PTW Freiburg Research Articles

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

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

SU-E-T-209: Independent Dose Calculation in FFF Modulated Fields with Pencil Beam Kernels Obtained by Deconvolution

Purpose: To obtain the pencil beam kernels that characterize a megavoltage photon beam generated in a FFF linac by experimental measurements, and to apply them for dose calculation in modulated fields. Methods: Several Kodak EDR2 radiographic films were irradiated with a 10 MV FFF photon beam from a Varian True Beam (Varian Medical Systems, Palo Alto, CA) linac, at the depths of 5, 10, 15, and 20cm in polystyrene (RW3 water equivalent phantom, PTW Freiburg, Germany). The irradiation field was a 50 mm diameter circular field, collimated with a lead block. Measured dose leads to the kernel characterization, assuming that the energy fluence exiting the linac head and further collimated is originated on a point source. The three-dimensional kernel was obtained by deconvolution at each depth using the Hankel transform. A correction on the low dose part of the kernel was performed to reproduce accurately the experimental output factors. The kernels were used to calculate modulated dose distributions in six modulated fields and compared through the gamma index to their absolute dose measured by film in the RW3 phantom. Results: The resulting kernels properly characterize the global beam penumbra. The output factor-based correction was carried out adding the amount of signal necessary to reproduce the experimental output factor in steps of 2mm, starting at a radius of 4mm. There the kernel signal was in all cases below 10% of its maximum value. With this correction, the number of points that pass the gamma index criteria (3%, 3mm) in the modulated fields for all cases are at least 99.6% of the total number of points. Conclusion: A system for independent dose calculations in modulated fields from FFF beams has been developed. Pencil beam kernels were obtained and their ability to accurately calculate dose in homogeneous media was demonstrated.

SU‐E‐T‐46: Application of a Twin‐Detector Method for the Determination of the Mean Photon Energy Em at Points of Measurement in a Water Phantom Surrounding a GammaMed HDR 192Ir Brachytherapy Source

Purpose:In a brachytherapy photon field in water the fluence‐averaged mean photon energy Em at the point of measurement correlates with the radiation quality correction factor kQ of a non water‐equivalent detector. To support the experimental assessment of Em, we show that the normalized signal ratio NSR of a pair of radiation detectors, an unshielded silicon diode and a diamond detector can serve to measure quantity Em in a water phantom at a Ir‐192 unit.Methods:Photon fluence spectra were computed in EGSnrc based on a detailed model of the GammaMed source. Factor kQ was calculated as the ratio of the detector's spectrum‐weighted responses under calibration conditions at a 60Co unit and under brachytherapy conditions at various radial distances from the source. The NSR was investigated for a pair of a p‐type unshielded silicon diode 60012 and a synthetic single crystal diamond detector 60019 (both PTW Freiburg). Each detector was positioned according to its effective point of measurement, with its axis facing the source. Lateral signal profiles were scanned under complete scatter conditions, and the NSR was determined as the quotient of the signal ratio under application conditions x and that at position r_ref = 1 cm.Results:The radiation quality correction factor kQ shows a close correlation with the mean photon energy Em. The NSR of the diode/diamond pair changes by a factor of two from 0–18 cm from the source, while Em drops from 350 to 150 keV. Theoretical and measured NSR profiles agree by ± 2 % for points within 5 cm from the source.Conclusion:In the presence of the close correlation between radiation quality correction factor kQ and photon mean energy Em, the NSR provides a practical means of assessing Em under clinical conditions. Precise detector positioning is the major challenge.

Elaboration of patient DRL in CBCT in dental practice

The aim of present study is to perform a national patient dose survey and to elaborate a DRL in Cone beam CT used for dental purposes. The patient dose was measured with calibrated KAP dosemeter Diamentor E2 (PTW Freiburg) with a standart head CT phantom. Also the size of the radiation field was measured with Gafchromic film. The pilot measurements was performed on two CBCT: NewTomVG and ILUMA – Imtec 3M. The result data was widely distributed, because of the different FOV and time of exposure. Complete set of ongoing measurements and analysis will be presented, as well as findings and conclusions.

National patient dose survey in dental CBCT and elaboration of national DRLs

National patient dose survey in dental CBCT and elaboration of national DRLs

SU-E-T-54: Dosimetric Verification of Monte Carlo Dose Calculations for a GammaMed HDR 192Ir Brachytherapy Source Using Various Detector Types, and Non-Reference Condition Correction Factors KNR

Purpose: To dosimetrically verify a Monte Carlo model of a GammaMed HDR brachytherapy source using various detector types, namely an unshielded diode (EDD‐5), a semiflex air‐filled ion chamber (PTW Freiburg), a TLD type 100H (LiF:Mg,Cu,P) and a radiochromic EBT3 film. The applicability of the sensitivity correction factor kNR, correcting the spectral dependence of the detector response, was investigated for the diode and TLD detectors. Methods: The EGSnrc Monte Carlo program was used to model the source and to compute lateral dose profiles and photon fluence spectra within a water phantom of dimensions (Radius = 20 cm, Height =40 cm). Special detector holders were constructed for the dosimetric verification of the source using four different detector types. Factor kNR was calculated using the ratio of the weighted responses Yt of a given detector t at radial distance r = 1 cm from the source and that under non‐reference conditions x (other off‐axis points); kNR = Yt(xref)/Yt(x). Gamma index 3%, 3mm was used to compare measured against Monte Carlo profiles. Results: Between radial positions from 1–10 cm from the source, the average deviation of measurements from Monte Carlo simulations using the gamma criterion were (values in braces); diode (0.46), semiflex (0.69), TLD 100H (1.6) and EBT3 (0.44). Considering measurement points closer to the source, the lowest average deviations were obtained with the semiflex chamber despite possible dose averaging effects. Precise dosimetry at the source surface is challenging, for example due to problems by water diffusion into the film. Conclusion: The modeled GammaMed HDR has been successfully validated in a medium‐sized cylindrical water phantom. This study demonstrates that the semiflex chamber is suitably applicable without kNR corrections, while the unshielded diode or TLD 100H are equivalently applicable for high‐precission measurements on application of kNR.

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.

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. 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. 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. 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-61: IMRT Patient QA with Dosimetry Check: The Effect of Detector Selection in Volumetric Dose Reconstruction

Purpose: to evaluate the effect of detector selection in conjunction with the Dosimetry Check system for CT based dose reconstruction from measurement of the fluence maps of IMRT beams. Methods: Twenty step and shoot IMRT patients were used to evaluate the performance of the Dosimetry Check software (Math Resolution, LLC, Maryland). Integrated images for each patient were taken with an aSi1000 EPID (Varian Medical Systems), Seven29 (PTW Freiburg, Germany), and an I'mRT MatriXX (IBA dosimetry, Germany). The treatments were delivered using a Varian Novalis Tx with an HD 120 MLC system. Each plan was delivered and measured by all detectors. An integrated image was taken for each individual field and imported into the Dosimetry Check software. Using the Pinnacle treatment planningsoftware, the CT scans, structure, plan, and dose files were imported in to the Dosimetry Check software. The software then used the imported fluence fields to calculate the absolute dose and dose volume histogram to compare with the reference data exported from the treatment planning system. Gamma analysis was used to compare the measured data with the reference data. A 3% ‐ 3mm criteria with a 5% dose threshold was used. Results: Good agreement was found between the calculated doses and the reference doses for all patients. For all detectors the gamma index calculations ranged from 90% to 100% pass rate. Similar results were seen for each patient using one of the ionization chamber arrays, while a 1–2% increase in the gamma index was seen when using the EPID. Conclusions: A slight increase in performance is observed with increasing detector resolution. It is desirable to use the aSi1000 EPID to get the best agreement between calculated and reference doses. However, the ionization chamber arrays both performed adequately and can be used for IMRT QA with the Dosimetry Check software.

SU‐E‐T‐151: Application of a 2D Ionization Chamber Array for Monthly and Annual Linear Accelerator QA

Purpose: An evaluation of a commercially available 2D array for monthly and annual linear accelerator QA. Methods: The 2D‐ARRAY seven29 (PTW, Freiburg, Germany) is a two‐dimensional ion chamber array, consisting of 729 (27 × 27) vented ion chambers, with separately housed evaluation electronics. The parallel‐plate chambers are 5 × 5 × 5 mm in size, and have a center to center spacing of 10 mm. To increase resolution, the array can be shifted three times in increments of 5 mm, and the resulting 2916 measurement points can be summed into one image. The array was placed under 10 cm of water‐equivalent material, and irradiated to evaluate: flatness and symmetry, static MLC positioning, dynamic MLC positioning, and output. It was also irradiated without any buildup for the light field and radiation coincidence test. All deliveries were completed using a Varian 2100 C/D linear accelerator with a Millennium‐80 MLC. For improved accuracy, the static and dynamic MLC fields were delivered 4 times each, and were then summed using the “merge” function in the PTW VeriSoft software. Results: Over one month of data acquisition, output fluctuated less than 2%. For light field and radiation coincidence, reproducibility is excellent, with a maximum of 1% variation in asymmetric jaw settings. When using the “merge” function on dynamic MLC measurements, results were less than 4% for transmission and 0.5 mm for positioning errors. Measurements for flatness and symmetry and static MLC positioning also had good reproducibility. Conclusions: The PTW 2D‐Array seven29 can be used to evaluate many areas of monthly linear accelerator QA. Particularly in light of the trend for departments to go filmless, it is a viable alternative for obtaining flatness and symmetry, static and dynamic MLC positioning, light field and radiation coincidence, and for measuring output.This work partially supported by PTW (Freiburg, Germany).

SU‐E‐T‐155: Evaluation of An in Vivo Device for Daily Patient Treatment Delivery Measurements Using a Varian Linear Accelerator

Purpose: A clinical evaluation of a commercially available dosimetric device for in‐vivo monitoring of IMRT and VMAT treatments. Methods: The Device for Advanced Verification of IMRT Deliveries (DAVID) system (PTW, Freiburg, Germany) is a transparent multi‐wire harp chamber that can slide into the wedge slot of a Varian linear accelerator. Its 40 measurement wires correspond to the positions of the leaf pairs of a Millennium‐80 MLC, with a projected spacing at isocenter of 5 mm. The DAVID was inserted into the beam during deliveries of clinically approved IMRT and VMAT treatments. The ability of the DAVID to detect errors in MLC positions was tested by introducing small errors into the plans. The reproducibility of measurements was tested for several disease sites over a series of deliveries. Results: The reproducibility of IMRT and VMAT plans is good. The maximum deviation of any one leaf among all plans and all deliveries was 3.11% (IMRT) and −2.09% (VMAT). The maximum total deviation among all plans and all deliveries was 3.41% (IMRT) and −1.99% (VMAT). The average total deviation among all plans and all deliveries for IMRT was 1.35% ± 1.06% and for VMAT was 0.40% ± 1.35%. When intentional errors were introduced, the DAVID could consistently detect leaf position errors when the resulting gap differed from the plan by great than or equal to 1 mm. Conclusions: The DAVID system is a useful, reliable tool for daily in‐vivo monitoring of patient treatments. It is capable of detecting errors as small as 1 mm, and requires no additional time or effort by the treatment staff for data collection. The DAVID system can provide assurance that every fraction of an IMRT or VMAT treatment is being executed as planned.This work partially supported by PTW (Freiburg, Germany).

Quality assurance of electron and photon beam energy using the BQ‐Check phantom

The BQ‐CHECK phantom (PTW Freiburg, Germany) has been designed to be used with a 2D ion chamber array to facilitate the quality assurance (QA) of electron and photon beam qualities (BQ). The BQ‐CHECK phantom has three wedges covering the diagonal axes of the beam: two opposed aluminum wedges used to measure electron energy and a single copper wedge used to measure photon energy. The purpose of this work was to assess the suitability of the BQ‐CHECK phantom for use in a routine QA program.A range of percentage depth dose (PDD) curves for two photon beams and four electron beams were measured using a MP3 plotting tank (PTW Freiburg). These beams were used to irradiate a STARCHECK array (PTW Freiburg) with and without the BQ‐CHECK phantom on top of the array. For photons, the ratio of the signals from two chambers underneath the copper wedge was used as an effective TPR measurement (TPReff) and, for electrons, the full width at half maximum of the profile (EFWHM) underneath the aluminum wedges was used as an electron energy constancy measurement. PDD measurements were compared with TPReff and EFWHM to assess the sensitivity of the BQ‐CHECK phantom.The clinical tolerances of TPReff were determined for 6 MV (0.634–0.649), and 10MV (0.683–0.692). For electrons, the clinical tolerances of EFWHM were determined for 6 MeV (94.8–103.4 mm), 8 MeV (105.5–114.0 mm), 10 MeV (125.4–133.9 mm) and 12 MeV (138.8–147.3 mm).Electron and photon energy metrics are presented which demonstrate that the BQ‐CHECK phantom could be used to form part of an efficient routine monthly QA program. Acceptable beam quality limits for various nominal beam energies were established and at these limits, modified profiles were acquired using the STARCHECK array. From the modified profiles, EFWHM and TPReff were determined for the electron and photon beams, respectively. It was demonstrated that both EFWHM and the TPReff have a linear relationship with conventional beam quality metrics.PACS numbers: 87.56.bd, 87.56.‐v

SU‐GG‐T‐342: Dosimetric Evaluation and Characterization of a MicroLion Liquid Ionization Chamber

Purpose: To evaluate the dosimetric characteristics of the microLion liquid ionization chamber. Evaluation criteria were dose rate dependence, energy and angular dependence, spatial resolution, and suitability for narrow field dosimetry. Method and Materials: Measurements were performed using a Varian 2300 C/D linear accelerator (Varian Medical System, Inc., Palo Alto CA). The microLion ionization chamber (PTW Freiburg, Germany) has a sensitive volume of 0.002cm3 with the centroid located at 0.975 mm from the entrance window. The spatial resolution, dose rate and energy dependence measurements were performed using the BluePhantom from Scanditronix‐Wellhofer (IBA, Memphis TN). A calibrated Unidoswebline electrometer (PTW Freiburg, Germany) was used to measure the charge and current. An external high voltage supply (PTW Freiburg, Germany) was used to provide the recommended bias voltage of 800V. Angular dependence was measured using a solid water phantom. Results: Dose rate analysis showed differences of <1.5%. Electron energy dependence showed differences of up to 3.0%. Output factors for field sizes smaller than 3.0×3.0cm2 showed differences when compared to a Semiflex detector. Small differences (<1.0%) in response due to angular dependence were observed. However, a 7.0% difference in response was noted when the detector was irradiated parallel, as opposed to perpendicular, to the beam direction. The microLion detector exhibited a sharper fall off in the penumbra region as well as a larger shoulder when compared to a Semiflex detector. Good reproducibility was demonstrated in percent depth dose curves. Conclusion: The dosimetric evaluation of the microLion detector, a liquid ionization chamber, showed that the detector demonstrates good relative and absolute dosimetry characteristics. When operated at 800V bias, the detector shows good signal response in addition to good stability. The extremely small sensitive volume allows the microLion to be used for small field dosimetry.

MO‐FF‐A2‐06: Feasibility of Using a Multi‐Array Ion Chamber Detector for Daily QA in Protons Therapy

Purpose: Feasibility of using a multiarray detector system for daily quality assurance checks in proton therapy. Method and Materials: The daily dose output of the passive scattering beamline for one range modulator wheel (RMW) is measured at 4 different locations along the depth (proximal, center and distal to the plateau and at the end of particle range) using a 2D‐multiarray detector system (Starcheck, PTW Freiburg, Germany). Different depths were generated along the SOBP by using a dedicated compensator. The flatness and symmetry of the beam were measured by irradiating the detector using a 15×15cm2 aperture without the compensator. Results: The diagonal profiles of the high‐energy medium‐scatter RMW were measured. The central part of each profile is influenced by scatter from the thicker part of the compensator into the thin proximal area. Therefore, the average dose value is determined 50 mm from the center by averaging the readings of 8 ion chambers. The dose output is in agreement with those from the commissioning data. This QA dosimeter system has a rather good reproducibility. The maximum relative deviation from the average measured dose with this system was 2.3% for all data, except for the end of range location. By using the ICRU78‐protocol, the symmetry and flatness was determined to be 0.23±0.07% and 1.8±0.2% respectively, which are within acceptable tolerance limits for this energy. Conclusion: The Starcheck with the dedicated compensator offers several advantages over conventional single point measurement with one ionization chamber for daily QA, namely, the ability to measure the depth dose and flatness and symmetry using the same device quickly. The use of this system allows both an independent flatness and symmetry check to be performed on a daily basis without increasing the time and efficient QA checks for all the RMWs used clinically.

SU‐GG‐T‐314: Daily Evaluation of in Vivo Dose Verification Device

Purpose: To evaluate daily and overall performance of an in vivo dose verification device. Methods and Materials: Five patients were used to evaluate the response of the in‐vivo measurement device DAVID (PTW Freiburg GmbH, Germany) against an ionization chamber array (I'mRT MatriXX, IBA dosimetry, GmbH, Germany). For each patient, two almost equivalent plans were created in Eclipse. The plans were optimized using 5 to 7 static IMRT beams delivered using sliding window and RapidArc. The treatments were delivered using a Varian Clinic 2100 C/D with a 120 leaf Millennium MLC. Each plan was measured by both detectors for six days. The PTW DAVID software automatically compares the reproducibility of each treatment to the reference measurement. Using the OmniPro‐IMRT software, the daily planar dose distribution was compared to the reference, and the gamma index was calculated using 2%‐ 2mm. For each type of IMRT treatment, the PTW DAVID day‐to‐day measurements were recorded and compared. Results: The day‐to‐day differences measured by PTW DAVID were up to 8% and 5% for RapidArc and IMRT sliding window deliveries respectively. The corresponding RapidArc gamma index calculations ranged from 91% to 100% pass rate but no correlation between PTW DAVID and I'm RT MatriXX measurements was observed. Overall, the dose measurements with the I'm RT MatriXX had less variability than with IMRT sliding window, while the opposite was observed for the PTW measurements. Conclusion: An in vivo measurement device for daily QA of the delivered dose was evaluated for its reproducibility. Our results show that the overall good reproducibility of the device is observed when used for RapidArc and IMRT sliding window plans. However, the in vivo daily variations did not correlate to similar dose variations as measured by an ionization chamber array. Research funded by PTW Germany

Praktische Dosimetrie und Konstanzprüfung zur Einführung der intraoperativen Bestrahlung mit Intrabeam ® (Zeiss)

The check of dosimetry of the intraoperative radiotherapy system Intrabeam ® is predefined by the manufacture (Zeiss). The purpose of the study was to develop and implement a method to verify the internal dosimetry of Intrabeam ® (Zeiss). Additionally the long-term stability of Intrabeam ® was checked for dose and isotropy. For dose to water measurements an Unidos ® was combined with a soft jet chamber (TM 23342) which was calibrated in water absorbed dose and as a phantom the type 2962 (PTW Freiburg) was used. RW1 plates were inserted as build up material. The applicators were placed in a bag filled with water to consider the side-scattering. At the surface of the applicator there was a mean difference of 3 percent between the dose to water measurement and the internal dosimetry. The constancy of the dose rate showed a mean deviation of 0.3% at the reference point. The analysis of the dose distribution perpendicular to the applicator axis z (reference z-axis) resulted in a mean deviation of −2.7% (x-direction) and −7,1% (−x-direction) for the x-axis and, respectively −4.1% (y-direction) and −5.3% (−y-direction) for the y-axis. The proposed method is suitable to verify the absolute dose of Intrabeam ®. The dose values measured by this method were congruent to the dosimetry of the manufacture (Zeiss). From our point of view it is sufficient to verify the absolute dosimetry only at time of commissioning of the system or in the case of changing the applicator. For the daily routine the check of constancy specified by the manufacture is adequate, because the dose rate is checked on a daily basis. Additionally the test of constancy showed a high long-term stability in terms of dose rate and symmetry.

Determining superficial dosimetry for the internal canthus from the Monte Carlo simulation of kV photon and MeV electron beams

This paper presents the findings of an investigation into the Monte Carlo simulation of superficial cancer treatments of an internal canthus site using both kilovoltage photons and megavoltage electrons. The EGSnrc system of codes for the Monte Carlo simulation of the transport of electrons and photons through a phantom representative of either a water phantom or treatment site in a patient is utilised. Two clinical treatment units are simulated: the Varian Medical Systems Clinac 2100C accelerator for 6 MeV electron fields and the Pantak Therapax SXT 150 X-ray unit for 100 kVp photon fields. Depth dose, profile and isodose curves for these simulated units are compared against those measured by ion chamber in a PTW Freiburg MP3 water phantom. Good agreement was achieved away from the surface of the phantom between simulated and measured data. Dose distributions are determined for both kV photon and MeV electron fields in the internal canthus site containing lead and tungsten shielding, rapidly sloping surfaces and different density interfaces. There is a relatively high level of deposition of dose in tissue-bone and tissue-cartilage interfaces in the kV photon fields in contrast to the MeV electron fields. This is reflected in the maximum doses in the PTV of the internal canthus field being 12 Gy for kV photons and 4.8 Gy for MeV electrons. From the dose distributions, DVH and dose comparators are used to assess the simulated treatment fields. Any indication as to which modality is preferable to treat the internal canthus requires careful consideration of many different factors, this investigation provides further perspective in being able to assess which modality is appropriate.

SU‐GG‐T‐144: IMRT Pre‐Treatment Plan and In‐Vivo Verification with 2D‐ARRAYs and Multi‐Wire Ionisation Chambers

Purpose: A concept for quality assurance and IMRT plan verifications with 2D‐ARRAYs and the DAVID‐ System is presented. Method and Materials: A 2D‐ARRAY (PTW‐Freiburg) and a multi‐wire ionization chamber (DAVID‐system, PTW Freiburg) are used in this work. The DAVID‐system is a translucent, multiwire transmission‐type ionisation chamber, placed in the accessory holder of the accelerator. Each detection wire is positioned in the projection line of a MLC leaf pair. The signal of each wire is proportional to the line integral of the ionisation density along this wire. The 2D‐ARRAY is used for daily dosimetrical checks (dose on central axis, MLC calibration, symmetry, flatness, energy) and for pretreatment IMRT plan verification. During the dosimetric verification of an IMRT plan reference values are measured with the DAVID‐system and stored in a patient specific database. During daily treatment the signals are re‐measured and compared to the reference values. Thus a direct connection to the IMRT plan verification is possible. A warning occurs if a deviation beyond a chosen threshold is detected. In an “expert” mode the physicist can analyse each single segment of the plan and detected errors are related to MLC pair. If necessary a re‐verification with 2D‐ARRAYs can be performed. Results: The application of the concept for standard IMRT cases and examples for error detection capabilities (e.g. de‐calibrated MLCs, neglected segments) are shown. For typical IMRT plans with field sizes beyond 10 cm × 10 cm the DAVID‐ system is able to detect positioning errors of MLC pairs in the sub‐millimeter region. Conclusion: The procedures can be used during daily routine with a minimum of additional time and are assuring a closed dosimetrical QA loop for IMRT.

Correction to BrainSCAN central axis dose calculations for 6-MV photon beams to lung with lateral electron disequilibrium

Purpose: To develop and evaluate a correction method for lateral electron disequilibrium and tissue inhomogeneities in lung tissues applicable to the BrainSCAN treatment planning system. Methods and Materials: Four noncoplanar 6-MV photon beams with different beam diameters were applied to the right lung of a thorax phantom. The measured/calculated dose value ratio was evaluated as a function of a parameter that describes the degree of the lateral electron disequilibrium based on the primary dose. Results: The dose ratio showed a clearcut linear dependency on the disequilibrium parameter. Applying the proposed correction method, only minor differences between the measured and calculated doses were found for lesions >1 cm. However, for lesions Conclusion: The data have indicated a relevant magnitude of the correction factor only for lung lesions

Sci‐Sat AM (2) Therapy‐03: Characterization of Intermediate Energy X‐Ray Photons (0.2–1.0 MeV) for stereotactic radiosurgery: experimental demonstration of reduced radiological penumbra

Introduction: Stereotactic radiosurgery is used to treat intracranial lesions with a high degree of accuracy. At the present time, x‐ray energies at or above Co‐60 gamma rays are used. We hypothesize that intermediate energy x‐ray photons (IEP's), combined with small field sizes, produce a reduced radiological penumbra leading to a sharper dose gradient. The purpose of this work is three‐fold: 1.) to produce IEP's using a medical linear accelerator 2.) to characterize the x‐ray energy 3.) to demonstrate reduced radiological penumbra for IEP's compared to 6MV x‐rays.Materials & Methods: A Siemens linear accelerator was modified to produce IEP's. PDD versus depth measurements were done in solid water using a Markus parallel plate ionization chamber (PTW Freiburg) at SSD=100cm for a 2×2cm2 field size. Monte Carlo computer simulations were done using MCNP‐4C. A penumbra measurement device was constructed to examine radiological penumbra for various photon energies, field sizes and depths. Film (Gafchromic EBT) was used to record field edge profiles. These films were scanned using a digital microscope (spatial resolution of 1.87 microns/pixel). Film irradiations were done using SSD=100cm, depth= 2cm, FS=1.1cm2.Results: For the IEP's, PDD values were 55.4%(surface), 62.6%(5cm), and 34.7%(10cm). Monte Carlo simulations suggest a nominal x‐ray energy of 800kV. The 80%–20% penumbra widths were 2.10mm (6MV) and 0.345mm (IEP's).Conclusions: A novel intermediate energy x‐ray beam (800kV) has been produced using a linear accelerator. There is more than a 5‐fold reduction in radiological penumbra for the IEP's versus 6MV x‐rays for the small field size examined.

SU‐FF‐T‐98: Application of DAVID, a Multi‐Wire Ionisation Chamber for in Vivo‐Verification of IMRT and Conformal Dose Distributions

Purpose: In this work we present first clinical results obtained with the DAVID chamber. Methods and Material: The DAVID system (PTW Freiburg, Germany) is a flat, translucent multi‐wire ionization chamber for daily in‐vivo verification of IMRT beams. It is placed in the accessory holder of the linear accelerator. Each detection wire of the chamber is positioned exactly in the projection line of two opposing leafs of the MLC. The signal of each detection wire is proportional to the line integral of the ionization density along this wire, therefore it is directly proportional to the opening of the associated leaf pair. The number of measurement channels equals the number of leaf pairs. The sum of all wire signals is a measure of the dose‐area product of the transmitted photon beam and of the total radiant energy administered to the patient. Results: After a successful dosimetric verification of an IMRT plan, the values measured by the DAVID system are stored as reference values. During daily treatment the signals are re‐measured and compared to the reference values. In case of a deviation beyond a threshold a warning occurs. Because the DAVID system operates as an ionization chamber, disadvantages which might be observed in other devices, such as aging, are not to be expected. Furthermore, the influences on the beam characteristics are negligible. Conclusion: Clinical examples demonstrate that the DAVID system is a relevant tool to improve the reliability of IMRT treatments. Conflict of Interest: The DAVID system was developed in cooperation with PTW‐Freiburg, Germany.