,Justus Liebig University | Zink K.,University of Marburg | Czarnecki D.,Justus Liebig University | Looe H.K.,Clinic for Radiation Therapy | And 3 more authors.
Medical Physics | Year: 2014

Purpose: The electron fluence inside a parallel-plate ionization chamber positioned in a water phantom and exposed to a clinical electron beam deviates from the unperturbed fluence in water in absence of the chamber. One reason for the fluence perturbation is the well-known inscattering effect, whose physical cause is the lack of electron scattering in the gas-filled cavity. Correction factors determined to correct for this effect have long been recommended. However, more recent Monte Carlo calculations have led to some doubt about the range of validity of these corrections. Therefore, the aim of the present study is to reanalyze the development of the fluence perturbation with depth and to review the function of the guard rings.Methods: Spatially resolved Monte Carlo simulations of the dose profiles within gas-filled cavities with various radii in clinical electron beams have been performed in order to determine the radial variation of the fluence perturbation in a coin-shaped cavity, to study the influences of the radius of the collecting electrode and of the width of the guard ring upon the indicated value of the ionization chamber formed by the cavity, and to investigate the development of the perturbation as a function of the depth in an electron-irradiated phantom. The simulations were performed for a primary electron energy of 6 MeV.Results: The Monte Carlo simulations clearly demonstrated a surprisingly large in- and outward electron transport across the lateral cavity boundary. This results in a strong influence of the depth-dependent development of the electron field in the surrounding medium upon the chamber reading. In the buildup region of the depth-dose curve, the in-out balance of the electron fluence is positive and shows the well-known dose oscillation near the cavity/water boundary. At the depth of the dose maximum the in-out balance is equilibrated, and in the falling part of the depth-dose curve it is negative, as shown here the first time. The influences of both the collecting electrode radius and the width of the guard ring are reflecting the deep radial penetration of the electron transport processes into the gas-filled cavities and the need for appropriate corrections of the chamber reading. New values for these corrections have been established in two forms, one converting the indicated value into the absorbed dose to water in the front plane of the chamber, the other converting it into the absorbed dose to water at the depth of the effective point of measurement of the chamber. In the Appendix, the in-out imbalance of electron transport across the lateral cavity boundary is demonstrated in the approximation of classical small-angle multiple scattering theory.Conclusions: The in-out electron transport imbalance at the lateral boundaries of parallel-plate chambers in electron beams has been studied with Monte Carlo simulation over a range of depth in water, and new correction factors, covering all depths and implementing the effective point of measurement concept, have been developed. © 2014 American Association of Physicists in Medicine. Source

Stelljes T.S.,Clinic for Radiation Therapy | Stelljes T.S.,Carl von Ossietzky University | Harmeyer A.,Carl von Ossietzky University | Reuter J.,Carl von Ossietzky University | And 7 more authors.
Medical Physics | Year: 2015

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 cm2 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 cm2. 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. © 2015 American Association of Physicists in Medicine. Source

Chofor N.,Clinic for Radiation Therapy | Chofor N.,Carl von Ossietzky University | Harder D.,University of Gottingen | Willborn K.C.,Carl von Ossietzky University | And 2 more authors.
Physics in Medicine and Biology | Year: 2012

In clinical photon beams, the dose outside the geometrical field limits is produced by photons originating from (i) head leakage, (ii) scattering at the beam collimators and the flattening filter (head scatter) and (iii) scattering from the directly irradiated region of the patient or phantom (internal scatter). While the first two components can be modified, e.g. by reinforcement of shielding components or by re-modeling the filter system, internal scatter remains an unavoidable contributor to the peripheral dose. Its relative magnitude compared to the other components, its numerical variation with beam energy, field size and off-axis distance as well as its spectral distribution are evaluated in this study. We applied a detailed Monte Carlo (MC) model of our 6/15 MV Siemens Primus linear accelerator beam head, provided with ideal head leakage shielding conditions (multi-leaf collimator without gaps) to assess the head scatter contribution. Experimental values obtained under real shielding conditions were used to evaluate the head leakage contribution. It was found that the MC-computed internal scatter doses agree with the results of our previous measurements, that internal scatter is the major contributor to the peripheral dose in the near periphery while head leakage prevails in the far periphery, and that the lateral decline of the internal scatter dose can be represented by the sum of two exponentials, with an asymptotic tenth value of 18 to 19cm. Internal scatter peripheral doses from various elementary beams are additive, so that their sum increases approximately in proportion with field size. The ratio between normalized internal scatter doses at 6 and 15 MV is approximately 2:1. The energy fluence spectra of the internal scatter component at all points of interest outside the field have peaks near 500 keV. The fact that the energy-shifted internal scatter constitutes the major contributor to the dose in the near periphery has a general bearing for dosimetry, i.e. for energy-dependent detector responses and dose conversion factors, for the relative biological effectiveness and for second primary malignancy risk estimates in the peripheral region. © 2012 Institute of Physics and Engineering in Medicine. Source

Looe H.K.,Clinic for Radiation Therapy | Looe H.K.,Carl von Ossietzky University | Uphoff Y.,Clinic for Radiation Therapy | Uphoff Y.,Carl von Ossietzky University | And 4 more authors.
Strahlentherapie und Onkologie | Year: 2012

Purpose. The quality of megavoltage clinical portal images is impaired by physical and geometrical effects. This image blurring can be corrected by a fast numerical two-dimensional (2D) deconvolution algorithm implemented in the electronic portal image device. We present some clinical examples of deconvolved portal images and evaluate the clinical advantages achieved by the improved sharpness and contrast. Materials and methods. The principle of numerical 2D image deconvolution and the enhancement of sharpness and contrast thereby achieved are shortly explained. The key concept is the convolution kernel K(x,y), the mathematical equivalent of the smearing or blurring of a picture, and the computer-based elimination of this influence. Results. Enhancements of sharpness and contrast were observed in all clinical portal images investigated. The images of fine bone structures were restored. The identification of organ boundaries and anatomical landmarks was improved, thereby permitting a more accurate comparison with the x-ray simulator radiographs. The visibility of prostate gold markers is also shown to be enhanced by deconvolution. Conclusion. The blurring effects of clinical portal images were eliminated by a numerical deconvolution algorithm that leads to better image sharpness and contrast. The fast algorithm permits the image blurring correction to be performed in real time, so that patient positioning verification with increased accuracy can be achieved in clinical practice. Source

Poppe B.,Clinic for Radiation Therapy | Poppe B.,Carl von Ossietzky University | Stelljes T.S.,Clinic for Radiation Therapy | Stelljes T.S.,Carl von Ossietzky University | And 6 more authors.
Medical Physics | Year: 2013

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 detector's 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. © 2013 American Association of Physicists in Medicine. Source

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