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 array's 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 array's 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 array's 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.

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 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.

SU-E-T-121: Investigating the Optimal Scanning Resolution for Radiochromic EBT-2 Films Using an Epson 10000XL Flatbed Scanner.

PURPOSE: The purpose of this work is to determine the optimal scanner resolution of an Epson 10000XL scanner for the analysis of radiochromic EBT-2 films. Using Fourier analysis and the Nyquist-Shanon sampling theory, the highest frequency component required to sufficiently reproduce a previously measured step dose profile was investigated. METHODS: A setup was created, in which one half of a 6×6cm2 EBT-2 film was shielded on exposure using a 15×5×10cm3 lead block to obtain sharp step dose profiles. The film itself was placed between two 6cm RW3 stacks on top of which the lead block was placed. Using a Siemens Primus linear accelerator operating at 6/15MV nominal energies, the setup was exposed to 400MUs at 6MV and 500MUs at 15MV respectively. Preliminary investigations were performed without RW3 between the lead and film. Initial image acquisition was performed at 600dpi to minimize information loss. Using the average of five line profiles, a uniformity correction algorithm provided by the manufacturer was implemented prior to the Fast Fourier Transform (FFT) operation. In an iterative process, all frequency components above a cut-off frequency wcut were successively removed and the original image reconstructed with the inverse FFT operation. The goodness of fit was evaluated by comparing the change in penumbra width on image reconstruction. RESULTS: The minimum scanning resolution required to analyze the step dose profiles created without build-up material was 52dpi for 6MV and 30dpi for 15MV. By adding build-up material, in the areas of secondary electron equilibrium the required resolution reduces to 12dpi for 6MV and 8dpi for 15MV. CONCLUSIONS: For sufficient image reproduction within any information loss, resolutions as low as 52dpi at 6MV and 30dpi at 15MV are sufficient for evaluating EBT-2 films. This is in compliance with 50dpi recommended by the manufacturer.

SU-E-T-126: Non-Reference Condition Correction Factor KNR of Typical Radiation Detectors for the Dosimetry of High-Energy Photons.

PURPOSE: To correct for the deviations of the detector response when typical radiation detectors are used under non-reference conditions, factor kNR was calculated from the known energy dependence of the detector response at photon energies from 10 keV upwards and from clinical photon spectra within a large water phantom beneath a Siemens Primus 6/15 MV linac. A Farmer type ion chamber (NE2571), two TLD detector types and two diodes were investigated. METHODS: Factor kNR was obtained as the ratio of the weighted responses Yt of a given detector t under reference conditions xref (axial distance r = 0 cm, depth d = 10 cm, field size 10 × 10 cm2 and SSD = 100 cm) and that under non-reference conditions × (off-axis points and depths for various field sizes); kNR = Yt(xref)/Yt(x). For small field (SF) dosimetry, we evaluated correction factor kNRSF, which refers to small field reference conditions (4 × 4 cm2 field). RESULTS: For all detectors investigated, the deviations of kNR from unity were highest outside the field, due to prevailing low-energy scatter contributions. For the Farmer chamber and EDP-10 diode, the kNR deviations did not exceed 2%, but were up to 60% for the EDD-5 diode, while kNR values for LiF:Mg,Cu,P and LiF:Mg,Ti deviated at most 15% and 5% respectively. kNR values appear as unique functions of the mean photon energy at the point of interest. CONCLUSIONS: Air-filled ion chambers show only small kNR variations, while for non-water equivalent detectors, kNR variations depend on the detector response at low photon energy. kNR can be presented as a unique function of the mean photon energy at the point of interest. A 4 × 4 cm2 reference field is recommended for small fields, with correction factor kNRSF varying almost negligibly from kNR except for unshielded Si diodes.

SU-E-T-224: Dose Distribution of Oesophagus Stents Measured by EBT2 Film Dosimetry.

PURPOSE: The purpose of this study is to investigate the dose enhancement at oesophagus stents made of nitinol. The material is a nickel titan alloy with an effective atomic number of 26. Because of the increased atomic number in comparison to the human body, dose enhancement in surrounding tissue is expected. METHODS: The relative dose distribution around the stent was measured in a water phantom. To simulate the air cavity within the oesophagus, a styrodur cylinder was placed inside the stent. The stent was held with a circular PMMA holder. An EBT2 film was wrapped around the stent to measure the relative radial dose distribution.The setup was irradiated with a 6MV photon beam (Siemens Primus) and a field size of 5cmx5cm. The distance between source and centre of the stent was 100cm.The EBT2 films were digitized at a scanning resolution of 72dpi using an Epson 10000XL flatbed scanner with a transparency unit. Furthermore, the films were fixed in a frame to prevent Newton rings in the scanned image. RESULTS: The dose increases in all directions around the stent. With approximately 18%, the highest increase is caused on the proximal side of the stent. On the backside the dose enhancement is approximately 10%. CONCLUSIONS: Dose enhancements around a stent are detectable and one should be aware of it's occurence in the radiotherapeutical treatment of oesophageal cancer. Because of the enhancement in all directions healthy tissue may be affected.

SU-E-T-244: High Spatial Resolution EBT2 Film Dosimetry.

PURPOSE: The purpose of this study was to measure depth dose curves and dose effects near high-Z interfaces with radiochromic EBT-2 films reaching a spatial resolution superior to conventional methods with no quality losses. METHODS: The setup is made of two 12cm stacks of RW3, fixing an EBT2 film in a vertical position. To measure a depth dose curve, the setup was irradiated with a 15MV photon beam (Siemens Primus). Since the film is positioned parallel to the beam propagation, the depth dose curve is measured with only one film per depth. Additionally, a dental gold alloy probe was inserted in the RW3 stack at 6cm depth and the dose enhancement in front of the probe was measured with the method described above. Hereby, the bottom edge of the film touches the probe's surface.The irradiated films were digitized with a resolution of 72dpi using an Epson 10000XL flatbed scanner with a transparency unit and alignment frames. With this setup, the spatial resolution is only limited by the scanning resolution. RESULTS: In order to verify the new measurement method, comparisons of the measured depth dose curves with the conventional method of placing the film orthogonal to the beam propagation showed deviations of lesser than 3%.The comparison of the dental gold measurements with Monte Carlo simulations shows a systematic lower measured dose which is still within 5% consistency. However attention has to be paid in the experimental setup and film preparation. CONCLUSIONS: The introduced method shows significant advantages to conventional orthogonal EBT2 film positioning. It shows a very high spatial resolution and the area of interest is only limited by the film size. The method will be used in further studies, to investigate dose profiles and dose effects near interfaces and in inhomogeneities.

SU-E-I-106: Description of Energy Dose Deposition Kernel for the Diagnostic Beam.

PURPOSE: Over the recent years, the employment of kV x-rays as a diagnostic tool into clinical routine has resulted in a significant increase in the patient's exposure to ionizing radiation. The accurate determination of the absorbed dose to patients during diagnosis is therefore necessary to avoid unnecessary exposure to the patient. This study presents an analytical model of the energy deposition kernel for the monoenergetic and polyenergetic kV beams for the fast calculation of dose in radiography. METHODS: The analytical model is based on the pencil beam kernels derived from Monte Carlo simulations. DOSXYZnrc code from the EGSnrc family was employed to simulate the pencil beam of 0.1 cm width for 80, 100 and 120 keV mono-energetic and polyenergetic beams. The lateral dose profiles were calculated at different depths within a homogenous water phantom of size 50×50×50 cm3 . The evaluated dose profiles showed a high amplitude primary component at the central axis and a long range low amplitude scatter component spanning a considerable distance from the central axis. The profiles were fitted analytically with a triple exponential decay function with an offset. All coefficients of the exponential function were further fitted with appropriate analytical functions to represent their behavior relative to depth and photon energy. The accuracy of the obtained kernel was checked by the convolution of a rectangular fluence profile and comparing the calculated dose distribution with the Monte Carlo simulated dose profiles for 2×2 cm2 field size. RESULTS AND CONCLUSION: In a homogeneous phantom, the comparisons of the convolution method and Monte Carlo simulations showed sufficient agreement except for largest depths (deviation approx. 15%). Future developments will focus on an implementation of the method for dose calculation in the patient.

SU-E-T-502: Dose Perturbation Effects Near Implant Surfaces Caused by Secondary Electron Transport in Photon-Beam Therapy.

PURPOSE: To investigate the dose perturbation effects at interfaces between water and a Titanium implant, attributable to secondary electron transport across the interface, during high energy photon radiotherapy. While dose enhancement is characteristic for the proximal interface of a high-atomic number implant, the dose perturbation at the distal interface varies from reduction to enhancement, requiring proper computation of secondary electron transport effects. The backward and forward perturbation factors pb and pf will be calculated. METHODS: Using DOSRZnrc, depth dose curves were computed in a water phantom using photon spectra of nominal energies 4, 6, 10, 15, 24 MV for conditions (i) homogeneous water without any insert, (ii) alternatively with Titanium inserts of thicknesses 3 and 5 cm placed at 10 cm water depth. Backscatter factor pb was computed as the ratio of the dose with implant against that without implant, whereas pf was calculated by first accounting for photon attenuation in the implant and then taking the ratio of the dose with implant against that without implant. RESULTS: At the front interface, pb is independent of the material thickness and varies slightly with beam energy and incident angle. On consideration of photon attenuation in the implant, pf was also found to be independent on material thickness, but strongly varying with energy, including change of sign. CONCLUSIONS: For 4-24 MV photon beams the maximum spread of the dose perturbation effect remains within only a few millimeters from the interface, with pb values ranging from 1.18-1.22, while factor pf ranges from 0.9-1.21 at normal incidence, indicating the extent to which planning systems may over- or underestimate the doses near implant interfaces. At inclined beam incidence the dose perturbation effects even increase, and for instance pb (1.24-1.25) and pf (0.85-1.32) were determined for 6 MV and 24 MV beams at 45° incidence.

Conversion coefficients for the estimation of effective doses in intraoral and panoramic dental radiology from dose-area product values.

Conversion coefficients for the estimation of effective doses in intraoral and panoramic dental radiology from dose-area product (DAP) values were determined by measuring organ-absorbed doses and the corresponding DAP values. Measurements were performed for all standard intraoral radiological projections and standard panoramic examination at different exposure parameters. Organ-absorbed doses were measured using thermoluminescent detectors and an adult anthropomorphic phantom specially designed for dosimetric study in dental radiology. Different techniques for the calculation of effective doses were evaluated. Conversion coefficients derived from this study range from 0.008 to 0.132 microSv mGy(-1) cm(-2) for intraoral radiography and 0.055 to 0.238 microSv mGy(-1) cm(-2) for panoramic radiography.

Characterization of the radiation quality of 60Co therapy units by the fraction of air kerma attributable to scattered photons.

In this work we present a new parameter for characterizing the emitted photon spectra of (60)Co radiotherapy units. It is intended to propose this parameter for the revised DIN standard 6809-1. In the previous DIN regulation, it had been sufficient to state the nature of the radioactive material within the source. However, scatter processes within the radioactive material as well as the source housing and the collimator system influence the shape of the photon spectrum, with a noticeable contribution in the low-energy portion. The fraction of the air kerma for a given distance from the source, position and beam size in air comprising all contributions by scattered photons up to an upper energy limit for the emitted spectrum from (60)Co decay, will be proposed as a typical parameter. The new quantity, which is termed the 'fraction of air kerma attributable to scattered photons', P(E)(Scatter), has been calculated for E = 1.17 MeV and compared for four different Monte Carlo-simulated spectra of used (60)Co devices. Not included in this new formalism is the air kerma contribution by scattered photons in between the two lines of the (60)Co spectrum. A simple measurement procedure based on the signal ratio of two Farmer chamber detectors with different wall materials is discussed and its feasibility shown.

Dose-area product measurements and determination of conversion coefficients for the estimation of effective dose in dental lateral cephalometric radiology.

A study has been carried out to propose diagnostic reference levels (DRLs) for lateral cephalometric radiology in Germany based on the dose-area product (DAP). DRLs were proposed separately for child and adult exposure settings which are 26.4 and 32.6 mGy cm2, respectively. Organ absorbed doses from lateral cephalometric radiology were also measured using thermoluminescence detectors (TLDs) and an adult anthropomorphic phantom specially design for dosimetric study in dental radiology. Effective doses were then calculated using three different techniques where the salivary gland and brain tissue were given different weighting factors. Conversion coefficients for estimating effective dose from DAP value derived in this study range from 0.042 to 0.149 microSv/mGy cm2.

Dose-area product measurements in panoramic dental radiology.

In this study, dose-area product (DAP) measurements in panoramic dental radiology have been performed in Germany. The results obtained in this study were proposed as diagnostic reference levels (DRLs). A representative number of dental panoramic units, both with digital and conventional image receptors, have been chosen. Common statistical parameters such as mean, standard deviation and 3rd quartile have been calculated. For four different standard programmes, 'large adult', 'adult male', 'adult female' and 'child', the proposed DRLs are 101, 87, 84 and 75 mGy cm(2), respectively. No clear tendency to a generalised dose reduction from the transition to digital techniques has been observed. Effective doses have been calculated from E/DAP conversion factors published in literature. Even though these values differ by a factor of approximately 3, upper limits of 15.8-21.2 microSv for the four different exposure settings were derived from the data.

Radiation exposure and dose evaluation in intraoral dental radiology.

In this study, dose area product measurements have been performed to propose diagnostic reference levels (DRLs) in intraoral dental radiology. Measurements were carried out at 60 X-ray units for all types of intraoral examinations performed in clinical routine. The third quartile values calculated range from 26.2 to 87.0 mGy cm(2). The results showed that there exists a large difference between the patient exposures among different dental facilities. It was also observed that dentists working with faster film type or higher tube voltage are not always associated with lower exposure. The study demonstrated the necessity to have the DRLs laid out as guidelines in dental radiology.

Radiation exposure to children in intraoral dental radiology.

In this study, dose area product (DAP) measurements have been performed aiming at establishing diagnostic reference levels (DRLs) in paediatric intraoral dental radiology. Measurements were carried out at 52 X-ray units for all types of intraoral examinations performed in clinical routine. Not all X-ray units have pre-set child exposure settings with reduced exposure time or in some cases lower tube voltage. Child examinations are carried out using adult exposure settings at these units, which increases the DAP third quartile values by up to 50%. For example, third quartile values for periapical examination ranges from 14.4 to 40.9 mGy cm(2) for child settings and 20.6 to 48.8 mGy cm(2) when the adult settings are included. The results show that there exists a large difference between the patient exposures among different dental facilities. It was also observed that clinics working with faster film type or higher tube voltage are not always associated with lower exposure.