bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐206: A Comparison of Four IMRT Quality Assurance Techniques
Authors: McKissick M , Duggan D , Gaiser J
Published: August 2010

A comparison of four methods of intensity‐modulated radiation therapy (IMRT) quality assurance (QA) is described. A single nine field IMRT plan was evaluated using two planar arrays, the MapCheck diode array and PTW seven29 ion chamber array, and two different types of film (GAFChromic radiochromic film and Kodak Verification silver‐halide film) with an ion chamber. Both field‐by‐field (in which the dose map for each field was measured and evaluated separately) and composite measurements (in which all fields were summed) were performed. The composite measurements were performed with each beam at the planned gantry angle. The PTW and MapCheck arrays were used for single beam and composite measurements. Each type of film was used for a composite measurement with planned gantry angles. For each type of film, two sheets of film and the ion chamber were placed in a stack of water‐equivalent plastic slabs, so that a relative dose map in two planes and the absolute dose at a single point in a third plane were measured simultaneously. All measurements were compared to the dose predicted by the Varian Eclipse treatment planning system, which was used to map the fluence to a CT image of each phantom. For measurements made with the planar arrays, the software provided by the vendor was used for the comparison. The software packages rt‐image and FilmQA were used for the film measurements. All the software packages calculate a gamma factor for each measured point from a relative dose difference (D) and distance‐to‐agreement (DTA). The pass rates (percentage of measured points with gamma ≤ 1) varied considerably among the software packages used for comparison. Nevertheless, all methods showed that the treatment was delivered as planned.
SU‐GG‐T‐209: A Novel Method for Further Analysis of IMRT QA
Authors: Lawrence M , Cullip T , Potter L , Lian J , Wang S , Xu Z , Deschesne K , Chang S
Published: August 2010

Purpose: IMRT QA is often performed with a commercial device that compares 2D intensity maps to determine if a field passes certain dose and position criteria. The effect of an intensity map error on 3D dosimetry and DVH is clinically relevant but often not considered in IMRT QA. We herein exam the 3D dosimetric impact of IMRT failures measured with a 2D IMRT QA tool. Methods and Materials: 22 IMRT plans had at least one field fail QA using MapCHECK 2, and these were further analyzed using in‐house software to perform a DVH analysis. The MapCHECK 2 passing criteria was 95% of measured points had to be within 5% of the planned dose and have a distance‐to‐agreement of 4mm. Software was installed in our treatment planning system, PlanUNC, that reads intensity maps measured with the MapCHECK 2 and creates DVHs for 3D dose analysis. The software allows for field segment adjustment of MLC positions to address field edge failures and modification of prescribed MUs to address detector points that failed QA. Results: Of the 22 plans that had a field fail initial QA with MapCHECK 2, it was determined that 16/22 and 3/22 plans were within 1% and 2% of the planned CTV and critical structure DVHs. 3/22 plans were found to have DVH differences greater than 2%, and MU values were therefore altered in the treatment plan. Conclusions: Further analysis following an initial IMRT QA with the MapCHECK 2 is often needed to address the 3D dosimetric effects of the measured plan. Software has been installed into PLUNC that allows for 3D analysis for IMRT QA. Our analysis shows that using the MapCHECK 2 passing criteria of dose within 5% and 4mm DTA is sufficient, since 19/22 plans failing this criteria are within 2% of the planned DVH.
SU‐GG‐T‐211: An Independent MU Calculation Program for Dynamic IMRT
Authors: Dimofte A , Zhu T , Ivanova T
Published: August 2010

Purpose: The performance of an independent monitor unit calculation program is evaluated to verify the monitor unit (MU) for dynamic IMRT fields. The MU calculation program runs on a PC and is based on an empirical pencil‐beam model to determine D/MU. A correction methodology is used to correct for the output (head‐scatter) change for the dynamic IMRT fields. Methods and Materials: The D/MU of IMRT fields at the same point (mostly the hot spot point) was compared between treatment planning system and the independent MU calculation program. Here MU for an IMRT field refers to the total MU delivered. The fractional MU of each segment of an IMRT field, defined as the MU for the segment per total MU, was assumed to be the same. The commercial treatment planning system is based on a pencil‐beam convolution algorithm with headscatter modeling. D/MU of a subset of IMRT fields, which is delivered to a Mapcheck, were compared among the treatment planning system calculation, the independent MU calculations, and measurements. Results: Among the 10 patients examined, the difference of D/MU for all IMRT fields delivered to a point agreed to within 7% for 90% of patients. This error is slightly worse then the measurement, where 100% of patients are within 7% limit. However, when each individual IMRT field is examined, 15.6% (25/160) of the fields have a discrepancy larger than 7%. All those fields (>7%) lie in the valley region of an IMRT field. Conclusion: The independent MU calculation program can effectively verify the dose calculation accuracy for most IMRT fields. Only in very rare instances, which invariably involves the calculation point to be in very low dose region, actual measurements are needed to further verify the accuracy of dose calculation for IMRT field.
SU‐GG‐T‐215: Respiratory Gated IMRT Quality Assurance with Motion in Two Dimensions for Varying Field Sizes
Authors: Daniels M , Massie T
Published: August 2010

Purpose: Currently, there is little baseline information available on how varying amounts of tumor motion in multiple dimensions affect our ability to deliver acceptable respiratory‐gated IMRT. We present a study that considers how varying amounts tumor motion in two dimensions as well as varying field sizes effect respiratory gated IMRT quality assurance (QA) results. By developing this baseline information, we can develop guidelines for acceptable limits on tumor motion and field size for respiratory‐gated IMRT.Method and Materials: Respiratory‐gated IMRT QA was performed with Sun Nuclear's Mapcheck and MotionSim products. Respiratory‐gated IMRT patient plans were utilized in this study for four different field sizes. Simulated tumor motion ranged from no motion (the baseline case) to 1 cm in both the superior‐inferior and left‐right directions. A QA plan was generated for all four patient plans and delivered with varying amounts of tumor motion. The treated and planned dose planes were then compared. An acceptance criteria of 3%/ 3mm was utilized to determine if an IMRT plan passed or failed. In addition, a new IMRT QA product (Sun Nuclear's 3DVH) was utilized to evaluate how tumor motion affected the dose delivered to the tumor and critical organs. Results: Smaller field sizes allowed for more tumor motion then large field sizes when meeting our acceptance criteria. In addition, it was found that motion in the left‐right direction plays a significant role in the acceptability of respiratory‐gated IMRT. Conclusions: Our results demonstrate that there are several factors in determining the acceptability of respiratory‐gated IMRT. Field size as well as motion in two dimensions must be evaluated in order to ensure adequate tumor coverage during treatment. We found that the 3DVH product demonstrated a unique ability to determine how errors in respiratory ‐ated IMRT delivery affect tumor coverage and critical organ sparing.
SU‐GG‐T‐281: Clinical Experience with an EPID‐Based Quality Assurance System for Linear Accelerators
Authors: Heaton R , Norrlinger B , Smale J , Islam M
Published: August 2010

Purpose: To report our experience with the implementation and clinical use of an electronic portal imaging device (EPID) based tool for the routine quality assurance (QA) of linear accelerator (Linac) radiation field geometry. Methods and Materials: The development of an EPID‐base QA tool, eQA, for routine QA of Linac radiation field geometry has previously been reported. Implementation of eQA began in 2007 for both Elekta and Varian Linacs in our clinic, with deployment completed by mid‐2008. The eQA is currently used to perform all radiation field geometry QA, including radiation field size, light vs. radiation field congruence, jaw symmetry, MLC leaf position, and gantry, collimator and couch isocentre. The eQA is routinely used to accurately set the lasers to the radiation isocentre and perform MLC calibrations. Efforts were made to automate much of the eQA functionality, using information in DICOM header tags to automatically identify the type of test and determine associated parameters in the analysis of results. Email summaries of QA results are sent automatically to appropriate staff. Compared to a manual assessment of film‐based measurements, eQA provides reproducible, non‐subjective analysis of the radiation field, reducing the variability in measurements. This has enabled a tightening of the tolerance for radiation field size measurements. Results: The eQA has successfully replaced film for all radiation field geometry tests in our department, and has improved the efficiency of our QA processes. The non‐subjective, reproducible results obtained with eQA have allowed a reduction of radiation field size tolerance from 2 mm to 1 mm. The eQA provides the unique functionality to map the locations of the gantry, collimator and table centre of rotations onto a common reference frame for determining the isocenter sphere. Conclusions: The EPID based software tool has been successfully implemented, improving the efficiency and overall performance of our QA processes.
SU‐GG‐T‐282: Commissioning Compensator‐Based IMRT in the Pinnacle Treatment Planning System
Authors: Opp D , Feygelman V , Forster K
Published: August 2010

Purpose: To describe a detailed method of commissioning Pinnacle treatment planning system (TPS) for compensator‐based IMRT and subsequently to validate a 3D bi‐planar diode array (Delta4, ScandiDOS AB) as a tool for compensator QA. Methods and Materials: 50 commercial (.decimal Inc.) compensators were evaluated for the most probable compensator thickness (MPCT). Output factors, scatter factor, effective attenuation, energy spectrum, and incident fluence are all Pinnacle parameters that were evaluated and optimized. Orthogonal dose profiles and point doses were measured in a water phantom to validate the model. The absolute calibration of the Delta4 was performed with the MPCT slab of brass in the beam. The absolute dose agreement was verified for a variety of field sizes and brass thicknesses. The energy dependence of the Delta4 diodes was investigated by comparison to the ion chamber measurements. Results: The thickness of the variable portion of the compensators ranges from 1 to 3 cm, with 2 cm being the most probable one. Relative output factors have to be measured for filtered beams, otherwise errors of up to 4% can be expected on the central axis. With the optimized parameters, the error does not exceed 1.1% for 2cm thick brass. Scanned cross‐beam profiles generally agreed with calculations to better than 2%. Delta4 measurements for the 4‐field box arrangements of different field sizes exhibited gamma(3%/2mm) passing rates >97%. The diodes at shallow depths in the large fields demonstrated an over‐response of 1.5% compared to the ion chamber. Conclusion: Beam model modifications are recommended for commissioning compensator‐based IMRT in the Pinnacle TPS. The Delta4 absolute calibration should be performed with the MPCT brass slab. The diode over‐response due to scattered radiation from brass is limited to the largest field size and shallow depths. Middle of the phantom error is minimal. Research sponsored by Dot Decimal
SU‐GG‐T‐288: Dosimetric Characterization of a New Prototype Varian Linear Accelerator
Authors: Naqvi S , Schinkel C , Jiang Z , Lasio G , D'Souza W
Published: August 2010

Purpose: To characterize the dosimetric properties of a novel Varian linac, Trilogy‐MX, and compare them with those of C‐series Trilogy. The new design employs digital control electronics with new embedded computer architecture. The energy switch, bending magnet, and carousel assembly are redesigned. Trilogy‐MX allows three flattened and two high‐dose‐rate (>1000 MU/min) flattening filter free (FFF) photons modes. Method and Materials: Depth dose, transverse profiles and output factors are obtained using a Scanditronix CC04 ion chamber (0.04 cm3) in a water tank. Depth doses are normalized at 10 cm depth. Photon output factors are measured at 10 cm depth, 90 cm SSD Results: The 6MV and 18MV modes of the two designs match in PDD to within experimental uncertainty for fields <20×20cm2. Differences of ∼1% are observed for depths >20cm in the 6MV mode, and in the buildup region for the 18MV beam for fields >30×30cm2. Output factors also match well, with maximum deviations ∼1% for 6MV and ∼1.5% for 18MV observed for fields > 20×20cm2. The PDD for 6MV‐FFF and 10MV‐FFF modes fall faster than their flattened counterparts due to lack of beam hardening and reduced scatter from off‐axis regions. At 10 cm depth for a 20×20 field, the transverse gradients are ∼1.5%/cm and ∼3.5%/cm for the 6MV‐FFF and 10MV‐FFF modes respectively. For fields < 5×5, the flattened and unflattened 6MV beams have similar transverse profiles. The electron beams match within 1mm in PDD, but deviate by ∼3% in transverse profiles for the largest field sizes for higher energies. Conclusion: The common modes between the C‐series and Trilogy‐MX are clinically interchangeable. Small adjustments in beam modeling can remove the small differences observed, particularly for electrons. The 6MV‐FFF mode is useful for stereotactic treatments because of their high dose rates and reasonably flat transverse profile for small fields.
SU‐GG‐T‐289: A Lifelike Breathing Phantom and Model Tumor for IGRT and 4D Dose Calculation Applications
Authors: Court L , Seco J , Lu X , Ebe K , Mayo C , Ionascu D , Winey B , Giakoumakis N , Aristophanous M , Berbeco R , Rottmann J , Bogdanov M , Schofield D , Lingos T
Published: August 2010

Purpose: To evaluate an anatomical breathing phantom and model lung tumor for IGRT and 4D applications. Method and Materials: The shape of a tumor was extracted from a patient's CT image, and a replica tumor created in a flexible resin using rapid‐prototyping techniques. A 5mm grid of fine holes was included in the model tumor to allow insertion of up to 30 micro‐mosfets. The physical characteristics of this tumor were compared with those of real patient tumors. This tumor was inserted into a commercial anatomic breathing phantom (RSD Inc.). This phantom includes spine, and a flexible ribcage. The tumor and phantom's chest motion were programmed using real patient traces. CT images (4D and free breathing), kilo‐voltage setup images (including fluoroscopy), and mega‐voltage portal images were compared with those from real patients. Twenty mosfets were inserted into the model tumor, and the phantom was irradiated using a step‐and‐shoot IMRT plan. The expected dose distribution was calculated using a previously validated 4D Monte Carlo technique. Results: The appearance of the phantom/tumor in kV and MV images was very similar to real patient images. The average CT number of the model tumor was 76 (at 120kVp), compared with 30 for the real patient (an effective difference in electron density of 2%). Its physical density and hardness was 1.3g cm−3 and 27durometers (range for real tumors: 0–60), respectively. The CT numbers of the bone and soft tissue regions of the breathing phantom were in the range found for real patients. The CT number of the lung material was the same as air. Agreement between the 4D Monte Carlo and experimental DVH was better than 3%. Conclusion: Rapidprototyping techniques can be used to produce realistic low cost model tumors that, in combination with an anatomical breathing phantom, can be used for IGRT and 4D applications.
SU‐GG‐T‐291: Evaluation of Two Formulations of Commercial Water‐Equivalent Plastic to Establish Suitability for Dosimetry of the ZEISS Intrabeam® 50 KVp X‐Ray Source
Authors: Dorgu A , Pavord D , Gong J , Sherouse G , Kriminski S
Published: August 2010

Purpose: The ZEISS Intrabeam® is a 50 kVp x‐ray source intended for interstitial brachytherapy. The purpose of this investigation is to establish suitability of two formulations of commercial water‐equivalent plastic for dosimetry measurements for this device. Method and Materials: Plastic Water® (CIRS, Norfolk, VA) is a water‐equivalent resin material available in two formulations optimized for low energy dosimetry: LR l(5keV to 8MeV) and DT (50keV to 25MeV). The bare x‐ray source with no applicator attachment was positioned in the center of a 30×30×30 cm volume of pure water with two Exradin A1SL ion chambers (Standard Imaging, Middleton, WI) held symmetrically 4 cm directly lateral of the nominal target position, one on either side. Without otherwise changing the geometry three sets of measurements were made for each material by interposing 0, 1 and 2 cm respectively of material (replacing some of the water) between the source and chamber on one side only. The chamber on the other side served as a constancy reference. Results: Both the Plastic water® LR and DT formulations yielded results which differed from water alone by less that 2%. LR measurements ranged from 1.5% to 0.4% corresponding to 1cm and 2cm width (of phantom material), while DT ranged from 0.5% to 1.5% respectively. Conclusion: Both LR & DT formulations of Plastic Water® appear to be suitable as water‐equivalent media for dosimetry of the Intrabeam® 50kVp beam.
SU‐GG‐T‐293: Development of National Radiotherapy Audit in the UK
Authors: Bolton S , Budgell G , Bradshaw E , Trainer M , Berresford J
Published: August 2010

Purpose: The use of audit within the radiotherapy community in the UK has developed over the last 20 years. A National Interdepartmental Audit Network was set up which comprised eight regional groups covering the 66 departments. Recently, national audits have been introduced, to set a baseline for all departments to achieve a dosimetry standard. Method and Materials: Standard photon and electron audits were developed which could be performed by departments within each regional group. The aim was to measure a clinically relevant situation using both host and auditors' equipment. The use of a spreadsheet meant that the results were available instantly to exclude set up errors, and enable investigation of any unexpected results. The uptake of IMRT has been slow in the UK so an audit method was developed in order to increase confidence in centers starting IMRT programs. The method required all institutions to export a plan on their treatment planning system (TPS) and then irradiate dosimetry film in a water equivalent phantom at a standard depth. In addition, a set of alanine dosemeters was used to measure absolute delivered dose at a defined point. The TPS calculated dose grid was exported in a standard format and films and alanine were processed centrally. Results: 33/58 centres in the IMRT audit have been analysed. 49/55 alanine measurements were within 3% of predicted dose. 20/21 prostate plans achieved better than 96% pixels passing a 3%/3mm gamma evaluation and 17/18 head and neck plans at 4%/4mm. Conclusion: Recent reporting of radiotherapy incidents in the press have shown that confidence needs to be established in the whole radiotherapy process and audit has a major part to play in achieving this. Participating in and developing new forms of audit which reflect the modern methods used in 21st century radiotherapy should be a major goal.
SU‐GG‐T‐295: Use of the PTW 729 for Routine QA of a TomoTherapy Unit
Authors: Pavord D , Dorgu A , Gong J
Published: August 2010

Purpose: To evaluate the use of the PTW 729 chamber array for routine quality assurance checks on a TomoTherapy unit. Method and Materials: The 729 was used to perform quality assurance tests for the following: Individual patient IMRT QA; beam profile, output, and energy constancy at the 4 cardinal angles; verification of the 70cm virtual isocenter distance. IMRT QA was performed by using the 729 in the Octavius phantom. Results were compared at specific points to measurements with a 0.1cc ion chamber. Beam profile and output constancy was performed at 0 degree gantry position at 1.5cm depth. Energy constancy was performed by placing the 729 in the Octavius phantom with the array parallel to the beam axis. This was done for the 4 cardinal angles. Output and profile constancy was similarly measured but with the array perpendicular to the beam. The virtual isocenter position was verified by aligning the 729 to the lasers. The table was then indexed to the true isocenter by the TomoTherapy control system prior to the exposure. Any deviation of the longitudinal profiles from the zero position would indicate an offset in the virtual isocenter laser position. Intentional offsets were introduced to determine the sensitivity of the test. Results: The IMRT QA results achieved results of > 90% of points passing 3%/3mm criteria for all deliveries checked. This is comparable with other reported results. In addition the individual points were within 2%. Beam profile, output, and energy constancy was within 1% for multiple readings. The beam profile was within 3% of water scans within the useful beam (+/−20cm) for all points. The average difference was 0.74%. The virtual isocenter was verified to within 1mm. Conclusion: The PTW 729 is suitable for routine QA measurements on a TomoTherapy unit. Research sponsored by PTW Corporation
SU‐GG‐T‐299: A Digital QA Solution Using 2D Ion Chamber Array
Authors: Dou Y , Yan Y , Velasco C , Zhang X , Chao M , Han E , Moros E
Published: August 2010

Purpose: To evaluate the feasibility of the extensive applications of an ionization chamber array in radiation therapy clinics. Methods and Materials: PTW Seven29 ion chamber array and the customized Octavius phantoms were primarily used to perform patient‐specific IMRT/IGRT QA procedures on either TomoTherapy units or conventional LINACs. BrainLAB iPlan, Philips Pinnacle3 and TomoTherapy Planning are the RTP systems used to generate treatment plans. Surprisingly the unique design of PTW Seven29 does not stop it from finding its way to other dosimetry verification tasks. In combination with solid water slabs, this ion chamber array has been utilized in many dosimetrical verification procedures. With the help of vendor‐supplied software VeriSoft and MultiCheck, it was utilized to check machine's performance consistency, IMRT beam fluence, dose distribution of special treatment techniques such as stereotactic body radiation therapy with either conical collimators or Micro MLC, and GRID therapy. Results: Dose distributions in the Octavius CT phantom from IMRT/IGRT plans were measured in the Octabius QA phantom after radiation delivery. A total of 107 IMRT/IRGT QA procedures passed the Gamma analysis criteria 5% and 5 mm, while 18% passed 3% and 3mm criteria. Failed points were mainly in low dose and high dose gradient regions. A strong agreement was found between two methods (ion chamber and film dosimetry vs. ion chamber array alone) for checking machine's performance consistency. It has also shown to be an effective tool for GRID QA procedures. Conclusion: The PTW Seven29 ion chamber array is a reliable accurate and efficient dose distribution verification tool for both patient‐specific and machine‐specific QA measurements. It is also one of the essential components of a paperless and filmless radiation therapy facility.
SU‐GG‐T‐308: Commissioning of PTW Seven29 for Helical TomoTherapy Quality Assurance
Authors: Myers P , Stathakis S , Gutiérrez A , Mavroidis P , Mihailidis D , Papanikolaou N
Published: August 2010

Purpose: The purpose is to commission the Seven29 inserted into Octavius for use with TomoTherapy. Materials and Methods: The PTW Seven29 2D‐ARRAY in conjunction with the Octavius octagonal phantom is intended for the dose validation of IMRT plans for which the dose delivery is from different angles. Cylindrical treatment plans were produced with the TomoTherapy Hi‐Art Treatment Planning System (TPS) using pitches 0.287, 0.433, 1, and 2 and field widths (FW) 1.06cm, 2.49cm, and 5.02cm; a total of twelve plans. Each plan was based on the same PTV but varying parameters to accomplish each plan. Most plans delivered 2Gy/fraction to the cylindrical PTV, but due to limits of the TPS, four plans were optimized with an intended dose of 0.5Gy/fraction. Results: Using the clinical gamma criteria, 3% and 3mm and suppressing doses below 20% of the maximum, all FW and pitch plans had a passing percentage of 90% or greater. Using gamma criteria 2% and 2mm, plans with FW of 5.02cm and 2.49cm passed with gamma greater than 93% for all pitches excluding a pitch of 2. All plans with a pitch of 2 failed, with a gamma of 81.50% for 5.02 FW, and an average of 80.20% for 2.49cm FW, except for 1.06cm FW which passed with a gamma percent average of 91.90%. The remainder of the 1.06cm FW plans failed with average gamma percent of 79.80%. Conclusions: Accurate dose validation is a key component in the successful delivery of patient treatments. The PTW Seven29 with Octavius is accurate with the clinical criterion of 3% and 3mm and is also accurate with more stringent criteria, 2% and 2mm, for clinically used field widths and pitches. With more obscure settings of 1.06cm FW and pitch of 2, higher discrepancies were observed using more stringent criteria. Research sponsored by PTW corporation.
SU‐GG‐T‐309: A Ratio‐Test for Routine Electron Energy Check for Linear Accelerators
Authors: Hossain M , Price R , Jin L , Ma C
Published: August 2010

Purpose: The AAPM TG‐40 report recommends that electron energies of linear accelerators be checked monthly for constancy. The goal of this work is to develop and implement a method for routine energy check for megavoltage electron beams. Method and Materials: The method is based on a percentage depth ionization (PDI) ratio‐test equivalent to 2%/2 mm deviation from the original electron energy. Two PDI curves are generated for the beam of each energy by shifting the original PDI curve by ±2 mm. The upper and lower limits of ratio are computed by taking the PDI at two different depths from the two shifted curves. These ratios are measured monthly for each electron energy in solid water phantoms. Monte Carlo simulations are performed to find the shift in energy which may produce a deviation of 2%/2 mm in the percentage depth dose (PDD) or PDI at the given depths. Results: The method is implemented in our clinic since September of 2008. We have results for 4 Varian and one Siemens Linacs. The ratio of PDI values falls within the allowed range consistently. For most Linacs the energy remains in a region narrower than the bounds of the allowed upper and lower limits. The position of this narrow range is determined by the energy set at the time of commissioning. The difference in the energy shift equivalent to a dose difference of 2% or a spatial shift of 2 mm differs for different energy beams. This is consistent with the well known relation between the mean energy and the depth of 50% ionization: E0=2.33R50. Conclusion: The implementation of the ratio method of checking electron energy shows that it is a viable and relatively simple method to perform routinely.
SU‐GG‐T‐320: The Correction Factors of a Cylindrical Ion Chamber in Heterogeneous Phantoms for Megavoltage Photon Beams
Authors: Araki F
Published: August 2010

Purpose: To investigate perturbation correction factors and inhomogeneity correction factors (ICFs) for a cylindrical chamber in heterogeneous phantoms which include solid water (SW), lung (LN), and bone (BN) materials. Methods and materials: The perturbation factors, Pgr, Pwall, Pstem, Pcel, and PQ for a cylindrical chamber in heterogeneous phantoms were calculated with the EGSnrc/cavity code for 6 and 15 MV photons. The PTW31010 (0.125 cm3) chamber was used for measurements and Monte Carlo (MC) calculations of depth‐ionization curves with field sizes of 3×3, 5×5, and 10×10 cm2 at SSD of 100 cm. ICFs were calculated from the ratio of the product of the stopping power ratios and PQ of LN or BN to SW. Finally, measured depth‐ionization curves were converted to depth‐dose curves by using ICFs and compared with those calculated by the MC method. Results: PQ for SW and LN at a 5×5 cm2 field for 6 and 15 MV photon beams was almost the same and ranged from 0.98 to 0.99. PQ for BN reached 1.037 for 6 MV and 1.030 for 15 MV. Pwall and Pstem for BN were 1.029 and 1.015 for 6 MV and 1.022 and 1.013 for 15 MV, respectively. ICFs for LN ranged from 1.005 to 0.995 for 6 MV and from 1.010 to 0.995 for 15 MV. In contrast, ICFs for BN reached 0.950 for 6 MV and 0.940 for 15 MV. The measured depth‐dose curves for heterogeneous phantoms agreed within 1% with those of MC calculations. Conclusions: The perturbation effects of the chamber stem and the wall in LN are insignificant but the effects in BN are more significant. It was found that ICFs for ionization measurements in heterogeneous phantoms are up to 2% for LN and 6% for BN, and depend on the photon energy and the field size.
SU‐GG‐T‐324: Experimental Evaluation of Dose Calculation Using Mega‐Voltage Cone Beam CT with MOSFET Detector in an Anthropomorphic Phantom
Authors: Matsubara K , Kohno R , Honmatsu K , Kitou S , Kawashima M , Ogino T , Saitoh H
Published: August 2010

Purpose: Recently, dose calculations using mega‐voltage cone beam CT (MVCBCT) images have been reported. The dose volume histograms of the dose plan using MVCBCT images have been already compared to those of kilo‐voltage CT (kVCT) for clinical cases. In the reposts, comparisons of the plan dose were made, however, experimental dose verification has not performed yet. In this report, we experimentally evaluated the usefulness of the dose calculation using MVCBCT images. Method and Materials: First, we corrected the uniformity of CT number on MVCBCT images. Next, dose verification was performed in a homogeneous PMMA phantom. Dose at multiple points were calculated using corrected MVCBCT images and kVCT images. Then, the calculated dose was evaluated with a MOSFET detector. Finally, we performed similar dose verifications in an anthropomorphic phantom. To consider the positional accuracy and precision of the detector in the phantom, another series of measurement was repeatedly performed 3 times. Results: The relative dose differences between the calculated dose and the measured dose were obtained up to 3.2 % for both images in the PMMA phantom. The dose differences for corrected MVCBCT images agreed with those for the kVCT images within 1 %. Therefore, we confirmed the usefulness of proposed uniformity correction for MVCBCT images. On the other hand, for dose verifications in the anthropomorphic phantom, the average of the dose differences was 5.2 % for the corrected MVCBCT images and 4.3 % for the kVCT images. We can explain the differences by measurement error and setup error of the MOSFET detector. Reproducibility was up to 1.4 % among the series of measurement using the detector. Conclusion: The dose calculations using the corrected MVCBCT images agreed well with those using the kVCT images in the anthropomorphic phantom. We will apply the dose calculation using MVCBCT images to clinical situation.
SU‐GG‐T‐337: Surface‐Dose Enhancement Characteristics of Brass Mesh for Breast Cancer Treatments
Authors: Utsunomiya S , Green OL Pechenaya , Klein E
Published: August 2010

Purpose: To characterize the surface and build‐up dose‐enhancement characteristics of a brass mesh material and compare it to widely‐used tissue‐equivalent bolus. Method and Materials: A parallel‐plate ionization chamber was used to measure the enhancement effect of brass mesh (Whiting & Davis) versus that of tissue‐equivalent bolus (“Superflab,” Mick Radio‐Nuclear Instruments) under varying layers of water‐equivalent plastic. The measurements were performed using 1 to 4 layers of brass mesh (1.78‐mm thickness, 2.69‐mm mesh spacing) and bolus of 0.5‐ and 1.0‐cm thickness. A Varian linear accelerator was used to deliver 6‐ and 10‐MV beams, and a Tomotherapy system was used for helical delivery of a 6‐MV beam. The distance between the measurement point and the phantom surface (located at 100 cm SSD) ranged from 0 to 30 mm; field sizes of 10x10, 5×5, and 20×20 cm were used for conventional linac measurements, and a field width of 2.5‐cm for Tomotherapy. Results: For a conventional linac with either 6 or 10 MV, the differences in enhancement by the brass mesh compared to tissue‐equivalent bolus were greatest at the surface, where the brass mesh considerably underdosed the surface even when four layers were used. The difference between brass mesh and bolus was negligible beyond 5 mm of depth. For Tomotherapy, the enhancement by 2 layers of brass mesh was greater than for 0.5‐cm bolus due to the tangential effect. Conclusion: The results indicate that the characteristics of the surface dose enhancement by brass mesh are not equivalent to 0.5‐cm bolus for the first 5 mm of depth. Therefore, despite anecdotal reports, a single or multiple layers of brass mesh are not adequate for treatments where bolus is routinely used specifically to enhance dose in the superficial region to prevent recurrence due to microscopic disease in and under the skin.
SU‐GG‐T‐348: Development of a Precise Position‐Controlled Moving Phantom
Authors: Mizuno K , Nakae K , Okamoto M , Sasaki H , Yoshiura K , Uozumi K , Mochiduki T , Demachi K , Uesaka M
Published: August 2010

Purpose: Current commercially available moving phantoms are limited in their motions and accuracy of positions. The purpose of this study is to develop a moving phantom with highly accurate position control (error less than 0.2 mm) in 3 directions so that it can reproduce most of patients' tumor motions obtained by 4D‐CT. Method and Materials: The moving phantom consisted of two parts: thorax phantom and 3D‐axial control unit. The thorax phantom was made of transparent resin, which has an electron density of 1.04. The plaster bones were structured by ink‐jet printing method, and were embedded in the resin. The 3D‐axial control unit was designed to achieve any motion less than 10 cm/sec in x, y and z directions in the region of 150×350×250 mm which was a cavity in the thorax phantom. Two acryl hemispheres with a radius of 1 cm were set on the tip of the 3D‐axial control unit. The motion imitating respiratory tumor motion or sine curve was input, and the motion of the target was measured by a laser displacement gauge to evaluate the accuracy. The target motion was also captured by conventional CT and CBCT to evaluate the performance. Results: The target motion was as accurate as less than 0.1mm in the case of sine waves (1 cm, 12 bpm). In the case of patient's tumor motion (up to 7 mm, 7.5 bpm), although the motion was not modified with smoothing, the error was less than 0.2 mm. The conventional CT and CBCT images reflected the motion of the target. Conclusions: We developed a novel moving phantom with a high accuracy in position control. This moving phantom could be used for evaluation of 4DRT techniques.
SU‐GG‐T‐351: Dosimetric Evaluation of an Implantable DVS‐HFT Dosimeter for Hypo‐Fractionated Radiation Treatment
Authors: Ding G , Coffey C , Malcolm A
Published: August 2010

Purpose: To evaluate the dosimetric accuracy and reproducibility of an implantable DVS‐HFT dosimeter for hypo‐fractionated radiation treatment in stereotactic radiotherapy. Method and Materials: The dosimeters are factory calibrated to read within the range of 340–950 cGy per fraction. Communication from the detector to the reading device is achieved telemetrically. Two sets of dosimeters were studied in a phantom and one set is implanted in a prostate cancer patient treated with five fractions. The planned doses to detectors were calculated based on CT images. The calculated doses were validated with ionization chamber measurements in the phantom. Both static and IMRT beams were studied. The uncertainty of dosimeter reading was estimated by Results: In the phantom studies, the uncertainty of daily readings were found to be from −0.1 to −4.9% (with a mean value of −1.6%) and from +1.4 to +4.8% (with a mean value of +2.5%) for five daily doses of 700 cGy delivered by static and IMRT beams respectively. For the dosimeters implanted in the prostate cancer patient, the daily discrepancies between dosimeter‐1 readings and calculated doses of 767 cGy range from −8.2% to +4.0% (with a mean value of +2.0%) during five fractions. However, the discrepancies of dosimeter‐2 implanted in a region with high dose gradients (calculated dose of 180 cGy) range from −4.3% to +35.0% (with a mean value of +3.4%). Conclusion: The accuracy of the DVS‐HFT dosimeter is shown to be within the manufacture stated uncertainties of < 5.5%. The study demonstrates that the dosimeter may be an effective tool to determine the actual dose delivered to treatment targets as well as organ at risk. The information obtained can be utilized to study actual dose fluctuations of daily treatment.

Conflict of Interest: Two sets of dosimeters used in phantom study were provided by Sicel.
SU‐GG‐T‐353: Light Scattering Properties of EBT/EBT2 Radiochromic Films and Its Application in Radiation Dosimetry
Authors: Lam K
Published: August 2010

Purpose: EBT/EBT2 film has been used as a point dosimeter. Some spatial variation in sensitivity has been observed, possibly due to variation in active layer thickness. Although it is well known that EBT/EBT2 films scatter light, the light scattering has not been directly studied and is not explicitly utilized in measurements. Since the amount of scattered light may be related to the thickness of the active layer and thus the sensitivity of the film to radiation, we attempt to quantify the angular distribution of light scattered from the film and the variation of this distribution within a batch of the film sample. Method and Materials: We used a Panasonic Lumix DMC‐GF1 digital camera body as the light detector. The sensor of this camera was exposed and was very accessible when the lens was removed. A 1 mm hole was drilled on the camera body cap to define the aperture for the incident light. The film sample was placed behind the 1 mm aperture facing the light sensor of the camera. Data were obtained with the film in landscape and portrait orientations. We used a red LED at 626 nm, and a blue LED at 430 nm as light sources. They were mounted at 25 cm from the aperture. The red pixel images and blue pixel images from the camera were extracted from the raw images with dcraw and analyzed with ImageJ. Results: Light scattering was observed on EBT/EBT2 films for red and blue light. The angular scattering showed significant anisotropy which followed the film orientation as the films were rotated. Quantitative analysis of the light scatter will be reported. Conclusion: Angular distribution of light scatter from EBT/EBT2 film can be measured with a consumer digital camera. There is significant anisotropy in the scattered light distribution.
SU‐GG‐T‐355: Theoretical Design and Performance of a Scintillating Fiber Detector for Real‐Time Quality Control of MLC‐Driven Radiotherapy Treatment
Authors: Goulet M , Gingras L , Beaulieu L
Published: August 2010

Purpose: To present the design and theoretical performance of a fluence monitoring detector based on the optical attenuation of scintillating fibers for the on‐line verification of radiotherapy treatments. Method and Materials: A theoretical model was constructed to predict the intensity of light collected on each side of a long (app. 30cm) scintillating fiber placed under a field of radiation of varying size, intensity and position. It can be shown that one can measure independently both the central position of the interaction (xc) and the integral fluence passing through the detector (Fint). The performance of this approach in term of detection capacity was quantified by the evaluation of the standard deviation (SD) on xc and Fint as a function of the irradiated length of the field and incident dose. Results: Examination of the SD of xc and Fint shows good agreement with Poisson statistics. SD below 1mm on xc is achieved for any field longer than 2cm (down to 1cGy). This precision is improved greatly as the dose to the fiber increases: for a dose greater than 5cGy any field longer than 2mm will result in a SD of less than 1 mm. Moreover, it can be seen that the SD on Fint does not exceed 0.5% for any field longer than 2mm (down to 1cGy). Conclusion: This work clearly demonstrates the potential of online fluence monitoring by a detector based on optical attenuation of scintillating fibers. This detector could be used in the accessory mount of a linac during a radiotherapy treatment, in order to compare (using statistical tests) the characteristics of the incident radiation field with those predicted by the treatment planning software. The sensitivity and specificity performances of the system would enable real‐time quality control of the incident fluence.
SU‐GG‐T‐363: 3D Dosimetric Verification of Ir‐192 HDR Brachytherapy Source Irradiation
Authors: Pierquet M , Oldham M
Published: August 2010

Purpose: Experimental verification of brachytherapy dose distributions, especially closer than 1 cm from the source, is very difficult with current dosimetry tools. PRESAGE, a 3D radio‐chromic plastic, has already shown promise in the realm of high energy clinical radiation dosimetry. We present a 3D verification of an IR‐192 HDR brachytherapy dose distribution based on the PRESAGE/Optical‐CT dosimetry system. Method and Materials: A 6cm diameter by 10cm height cylinder of PRESAGE was made with a 7cm long by 2mm diameter channel in the center. Irradiation took place using a GammaMed 12i HDR source, and one dwell position for total irradiation time of 37.8 seconds. Measurement of the dose distribution consisted of using an in‐house optical‐CT scanner with reconstructed pixel size of .6mm. The following parameters were used for image acquisition: 900 projections at .2 degree spacing, 5 exposure averages per projection, and 0.0774mm pixel size. Reconstructed resolution was 1.2mm. Comparisons were made of the measured dose distribution from PRESAGE with Eclipse treatment plan for a single dwell irradiation. Results: Eclipse data and PRESAGE data were normalized to relative dose distributions to the point 1cm from the transverse bisector. PRESAGE profiles in transverse plane agree with Eclipse profiles to ∼5% between .6 cm and 2 cm from the source position. There was a loss in PRESAGE measurement data due to edge reconstruction artifacts extending to 3mm on either side of the channel. Conclusion: We have shown that the PRESAGE/Optical‐CT system is capable of providing a 3D dose distribution Eclipse verification of a simple Ir‐192 HDR irradiation. The unique features of the system are very high spatial resolution isotropically in 3D, simplicity (lack of need for an external container) and robustness (e.g. lack of sensitivity to oxygen contamination).
SU‐GG‐T‐367: Quantification and Reduction of Peripheral Dose from Electron Applicator Leakage
Authors: Yeboah C , Karotki A
Published: August 2010

Purpose: To quantify peripheral dose due to electron applicator leakage and to propose techniques for minimizing the leakage. Method and Materials: Peripheral dose was measured as a function of off‐axis distance from field edge and of depth in water using an electron diode and 10×10−25×25 cm2 applicators on Siemens primus accelerators. Second, the effect of oblique beam incidence on peripheral dose was investigated by comparing peripheral doses measured for normal beam incidence to those measured for obliquely incident beams. Third, leakage transmitted through various thicknesses of Xenolite‐NL was measured at depth 0.2 cm in solid water for 6×18 MeV electron beams. Finally, to reduce leakage escaping through applicator sidewalls, each applicator was modified by lining the outer surface of its sidewalls with a 1mm thick sheet of lead. Results: The applicator leakage gave rise to a broad peripheral dose‐off‐axis distance peak in patient plane. Maximum peripheral dose from normally incident beams at depth 1 cm in water was observed to be 5% of central‐axis dose maximum and as high as 9% for obliquely incident beams with angles of obliquity ≤40°. Measured depth‐peripheral dose curves showed the “practical range” of leakage electrons in water varies from approximately 1.4 to 5.7 cm for 6 to 18 MeV beams. The leakage transmission measurements showed a 4 mm thick sheet of Xenolite‐NL is required to attenuate the leakage from 9 MeV beams by two‐thirds. For each additional 3 MeV increase in electron energy, an additional Xenolite‐NL thickness of 2 mm is needed for shielding on skin. Finally, modification of applicator sidewalls resulted in reduction of peripheral dose by up to 80%. Conclusion: Peripheral dose from leakage can be reduced by shielding with an appropriate thickness of Xenolite‐NL. Alternatively, electron applicator leakage can be minimized by modifying the applicator sidewalls with a sheet of lead.
SU‐GG‐T‐369: An In‐House Developed MOSFET Dosimeter with Reset Capabilities
Authors: Verellen D , Van Vaerenbergh S , Tournel K , Duchateau M , Linthout N , Gevaert T , Reynders T , Van de Vondel I , Coppens L , Depuydt T , Storme G
Published: August 2010

Background and purpose: To report the feasibility and clinical validation of an in‐house developed MOSFET dosimetry system and describing an integrated non‐destructive reset procedure. Material and Methods: Off‐the‐shelf MOSFETs are connected to a common PC using an 18bit/analogue‐input and 16bit/output data acquisition card. A reading algorithm was developed defining the Zero‐Temperature‐Coefficient point to determine the threshold voltage. A wireless interface was established for ease‐of‐use. The reset procedure consists of an internal circuit generating a local heating induced by an electrical current. Sensitivity has been investigated as a function of bias voltage (0V–9V) to the gate. Dosimetric properties have been evaluated for 6MV and 15MV clinical photon beams and in vivo benchmarking was performed against TLD for conventional treatments (2 groups of 10 patients for each energy) and TBI. Results: MOSFETS were pre‐irradiated with 20Gy. Sensitivity of 0.08mV/cGy can be obtained for 200cGy irradiations at 5V bias voltage. Ten consecutive measurements at 200cGy yield a SD of 2.08cGy (1.05%). Increasing dose in steps from 5cGy to 1000cGy yields a 1.00 Pearson correlation coefficient and agreement within 2.0%. Dose rate dependence (160–800cGy/min) was within 2.5%, temperature dependence within 2.0% (25–37°C). A strong angular dependence has been observed for gantry incidences exceeding ±30°. Dose response is stable up to 50Gy (saturation occurs at approximately 90Gy), which is used as threshold dose before resetting the MOSFET. An average measured‐over‐calculated dose ratio within 1.05 (SD: 0.04) has been obtained in vivo. TBI midplane‐dose assessed by entrance and exit dose measurements agreed within 1.9% with ionisation chamber in phantom, and within 1.0% with TLD in vivo. Conclusions: An in‐house developed resettable MOSFET‐based dosimetry system is proposed. The system has been validated and is currently used for in vivo entrance dose measurement in clinical routine for simple (open field) treatment configurations.