Quantitative signal intensity ratios to distinguish between subfascial lipoma and atypical lipomatous tumor/well-differentiated liposarcoma using short-tau inversion recovery (STIR) MRI.

PURPOSE: To establish simple quantitative variables at short-tau inversion recovery (STIR) magnetic resonance imaging (MRI) to identify lipomas with high specificity in patients with indeterminate subfascial lipomatous tumors. MATERIALS AND METHODS: The MRI examinations of 26 patients (14 men, 12 women; mean age 63±12.5 [SD] years; range: 40-84years) with histopathologically proven subfascial atypical lipomatous tumors/well-differentiated liposarcomas (ALT/WDLs) and those of 68 patients (32 men, 36 women; mean age, 56±13.5 [SD] years; range: 21-83years) with lipomas were retrospectively reviewed. Ratios derived from region of interest based signal intensity (SI) measurements of tumors and adjacent fat on STIR images were calculated and maximum tumor diameters were noted. Diagnostic parameter capabilities were assessed using ROC curve analysis. Interreader agreement was evaluated by calculation of intraclass correlation coefficients (ICC). RESULTS: Using a cut-off value of 1.18, STIR-SI ratios allowed discriminating between lipoma and ALT/WDL (AUC=0.88; P<0.001) yielding 93% specificity (95% CI: 77-99%) and 74% sensitivity (95% CI: 61-84%) for the diagnosis of lipoma. Interreader agreement was excellent (ICC=0.93). A significant difference in maximum tumor diameter was found between ALT/WDLs (mean: 18.1±6.0 [SD] cm; range: 5.6-33.1cm) and lipomas (mean: 9.7±5.0 [SD] cm; range: 2.9-29.1cm) (P<0.001). Using a cut-off of 11cm, maximum tumor diameter allowed discriminating between lipoma and ALT/WDLs with 92% specificity (95% CI: 75-99%) and 69% sensitivity (95% CI: 57-80%). The combination of a STIR-SI ratio<1.4 and maximum tumor diameter<11cm yielded 100% specificity (95% CI: 87-100%) and 65% sensitivity (95% CI: 54-77%) for the diagnosis of lipoma. CONCLUSION: The combination of STIR-SI ratio and maximum diameter allows discriminating between lipoma and ALT/WDL in initially indeterminate lipomatous tumors.

Systematic Radiation Dose Reduction in Cervical Spine CT of Human Cadaveric Specimens: How Low Can We Go?

BACKGROUND AND PURPOSE: While the use of cervical spine CT in trauma settings has increased, the balance between image quality and dose reduction remains a concern. The purpose of our study was to compare the image quality of CT of the cervical spine of cadaveric specimens at different radiation dose levels. MATERIALS AND METHODS: The cervical spine of 4 human cadavers (mean body mass index; 30.5 ± 5.2 kg/m2; range, 24-36 kg/m2) was examined using different reference tube current-time products (45, 75, 105, 135, 150, 165, 195, 275, 355 mAs) and a tube voltage of 120 kV(peak). Data were reconstructed with filtered back-projection and iterative reconstruction. Qualitative image noise and morphologic characteristics of bony structures were quantified on a Likert scale. Quantitative image noise was measured. Statistics included analysis of variance and the Tukey test. RESULTS: Compared with filtered back-projection, iterative reconstruction provided significantly lower qualitative (mean noise score: iterative reconstruction = 2.10/filtered back-projection = 2.18; P = .003) and quantitative (mean SD of Hounsfield units in air: iterative reconstruction = 30.2/filtered back-projection = 51.8; P < .001) image noise. Image noise increased as the radiation dose decreased. Qualitative image noise at levels C1-4 was rated as either "no noise" or as "acceptable noise." Any shoulder position was at level C5 and caused more artifacts at lower levels. When we analyzed all spinal levels, scores for morphologic characteristics revealed no significant differences between 105 and 355 mAs (P = .555), but they were worse in scans at 75 mAs (P = .025). CONCLUSIONS: Clinically acceptable image quality of cervical spine CTs for evaluation of bony structures of cadaveric specimens with different body habitus can be achieved with a reference mAs of 105 at 120 kVp with iterative reconstruction. Pull-down of shoulders during acquisition could improve image quality but may not be feasible in trauma patients with unknown injuries.

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.

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.

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.

Monte Carlo study of the depth-dependent fluence perturbation in parallel-plate ionization chambers in electron beams.

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.

Systematic survey of the dose enhancement in tissue-equivalent materials facing medium- and high-Z backscatterers exposed to X-rays with energies from 5 to 250 keV.

The present study has been inspired by the results of earlier dose measurements in tissue-equivalent materials adjacent to thin foils of aluminum, copper, tin, gold, and lead. Large dose enhancements have been observed in low-Z materials near the interface when this ensemble was irradiated with X-rays of qualities known from diagnostic radiology. The excess doses have been attributed to photo-, Compton, and Auger electrons released from the metal surfaces. Correspondingly, high enhancements of biological effects have been observed in single cell layers arranged close to gold surfaces. The objective of the present work is to systematically survey, by calculation, the values of the dose enhancement in low-Z media facing backscattering materials with a variety of atomic numbers and over a large range of photon energies. Further parameters to be varied are the distance of the point of interest from the interface and the kind of the low-Z material. The voluminous calculations have been performed using the PHOTCOEF algorithm, a proven set of interpolation functions fitted to long-established Monte Carlo results, for primary photon energies between 5 and 250 keV and for atomic numbers varying over the periodic system up to Z = 100. The calculated results correlate well with our previous experimental results. It is shown that the values of the dose enhancement (a) vary strongly in dependence upon Z and photon energy; (b) have maxima in the energy region from 40 to 60 keV, determined by the K and L edges of the backscattering materials; and (c) are valued up to about 130 for "International Commission on Radiological Protection (ICRP) soft tissue" (soft tissue composition recommended by the ICRP) as the adjacent low-Z material. Maximum dose enhancement associated with the L edge occurs for materials with atomic numbers between 50 and 60, e.g., barium (Z = 56) and iodine (Z = 53). Such materials typically serve as contrast media in medical X-ray diagnostics. The gradual reduction in the dose enhancement with increasing distance from the material interface, owed to the limited ranges of the emitted secondary electrons, has been documented in detail. The discussion is devoted to practical radiological aspects of the dose enhancement phenomenon. Cytogenetic effects in cell layers closely proximate to surfaces of medium-Z materials might vary over two orders of magnitude, because the dose enhancement is accompanied by the earlier observed about twofold increase in the low-dose RBEM at a tissue-to-gold interface.

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.

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.

The protective effect of astrocyte-derived 14,15-epoxyeicosatrienoic acid on hydrogen peroxide-induced cell injury in astrocyte-dopaminergic neuronal cell line co-culture.

Astrocytes perform several functions that are essential for normal neuronal activity. They play a critical role in neuronal survival during ischemia and other degenerative injuries and also modulate neuronal recovery by influencing neurite outgrowth. In this study, we investigated the neuroprotective effects of astrocyte-derived 14,15-epoxyeicosatrienoic acid (14,15-EET), metabolite of arachidonic acid by cytochrome P450 epoxygenases (CYP), against oxidative stress induced by hydrogen peroxide (H(2)O(2)). We found that dopaminergic neuronal cells (N27 cell line) stimulated with two different doses of H(2)O(2) (0.1 and 1mM) for 1h showed decreased cell viability compared to the control group, while astrocytes showed less cell death after stimulation with the same doses of H(2)O(2) for 1h. Dopaminergic neuronal cells (N27 cell line) pretreated with different doses of 14,15-EET (0.1-30 µM, 30 min) before H(2)O(2) stimulation also showed increased cell viability. Furthermore, pre-treatment of the co-cultured cells with 12-(3-adamantan-1-yl-ureido)-dodecanoic acid, an inhibitor of the EET metabolizing enzyme, soluble epoxide hydrolase (sEH), before H(2)O(2) stimulation (1mM, for 1h) increased cell viability. It also increased the endogenous level of 14,15-EET in the media compared to control group. However, pretreatment with the CYP epoxygenase inhibitor miconazole (1-20 µM, 1h) before H(2)O(2) (1mM, 1h) stimulation showed decreased cell viability. Our data suggest that 14,15-EET which is released from astrocytes, enhances cell viability against oxidant-induced injury. Further understanding of the mechanism of 14,15-EET-mediated protection in dopaminergic neurons is imperative, as it could lead to novel therapeutic approaches for treating CNS neuropathologies, such as Parkinson's disease.

Numerical deconvolution to enhance sharpness and contrast of portal images for radiotherapy patient positioning verification.

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.

WE-G-BRB-03: On the Detectability of Jaw Decalibrations with the DAVID System.

PURPOSE: To evaluate if the recently described problems with the calibration of jaws at Varian linear accelerators can be detected with the DAVID system. METHODS: Recently Varian Clinac field size issues resulted in a widespread medical device correction notice, stating that incorrect jaw positioning due to excessive connector resistances may result in clinically relevant deviations from the intended field size. Since field size checks via the light field, separately for each administered field segment, are not practicable, it might be helpful to know that an independent, permanently active field size surveillance system is routinely used in our department since four years. The DAVID system (PTW-Freiburg, Germany) is a translucent multiwire transmission ionisation chamber, placed in the linac accessory holder. Each detection wire of the chamber is positioned in theprojection of the midline of the associated MLC leaf pair, resulting in a signal proportional to the leaf pair aperture. High lateral resolution has been achieved by a deconvolution software (Looe et al, PMB 55, 2010, 3981- 3992). MLC decalibrations are detected by comparison between measured and reference signal profiles. RESULTS: The side wings of the DAVID system signal profiles are sensibly influenced by the position of the backup jaw blocks limiting the field size in the direction at right angles to the MLC travel direction. Systematic variation of the backup jaw positions for several field sizes and IMRT plans has revealed high sensitivity of the signal profile side wings with respect to jaw decalibrations. CONCLUSIONS: The DAVID system is able to detect, without any delay, suddenly appearing jaw decalibrations down to positioning errors less than 1mm. Use of the DAVID system provides the independent and permanent surveillance of correct field sizes including the position of the backup jaws, making checks of light-field congruence for each field segment unnecessary.

SU-E-T-114: Characterization of the Spatial Response Functions of Ionization Chambers for Photon Beam Dosimetry.

PURPOSE: The finite extension of an ionization chamber gives rise to a spatial averaging effect, known as the "volume effect". In order to provide the appropriate corrections, the response functions along its lateral and longitudinal directions are characterized using Gaussian distributions, whose standard deviations slat and slong have been determined for a large set of clinical dosimeters. METHODS: Nine cylindrical ionization chambers, two parallel-plate chambers and two 2D ionization chamber arrays have been examined by scanning rectangular photon fields along their short axes. The true profiles D(x) were known from scans with a small Si diode. The ionization chambers were aligned with their symmetry axes either perpendicular or parallel to the scan direction in order to obtain slat and slong separately. In a search process, D(x) was numerically convolved with normalized one-dimensional Gaussian kernels K(x) of varying s. The best fit between the convolution product D(x) * K(x) and the measured profile M(x) of the ionization chamber was used to determine parameters slat and slong of the Gaussian kernels. RESULTS: For both the lateral and longitudinal directions, very good agreement was found between M(x) and the convolution products of D(x) with Gaussian kernels K(x). For all chambers, their 2s values are similar to the cavity dimensions, which means that the "tails" of the Gaussian response functions reach into the exterior of the chambers, - an effect of the ranges of the secondary electrons. At higher photon energies response functions K(x) are slightly wider, but no detectable depth dependence has been observed. CONCLUSIONS: We have shown that the response functions of ionization chambers can be described by Gaussian distributions, confirming earlier observations, and we determined their standard deviations in both the lateral and longitudinal directions. Using these response functions, appropriate correction methods determined to eliminate the volume effect can be applied.

SU-E-T-129: Rescaling of IMRT Verification Deviations from Detector Arrays into the Patient.

PURPOSE: To discuss a method of "rescaling" failures detected in a Gamma-Index analysis with detector arrays into the patient as a re-evaluation method in IMRT verifications. METHODS: In a homogeneous phantom, plane signals measured with ionisation chamber arrays during IMRT field-by-field verifications can be understood as resulting from the convolution of the incident photon fluence with two spatially invariant kernels (dose deposition in the measurement plane and spatial detector response). In principle, Gamma-Index deviations between planned and the measured plane signal profiles can therefore be "back-projected" into a deviation of the photon fluence profile from the planned one which can be "forward-projected" as a dose deviation at test points in the patient. Assuming this model for each field under investigation, all Gamma-Index failures in a certain array region can thus be "rescaled" as deviations of the patient dose and evaluated by with patient related tolerances. The method is evaluated in the prostate and H&N region by use of the 2D-ARRAY729 and VeriSoft5.0. The software offers the possibility to project the position of the ionization chambers onto the patient's CT. Thereby, the field specific analysis of deviations between measurements and plan is supported by suitable imaging procedures. Simulated Gamma-Index failures (3mm/3%) have been evaluated according to the method described above and compared with direct dose calculations. RESULTS: The degree of consistency between relative dose deviations predicted from 2D-ARRAY evaluations and corresponding relative dose deviations calculated within the patient is found to be in an acceptable range. The applicability in regions of inhomogeneity boundaries (air-tissue) and out of field regions was not included in this study and will be analyzed in the future. CONCLUSIONS: In combination with the described visualization tools, the method offers the possibility to re-evaluate the dose deviations inside the patient when Gamma-Index failures have been detected with 2D-arrays.

SU-E-T-113: Volume Effect Correction Factor KV for Small-Field Photon Dosimetry with Ionization Chambers.

PURPOSE: The volume effect of ionization chambers gives rise to a spatial averaging effect that can be expressed mathematically as the convolution of the true dose profile with the detector's response function. The latter has been shown to be best described by Gaussian distribution. Based on this knowledge, the volume effect correction factor kV is derived. METHODS: To derive kV, a sixth degree polynomial is fitted to the true dose profile: D(x) = a0 + a2×2 + a4×4 + a6×6. The measured dose profile M(x) is calculated as the convolution product of D(x) with a one-dimensional normalized Gauss function with standard deviation s. Therefore kV at the dose maximum has the value D(0)/M(0), which is a function of the coefficients a0,2,4,6 and the detector specific s. In the case where D(x) is unknown, kV can be derived analogously from M(x) so that M(x) = b0 + b2×2 + b4×4 + b6×6, where kV can now be expressed as a function of the coefficients b0,2,4,6 and s. RESULTS: The magnitudes of kV,lat and kV,long were calculated for 1 to 5 cm dose profiles using measured s values, both in the lateral and the longitudinal directions, for a set of common ionization chambers. At field widths above 2 cm, the values of kV,lat fall below 1.01 for all the chambers evaluated, whereas it needs field widths above 4 cm to get all values of kV,long below 1.01. Since the detector's signal is integrated over the sensitive volume, the total kV can be calculated as kV,total = kV,lat . kV,long. CONCLUSIONS: In this work, a correction is developed to eliminate the volume effect of ionization chambers when they are positioned in the maxima of dose profiles, particularly for the performance of output factor measurements for the calibration of narrow photon beams.

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-174: Synopsis of the Experimental and Monte Carlo Calculated Values of the Radial EPOM Shift of Cylindrical Ionization Chambers for Photon-Beam Dosimetry.

PURPOSE: According to the concept of the effective point of measurement (EPOM), the values of depth z serving as the variable of photon-beam depth dose distributions in water measured with cylindrical ionization chambers are commonly corrected according to the formula zeff=zR+▵z, were zeff is the depth of the EPOM, zR the depth of the reference point of the chamber, i.e. of its symmetry axis, and Az the EPOM shift. Agreed-upon values of the EPOM shift are for instance ▵z=-0.6r (IAEA TRS 398, 2000) and ▵z=-0.5r (DIN6800-2,2008) with r=radius of the air-filled volume. The EPOM concept holds in the falling as well as in the rising curve branch. The Az versus r relationship is currently reviewed. METHODS: Measurements of the EPOM shift of cylindrical ionization chambers for 6/15 MV photons had been based upon a comparison with depth dependent dose distributions in water measured with the Roos chamber (PTW Freiburg). Its EPOM position (1.5 mm below the front face) had been determined by comparison with radiochrome films. (Looe et al, Phys.Med.Biol. 2011;56:4267-4290). Available are also the experimental EPOM shift values by Huang et al, Physica Medica 2010;26:126-131 and the classical value of Johansson et al, IAEA -SM-222/35,243-270(1978). Accurate Monte Carlo values had been supplied by Tessier and Kawrakow, Med.Phys. 2010;37:96-107. RESULTS: As shown by the graphical display of Az versus r, the relationship is nonlinear, shaped as a hockey stick, with values around -0.2 mm for the "pinpoint" chambers and with values near -1.4 mm for the often-used chambers with radii near 3 mm. For r ranging from 1 to 4 mm, the relationship can be approximated by ▵z = -(2.25 mm)×[1-exp(-0.0122 r̂4)] with r in mm. CONCLUSIONS: Time has come for a nonlinear updating of the EPOM shift versus radius relationship of cylindrical ionization chambers applied in photon-beam dosimetry.

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 detector's 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.

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.