http://nova.newcastle.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:7064 Purpose: Commercial EPIDs are normally used in indirect detection mode (iEPID) where incident x-ray photons are converted to optical photons in a phosphor scintillator, which are then detected by a photodiode array. The EPIDs are constructed from a number of nonwater equivalent materials which affect the dose response of the detector. The so-called direct detection EPIDs (dEPIDs), operating without the phosphor layer, have been reported to display dose response close to in-water data. In this study, the effect that different layers of materials in the EPID have on the dose response was experimentally investigated and evaluated with respect to changes in field size response and beam profiles. Methods: An iEPID was disassembled and the different layers of materials were removed or replaced with other materials. Data were also obtained on and off the support arm and with a sheet of opaque paper blocking the optical photons from the gadolinium oxysulfide (Gd2S2O: Tb) phosphor layer. Field size response was measured for field sizes ranging from 2 x 2 to 25 x 25 cm(2), and profiles for the 25 x 25 cm(2) beams were extracted from the data. Results: The iEPID configuration was found to be very sensitive to backscatter. The increases in output with solid water backscatter compared to the no backscatter case were 14.7% and 6.6% at the largest field size investigated for the 6 and 18 MV beams, respectively. The Gd2S2O: Tb phosphor layer had a large influence on field size response as well as beam profiles for 6 MV photons, while no major effects were observed for the 18 MV beam. For 18 MV large differences in dose response were found when the standard 1 mm Cu buildup was changed for d(max) equivalent Cu or solid water buildup, indicating that head scatter largely influences dose response for this energy. When the optical photons originating in the Gd2S2O: Tb layer were blocked from reaching the photodiodes, both field size output data and beam profiles corresponded well with data obtained in the dEPID configuration as well as reference ion chamber data for both energies. Conclusions: As expected, changing the layers of material in the EPID had a dramatic effect on dose response, which was often quite complex. For 6 MV, the complex dose response is mainly caused by the optical photons from the Gd2S2O: Tb layer, while insufficient filtering of scattered radiation largely affects the dose response for the 18 MV beam. The iEPID was also found to be very sensitive to backscatter for both energies. Blocking the optical photons created in the Gd2S2O: Tb layer essentially changed the iEPID configuration into the dEPID configuration, thus demonstrating great potential for a system that can be optimized for both imaging and dosimetry. 2012-01-30T05:02:29.127Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:7317 The routine use of electronic portal imaging devices (EPIDs) as dosimeters for radiotherapy quality assurance is complicated by the non-water equivalence of the EPID's dose response. A commercial EPID modified to a direct-detection configuration was previously demonstrated to provide water-equivalent dose response with dmax solid water build-up and 10 cm solid water backscatter. Clinical implementation of the direct EPID (dEPID) requires a design that maintains the water-equivalent dose response, can be incorporated onto existing EPID support arms and maintains sufficient image quality for clinical imaging. This study investigated the dEPID dose response with different configurations of build-up and backscatter using varying thickness of solid water and copper. Field size output factors and beam profiles measured with the dEPID were compared with ionization chamber measurements of dose in water for both 6 MV and 18 MV. The dEPID configured with dmax solid water build-up and no backscatter (except for the support arm) was within 1.5% of dose in water data for both energies. The dEPID was maintained in this configuration for clinical dosimetry and image quality studies. Close agreement between the dEPID and treatment planning system was obtained for an IMRT field with 98.4% of pixels within the field meeting a gamma criterion of 3% and 3 mm. The reduced sensitivity of the dEPID resulted in a poorer image quality based on quantitative (contrast-to-noise ratio) and qualitative (anthropomorphic phantom) studies. However, clinically useful images were obtained with the dEPID using typical treatment field doses. The dEPID is a water-equivalent dosimeter that can be implemented with minimal modifications to the standard commercial EPID design. The proposed dEPID design greatly simplifies the verification of IMRT dose delivery. 2011-09-13T00:11:32.288Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:1021 In this study we investigate the characteristics of a rounded leaf end multileaf collimator (MLC) that is used for delivering intensity-modulated radiotherapy (IMRT) with a Varian linear accelerator. The rounded leaf end MLC design results in an offset between the radiation field edge (the physical leaf position) and the light field (the geometric leaf position). We call this the radiation field offset (RFO). The leaf position is calibrated to the leaf tip at the mid-leaf plane. There is an additional offset between the geometric leaf position and the projected leaf tip position that varies as a function of distance from the collimator central axis due to the MLC geometry. We call this the leaf position offset (LPO). There is a lack of consistency in the interpretation and implementation of the RFO and the LPO in the literature. We investigated the RFO and the LPO on Varian's 600 C/D and 21 EX linear accelerators. We used a combination of film and ion chamber measurements of static, segmental MLC (SMLC) and dynamic MLC (DMLC) fields to quantify the leaf offsets across the range of leaf positions. We were able to improve the dosimetry at large off-axis positions with minor adjustments to the vendor's LPO file. The RFO was determined to within 0.1 mm accuracy at the collimator central axis. The measured RFO value depends on whether the method is based on the radiation field edge position or on an integral dose measurement. The integral dose method results in an RFO that is approximately 0.2 mm greater than the radiation field edge method. The difference is due to the MLC penumbra shape. We propose a methodology for measuring and implementing MLC leaf offsets that is suitable for both SMLC and DMLC IMRT. In addition, we propose some definitions that more clearly describe the MLC leaf position for accurate IMRT dosimetry. 2011-09-13T00:00:07.920Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:5040 The purpose of this study was to experimentally quantify the change in response of an amorphous silicon (a-Si) electronic portal imaging device (EPID) to dynamic multileaf collimator (dMLC) beams with varying MLC-transmitted dose components and incorporate the response into a commercial treatment planning system (TPS) EPID prediction model. A combination of uniform intensity dMLC beams and static beams were designed to quantify the effect of MLC transmission on EPID response at the central axis of 10×10 cm² beams, at off-axis positions using wide dMLC beam profiles, and at different field sizes. The EPID response to MLC transmitted radiation was 0.79±0.02 of the response to open beam radiation at the central axis of a 10×10 cm² field. The EPID response to MLC transmitted radiation was further reduced relative to the open beam response with off-axis distance. The EPID response was more sensitive to field size changes for MLC transmitted radiation compared to open beam radiation by a factor of up to 1.17 at large field sizes. The results were used to create EPID response correction factors as a function of the fraction of MLC transmitted radiation, off-axis distance, and field size. Software was developed to apply the correction factors to each pixel in the TPS predicted EPID image. The corrected images agreed more closely with the measured EPID images in areas of intensity modulated fields with a large fraction of MLC transmission and, as a result the accuracy of portal dosimetry with a-Si EPIDs can be improved. Further investigation into the detector response function and the radiation source model are required to achieve improvements in accuracy for the general case. 2011-02-02T05:20:01.399Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:5384 Electronic portal imaging devices (EPIDs) integrated with medical linear accelerators utilize an indirect-detection EPID configuration (ID-EPID). Amorphous silicon ID-EPIDs provide high quality low dose images for verification of radiotherapy treatments but they have limitations as dosimeters. The standard ID-EPID configuration includes a high atomic number phosphor scintillator screen, a 1 mm copper layer, and other nonwater equivalent materials covering the detector. This configuration leads to marked differences in the response of an ID-EPID compared to standard radiotherapy dosimeters such as ion chambers in water. In this study the phosphor and copper were removed from a standard commercial EPID to modify the configuration to a direct-detection EPID (DD-EPID). Using solid water as the buildup and backscatter for the detector, dosimetric measurements were performed on the DD-EPID and compared to standard dose-in-water data for 6 and 18 MV photons. The sensitivity of the DD-EPID was approximately eight times less than the ID-EPID but the signal was sufficient to produce accurate and reproducible beam profile measurements for open beams and an intensity-modulated beam. Due to the lower signal levels it was found necessary to ensure that the dark field correction (no radiation) DD-EPID signal was stable or updated frequently. The linearity of dose response was comparable to the ID-EPID but with a greater under-response at low doses. DD-EPID measurements of field size output factors and beam profiles at the depth of maximum dose (dmax), and tissue-maximum ratios between the depths of 0.5 and 10 cm, were in close agreement with dose in water measurements. At depths beyond dmax the DD-EPID showed a greater change in response to field size than ionisation chamber measurements and the beam penumbrae were broader compared to diode scans. The modified DD-EPID configuration studied here has the potential to improve the performance of EPIDs for dose verification of radiotherapy treatments. 2011-02-02T05:20:01.273Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:3149 The aim of this work was to experimentally determine the difference in response of an amorphous silicon (a-Si) electronic portal imaging device (EPID) to the open and multileaf collimator (MLC) transmitted beam components of intensity modulated radiation therapy (IMRT) beams. EPID dose response curves were measured for open and MLC transmitted (MLCtr) 10×10 cm² beams at central axis and with off axis distance using a shifting field technique. The EPID signal was obtained by replacing the flood-field correction with a pixel sensitivity variation matrix correction. This signal, which includes energy-dependent response, was then compared to ion-chamber measurements. An EPID calibration method to remove the effect of beam energy variations on EPID response was developed for IMRT beams. This method uses the component of open and MLCtr fluence to an EPID pixel calculated from the MLC delivery file and applies separate radially dependent calibration factors for each component. The calibration procedure does not correct for scatter differences between ion chamber in water measurements and EPID response; these must be accounted for separately with a kernel-based approach or similar method. The EPID response at central axis for the open beam was found to be 1.28±0.03 of the response for the MLCtr beam, with the ratio increasing to 1.39 at 12.5 cm off axis. The EPID response to MLCtr radiation did not change with off-axis distance. Filtering the beam with copper plates to reduce the beam energy difference between open and MLCtr beams was investigated; however, these were not effective at reducing EPID response differences. The change in EPID response for uniform sliding window IMRT beams with MLCtr dose components from 0.3% to 69% was predicted to within 2.3% using the separate EPID response calibration factors for each dose component. A clinical IMRT image calibrated with this method differed by nearly 30% in high MLCtr regions from an image calibrated with an open beam calibration factor only. Accounting for the difference in EPID response to open and MLCtr radiation should improve IMRT dosimetry with a-Si EPIDs. 2010-04-27T05:06:10.335Z ]]>