bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐268: Implementation of Good Laboratory Practice Standards for Improving Dosimetry in Radiobiology Lab
Authors: Morrow N , Chen G , Tai A , Li X
Published: July 2006

Purpose: Accuracy of radiation dosimetry in radiobiology lab needs to be improved so that the results can be reproduced at different labs and translated into clinic. In this work we establish dosimetry procedures and improve accuracy of radiation delivery in a radiobiology lab based on the Good Laboratory Practice (GLP) requirements. Method and Materials: GLP guidelines and AAPM protocols were used to collect dosimetry data and to establish dosimetry procedures for routine quality assurance and for documenting delivery dose. A radiation field analyzer (Scanditronix) was used to characterize x‐ray beams produced by a Pantak HF320 orthovoltage system in the Radiobiology lab of the Medical College of Wisconsin. The relative dosimetry data were measured with an ionization chamber suitable for the kilovoltage x‐rays. A software method was developed to generate 3D isodose distributions. The absolute dosimetry measurements were performed using a Farmer ionization chamber calibrated in a standard dosimetry lab. The absolute doses were determined using the AAPM TG‐61 protocol. The QA procedures were established according to the GLP standards and based on AAPM TG‐40 protocol. Results: Accurate relative dosimetry data including depth doses, profiles, and isodose distributions for a variety of energies, filters, collimators and source‐to‐surface distances were obtained. Isodose distributions were generated for various opposite parallel fields irradiating phantoms with different thicknesses. Absolute doses were determined for a series of irradiation situations. These data facilitated accurate planning of animal irradiations and documentation of the delivered doses. Periodic QA procedures were established. Conclusion: We have collected essential dosimetry data and have established procedures for routine QA and dose documentation under the GLP standards. Thereby we improved the dosimetry in the Radiobiology lab to the level seen in the clinical setting. This work will facilitate accurate dose delivery, reproducibility of the results, and direct transfer of animal data into the clinic.
SU‐FF‐T‐270: Implementing a Monte Carlo Algorithm for Fields with Dynamic Beam Modifiers
Authors: Siljamäki S , Tillikainen L , Helminen H , Pesola K , Volken W , Frei D
Published: July 2006

Purpose: Monte Carlo methods for photons are slowly becoming available for normal clinical use. Especially for dynamic or otherwise complex fields Monte Carlo methods can be superior to traditional methods. The time to compute a Monte Carlo dose distribution up to a given accuracy is nearly independent of the complexity of the whole plan. Instead, traditional methods usually require computation time directly proportional to the complexity of the treatment. In this work, VMC++ (Voxel Monte Carlo, National Research Council, Canada) algorithm for fields with dynamic beam modifiers has been implemented. Method and Materials: A Monte Carlo implementation of an accelerator head model for dynamic fields is presented. Cases studied include dynamic wedges, sliding window IMRT fields, and complex arc fields. Except for dynamic wedges, the transport through the relevant components is modeled using VMC++. Tongue‐and‐groove, air cavities, divergent leafs, and rounded leaf tips in a multi‐leaf collimator are taken into account. In the case of dynamic wedges the jaws are modeled as impenetratable blocks. The final patient dose calculation is also done using VMC++ algorithm. Results: Results are presented for each of the dynamic components by comparing the Monte Carlo calculated data to measurements. The agreement is excellent in all cases. The calculation time varies from a few minutes to a few hours, depending on the required accuracy and target volume. Conclusion: Monte Carlo is shown to be a very competitive alternative for traditional methods in modeling accessories, especially for dynamic treatments. The results of the corresponding dose calculation match very well with measured beam data. In addition, the obtained execution times are suitable for routine clinical use. Conflict of Interest: This work has been funded by Varian Medical Systems Inc.
SU‐FF‐T‐271: Implementing a Parallel Monte Carlo System as a Clinical Treatment Planning System Validation Tool
Authors: Kirkby C , Field G , MacKenzie M , Fallone B
Published: July 2006

Purpose: We present the implementation of an automated EGSnrc‐based Monte Carlo (MC) system within a parallel computing environment as a tool for validating the dose distributions produced by a clinical treatment planning system (TPS) for external beam radiotherapy. Method and Materials: A clinical plan is generated on Varian's Eclipse TPS, which produces a set of DICOM RT data objects: RT Plan, RT Structure Set, RT Dose and a CT image set. MC simulation parameters are read and calculated from the DICOM RT objects. The CT image set is then used to generate a virtual phantom in a manner analogous to that of CTcreate (the relevant EGSnrc program), but with the added advantage of incorporating the structures in RT Structure Set for volume definition within the phantom. The MC simulation is broken into stages that progress from the target in the head of the accelerator (modeled explicitly), through the various stages of collimation, and into the phantom using both BEAMnrc and DOSXYZnrc on a dedicated cluster of 18 processors. Using RT Dose, we compare absolute dose differences between the TPS and MC quantitatively in 3D using a χ metric. To illustrate the implementation, we compare dose distributions for an unblocked four field pelvis plan at 15 MV. Results: Simulation of the complete plan took < 7 hours, achieving statistical uncertainties of ∼1.6% for each field (at isocenter) in 2.5 ×.2.5 × 3 mm3 voxels. Within the body contour 98.4% of the MC dose voxels agree with the TPS (2%/2.5 mm criteria). Significant differences (> 4%) are observed around the bony anatomy. Conclusion: This work establishes a framework for the full clinical implementation of MC as a tool for TPS verification.
SU‐FF‐T‐272: Implication of CT Couch Sag On Geometrical Accuracy During Virtual Simulation
Authors: Zullo J , Kudchadker R , Wu R , Lee A , Prado K
Published: July 2006

Purpose: Advances in radiation therapy have increased the need for better patient positioning accuracy. We have measured and compared the degree of couch sag exhibited by various CT scanners at our institution, and have evaluated its implication. Methods and Materials: Six CT scanners, a GE LightSpeed RT wide bore scanner, a GE LightSpeed RT standard bore scanner, a GE LightSpeed PET/CT scanner, a Philips wide bore scanner, a Philips MX‐8000 IDT scanner and a Picker PQ 5000 scanner were evaluated for couch sag. In order to simulate the weight distribution of a patient, five large water bottles, each weighing 43.3 lbs, were placed evenly across the couch. To quantify sag, a laser QA phantom was placed at locations on the couch, corresponding to the approximate location of three commonly scanned regions of the body: head/neck, thorax and pelvis. Using virtual simulation software, the vertical position of the phantom alignment point inside the CT bore was determined, and then compared to the starting vertical position of the external reference laser. This comparison yielded the amount of couch sag under a clinically representative weight distribution. Results: Results indicate that the amount of couch sag ranged from approximately 0.7 mm to 6.6 mm. As expected, couch sag varied with anatomical region. In addition, the amount of sag varied between different manufacturers, and between couches of the same model as well. Conclusion: Failure to recognize this problem could lead to incorrectly derived isocenter localization, and subsequent patient positioning errors. The amount of sag should be measured under a clinically representative weight load upon CT‐simulator commissioning. We are evaluating ways of incorporating these couch displacements into our virtual simulation process.
SU‐FF‐T‐276: Improving the Accuracy of Linac‐Based Stereotaxy
Authors: Grimm J , Grimm S , Simpson L , Jacob D
Published: July 2006

Purpose: Stereotactic alignment accuracy of couch mount linac systems can be improved by analyzing the film test with a computer program and repositioning the setup appropriately. Method and Materials: The Winston‐Lutz style film test is scanned and analyzed with a computer program that can estimate the 3D alignment error of the stereotactic quality assurance (QA) setup to within 0.1 mm. The program gives repositioning advice, and digital cameras are used by another program to provide laser guided repositioning. A final film test is taken whenever the couch mount needs to be repositioned. The programs remember laser, cone, and gantry offsets, so if the system is used frequently enough the laser guided repositioning often makes it possible to get within 0.2 mm of isocenter on the first couch mount film test. This system can make the couch mount more accurate than the mechanical isocenter standard (MIS). When the MIS has alignment problems it typically takes weeks or months to resolve, and often a site visit from a field service engineer is required. Conversely, our new approach adapts every treatment day to slight gantry, couch, and collimator movements, and always adapts to minimize the overall alignment error. Results: Clinical tests with a Varian 600c at Mercy Hospital in Scranton, Pennsylvania, and a Siemens Mevatron MXE 2 at Christiana Care in Newark, Delaware have shown that 0.2 mm isocenter alignment of the couch mount QA setup can be achieved using this system. This is 7.5 times more accurate than the current 1.5 mm radial alignment specification of a commercial linear accelerator based stereotactic system. Conclusion: This research shows the potential of linac based stereotaxy to approach the accuracy of the dedicated stereotactic systems.
SU‐FF‐T‐279: IMRT Plans Can Be Simplified Using One‐Step Optimization
Authors: Ludlum E , Akazawa C , Xia P
Published: July 2006

Purpose: To compare the effectiveness of one‐step optimization (DMPO) to traditional two‐step optimization for IMRT planning for prostate cancer, and to test the feasibility of simplified IMRT plans for reduction of delivery time and background dose without significant degradations in plan quality. Method and Materials: Clinical plans for five patients were created using DMPO with the maximum number of segments set at 40. Three additional plans were created using a leaf sequence converter with the same planning objectives. The number of levels was set to 10, 5 and 3 with corresponding number of extracts set at 5, 3 and 2, resulting in plans with decreasing numbers of segments. All plans were normalized so 95% of each target volume (prostate, lymph nodes and seminal vesicles) met the prescribed dose. Plan quality was assessed based on tumor coverage, defined endpoints for sensitive structures, plan dose conformality and dose homogeneity. Results: The DMPO plans resulted in the lowest average number of segments at 38.8, compared to 160.4, 77.2 and 45.4 for the two‐step plans, and also had the highest overall plan quality. The DMPO plans resulted in the lowest D5 and D20 to the bladder, the lowest D20 to the rectum and the lowest V45 to the small bowel. In addition, the DMPO plans had the lowest number of monitor units at 745.8, compared to 910.6, 893.6, and 823.2 MU respectively for the two‐step plans. The DMPO plans had an average homogeneity index of 89%, compared to 90%, 88% and 83%, and a conformal index of 2.9 compared to 2.8, 3.9 and 4.8. Conclusion: One‐step optimization is an efficient and effective way to create simplified IMRT treatment plans. For prostate cancer patients treated with IMRT, clinical requirements can be met with 40 or fewer segments using DMPO.
SU‐FF‐T‐281: In Vivo Dosimtery for IMRT Treatments Generated by Pinnacle Treatment Planning System
Authors: Alaei P , Higgins P , Gerbi B
Published: July 2006

Dose verification using diodes has recently been proposed for IMRT treatments and has been evaluated for IMRT deliveries planned using the Eclipse treatment planning system. The Pinnacle treatment planning system generates plans that are delivered in a different fashion than Eclipse. Whereas the Eclipse‐generated plans are delivered by scanning the treatment area from one side to the other, Pinnacle‐generated plans are delivered in a seemingly random fashion, treating multiple small areas within the field. This makes diode measurements at a point potentially more uncertain since the diode may be exposed fully or partially to multiple small fields during one single field's treatment as opposed to being exposed to very few segments scanning the point during an Eclipse‐generated delivery. We have evaluated in vivo dosimtery for Pinnacle‐generated IMRT plans, characterized diode response to IMRT deliveries involving various beamlets and evaluated its response due to full or partial exposure to radiation. We will also present the results of diode measurements performed for 150 IMRT fields on patient and phantom.
SU‐FF‐T‐290: Integration of AAPM TG‐43 Formalism Into a New Plastic Scintillator Dosimeter System
Authors: Mitch M , McQuighan K , Kennedy R , Sliski A , Soares C
Published: July 2006

Purpose: To integrate the AAPM Task Group 43 (TG‐43) brachytherapy dose calculation formalism directly into a new plastic scintillator dosimeter system. Method and Materials: A novel plastic scintillator system specifically designed to obtain the dose distribution in three dimensions in real time around low‐energy x‐ray‐emitting prostate brachytherapy seeds has been constructed. The small sensitive volume (0.5 mm diameter × 0.5 mm thick) allows unprecedented resolution in dose‐mapping of brachytherapy sources. High sensitivity is achieved by proprietary electronic signal acquisition and processing methods, allowing measurements of the very low dose rates (down to 1 mGy / h) at distances of up to several centimeters from a seed mounted in a water phantom. The computer program that controls this automated dosimeter system has been designed to acquire and display data directly in the format specified by the AAPM TG‐43 protocol for calculating brachytherapy seed dose distributions in terms of radial dose function and anisotropy function. Results: The dose distributions around typical I‐125 and Pd‐103 prostate brachytherapy seeds have been measured. A comparison of the results to published TG‐43 data for several seed models shows excellent agreement in most cases. Anisotropy functions at distances closer to the seed than those published for thermoluminescent dosimeter (TLD) measurements of an I‐125 seed have been measured. In one case, use of this dosimeter allowed the rapid identification of a seed with an anomalous anisotropy function, later confirmed by a radiochromic film contact‐exposure measurement. Conclusions: The automation of this new, high sensitivity, high‐resolution plastic scintillator dosimeter, incorporating the TG‐43 dose calculation formalism, has allowed real‐time characterization of the dose distribution around prostate brachytherapy seeds in a liquid water phantom. Conflict of Interest: This work supported by NIST SBIR Grant SB1341‐03‐W‐0815 and NSF Grant Nos. 0097450 and 0453430.
SU‐FF‐T‐293: Inter‐Linac and Intra‐Linac Variability of X‐Ray and Electron Dose Distributions
Authors: Chen J , Schreiber E , Lehmann J , Faddegon B
Published: July 2006

Purpose: To determine the maximum expected variation in fluence, differential in energy and position, with an accuracy of 1% for a single linac model, and eventually, combining with Monte Carlo beam models, to develop gold standard beam model benchmarks. Method and Materials: The 6 and 18 MV beams of the Siemens Primus linear accelerator (linac) were tuned through the manufacturer acceptance range in field flatness by adjusting beam energy. Dose profiles were measured at the low, high and average energy in this range for small, medium and large field sizes to quantify the intra‐linac variations caused by energy adjustment. The 6 and 18 MV x‐ray beams, and 6–21 MeV electron beams on three Primus machines, with the same treatment head, were tuned to give the same central axis depth dose curves to better than 0.5% for the largest fields available: 40×40 cm for photons, and 25×25 cm for electrons. Profiles were measured on these linacs to investigate the inter‐linac variations. Results: For the intra‐linac variations, the energy tuning has the largest effect of 7% at 10 cm on beam penetration for small fields. Scattering in the field reduces this effect for larger fields. Wedged‐field profiles are affected more than the open field, due to the dependence of transmission on energy. For inter‐linac variation, profiles measured from three different machines vary more than 2% due to beam tuning differences, lateral displacement of flatteners or scattering foils, and differences in position of components relative to the source. Conclusion: The range of inter‐linac and intra‐linac variability has been measured on a Primus linac and is clinically significant. This data will be used with Monte Carlo treatment head simulation to calculate gold standard beam model benchmarks for the development of tunable beam models of high accuracy and detail in the dose‐critical components of fluence.
SU‐FF‐T‐297: Investigating Using IMRT Convert Parameter Settings to Improve the Agreement Between Planned and Delivered Dose Distributions
Authors: Robisch D , Davis M , Elson H , Lamba M , Kumar N
Published: July 2006

Purpose: To improve the IMRT quality assurance process. Method and Materials: The linear accelerator used in this study was a Varian model 21EX with a 120 multi‐leaf collimator. IMRT planning was conducted on an ADAC Pinnacle3, v6.2b, P3 IMRT treatment planning system. Fluence measurements were analyzed using a MapCHECK™ unit consisting of 445 N‐type diodes. Coplanar dose composites from thirty‐two converted IMRT plans were analyzed using an ANOVA‐TM v.3.0.0.2 software package. The software, containing design of experiments (DOE) techniques, utilizes analysis‐of‐variance methods and orthogonal array designs for conducting efficient experimentation. Results: From the ADAC Pinnacle3 convert parameters which were selected for this study, only the minimize tongue and groove parameter had a notable contribution toward influencing the percent agreement between the planned and the delivered dose distributions, (rho = 5.50%). Statistically, the calculated value “rho” indicates a parameter's unique contribution toward changing the percent agreement; the bigger the rho, the bigger the parameter's contribution. Field size, which was considered to be an uncontrolled noise factor, was found to be a significant noise factor influencing the agreement output, (rho = 70.10%). Beam energy, also an uncontrolled noise factor, was found to have a smaller contribution influencing the agreement output, (rho = 6.09%) Conclusions: Changing the convert parameters cannot be used to significantly improve the IMRT quality assurance process. While the convert parameters do affect treatment plan dosimetric results and delivery times, they will not significantly affect the outcome of the IMRT QA results. DOE techniques can be very useful when analyzing the contributions from experimental factors.
SU‐FF‐T‐299: Investigation of Near Surface Doses in Mesothelioma Intensity Modulated Radiation Therapy
Authors: Both S , Hahn S , Circiumaru O , Kassaee A
Published: July 2006

Purpose: To investigate the measured and computed near surface doses for intensity modulated radiation therapy (IMRT) of mesothelioma. The near surface dose value is important since in most cases the intent is to deliver about 90% of the prescribed dose to the surgical scar region. Material and Methods: Two planning CT scans (3mm slice thickness) of a humanoid Alderson phantom with 5 mm and 10 mm bolus placed on the thoracic region were acquired with our Philips CT scanner. At the time of CT, opaque markers are placed under bolus to designate 10 different locations throughout thoracic region for calculations and measurements. IMRT plans are generated using the Nucletron OTP treatment planning system. IMRT calculations are performed for a 3 mm × 3 mm calculation grid and inhomogeneity correction with pencil beam algorithm. Plans are delivered to the phantom with a Siemens Oncor linac. Doses are measured at each marked location under each bolus, mimicking depths of 5 mm and 10 mm, using a pair of LiF thermoluminescence dosimeters (TLD). Results: The average dose difference between the measured and calculated data at depth of 5 mm is 8.6% with percent differences ranging from 3.6% to 18.3%. For the points at 10 mm depth this average is about 2.7 % and the percent difference range is 0.1% to 4.7%. Conclusion: Our results indicate that dose calculated for mesothelomia IMRT cases at depth of 5mm show a larger uncertainty relative to the measurements than dose calculated at deeper depth of 10 mm for OTP. The measurements show good agreement at 10 mm depth; on average, results are within accuracy of the TLD measurements (within 3%). It is therefore important for patient treatment to carefully evaluate the near surface dose (i.e.5 mm depth) via measurements.
SU‐FF‐T‐300: Investigation of Respiratory Motion Effect On Lung Tumor Radiotherapy Using 4D Monte Carlo Treatment Planning and 4D CT
Authors: Ding M , Xing L , Newman F
Published: July 2006

Purpose: To investigate the respiratory motion effect on lung tumor radiotherapy using 4D Monte Carlo treatment planning and 4D CT. Method and Materials: 4D CT images for four lung patients (two upper lung tumors and two lower ones, with different volumes) were acquired by using a GE LightSpeed‐QX/I scanner. Ten phase bins were used in the 4D‐data acquisition. A 4D Monte Carlo treatment planning system based on the EGS4/MCDOSE code was developed to calculate the 3D dose and map the 3D dose of the CT at each phase to the inhale CT (as reference). CT images at different phases were registered with the inhale CT image using a BSpline deformable registration model. Isodose lines and the DVHs of tumor and normal lung were used to compare the 4D plan (3D dose mapped from the CT at 10 selected phases to the inhale CT) and the 3D reference plan (3D dose for the inhale CT) for each patient. Respiratory motion effect was investigated for the different tumor volumes and locations. Results: In our study, for the upper lung tumors, the respiratory motion effect on target dose coverage was insignificant (<2% difference between the 4D plan and the 3D reference plan). However, for the lower lung tumors, the motion effect was clinically significant (>3% difference between the 4D and 3D plans). For the same PTV margin, less motion effect was observed for larger tumor volumes. The motion effect for the normal lung volume receiving dose was not correlated with the tumor volume or the location. Conclusion: Respiration motion may significantly influence the tumor dose in lung radiotherapy and 4D dose calculation may be necessary when treating tumors in the lower lung or when the tumor volume is small.
SU‐FF‐T‐301: Investigation Of The Accuracy Drift For The CyberKnife Systems
Authors: Yu C
Published: July 2006

Purpose: During a routine quality assurance check, it was found that the total accuracy was drifted out, or deteriorated to 3.5mm and greatly exceeds the manufacturer's specification of less than 1mm. In this report, observations and the preliminary results of investigation on the issue will be presented. Lessons learnt from the incident, suggestions and precaution measures to prevent from its happening will also be discussed. Method and Materials: Previous records of the routine QA and system logs were carefully reviewed. All evidence and all likely cause for the drift, such as the imaging system out of calibration, physical contacts, earthquakes, hospital vibration and room settling were thoroughly examined. Personnel who have access to the CyberKnife Systems were interviewed. Results: The imaging system required no adjustment or recalibration. The system log files seam not corrupted. The robot appeared to be off from its original position. Two spots, one dent and one scratch, on the linac cover occurred over two years ago and were excluded from the likely cause list. The accuracy was re‐confirmed within the specification by end‐to‐end tests after several adjustments, including the robot re‐mastering and the secondary calibration of the treatment paths. The likely causes suggested by the manufacturer were not very convincing. Conclusion: By nature, any mechanical system can fail at any time. Failure to identify the true cause of the problem could result in its happening again at any time no matter how often QA is performed and could potentially expose patients to great risks. Rigorous QA such as the end‐to‐end should be performed monthly; the iso‐post test should be performed daily and the laser spot at the perch position should be performed prior to each patient treatment. Some kind of interlock systems should be developed to self‐check or detect any deviation for each subsystem.
SU‐FF‐T‐303: Ion Loss Measurements for a Large‐Volume Free‐Air Ionization Chamber
Authors: Culberson W , DeWerd L
Published: July 2006

Purpose: To extend the capabilities of large‐volume ionization chambers to higher‐fluence sources by thoroughly evaluating ion loss in preparation for Electronic Brachytherapy air‐kerma strength measurements. Method and Materials: Electronic Brachytherapy was recently approved by the FDA as a low‐energy photon‐emitting source for the treatment of breast tumor cavities after lumpectomy surgery. The dosimetry for these types of treatments is based on well‐established AAPM TG 43 protocols for low‐energy photon‐emitting brachytherapy sources. This task group recommends that source strength for low‐energy brachytherapy sources be measured with a large‐volume free‐air ionization chamber to determine the air‐kerma rate absolutely. Until now, using such a chamber for Electronic Brachytherapy sources has not been possible because of the higher x‐ray energies of Electronic Brachytherapy sources (up to 50 keV) as well as their significantly greater exposure rates. There is convincing evidence that the air‐kerma strength of Electronic Brachytherapy sources may be measured with the Variable‐Aperture Free‐Air Chamber (VAFAC). To this end, a systematic investigation of the mechanisms of ion loss has been completed by varying exposure rates, plate separations, and high‐voltage biases. Results: Measurements of ion‐loss agreed with empirical predictions to within 0.2% using the VAFAC for low‐energy photon‐emitting brachytherapy sources with strengths up to 5000 U, more than 50 times higher than the upper limit of the current U.S. national standard (100 U), and within the estimated source strength of new Electronic Brachytherapy sources. Measurements of two coefficients for conventional ion‐loss equations are and , which are within the range of values published in the literature. Conclusion: The VAFAC's measurement capabilities have been extended to higher‐fluence sources (up to 5000 U) in preparation for air‐kerma strength measurements of Electronic Brachytherapy sources. Measurements of empirical coefficients are within the range of values listed in the literature.
SU‐FF‐T‐304: Is 0.5 Cm Leaf Width of MLC Beneficial in IMRT?
Authors: Su Z , Liu C , Li J , Yang C , Palta J , Kim S
Published: July 2006

Purpose: Evaluate treatment plan quality and quantify dosimetric accuracy of IMRT for different disease sites with MLC of 0.5 and 1.0 cm leaf width. Method and Materials: We tested a hypothesis, quality of IMRT treatment plans with smaller MLC leaf width is better, by developing treatment plans on Pinnacle treatment planning system (TPS) for multiple patients (total 60) with tumor sites located in Head and Neck (H&N), Vertebral Body and Prostate. For each patient, two plans are created using same objective functions and optimization parameters and algorithm (Direct Machine Parameter Optimization): one with 0.5 cm leaf width; the other with 1.0 cm leaf width. The TPS beam modeling of the two virtual machines was exactly the same except for the MLC leaf width. The dose volume histogram (DVH) is used in the evaluations of plan quality. All target coverage is normalized to the criteria of 95% of the target volume receiving prescribed dose. A diode array (Mapcheck) was used to quantify the dosimetric accuracy of all plans created with two MLC leaf width resolutions. Results: DVHs of all the patient plans reveal very minor dosimetric differences between the IMRT plans of the two MLC systems for the same patient. There are almost identical target coverage and similar dose distributions among the critical structures in H&N patients and vertebra body patients. For prostate, plan of 1 cm MLC leaf width has slightly more hot spots in the target with the same dose coverage criteria. Planar doses comparisons with Mapcheck measurements indicate slightly larger uncertainty in 0.5 cm leaf plans. Conclusion: There are negligible dosimetric differences in IMRT plans created with 0.5 cm and 1.0 cm leaf width resolution. However, the dosimetric accuracy of plans generated with 1.0 cm leaf width is better than the plans generated with 0.5 cm leaf width.
SU‐FF‐T‐307: Is PTV Coverage a Valid Indicator for Plan Evaluation in Shallow Tumor Cases Where the PTV Extends Into the Build‐Up Region?
Authors: Nguyen TB , Hoole ACF , Thomas SJ , Burnet NG
Published: July 2006

Purpose: The Planning Target Volume (PTV) is defined by ICRU report 50 as a geometrical concept, used to select appropriate beam sizes and beam arrangements. Clinically, a plan is normally acceptable if the 95% isodose surface covers the PTV. The goal of this study is to investigate the validity of using the PTV coverage for plan evaluation in shallow tumors where the PTV extends into build‐up region. Method and Materials: A conventional 3‐D conformal technique with a 10mm CTV‐PTV margin was used to produce 3‐field plans for a deep tumor (Prostate) and a shallow tumor (Head&Neck). For the H&N case, as the CTV is normally at a fixed distance below the skin, the geometric uncertainties are mainly from patient‐beam positioning errors. The CTV coverage with these errors was assessed by shifting the isocenter to 200 separate positions. The isocenter offsets were randomly selected using 3 uniform random Cartesian deviations between −10mm and 10mm, with a 3D distance constraint of 10mm. For the shallow tumor case, another plan with a 5mm larger field was made. The DVH of each plan with 200 shifted isocenters was analyzed. Results: In the H&N case, an “unacceptable plan” was obtained, which has V95=81% for the PTV. In the prostate case, we obtained an acceptable plan which has V95=94.9% for the PTV. However, both cases gave the same good CTV coverage for 200 shifted isocenter plans. For the H&N case, PTV coverage (V95) can be improved (from 81% to 83%) by increasing field sizes, but without improvement in CTV coverage, and larger fields increase skin dose. Conclusion: The current method of evaluating plan quality by means of PTV coverage is adequate for deep tumors, but is not adequate for shallow tumors. A new indicator for CTV coverage evaluation in shallow tumor cases needs to be developed.
SU‐FF‐T‐309: Laser‐Accelerated Proton Therapy: Target Chamber Design and Shielding Requirements
Authors: Ma C , Veltchev I , Fourkal E , Li J , Fan J , Lin T , Luo W , Stathakis S
Published: July 2006

Purpose: Recent advances in laser technology have facilitated proton (light ion) acceleration using laser‐induced plasmas. In this work, we investigate target chamber designs and their shielding requirements for a laser‐proton therapy system. Method and Materials: This new therapy system employs laser‐accelerated protons. If successfully developed, it may provide a compact and cost‐effective solution to energy‐ and intensity‐modulated proton therapy (EIMPT). Since laser‐accelerated protons have broad energy and angular distributions, which are not suitable for radiotherapy applications directly, we have designed a compact particle selection and beam collimating system for EIMPT beam delivery. The target chamber contains the laser focusing and target assembles and is connected to the proton beam collimation and particle selection device. Monte Carlo simulations using MCNPX and FLUKA have been performed to verify the shielding walls for the experimental setup, which consist of stainless steel, polystyrene and lead. Results: The primary particles resulting from the laser‐target interaction are protons and electrons. Our particle in cell simulation predicted energy spectra with 300 MeV maximum energy for protons and 20 MeV for electrons for a laser intensity of 1021 W/cm2. The maximum number was 1011 and 1012 per pulse for protons and electron, respectively. Our Monte Carlo simulations showed that a combination of 1/4″ 304 stainless steel, 6″ polystyrene and 3″ lead will reduce the combined leakage dose equivalent to 0.32 μSv (0.032 mrem) per laser pulse at 1 m from the chamber, which included the effect of all primary and secondary particles. Conclusion: We have designed an experimental setup to accommodate the laser focusing mirror, the target assemblies, the beam collimator and the particle selection system. Different shielding walls are designed to ensure the leakage dose equivalent to within 20 μSv (2 mrem)/week.
SU‐FF‐T‐310: LINAC Dosimetry: Benchmark Data Set Requirements
Authors: Simon W , Liu C , Palta J , Dempsey J , Bayouth J , Pavord D , Ibbott G , Tailor R , Followill D
Published: July 2006

Purpose: Provide accurate basic acceptance data needed for modeling dose calculation algorithms. Provide accurate dosimetry data for special geometries and heterogeneities that are frequently encountered in IMRT planning systems. Method and Materials: Develop requirements for the Benchmark Datasets that fully characterize the modern day delivery systems in commercially available 3D treatment planning systems (TPS), consistent with TG‐53 report on TPS QA. The hypothesis is that high quality benchmark data can be acquired by comprehensively characterizing single linacs of each make. The RPC database was used to demonstrate that hypothesis. Results: A measurement template that defines basic data for beam shape, penumbra, and radiation dosimetry characteristics, such as depth dose and scatter factors, has been defined for LINACs from the three major manufacturers: Elekta, Siemens, and Varian. Measurements will be made on a Varian Clinac 21EX, a Siemens Oncor and an Elekta Precise. The x‐ray beam energies measured will be 6, 10 and 18 MV. These measurements will be made in a 3D water phantom scanning system, using detectors that will enable high spatial resolution of dose gradients resulting from beam limiters. These detectors will include small volume ion chambers, diodes, and film. In addition, radiation dose in and around various heterogeneities will be measured in a water‐equivalent solid phantom that will allow the insertion of heterogeneous components. A comprehensive measurement uncertainty analysis will complement the data. Software has been developed for importing the Benchmark Data into structured database following the TG11 format. Conclusion: Software to streamline routine TPS QA, using the benchmark data, will be made commercially available, enabling recommendations from TG53. Conflict of Interest: This project has been funded in part with Federal funds from the National Cancer Institute, National Institutes of Health, Department of Health and Human Services, under Contract No, HHSN261200522014C and by Sun Nuclear Corporation.
SU‐FF‐T‐311: LINAC Dosimetry: Benchmark Data Set Uncertainty
Authors: Simon W , Kozelka J , Rose M , Liu C , Palta J , Dempsey J , Lynch B , Bayouth J , Pavord D , Ibbott G , Followill D
Published: July 2006

Purpose: Determine sources of error in the collection of a Benchmark Data set for LINAC dosimetry and provide methods of error correction that will ensure the highest possible accuracy of the dosimetric data. Method and Materials: Guided by the measurement requirements for the Benchmark Datasets, the sources of experimental error can be divided into 3 sources: 1) discretization and volume averaging errors; 2) Stochastic errors; and 3) Systematic or artifactual errors. Measurements will be made in a 3D water phantom scanning system and in a water‐equivalent solid phantom that will allow the insertion of heterogeneous components. Results: We present theoretical analyses of the expected errors associated with ion chamber, radiochromic film, and diode measurements and provide specific techniques that will enable high spatial resolution of dose gradients resulting from beam limiters and dose perturbations in and around heterogeneity interfaces, such as air/tissue, lung/tissue, bone/tissue. These techniques include de‐convolution of chamber response and a 3D correction matrix of a film scanner. Conclusion: Theoretical limits of spatial resolution in LINAC dosimetry are achievable and demonstrable using dosimetric tools available today. These tools and methods will be used to collect accurate Benchmark Dosimetry data for future research in Monte Carlo techniques for treatment planning as well as automated software tools that will enable systematic QA of modern treatment planning systems. Conflict of Interest: This project has been funded in part with Federal funds from the National Cancer Institute, National Institutes of Health, Department of Health and Human Services, under Contract No, HHSN261200522014C, NCI grants R01‐CA‐100636, and by Sun Nuclear Corporation.
SU‐FF‐T‐312: Liquid Helium Calorimeter for Brachytherapy Source Strength Measurement
Authors: Holmes S , Palmer B , DeWerd L , Micka J , Stump K
Published: July 2006

Purpose: To construct and demonstrate the utility of a new liquid helium calorimeter for the determination of contained and emitted power of low dose rate (LDR) and beta emitting brachytherapy sources Method and Materials: A liquid helium adaptation of the UWMRRC Stump calorimeter (Rev. Sci. Instrum., 76, 2005) was constructed for use with LDR and beta emitting encapsulated brachytherapy sources. Silver absorber construction provides 4π measurement geometry for contained power measurements, eliminating the need for geometric or solid‐angle corrections and removing uncertainty stemming from source anisotropy. Temperature of the absorbers as well as the detector housing is actively controlled, and an electrical substitution measurement technique is used. This technique does not require corrections for thermal coefficients that change with temperature, thus eliminating the uncertainty of the absorber mass from the measurement uncertainty. Temperature is monitored using germanium resistance thermometers, and overall system temperature stability is improved by placing the liquid helium reservoir inside a vacuum chamber that is in turn surrounded by liquid nitrogen. Results: The decreased noise floor of the liquid helium calorimeter will permit more accurate measurements of low activity sources than was possible with the Stump calorimeter. Preliminary MCNP5 Monte Carlo modeling indicates that the contained power collectors will be 99.99% efficient for the LDR sources for which the calorimeter was designed, and that for the 90Sr/90Y beta emitting source, the efficiency decreases to 98.13% as a result of the more energetic bremsstrahlung photons. Conclusion: Knowledge of the contained and emitted power for brachytherapy sources can yield a direct source strength value for use as a benchmark in Monte Carlo statistical models. The emitted fraction, which is the quotient of the emitted power and the total contained power, can also be utilized in determining the validity of geometric and atomic source composition used in statistical models.
SU‐FF‐T‐316: Measured and Calculated Dose Distribution Around 125I Brachytherapy Seeds in a Breast Phantom
Authors: Bertrand M‐J , Reniers B , Verhaegen F
Published: July 2006

Purpose: To compare measurement with Gafchromic® EBT films and Monte Carlo (MC) calculations of the dose distribution around LDR brachytherapy 125I seeds in a breast phantom and to analyze the effect of tissue elemental composition. Method: MC simulations of IBt Interseed125 seeds were performed, using MCNP4C. In view of the low energy of the photons, the detailed physics treatment was used, with libraries from EPDL97 and no electron transport. In order to evaluate the perturbation of the film, simulations were carried out to compare the dose distributions in the film and in water or acrylic. The radial dose function of tissues and potential substitutes were determined using MC and compared to associate the best substitute to the desired tissue. Measurements for 5 seeds in an acrylic breast phantom were performed and compared with the MC simulations for the same geometry. Results: Our simulations show no significant differences between dose distributions calculated in films and in water (average difference 0.13%). Similar results can be observed for simulations in acrylic. The radial dose function of acrylic is very close to the one of breast tissue (2/3 mammal gland, 1/3 fat). However, there is a significant difference in the dose distributions in acrylic and water (average 7%, maximum 25%). The comparison between the measurements in the acrylic phantom and the calculations shows a good agreement between the isodose distributions (within 12%). Conclusion: Both in water and in acrylic, EBT films appear to introduce no significant perturbation to the measured dose distribution around 125I LDR brachytherapy seeds. These films are very useful tools for measurements around 125I seeds. There is a significant tissue composition effect that should be taken into account in LDR brachytherapy dosimetry. Acrylic appears to be the best substitute for breast tissue. Seeds furnished by IBt.
SU‐FF‐T‐318: Measured Versus Calculated Dynamic Wedge Dose Distribution Using Anisotropic Analytic Algorithm (AAA) and Pencil Beam Convolution (PBC) Algorithms
Authors: Caprile P , Venencia C , Besa P
Published: July 2006

Purpose: The purpose of this work is to compare measured and calculated 2D EDW dose distributions using AAA and PBC calculations algorithms. Method and Materials: A 6MV photon beam from a Clinac 2300C/D Linac equipped with 7 EDW was used. Dose distributions were calculated for square symmetric fields by a Varian Eclipse v7.3.1 3DTPS with AAA and PBC algorithms. EDW were commissioning in the TPS. Dose distributions were measured using Kodak EDR2 films in a perpendicular configuration at 5cm depth in a solid water phantom for 4×4, 10×10, 15×15 and 20×20 cm field size settings. TPS calculations were performed for the same conditions as measurements, using a phantom with the same geometry. Calculation grid was 2.5×2.5 mm for PBC and 1×1 mm for AAA. Dose distributions were compared using RIT v4.3 software with gamma evaluation using 3% dose variation and 3mm DTA criteria. Results: Measured versus calculated percentage depth doses and wedge factors agree within 1%. Profiles in the non wedged direction, exhibit variations lower than 2% of dose or 2 mm DTA. In wedged direction both algorithms reproduces the measured profiles with deviations smaller than 2% of the normalization dose except in the penumbra region. AAA algorithm models better the penumbra region. EDW with angle lower than 45°, both algorithms model the distributions within the acceptance criteria. Some differences could be seen in the corners of the beams. For EDW with 60° angle, the percentage of pixels that not passes the acceptance criteria using AAA is acceptable, but using the PBC algorithm the percentage must be considered (reaching 28.5% of the distribution), representing regions of disagreement that may be of clinical interest. Conclusions: AAA algorithm models EDW dose distributions better than PBC. Differences between both algorithms increase with field size.
SU‐FF‐T‐321: Micro‐Computed Tomography: A Tool for the Determination of the Sensitive Volume of Cylindrical and Plane Parallel Ion Chambers
Authors: McNiven A , Umoh J , Holdsworth D , Kron T , Battista J
Published: July 2006

Purpose: To use micro‐computed tomography (micro‐CT) as a tool for non‐destructive imaging of air ionization chambers for independent sensitive volume determination and quality assurance of the chambers. Methods and Materials: A GE Locus Micro‐CT imaging system was used to acquire high‐resolution images of several common small‐volume chambers (Exradin T11 plane‐parallel chamber and Exradin A1SL cylindrical chamber). Initial scans were taken of these chambers with a 4cm field of view, 80kVp energy, 450μA tube current, 399 views, and 400ms exposure time per view. Images were reconstructed with 89μm pixel size. GE MicroView visualization software was used to determine the chambers' air volumes for comparison with the manufacturer's specifications. The ability of micro‐CT to differentiate materials and the effect of high density materials on image quality was assessed. In addition, four Exradin A1SL cylindrical chambers were imaged with an increased number of views (720) and acquisition angles (360°) and reconstructed with 20μm pixel size. Air volumes from all of the chambers were compared to the chamber relative ionization signals generated in a 10×10cm2 reference field from a Varian 2100C/D linac. Results: The air cavity volumes derived from the micro‐CT images agreed with nominal volumes given by the manufacturers within 5% for both the cylindrical and plane parallel chambers. The relative response of the cylindrical chambers agreed with the relative volumes (semi‐automatic method) within 2%. The presence of the connecting cables or pins within the chambers did not affect the ability to accurately visualize the sensitive air volumes. Conclusions: Micro‐CT is a promising tool for the measurement of ion chamber air volume and potentially for determining calibration factors for use in dosimetry. These high resolution images could also prove useful as input to Monte Carlo simulations and the calculation of ion chamber response factors from first principles.
SU‐FF‐T‐323: MLC IMRT Vs. Solid Compensators Based IMRT — Comparison of Various Clinical Cases
Authors: Kuros‐Zolnierczuk J , Papiez E , Papiez L , Lu Xiaoyi
Published: July 2006

Purpose: To compare multileaf collimator (MLC) intensity modulated radiation therapy (IMRT) with solid compensator based IMRT in order to demonstrate the feasibility and accuracy of solid compensator based IMRT. Method and Materials: Using the CMS radiotherapy planning system a sequence of two separate IMRT plans were prepared using superposition algorithm. First plan in each sequence delivered IMRT by means of the MLC, whereas the second one delivered IMRT by means of solid compensator. Quality Assurance (QA) IMRT was performed on a 2100CD Varian linear accelerator for both plans. For the QA measurements we utilized a 120‐leaf Millennium MLC with 0.5 cm resolution and brass compensators (milled by DECIMAL INC.). Cases for different sites, e.g. head and neck, chest, abdomen, pelvis were investigated using 6 or 16 MV photons. Number of delivered monitor units, intensity map shapes and the dose distributions for GTV, CTV and OAR were compared for both plans. Results: Preliminary results indicate that solid compensator plans use significantly less monitor units and deliver smaller dose to the OAR as compared to the MLC plans. Moreover, solid compensators offer generally better resolution of intensity maps than MLC. Finally, in the process of segmentation, MLC IMRT changes DVH. In contrast, solid compensators by avoiding segmentation process of intensity maps, leave DVH unchanged between original, optimized plan and deliverable plan. Conclusion: The presented comparison demonstrates that solid compensator based IMRT is feasible. In addition, solid compensator based IMRT can provide clinical advantage over MLC IMRT for some treatments.