bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐120: Optimization of Energy in Inverse Planning Process
Authors: Sévigny C , Beaulieu L
Published: June 2005

Purpose: To simultaneously optimize the photon beam energy, orientations (gantry, table), wedge filters and beam weights in aperture‐based IMRT, a new inverse planning system has been developed. This system, Ballista, uses anatomy‐based MLC fields as aperture. It is an alternative for beamlet‐based IMRT. The aim of this study is to include the photon energy in the optimization process. Method and Materials: Ballista uses a fast simulated annealing algorithm to select the optimal configuration of beams with respect to the objectives specified by the planner. To include the energy as a free parameter in the existing overall optimization system, this variable is interpreted by the algorithm as an angle and competes with gantry and table angles. That is, each eligible energy is assigned to a circular section of the space of all possible solutions. In addition to varying the energy of the individual beams, an option was added to the process which allows associating a combination of energies to various segments of a single field. The coverage margin overlap of the fields is naturally adapted as a function of the energy used. Results: The effectiveness of energy optimization has been tested on several sites. The lung cancer, which is an interesting example, has provided good results. The value of V20 (29,6% vs. 35%) is reduced as well as the mean dose to the heart (14,5% vs. 21,5%). Also, a better tumor coverage (V95% = 97,8% vs. 96,6% and IH = 1,07 vs. 1,10) is obtained in the case of a non‐coplanar plan realized with this new approach in comparison with a standard plan (only one energy for the whole plan, 23MV). Conclusion: The inclusion of energy gives the optimization process an additional advantage over standard planning, and better close the gap between the optimized approach and the clinical treatment methods.
SU‐FF‐T‐121: Optimizing IMRT Plans with Geometric Uncertainty
Authors: Martin B , Bortfeld T , Trofimov A , Craft D
Published: June 2005

Purpose: To examine the use of robust optimization methods for IMRT planning with geometric uncertainties. Method and Materials: A simulation study was performed to compare several approaches of optimizing under uncertainty. We used simple example cases where the optimization produced treatment plans that appear excellent on average, but are clinically unacceptable for some fraction of patients. To remedy this, we first defined an objective function and minimized the objective of the expected dose. Next we generated a sample patient population, computed the objective for each patient individually, and minimized either the average or the maximum of these objectives. Results: Cases were identified in which optimizing the expected dose distribution yields unacceptable dose distributions when the dose is delivered. This was particularly evident if a small number of fractions were delivered or if there were systematic errors present. Using robust optimization techniques, we were able to produce acceptable dose distributions for a high percentage of simulated deliveries. For cases with a small number of fractions or large systematic error, the optimal result was similar to a margin approach. Using the same technique with a high number of fractions and small systematic errors yields a better dose distribution than is possible with a margin approach. Conclusion: This work indicates that it should be feasible to replace a standard planning treatment volume approach with a robust optimization that automatically takes geometric uncertainty into account. In cases with limited information of systematic errors, such a solution would likely be very similar to the manually designed margins used today. If the uncertainties are random with well known statistics, however, significantly improved tumor coverage and normal tissue sparing can be achieved.
SU‐FF‐T‐124: Time‐Resolved Aperture Modulated Radiation Therapy
Authors: Wang C , Yang Y , Xing L
Published: June 2005

Purpose: To develop a method to incorporate organ motion into aperture modulated arc therapy treatment plan optimization and evaluate the performance of this method by applying it to prostate cancer cases. Method and Materials: A 36 beams treatment plan was prepared using PLUNC treatment planning system to simulate arc therapy. To take into account the effect of organ motion, a 3D distribution function was supposed to describe the displacement resulted from organ motion at different breathing phases. Doses to voxels were calculated using the displacement information at any phase; the deformed doses could be mapped back to the primary breathing phase which is a deformation free phase. Then an optimization objective function about multileaf collimator leaf positions could be defined based on the mapped doses; and adapted simulated annealing (ASA) approach was employed to minimize the objective function subject to clinical prescription requirements. After optimization, an aperture (leaf positions) optimized treatment plan could be gotten. A conventional IMRT treatment plan was also designed for comparison. The aperture optimized treatment plan could be exported to Eclipse treatment planning system and recalculated to observe its performance on Varian accelerators. Results: Compared with IMRT treatment plan, aperture optimized treatment plan can produce more conformal dose distribution around the target and critical structures can be spared more effectively, the delivery time and machine MU are also reduced significantly. Conclusion: Organ motion can be incorporated into the aperture modulated arc therapy treatment plan optimization with the information of displacement information. A more realistic distribution function can be employed conveniently to produce more significant clinical result.
SU‐FF‐T‐126: 3D Gel Dosimetry of IMSRT Using Normoxic MAGAT Polymer Gel
Authors: Beiki‐Ardakani A , Heydarian M , Vitkin A , Kirilova A , Jaffray Princess DA
Published: June 2005

Purpose: To investigate application of 3D gel dosimetry in verification of intensity modulated stereotactic radiotherapy (IMSRT) of intracranial lesions. Method and Materials: Radionics treatment planning (XK‐RT3) and delivery systems, including a Mini Multileaf Collimator (MMLC) were used in this study. A PVC Head phantom filled with MAGAT polymer gel (Normoxic MAGIC gel with THP as anti oxidant) attached to the GTC frame and stereotactic localizer. The phantom was CT and MR scanned using stereotactic protocols and image fusion was performed for the localization. A four field non‐coplanar IMSRT plan was generated using a 6 MV beam with the Radionics planning system and delivered according to our routine stereotactic treatment procedures. For comparison with the planned isodose distribution, the R2 maps were converted to relative dose maps using an R2‐dose calibration curve. The polymer dose maps were then co‐registered with the planning dose distributions in an in‐house Matlab program which provides reconstructed sagittal and coronal images for 3D evaluation of measured and planned dose. Various isodose levels in the 3D dose map have been compared between the treatment plan calculations and gel measurement. Results: Results of this study have shown good agreement between the gel dosimetry and treatment planning calculation. The 2D and 3D agreement was overall better than 5% in dose or to within 3 mm distance for the 80% and 95% isodose levels. The maximum difference for these isodose levels was 10%. Conclusion: In this work we have demonstrated that the Polymer gel dosimetry have the ability to accurately localize the high dose regions delivered by intensity modulated stereotactic radiotherapy.
SU‐FF‐T‐127: A Monte Carlo‐Based IMRT Plan Re‐Calculator
Authors: Zakarian K , Alaly J , Low D , Wood R , Zhang L , Deasy J
Published: June 2005

Purpose: IMRT QA currently is currently a time consuming activity. We hypothesize that IMRT QA labor would significantly decrease if a software tool were available which conveniently re‐checked IMRT treatment plans by independently re‐computing the expected dose distribution based on the leaf‐instructions generated by the treatment planning system. Such a system should have the capability to make detailed comparisons with the original treatment plan. Method and Materials: Our research treatment planning system CERR (Computational Environment for Radiotherapy Research) was modified and extended to include the capability of recomputing dose based on leaf‐sequences received via DICOM. Three dose computation algorithms have been implemented: (1) a simple pencil beam model which corrects for changes in central axis attenuation, but includes realistic scatter tails, (2) the Monte Carlo code VMC++, and (3) the open source Monte Carlo code DPM (dose planning method). GUI tools were developed to allow for side‐by‐side dose comparisons and comparative profile dose plots, in addition to DVH comparisons. Initial tests were performed using data generated via the Varian Helios planning system. Comparisons were made beam‐by‐beam in a simplified QA geometry as well as for total dose. Results: Initial results indicate reasonable agreement between the Helios planning system dose distributions and the pencil beam method. Differences with Monte Carlo results were greater, but energy spectral effects have not yet been added to the model. Conclusion: CERR provides a powerful and convenient environment to develop an IMRT plan re‐calculator. Initial results indicate the basic correctness of data being used for the dose recalculation. We expect to fully develop this system as a helpful tool for IMRT QA. Conflict of Interest: This research was supported by a grant from Sun Nuclear, Inc.
SU‐FF‐T‐128: A Practical Method for a Community Radiation Therapy Center to Evaluate the Quality of a Prostate IMRT Program
Authors: Lasher D , Wojcicka J , Malcom R
Published: June 2005

Purpose: To present a practical method for a community radiation therapy center to evaluate the quality of a prostate IMRT program. Method and Materials: The center transitioned from treating prostate cancer with 3DCRT to IMRT in January of 2004. Our quality assurance program included point dose verification via an ionization chamber measurement, planar dose distribution check with Kodak EDR2 film, and a dose calculation check with an independent monitor unit calculation program. RPC phantom measurements found agreement between planned and delivered doses of 1% and 2 mm. After treating approximately 100 patients with IMRT, we reviewed a sample of patient records that received the intermediate to high‐risk dose from each technique (73.8 Gy 3DCRT, 76 Gy IMRT). Results: Ionization chamber measurements were within 1.1 +/−0.7% of the plans' predictions, point dose calculated by the independent MU check software agreed with the treatment plans to within 0.8 +/−0.7%, and the film measurements demonstrate a dose distribution agreement within 2.3 +/−0.8 mm for the 85% isodose line and 2.0 +/−1.3% for the 50% line. The mean dose to the prostate increased from 75.87 Gy for 3DCRT to 80.04 Gy with IMRT (p<0.0005). The percent of the rectum receiving more than 65 Gy decreased from 21% to 12% with IMRT (p<0.0005). The percent of the bladder receiving more than 65 Gy was unchanged (16% for IMRT and 15% for 3DCRT, p=0.2651). The records indicate that the frequency of GI Grade II toxicity decreased from 22% for 3DCRT to 9% for IMRT. Grade II GU toxicity increased from 30% with 3DCRT to 56% with IMRT, perhaps because of the higher dose to the urethra within the prostate volume. Conclusion: This efficient study of patient records can provide a community center with an objective evaluation of the quality of a prostate IMRT program.
SU‐FF‐T‐131: An Analytic Approach to Film Based Absolute Dosimetry for IMRT
Authors: Shi C.Y. , Papanikolaou N. , Yan Y. , Weng X.
Published: June 2005

Purpose: To analyze the uncertainties in film based absolute dosimetry for patient specific quality assurance in IMRT. Method and Materials: In our institution, we use EDR2 film for the planar dose quality assurance measurements in patient specific IMRT. Our film dosimetry system was calibrated using a step wedge pattern for the linac and a step valley pattern for HiArt Tomotherapy machine. The dose to each step was measured using a calibrated ion chamber. All films were processed using a Kodak processor and subsequently scanned using the VXR‐16 Vidar scanner driven by the RIT software. The film optical density to dose response curves for six months were collected and analyzed. A fitting equation was derived and a systematic analysis was performed to evaluate whether a film calibration needs to be performed daily for the film dosimetry system. Results: Absolute dosimetry with film is subject to machine output fluctuations, film processing, film scanning and other sources of uncertainty. Based on our analysis, we found that the film processing error can be up to 7% (1SD), which corresponds to 1% (1SD) error in absolute dose (as determined from film dosimetry). The film scanner number can be correlated to absolute dose using a logarithmic fit to our measured data. Several step‐and‐shoot and TomoTherapy IMRT QA plans were evaluated using this method. The QA compares very well against the daily film calibration technique. Conclusion: Absolute film dosimetry for IMRT is feasible, and when the processor is properly maintained, a less than 3% error can be achieved in the calculation of dose. Once several dose response curves for the film are collected, an analytical fitting curve can be used in lieu of daily film calibration.
SU‐FF‐T‐132: An Investigation of the Accuracy of Film Dosimetry Using EDR2 Film for the Point Dose Determination for IMRT‐QA
Authors: Yu M. K. , Gladstone D. J. , Hug E. B.
Published: June 2005

Purpose: To investigate the accuracy of film dosimetry using EDR2 film for point dose determination for IMRT‐QA. Method and Materials: This work compared point doses measured by EDR2 film with those by chamber for 135 IMRT cases. The dose to the measurement point was about 200 cGy. EDR2 films were read and analyzed using a Vadar Scanner and the RIT Film Dosimetry System. In order to reduce the uncertainty of film dosimetry, the following measures were taken: (1) the H&D curve was obtained by placing the calibration films at 4 cm in solid water, and at the same depth of solid water EDR2 films were irradiated with planned IMRT fields for QA purpose; (2) each photon beam energy has its own H&D curve; (3) the length of time between film exposure and processing was always kept within 10 minutes since processing time delay affects the dose response of the film; and (4) the measurement point was chosen to be at low dose gradient region. For chamber dosimetry, a 0.3cc PTW ion chamber (type 31003) was used in solid water. Results: Comparisons of point doses measured by chamber and those by film show that, for 135 IMRT‐QA measurements, 51% of point dose values from EDR2 films match corresponding dose values from chamber to within ±3%, 72% of dose values to within ±5%. However, for 28% of measurements, the deviation between dose values from EDR2 film and those from chamber is larger than ±5%, the maximum deviation being 9.3%. Conclusion: If the dose value determined by chamber can be considered as the “true value”, then we believe that the accuracy of EDR2 film still needs to be improved for point dose determination although the film is good for verification of relative dose distribution in our experience.
SU‐FF‐T‐133: Analysis of the Correlation Between MLC File Parameters and IMRT QA
Authors: Kirk M , Chu J , Zhang Y
Published: June 2005

Purpose: We examined IMRT beams for potential correlations between properties of the MLC files and the agreement between the expected and measured dose distributions for patient specific QA. Method and Materials: We studied the significance of the incomplete modeling of the MLC rounded leaf edge in the Pinnacle‐v6.2b TPS. Test beams were generated with opposed leaf separation widths of 0.5, 0.7 and 1.0 cm. Different percentages of the total MU were delivered with these small separation segments with the remaining MUs delivered via an open field. A diode array was used to measure the delivered dose. The agreement between the planned and delivered beams (pass‐rate) was based on a 3%‐3mm dose‐difference and distance‐to‐agreement criteria. A software program examined the MLC files and identified opposed leaves separations less than 0.5, 0.7 and 1.0cm and calculated the fractional area (Area) exposed by the separated leaves as well as the percentage of MUs (%MU) delivered via that segment. Correlations between the product of the %MU and Area and the pass rate were examined. Results: For the test beams, strongly statistically significant Spearman correlations were found for %MU*Area and pass rate for leaf separations of 0.5 and 0.7cm (ρ=0.9 and ρ=0.8). An analysis of the 350 step‐and‐shoot beams indicates a possible correlation between the pass rate and the %MU*Area value for separation thresholds of 0.5 and 0.7cm (ρ=0.5 and ρ=0.51). Conclusion: The pass rates and leaf separation were correlated in the test beams, however the ability of the small separations to predict pass rates in real beams was not significant. For real beams, there are contributing factors such as interleaf leakage and leaf position errors which can cause the low pass rates. For 1.0cm separations, the pass rate was 100% indicating that the systematic errors for leaf separation greater than 1cm were not significant.
SU‐FF‐T‐136: Correlation Between IMRT Plan and Delivery Quality for 9 Treatment Planning Systems
Authors: Olch A , Chopra K , Jiang Z , Mitra R , Hasson B , Chang S , Cheng C , Das I
Published: June 2005

Purpose: This paper inter‐compares measured vs. calculated IMRT verification data with the planning quality data from 9 treatment planning systems (TPS) to determine if the benefit of the TPS that generates the best plans on paper is realized when the dose is actually delivered. Method and Materials: A collaboration of 8 centers with 9 TPS has previously presented results of a dosimetric comparison of IMRT plans using a consistent CT dataset for prostate, head and neck, and lung cases. Now these plans have been exported to phantoms and irradiated with the planned beams by each participating institution. Measurements have been made with an ion chamber at isocenter and film in a coronal plane 1 cm anterior to isocenter to determine the degree of agreement between the TPS dose and the measured dose. Also, the percentage of pixels exceeding a Gamma of 1.0 were tabulated for each system and site. A point system based on rank was applied to both Gamma values and absolute dose agreement for each system and site. The quality of the patient‐based treatment plans was also quantified by a point system based on the ability to meet the stated dose‐volume goals. The total points earned by the plans were correlated to the points earned by the QA results. Results:. Both the mean and standard deviation of the differences between measured and calculated absolute dose were within 3%. There was no trend for the Gamma values by site and TPS. The correlation of plan quality and dose calculation accuracy was weak. Conclusion: Although there were differences found in dose calculation accuracy between 9 TPS in both absolute dose and isodoses, there does not appear to be a correlation between plan quality and ability to deliver the planned doses.
SU‐FF‐T‐140: Dosimetric IMRT Plan Verification and Daily Quality Assurance with a Two‐Dimensional Ionization Chamber Array
Authors: Poppe B , Blechschmidt A , Djouguela A , Kollhoff R , Rubach A , Harder D , Schuele E
Published: June 2005

Purpose: IMRT requires a more specified quality assurance program than the conventional techniques. In this work we present our solution for a full IMRT quality assurance program with two‐dimensional ionization chamber arrays (2D‐ARRAY, PTW‐Freiburg) containing daily checks and individual dosimetric plan verifications. Method and Materials: The used array (type 10024) has 27 × 27 ionization chambers arranged in a plane, with an entrance window of 5 mm × 5 mm each. The centers of the 729 single chambers are positioned at 10 mm distance from each other. Results: Our quality assurance program is divided into two parts: On a daily basis, as a morning check, the dose at the central axis, the flatness and symmetry as well as the MLC calibration and light/radiation field congruence are evaluated by a single measurement. For a patient specific IMRT plan verification, the calculated dose distribution of the patient is exported to a CT containing the phantom set‐up with the 2D‐ARRAY. The corresponding IMRT sequence is exported to the linear accelerator. The values calculated for the plane of the 2D‐ARRAY and the values measured with it are then compared. Conclusion: The daily QA program has been extensively tested. All important field parameters can be obtained in a single measurement per energy. Furthermore MLC misalignments can be detected with an accuracy of less then 1.0 mm, allowing an early warning for a necessary MLC recalibration. The described program for IMRT plan verification has been proved to be very useful for an easy and fast pre‐treatment quality assurance. The error detection capabilities will be discussed in detail and shown to be sufficient for standard IMRT plans. Conflict of Interest: This work was performed in collaboration with PTW‐Freiburg Dr. Pychlau GmbH, Freiburg, Germany.
SU‐FF‐T‐144: Feasibility of a Novel Gafchromic Film for Routine IMRT QA
Authors: Kapulsky A , Hanley J
Published: June 2005

Purpose: To assess the feasibility of using an improved Gafchromic film, EBT (International Specialty Products), for routine IMRT QA. Method and Materials: EBT films were evaluated for clinical use in IMRT QA by comparing their performance to the widely used Kodak EDR2 film. As a clinical test, one hybrid phantom plan, consisting of seven IMRT fields, was calculated on a commercial treatment planning system with prescription levels of one to five Gray. The plans were delivered to EBT and EDR2 films loaded in a Bluebox film phantom. The films were digitized on Vidar VXR‐12 film scanner and analyzed using the RIT film dosimetry system. The percentage of pixels (exceedence) with plan/film dose difference above 5% tolerance level was used as a metric for film comparison. The symmetry and flatness for the same dose range were also determined for both types of films. Results: For EBT films, flatness and symmetry in two orthogonal directions improved as dose increased. Correlation between the plan and EBT film improved with increased dose, as indicated by a decrease in the exceedence (%): 22.9(1Gy), 11.9(2Gy), 11.2(3Gy), 6.6(4Gy), 4.1(5Gy). EDR2 films exhibited a different trend: 12.7(1Gy), 7.7(2Gy), 12.6(3Gy), 13.4(4Gy), 26(5Gy). Delivery time increased from 6 minutes for 1Gy plan to 9.5 minutes for 4Gy plan using dose rate of 600MU/min. Conclusion: At higher doses, EBT films show results comparable to, or improved over, EDR2 films. They demonstrate improved flatness and symmetry and IMRT QA results with increased dose. The additional 3.5 minutes necessary to deliver 4Gy to EBT film in order to achieve better results than with the EDR2 film is more than compensated by the convenience of using Gafchromic film. Further improvement of the digitization procedure and increase of the signal‐to‐noise ratio of EBT film should allow even lower dose for routine IMRT QA.
SU‐FF‐T‐145: Finding the Optimal Digitizer for Use with Gafchromic EBT Film
Authors: Baker J , Reinstein L , Gluckman G
Published: June 2005

Purpose: When performing a QA procedure using film the accuracy of the calculated dose is dependant on the digitizer. The ideal digitizer would have high sensitivity, density stability and uniformity. Our goal is to find the best scanner for use with the newly introduced Gafchromic EBT film. Method and Materials: We first exposed the EBT and EDR2 films to IMRT patterns with a maximum dose of 200cGy and generated a calibration film, which consists of 26 dose steps from near zero to 250cGy. The films were scanned on our Vidar Dosimetry Pro, as well as two Epson and two Microtek flatbed scanners. Film dosimetry software was used to analyze the films, by comparing the generated dose maps from EBT films to EDR2 films. Results: In comparison we found that the EBT film performed best on the Epson scanners and yielded a sensitivity 1.5 times greater than the Vidar, due to the film's absorption peak matching the red color channel of the Epson. Microtek's sensitivity equals the Epson, but noise levels are up to 5% compared to 1% on the Epson and Vidar. In comparing EBT to EDR2 we found there is no difference when scanned on the Epson or Microtek, but the Vidar showed high dose levels to be 8% lower with EBT due to a large point spread function altering the calibration curve. Comparing scanners shows that the noise level in dose is reduced on the Epson by 1.5 with EBT over the Vidar, while EDR2 remains the same and equal to the EBT on Epson. Microtek yields noise up to five times greater than the Epson. Conclusion: Digitizing on the Epson scanners utilizing the red color channel yields the best results. Similar results are attainable if an average scanning technique is utilized with Microtek scanners.
SU‐FF‐T‐146: How Monthly QA Affect the IMRT QA Result
Authors: Grimm S , Zhou F , Grimm J , Baker A
Published: June 2005

Purpose: According to TG 40, film demonstrates the width and length of field between 9.8cm to 10.2 cm are considered acceptable for a 10×10 cm field size. This work is to demonstrate under these two accepted scenarios, how IMRT QA results are affected. Method and Materials: Five IMRT QA plans are generated with each gantry angle set to the nominal angle (down onto phantom surface) and isocenter located at 100cm SAD, depth 10 cm. A 2D diode array MAPCHECK was set up and radiated according to the QA plan. These five QA plans were repeatedly radiated on four different days. The first exposure was done prior to the field adjustment with field size 9.8×9.9 cm for a 10×10 cm field size; the subsequent three measurements were done in three different days after the field adjustment to 10.12×9.9cm. Absolute Measured dose is compared to planned absolute dose for each detector. We also compared the measurement data with the different beam modeling sigma parameter. Results: Prior to the field adjustment, the passing rate for these five QA plans ranged from 42.1% to 78.3 % with 3% Dose difference and 2mm of DTA. The passing rate increased from 64.5% to 91.8% with 3% Dose difference and 3 mm of DTA. Table I present these comparisons. After the field adjustment, the passing rate range from 91.5% to 98.3 % with 3% Dose difference and 2mm of DTA. Table II, III and IV present the comparisons. Conclusion: Even though the field size for 9.8×9.9 cm is acceptable for a 10×10 field size for monthly QA, the consistent reduction of the field size (2mm) all the way to 1×1 dramatically impacts IMRT QA results much more so than the adjusting the sigma parameter on the beam modeling data.
SU‐FF‐T‐151: In‐Vivo Diode Dosimetry for IMRT Treatment Dose Verification
Authors: Tong S , Li T , Lei S , Grant D , Mazzilli T , Papadopoulos D , Chu K
Published: June 2005

Purpose: To investigate how diodes respond to various IMRT beam conditions and to evaluate the feasibility of using in‐vivo diode dosimetry for IMRT treatment delivery verification. Method and Materials: A series of dynamic MLC files were created to simulate IMRT fields. Diodes and a Farmer chamber were used to measure IMRT doses at dmax in a phantom. Dose dependence on field size (FS), sliding window MLC leaf gap width, SSD and energy was investigated. In‐vivo diode dosimetry was also used for patient IMRT treatment dose verification and compared with IMRT plan calculations. Results: Measured doses decreased with decreasing FS and MLC leaf gap. Diode readings agreed with ion chamber at dmax to within 3%. All measured data points were fitted to a straight line of “Measured Dose” vs. “%Primary Beam”, with a slope of 0.945, intercept 0.04 cGy/MU, and correlation coefficient 0.997. For a given FS and leaf gap, measured doses increased with decreasing SSD. SSD dependence for ion chamber agreed with Inverse Square Factor (ISF) to ∼1%. Diode SSD dependence deviated ISF by 1∼4%. Conclusion: In measuring IMRT dose at dmax in phantom, diode readings agreed with ion chamber to within a few percent. Diode correction factors for SSD, FS, leaf gap, and energy dependence in IMRT treatment fields are small. Our study has shown that diode dosimetry can be used for in‐vivo patient IMRT treatment delivery verification. In most cases, in‐vivo diode measurements and IMRT plan calculations should agree to within ±7%. It is important to select a low dose gradient region from IMRT plan for diode placement. Larger discrepancies between measurements and calculations usually can be attributed to high dose gradient, errors in patient setup and diode positioning, and SSD change.
SU‐FF‐T‐155: Monte Carlo Dosimetric Verification for IMRT QA Using MLC Log Files and EPID
Authors: Luo W , Li J , Fourkal E , Fan J , Yang J , Price R , Ma C
Published: June 2005

Purpose: EPID is a useful tool for pre‐treatment patient setup and target localization and post‐treatment dosimetry verification. However, EPID images are significantly affected by photon attenuation and scattering in the patient and the detector and therefore cannot be used directly for dose reconstruction. This work investigates a Monte Carlo dosimetric verification method based on MLC log files and the electronic portal imaging device (EPID) for IMRT quality assurance (QA). Method and Materials: We have developed Monte Carlo based software to derive accurate intensity‐modulated fluence maps behind the patient using MLC log files, which take into account the accelerator head geometry, the MLC leaf movement accuracy and the patient attenuation and scatter. Patient initial simulation and pre‐treatment CT data were used to simulate the patient anatomy for interfraction dose comparison. The phase space data behind the patient were used in the EPID response simulation and compared with measured EPID images to investigate patient setup accuracy and dose reconstruction uncertainty. Results: Ten previously treated prostate IMRT plans and patient CT data are included in this study. The MLC leaf position accuracy of the dynalog files from a Varian 21Ex accelerator is verified to within 1mm. The dose distributions based on the leaf sequences from the treatment planning system and the fluence maps rebuilt from the dynalog files are consistent to within 2%, validating our software implementation. The primary photons and scatters are recorded separately behind the patient for different applications as described above. Energy spectrum, fluence distribution and angular distribution are derived to facilitate dose calculation in the EPID. Conclusion: We have developed a log file based Monte Carlo method to generate phase space and fluence maps for pre‐treatment patient setup and post‐treatment dose verification with EPID. This work is implemented as part of our IMRT QA procedure.
SU‐FF‐T‐158: Optimized Sampling Pattern for Step and Shoot IMRT QA with a Diode Array
Authors: Letourneau D , Yan D , Keller H , Jaffray D
Published: June 2005

Purpose: Diode arrays with sparse detector density are now widely used for IMRT QA. The goal of this work is to quantify and optimize the sampling of dose maps prior to the measurement in order to insure optimum sensitivity to potential delivery errors. Method and Materials: The diode pattern was first overlaid with the dose map and the fractional area sampled by at least one diode was separated into low and high gradient regions. The area sampled by a diode depended of the local dose gradient, which should not exceed a user‐defined threshold. The number of leaf and jaw positions sampled by at least one diode was also obtained from the area of gradient sampled and the known segment shapes. The dose map and the collimator sampling were optimized using a simulated annealing algorithm to select the position of the diode array in the beam. The sampling improvement with multiple measurements at optimized detector positions was also studied. Results: For a 7 beam head and neck IMRT plan and the diode array centered on the central axis, the average fraction of the high and low gradient regions sampled at least by one diode was 13.9% and 79.6% respectively. In average, 85.8% and 83.5% of the leaf and jaw positions were sampled with a minimum reaching 58%. The optimization of the detector position improved the collimator sampling to more than 94% for all beams. The use of two array positions per beam improved the sampling in the high gradient areas. Conclusion: This method allows to identify the beams for which conventional diode array sampling would be suboptimal. The optimization of the detector position and the number of measurements insure adequate sensitivity to delivery errors related to dose and collimator calibration. Conflict of Interest: Supported by Sun Nuclear Corporation.
SU‐FF‐T‐160: Patient Specific QA for Prostate and H&N IMRT Using MapCheck
Authors: Luo C , Yang C , Wu C , Stern R , Perks J , Purdy J
Published: June 2005

Purpose: To evaluate the MapCheck system as a device for routine IMRT QA. Method and Materials: 20 prostate and 2 nasopharyngeal cases are used for this study. IMRT treatment planning was performed with Varian Eclipse/Helios planning system using sliding window technique. The measured dose distributions (both absolute and relative) are compared with the generated ones from the planning system. Results: For prostate IMRT QA, the measured dose distribution agrees well with the calculated ones by using a 3% difference and 3 mm distance to agreement (DTA), 10% threshold criteria. The pass rate is over 90% (20 cases) for both relative and absolute dose comparison. For nasopharyngeal head & neck IMRT, the relative pass rate is over 90%, while the absolute pass rate is around 40–60% by using a 5% difference and 3 mm DTA. Most of measured points are 5–10% higher than planned in the absolute comparison. In addition, we made ion chamber point dose verification measurements at carefully selected points in the relatively low dose gradient area at the same gantry angle of each beam as delivery of the treatment. Chamber measurement results have shown an agreement of within 3% in total dose compared to the calculated ones. Conclusion: Our clinical experience shows that MapCheck device works well for prostate IMRT QA, for both absolute and relative dose verification, which dramatically increases our IMRT QA efficiency. However, from the preliminary work from our 2 nasopharyngeal head & neck IMRT cases, the measured dose by MapCheck appears to be 5–10% higher than the planned dose for most points, while the relative dose map matches well with the planned data (with criteria 10% threshold, 5% difference, 3mm DTA) which is inconsistent with ion chamber measurements. The discrepancy is under investigation.
SU‐FF‐T‐162: Quality Assurance for a Tomotherapy Machine: New Procedures and Comparison to TG‐40 Recommendations for Conventional Linear Accelerators
Authors: Jursinic P
Published: June 2005

Purpose: To compare daily and monthly quality assurance, QA, procedures on a TomoTherapy machine with TG‐40 recommendations for a conventional linear accelerator. Method and Materials: A commercially available diode check‐unit is retrofitted for use in daily and monthly TomoTherapy QA. Output consistency is checked with this unit. The housing of the check unit is equipped with 1‐mm diameter lead markers that provide for checks of megavoltage CT imaging, laser alignment, and couch motion. On a monthly basis more quantitative checks of laser alignment, beam energy, beam profiles, and couch motion are done with a diode array. A very efficient test procedure has been developed that uses this array. Results: On a daily schedule, output consistency, laser alignment, and couch motion are check with a single measurement that takes approximately 10 minutes. Daily output checks over 118 days of operation show the output to vary with a 1‐standard deviation of 2%. Based on megavoltage imaging of the morning check unit, the vertical and superior‐inferior green‐laser alignment and couch motion are found to vary by less than 1 mm. In this time period, the left‐to‐right green‐laser drifted past a 2 mm limit and adjustments were made to correct the alignment. On a monthly schedule absolute beam output was measured for 1, 2.5 and 5.0‐cm jaw widths and found to vary by ±1%. Beam energy shifts were found to be ±1.2%. Monthly comparisons to commissioning profiles indicate small changes in the left‐to‐right direction, which are indicative of small changes in beam energy. Conclusion: New procedures and tolerances had to be developed for daily and monthly checks of the TomoTherapy unit. Over a five month period the machine stability has been acceptable. Many TG‐40 checks for conventional linear accelerators are not applicable to a TomoTherapy machine.
SU‐FF‐T‐167: Systematic Approach for Predicting DMLC Motor Failure: A Prognostic Strategy
Authors: Song Y , Chan M , Burman C
Published: June 2005

Purpose: Dynamic MLC and segmental MLC are the two competing delivery modes for IMRT plans. Each has its merits and limitations. Dynamic MLC provides a higher temporal resolution and more accurate dose delivery. However, it exerts extra stress on MLC driving motors, leading to a high motor failure rate. This stems from frequently alternated leaf acceleration and deceleration. The excessive heat generated by high instantaneous torque of the driving motors eventually leads to motor failure. This is particularly true for complicated head and neck cases, where many irregularly‐shaped critical structures are involved and steep dose gradients are required. Frequent machine down‐times create serious logistical problems and delay patient treatment. In this study, an attempt was made to decipher the DMLC motor failure pattern and design a practical prognostic strategy. Method and Materials: A new VARIAN CLINAC 21EX was installed in our center in June 2004. Each evening, a “stress‐test” DMLC file was run at gantry angles 0°, 90°, 180°, and 270°. This procedure was repeated after MLC reinitialization. All Dynalog files recording the leaf positions were saved for postprocessing. The “stress‐test” file was created to intentionally “exercise” the leave at high speed so that those “sick” leave could be identified. The Dynalog files were then analyzed using ARGUS IMRT software. Based on our past seven months' data, a prognostic most probable motor failure threshold was established. Results: The motor failure patterns in terms of leaf position RMS errors were diverse, complicated, and disease site specific. An RMS value of 0.15 cm could be a good prognostic index for potential motor failure. However, “false positive” cases could still occur. Conclusion: The prognostic strategy presented here is practical and easy to implement. However, a large sample of failed motors is needed to further validate the established threshold.
SU‐FF‐T‐169: The Use of Diode in In‐Vivo Dosimetry Quality Assurance in IMRT
Authors: Kim H , Wang Z , Lalonde R , Sontag M , Chen H , Li F , Smith R , Huq M , Heron D , Yue N
Published: June 2005

Purpose: In IMRT treatments, the ultimate QA procedure is to carry out in‐vivo dosimetry measurement to ensure the accuracies of both patient setup and beam delivery. This study was designed to explore the use of in‐vivo diode dosimetry measurement for QA of IMRT treatments. Method and Materials: IMRT plans were generated based on a set of CT scans of a head & neck anthropomorphic phantom. Corresponding IMRT QA verification plans were also generated. Diode calibration readings (Rc) were obtained for each beam during the routine dose verification QA process. During verification, a diode was placed along the beam central axis on the surface of a flat QA phantom at the SSD specified in the QA plan. Radiation was delivered dynamically using the same dynamic MLC files that were to be used for the patient treatment. For in‐vivo measurements, the anthropomorphic phantom was setup according to the treatment plan. For each beam, a diode was placed along the central axis at the beam surface entry point. Radiation was then delivered according to the plan and the diode reading (Ri) was recorded. If both the setup and the beam delivery were correct, Ri should be in agreement with quantity Rc*fSSD within certain uncertainty (fSSD is SSD correction factor); otherwise, it would be an indication of incorrect patient setup or incorrect beam delivery. Results: It was found that the calibration diode readings followed the SSD inverse square law within an uncertainty of 0.4%.
urn:x-wiley:0094-2405:media:mp7840:mp7840-math-0001 The derived in‐vivo diode readings (Rc*fSSD) were in agreement with those measured ones within 3.6% for three beams at different gantry angles, with an average difference of 1.8%. Conclusion: With a proper calibration method, diode verification can be used relatively accurately for in‐vivo measurements to check on the accuracies of patient setup and beam delivery for IMRT treatments.
SU‐FF‐T‐170: Tomotherapy Daily Quality Assurance Phantom
Authors: Jaradat H , Paliwal B , Mehta M
Published: June 2005

Purpose: To present a phantom specifically designed for daily quality assurance measurements of the radiation beam characteristics of the tomotherapy Hi‐Art machine over a large angular range of the beam. Method and Materials: The phantom is made out of solid water carved in the shape of two co‐axial hemi‐cylinders with two different radii, and the same depth. The radii are chosen to be 5.0 and 10.0 cm. The depth of the phantom is chosen to be 6.0 cm. The two hemi‐cylinders are supported on top of a 16.0 cm high stand also made out of solid water. A hole along the phantom's central axis accommodates a PTW ion chamber which is connected to an electrometer for current or charge reading. A delivery plan utilizing the synchrony between the MLC and the gantry rotational speed is specifically designed to run while the phantom is placed at the machine isocenter. The electrometer reading can be manually or automatically entered into a computer program that will calculate the instantaneous or average rotational output and energy consistency of the machine. Results: When collecting the current from the electrometer, six output reading segments (each segment covers 120 degrees) at two different depths, can be collected in six minutes. If the electrometer is setup to collect charge, six output readings will be collected; each reading is the cumulative charge from 120 degrees of the rotational beam. Conclusion: Using this phantom, daily quality assurance of the Hi‐Art Tomotherapy machine can include measurements of the consistency of rotational output, rotational beam energy, gantry rotational speed, and MLC synchrony with the gantry speed all packed within on six‐minute procedure.
SU‐FF‐T‐173: A Comparison of Techniques for Effective SSD Measurements and Evaluation of Data for Accelerators From Different Manufactures
Authors: Xu D , Xiao Y , Nawaz O , Galvin J
Published: June 2005

Purpose: Investigating and comparing the effective SSDs of electron beams measured by ionization chamber and electron diode for Elekta linear accelerators. Method and Materials: The effective SSDs for different energies and field sizes are measured for three Elekta linear accelerators. In order to measure effective SSD (SSDeff), two methods are used. The first method is placing a Farmer ionization chamber in a solid water phantom. The second method is using an electron diode placed on top surface of a solid water phantom. Results: The data shows that effective SSDs are energy and insert field size dependent. For both ionization chamber and electron diode measurements, the effective SSDs of three Eleckta linear accelerators, 1) increase with increasing of energy and insert field sizes which can be explained by less lateral scattering with higher energy and more lateral scattering equilibrium with lager field sizes, 2) agree well between the ionization chamber and diode measurements except for under the combinations of low energy and small field size. In most cases, the diode measurements are about 8 to 12 percent lower than ion chamber measurements for the same energy and insert field, except low energy and small insert combinations. Interestingly, for small field size, the effective SSD of diode measurement is larger than that of ion chamber measurement. Conclusion: The effective SSDs of three Elekta Linear accelerators agree well under most conditions except low energy and small insert field size combinations. The data also indicate that ion chamber measurements give a more accurate measurement over diode measurement and we recommend that ionization chamber should be used to measure effective SSD. The data also show that effective SSDs for Eleckta accelerators are different but comparable to that of Varian machines.
SU‐FF‐T‐175: A New Linac QA Procedure for the Characterization of Gantry Radiation Isocenter
Authors: Rosca F , Zygmanski P , Lorenz F , Hacker F , Chin L , Friesen S , Petsuksiri J , Shanmugham L , Ramakrishna N
Published: June 2005

Purpose: A new linac QA method is developed to characterize the size and shape of gantry radiation isocenter. The method accurately measures gantry sag as a function of gantry angles and the size of gantry radiation isocenter in the gantry rotation plane. Method and Materials: In order to determine both sag and radiation isocenter size in the gantry rotation plane we wrap a film (or a phosphor imaging plate) on a cylinder of circumference equal to the film length. We designed a static MLC “dashed line” pattern consisting of regions of radiation along a line. The pattern is designed such that the center of the irradiation line can be easily determined. The cylinder is positioned such that its axis is the same as the gantry rotation axis. The film is exposed to the MLC line pattern for multiple gantry angles. Both entry and exit patterns are seen on the film. A special MLC pattern was designed to accurately characterize the gap between the two ends of the film. An automated analysis program determines the MLC line angles and centers on the film, characterizes the cylinder position and generates the desired gantry radiation isocenter information. Results: The gantry sag is observed to have a smooth dependence on gantry angle with extreme values for 0° and 180°. The smallest circle crossing or tangent to all rays in the gantry rotation plane determines the gantry radiation isocenter size in this plane. The inherent accuracy of the method is estimated at ±/− 0.1 mm. The total irradiation time is about 15 min. The data analysis is less than 5 min. Conclusion: The designed method characterizes both gantry sag and wobble in one simple and accurate test.