bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐241: High Precision Data Set For Benchmarking of Electron Beam Monte Carlo
Authors: McDonald A , McEwen M , Faddegon B , Ross C
Published: July 2007

Purpose: In treatment planning, and particularly in beam commissioning, Monte Carlo dose calculation algorithms are finding increasing application. This research aims to create high accuracy reference data, which will be used for benchmarking of Monte Carlo models. Method and Materials: Using the Vickers research linac with beam energy known to 0.5 % and a translational stepping slide with positioning accuracy of 0.2 mm in‐air beam profiles were obtained. The measurements were made using an Exradin #a16 microchamber and PTW Electron Diode type 60012 and these were compared to a BEAMnrc Monte Carlo simulation and also to ICRU report 35 mass scattering powers. Results: An experimental data set has been obtained using a well‐known beam energy, spot size and irradiation geometry. Measurement data was acquired for a range of scattering foils and with different scattering gases between foil and detector. The uncertainty in the detector position is 0.2 mm and the uncertainty in the measured fluence is 0.1 %.

i) There was good agreement between the profiles measured by the two detectors.

ii) The initial Monte Carlo results indicated that a divergence of the incident beam must be included in the simulation to give agreement with the experimental data.

A simple scattering model, using mass scattering powers from ICRU report 35, was used to obtain an estimate of the initial beam divergence. A value of 10 – 15 mrad was obtained, which is in good agreement with the Monte Carlo results. Conclusion: We now have a high accuracy data set for the benchmarking of electron beam Monte Carlo models at better than 1 % /1 mm level. An initial BEAMnrc calculation has indicated that the angular divergence of the beam has a significant affect on the width of the in‐air beam profile.

Acknowledgment: Support from NIH grant R01 CA104777‐01A2.
SU‐FF‐T‐249: Improved Homogeneity with Fourth Generation Prostate Brachytherapy Implants — A Revised I‐125 Seed Nomogram
Authors: Kudchadker R , Johnson J , Bruno T , Frank S
Published: July 2007

Purpose: Fourth generation stranded implants permit periprostatic placement of seeds with improved homogeneity across the target volume. The purpose of this study was to compare Mick nomogram seed implants with fourth generation stranded implants and determine if a revision in the activity per volume nomogram is warranted. Methods and Materials: Eighty‐four patients who had undergone prostate brachytherapy were selected for this study and were split into two arms of forty‐two patients each. The first arm, were treated in the year 2000 with loose seeds. Treatment planning for these set of patients was performed intraoperatively using a nomogram based on the volume of the prostate. The second arm, were treated in the year 2006 with stranded seeds. Pre‐plans determining the number of seeds and needle loading were generated using Variseed treatment‐planning system. Approximately 20 – 25 % of the seeds were implanted within the PTV in the periprostatic tissue. The prescribed dose was 145 Gy for all eighty‐four patients. Also, all patients were implanted with I‐125 (Model 6711) seeds with an activity of 0.497 U/seed (0.391 mCi/seed). Results: The total implanted activity and the number of seeds used in our stranded seed prostate implants was significantly less than that used in our loose seeds implants. Mean V100 values for the stranded seeds were greater than those for loose seeds. The stranded seeds were also associated with higher mean D90 values and lower V150‐uretheral doses. Conclusions: We attribute this reduction in activity and the number of seeds to effectively treat the same prostate volume, to the efficient seed distribution of the stranded seeds. Loose seeds inside the prostate are subject to variable migration throughout the prostate resulting in a more heterogeneous dose distribution, while stranded seeds are systematically positioned inside the target volume resulting in a more homogeneous dose distribution.
SU‐FF‐T‐251: Improvement in IMRT Dose Calculation Accuracy with Megavoltage CT Imaging in the Presence of High Z Materials
Authors: Chen C , Meadows J , Bichay T
Published: July 2007

Purpose: The purpose of this study was to evaluate the dosimetric uncertainties in TomoTherapy IMRT treatment planning due to the presence of high Z materials. Treatment plan calculations, and chamber measurements, were compared for accuracy with kilovoltage (KVCT), and megavoltage imaging (MVCT). Method and Materials: Electron density calibration curves were created using a Siemens Open Sensation unit for KVCT, and a TomoTherapy Hi‐Art II unit for MVCT. Treatment plans were compared for a pelvic patient with bilateral hip replacements. In addition, for chamber measurements, treatment plans were also carried out on a phantom containing two high Z cerrobend inserts. The TomoTherapy Planned Adaptive module was used for IMRT calculation comparisons of KVCT and MVCT. Film profiles in the region of the heterogeneity were analyzed and compared using MatLab 7.0. Results: IMRT treatment planning calculations in the clinical image dataset resulted in a dose calculation discrepancy of 2.5% between KVCT and MVCT images. In addition, treatment planning with the phantom dataset, containing the high Z material, resulted in a 3.4% higher dose in the KVCT calculation compared to the MVCT calculation. Chamber measurements confirmed that the MVCT calculated treatment plan was within 0.05% of expected, while the KVCT calculated treatment plan was higher by 3.4% of actual measurement. Rendering from MatLab 7.0 showed that the KVCT film image was much noisier than MVCT film. Conclusion: These results demonstrate that KVCT IMRT treatment planning may be prone to error in the presence of high Z prosthetics. Under these conditions, MVCT imaging may result in more accurate treatment planning calculations.
SU‐FF‐T‐253: Improvement of the Convolution/Superposition Algorithm for IMRT Dose Calculations
Authors: Acosta E , McShan D , Fraass B
Published: July 2007

Purpose: To study improvements to the Convolution/Superposition (CS) dose calculation algorithm which enhance IMRT dose calculation accuracy. Method and Materials: Several aspects of typical CS calculation algorithms can be improved in order to increase the accuracy of IMRT dose calculations. To improve beam modeling and fitting, CS polyenergetic kernels were based on newly generated individual energy deposition kernels which are calculated on a high resolution grid using the Monte Carlo simulation package PENELOPE and relative contributions obtained from a detailed simulation of the accelerator head using the Monte Carlo code BEAM. Further improvement resulted from beam fitting based on data carefully measured using different detectors to better model dose in different regions of the dose distribution (e.g. beam penumbra, leaf and jaw transmission, buildup region, etc). Additional features added to enhance the CS algorithm included calculation of collimator scatter (for shaped and IMRT fields), an electron contamination correction for the buildup region for both the individual beamlet and final dose calculations, and modeling of the complete SMLC delivery sequence, allowing a more realistic final dose calculation after the optimization process is completed. Results: Comparison of output, depth dose and various dose profiles for square and shaped fields demonstrate the improved results for basic beam modeling. Ion chamber and 2‐D dose distributions measured for a series of IMRT fields illustrate the increased accuracy possible with the enhanced algorithm. Routine IMRT QA accuracy showed an average improvement of 3% with respect to the more standard CS implementation. Conclusion: Enhancements to standard convolution/superposition algorithm implementations can significantly improve accuracy for shaped field and IMRT dose calculations throughout the patient, including the buildup region, penumbra and outside the field.
SU‐FF‐T‐254: IMRT — Tomotherapy Vs Conventional Linear Accelerator
Authors: Ku P , Pino‐y‐Torres J , Chang S , Hsieh H , McKeague J
Published: July 2007

Purpose: To compare IMRT plans delivered with a conventional linear accelerator (Varian 2100EX) and a Helical Hi‐Art Tomotherapy accelerator and the dose differences to critical organs and structures surrounding the tumor volume. Method and Materials: Five patient plans were generated using both the Varian Eclipse treatment planning system and the Tomotherapy treatment planning system. A sampling of different treatment sites were chosen for the study, which include an esophagus, a hilum/lung, a para‐aortic, a pelvis, and an orbital case. IMRT plans were created with the goal of reducing dose to surrounding critical organs and structures while achieving similar satisfactory target coverage. Radiation doses were taken at three volumes of 25%, 50% and 75% for comparison. Also a mean dose and a max dose for spinal cord are provided. The plans generated represent the best dosimetric effort within clinically permitted time frame. Results: The critical organs for each case are highlighted in yellow in the supporting documentation. Tomotherapy plans show an average improvement of mean doses to the critical organs by 31.83% over the conventional linear accelerator plans. For spinal cord doses, the average improvement of maximum doses shows an 18.75% advantage to the Tomotherapy plan over the conventional linac. Conclusions: Examination of the dose volume histograms determined that the Tomotherapy unit was superior for thoracic cases, significantly reducing total lung doses. In most cases, the data show that the Tomotherapy plans provide better overall dosimetric characteristics. Also note that only five cases were compared in this study but similar results can be expected in most other cases.
SU‐FF‐T‐255: IMRT Dose Calculation Comparison Between BrainSCAN and Eclipse
Authors: Liu H , Schell M , Clark D , Fu L , Rosenzweig D
Published: July 2007

Purpose: To evaluate the dose calculation difference between BrainSCAN (5.31) and Eclipse (6.5) for the same IMRT plan. Method and Materials: 9 IMRT plans (3 Head/Neck, 3 Brain, and 3 prostate) were developed using BrainSCAN and exported to Varis RV system. The same patient CT scan was imported into Eclipse and the same isocenter position was selected with the help of AP and lateral radio‐opaque markers. The field parameters were brought back into Eclipse from the treatment field in Varis, using the “convert to planning field” function. The MLC leaf motion information was brought back using the “convert to actual fluence” function for each field, with the same MU as the BrainSCAN plan. Dose distribution was calculated using Eclipse PBC algorithm. Isocenter dose was compared to that from BrainSCAN. The 2D isodoses of the 3 orthogonal planes across the isocenter were exported from both TPS and compared using RIT. The plan was mapped to the same phantom from each TPS separately, and a Kodak EDR2 film was exposed so that it can be used as a judge to the isodose comparison. A Varain 21EX with MLC80 was used for IMRT delivery. Both dynamic and step‐and‐shoot were included. Results: The isocenter dose shows a −0.1±1.1% difference. The isodose comparison also shows good general agreement. However, we do notice that the Eclipse calculated isodose tends to smooth out the step‐shapes created by the MLC leaves, while the BrainSCAN preserves those shapes, which are closer to the actual situation shown on the film. Conclusion: This comparison study showed that the IMRT dose calculations of BrainSCAN and Eclipse are generally in good agreement. Eclipse tends to smooth out the step‐shapes, which is very clear in our study because 1‐cm leaf width MLC was used. A comparison with Eclipse AAA algorithm is on progress.
SU‐FF‐T‐259: In‐Phantom Measurements of the Bremsstrahlung Dose for Megavoltage Electron Beams Incident On High Z Materials for Use in An Electron Multileaf Collimator
Authors: Reft C
Published: July 2007

Purpose: There is a growing interest in the use of a multileaf collimator (MLC) for electron beam therapy to shape the fields as well as provide intensity modulated beams. One important consideration is the material and thickness of the collimating leaves. They must be sufficiently thick to stop the highest electron energy as well as minimize the bremsstrahlung radiation produced by the high Z material. In this work the total bremsstrtahlung dose is measured for megavoltage electron beams incident on three high Z materials used to simulate collimating leaves. Materials and Methods: A Varian Cl 2100 C is used to provide 6, 9, 12, 16 and 20 MeV electron beams. At the end of the standard cone are positioned high Z materials with thickness appropriate for multileaves. The high Z materials studied are cerrobend, brass and tungsten. Thermoluminescent dosimeters, radiographic film and a parallel‐plate ionization chamber are used to measure the x‐ray dose along the central beam axis in a solid‐water phantom at a source to surface of 100 cm. Film is used to obtain beam profiles at selected depths as well as provide an estimate of the beam quality. Results: The results show that for 20 MeV electrons the leakage as a percentage of the central‐axis (electron and x‐rays) dose is 4.4, 2.0 and 1.0 % for cerrobend, brass and tungsten, respectively, and 4.5% for the open field. The estimated beam quality of the x‐ray spectra inferred from the percentage depth dose measurements are 0.32, 0.63 and 0.86 MeV for brass, cerrobend and tungsten, respectively. Conclusion: The data show that 2 cm thick tungsten stops the 20 MeV electrons while reducing the bremsstrahlung dose to levels lower than that produced by the source in the unblocked field, and they also provide measurements for comparison with Monte Carlo calculations.
SU‐FF‐T‐261: Independent Point Dose Verification Using TomoTherapy Quality Assurance Phantom
Authors: Cheek D , Hogstrom K , Robertson C , Rosen I , Gibbons J
Published: July 2007

Purpose: To detect TomoTherapy planning errors by relating a point dose in the patient treatment plan to the dose calculated at the corresponding point in the quality assurance phantom. Methods and Materials: Standard TomoTherapy patient‐specific quality assurance applies the patient treatment plan to a cylindrical QA phantom and then compares measured phantom dose to calculated phantom dose. However, there is no check of dose in the phantom to dose in the patient. We applied a ratio of TMRs to a point dose in the patient treatment plan in order to estimate the dose to the corresponding point in the phantom. Rotational delivery of TomoTherapy was approximated using either a 360° arc, 8 equally‐spaced angles, or 4 equally‐space angles. For each approximation, calculations were done using average physical depth and average radiological depth. The method was tested using data from 87 TomoTherapy patients. TMR ratio method doses were compared to treatment planning system doses. Agreement within ±5% was considered clinically acceptable. Results: Using average radiological depth, the pass rates were 70.6%, 69.4%, and 65.9% for the 360°, 8−, and 4‐;angle approximations, respectively. Using average physical depth, the pass rates were 63.5%, 56.5%, and 58.8% for the 360°, 8‐, and 4‐angle approximations, respectively. The best results were obtained for centrally‐located tumors such as prostate (83.9% pass rate for radiological depth and 360° arc approximation). The pass rates were worst for superficial tumors (50.0% pass rate for radiological depth and 360° arc approximation). Conclusions: The TMR ratio method was used to relate dose in the QA phantom to dose in the patient. For each beam approximation, radiological depths gave better results than physical depths. The site‐specific pass rates could be used to determine action levels for implementing this method in the clinic.

Supported in part by a research agreement with TomoTherapy, Inc.
SU‐FF‐T‐262: Indications of Effective Depth Reversal for Cylindrical Ion Chambers in Electron Beams
Authors: Lightfoot D
Published: July 2007

Purpose: Cylindrical ionization chamber measurements of electron beam depth dose are corrected for the effective depth of measurement and water/air stopping power ratio. The purpose of this presentation is to examine whether a unidirectional upstream shift from physical to effective depth of measurement as often used in data acquisition systems makes sense. Method and Materials: Ionization chamber measurements of depth dose corrected for effective depth by a unidirectional shift and water/air stopping power ratio at depth are compared with diode and EBT film measurements for standard electron beam cone sizes over the nominal energy range of 6 MeV to 20 MeV. The diode results are obtained with a NIST traceable electrometer and are used without correction for any change in water/silicon stopping power ratio vs depth. The EBT film density is converted to dose based on a characteristic curve from films exposed in plastic water at the nominal depth of dose maximum. EBT film results are used without additional corrections for any change in water/EBT film stopping power ratio vs depth. The EBT film density is evaluated with a pin‐hole densitometer as well as a VIDAR film scanner. Results: Good agreement of all measurements at depth from 80 to 10 percent suggest ignoring variation in stopping power ratio for both diode and EBT film has little impact on the results. Both diode and film suggest a slightly deeper depth of dose maximum and a more rapid drop in dose from Dmax towards the surface. Conclusion: Agreement between ionization chamber results and the high resolution solid detector results would improve if the shift in the effective depth of measurement of the ionization chamber is based on dose gradient and reverses as the gradient reverses with perhaps no shift at the zero gradient position. This may impact on calibration protocols.
SU‐FF‐T‐263: Influence of Implanted Gold Markers On the Dose Distributions for Therapeutic Proton Beam
Authors: Kwak Jungwon , Lee Se Byeong , Shin Dongho , Shin Jungwook , Park Sung Yong , Park Soah , Yoon Myonggeun , Cho Kwan Ho
Published: July 2007

Purpose: The aim of this work is to examine the influence of implanted gold markers on the dose distributions in proton therapy and to investigate the dependency on the marker position in Spread Out Bragg Peak (SOBP) proton beam. Methods and Materials: The implanted gold marker has a cylindrical shape of 3mm length and 1.2mm diameter. The dose influence of the gold marker was verified in the condition of various locations of the proton beam field, which has 20 cm range in patient and 5cm of SOBP. Spatial dose distributions were measured using 3‐Dimensional moving water phantom and a stereotactic diode detector with 0.5 mm of effective diameter. Also, a film dosimetry using EBT film was performed. For Monte Calro simulation, GEANT4 simulation tool kit was used to confirm the measurement and to simulate the dose influence of multi‐implanted condition. Results: Gold is high‐Z material, which is good for CT marker but give more proton beam divergency and more under‐dosed volume. The under‐dosed volume of cold spots due to the single gold marker is measured to be 0.035 cc which has less than 80% of a prescription dose. The effect on the volume‐dose relation has been studied. Monte Calro simulation made us enable to study the various conditions of marker position. Conclusion: Cold spots inside the target volume affect the probability of the local tumor control. Cold volume due to the gold marker is very small but is not trivial in the view of the local control probability in proton therapy. The position of gold markers and the conditions of the proton beam field give the effect on the dose distribution at the target region implanted gold markers.
SU‐FF‐T‐264: Influence of Monte Carlo Beamlet Smoothness On Plan Metric Accuracy
Authors: Wattson D , Cui J , Wilkens J , Deasy J
Published: July 2007

Purpose: Monte Carlo (MC) may be advantageous as a basis for IMRT dose calculations as it provides for reliable heterogeneity corrections even in complex media. However, the upper‐limit of noise in pre‐computed beamlets has not been previously investigated. Method: We investigated convergence in plan metrics for optimized IMRT treatment plans as a function of MC noise in the input pre‐computed beamlet influence matrices. We analyzed seven individual patient cases of head and neck cancer. Each radiotherapy dose calculation involved nine 6 MV beams divided into beamlets of 1cm × 1cm cross‐section. The VMC++ Monte Carlo dose calculation engine was used as a basis for the calculations. A prioritized prescription optimization process was utilized to produce the final plan. As a function of beamlet noise levels, we monitored minimum combined‐PTV dose, D95 (minimum dose to hottest 95% of the combined PTVs), maximum dose to the brain stem, and mean dose to the parotid glands. For all metrics, we took a convergence within 1% of the low‐noise limit as being adequate. Results: For D95, 1% accuracy was typically achieved with an individual beamlet noise level of 2.4% (std. error); minimum target dose was accurate only when very noise levels were reached (< 0.5%); maximum dose to brain stem was accurate when beamlet noise was typically less than approximately 2%; mean dose to the parotid glands was accurate even at relatively noise beamlet levels (5%). Conclusions: Adequate MC beamlet smoothness depends critically on the plan review metrics. Metrics which partially or fully average over voxels (D95 of targets, or mean doses to organs) converge rather quickly compared to metrics which are sensitive to single‐voxel excursions (e.g., maximum organ dose or minimum target volume dose).

This research was partially supported by NIH grant R01 CA90445 and a Siteman Cancer Center fellowship.
SU‐FF‐T‐266: Intensity Modulated Radiotherapy of Breast Cancer Using Direct‐Machine‐Parameter‐Optimization
Authors: Mihailidis D , Plants B , Fischer T , Tomara P
Published: July 2007

Purpose: To design an efficient step‐and‐shot IMRT technique for breast CA and compare the results with other established treatment planning techniques. Methods and Materials: The technique was based on a modified Direct Machine Parameter Optimization (DMPO) method, where some of the inversely optimized segments were adjusted by the user in order to allow for skin “fall‐off”. During DMPO, the optimized fluence was converted into segments where the segment shape and weight became part of the optimization; thereby, forcing segments that did not satisfy the requirements to be discarded. A set of 20 breast CA patients was used to develop the technique and compare it with a 3D‐conformal with optimized wedges and a forward‐optimized technique with manually defined segments. Results: Dose distribution and DVH analysis of all patient plans resulted superior PTV coverage with better dose homogeneity and lower lung and heart doses for our IMRT technique, compared to the two other techniques. The number of segments can be controlled by the user and the resulted MUs for delivery are less that those needed for the 3D‐conformal with optimized wedges technique. Conclusions: DMPO can be effectively used on a routine breast IMRT treatment planning since it allows the user to modify the number and shapes of selected segments quickly and efficiently. Thus, this method solves the issue of patient and PTV setup uncertainty and motion by allowing the creation of “fall‐off” defined by the user, on patient‐to‐patient basis. The technique can be easily extended to 4‐field plans were the two anterior and posterior oblique fields are added for more complex PTVs. Dose uniformity to the PTV (due to the electronic compensation) and lower doses to the lungs and heart are the major advantages of our method.
SU‐FF‐T‐271: Investigation of Convergence and Systematic Dosimetric Errors Resulting From Endorectal Balloon‐Based Heterogeneity in Conformal and IMRT Planning of Prostate Cancer
Authors: Li S , Solberg TD , Rosu M , Foster R , Enke C , Chetty IJ
Published: July 2007

Purpose: To investigate dosimetric errors caused by endorectal balloon‐based heterogeneity in prostate cancer forward and inverse planning.

Method and Materials: Five prostate cancer patients, treated with air‐filled rectal balloons, were retrospectively recalculated assuming air‐equivalent density and were compared to plans in which unit‐density was assigned to the air. Each plan was performed using 3D‐CRT (4‐ and 6 field) and IMRT techniques. For IMRT plans, systematic dose errors were estimated as follows: , where the superscript, “air” assumes the true air density in the rectal balloon and indicates the dataset upon which beam weights were optimized. The subscript represents the final dose calculation dataset based on the optimized beam weights. The convergence dose error may be approximated as follows: , where the final dose calculation dataset is equivalent (assumes air density) while the initial dataset, upon which beam weights are optimized, is changed between the water‐based and air densities. DVHs, and biological dose indices, generalized EUD (gEUD), and NTCP were used to assess the dosimetric consequences of assigning unit‐density to the air in the rectal balloon. Results: Although the posterior aspect of the prostate (prostate/rectum junction) was consistently under‐dosed when air‐densities were assigned to the rectal balloon, these differences did not significantly affect the prostate gEUD. Average differences were approximately 0.5 Gy, with a maximum difference of 1.5 Gy observed in one IMRT plan. Rectal %NTCPs sometimes differed significantly (up to 7%) between the water‐ and air‐density assignment and were more pronounced for the 3D‐CRT plans. IMRT systematic dose errors were generally found to be larger than convergence errors, with the greatest effect observed in the rectum where NTCP differences ranged from 1 to 5%. Conclusion: Tissue heterogeneity effects resulting from use of endorectal balloons may be dosimetrically important and must be considered for accurate 3D‐CRT and IMRT prostate cancer planning.
SU‐FF‐T‐272: Investigation of Small IMRT Beamlets with a Wellhöfer CC01 “Pinpoint” Chamber
Authors: Simmons S , Cheng J , Ashburn J , Awan S , Meigooni A
Published: July 2007

Introduction: Most investigators of small IMRT beamlets rely heavily on ion chamber measurements. The PTW Pinpoint chamber (0.015 cm3) has recently been reported1 to be overly sensitive to low energy scatter X‐rays. It is therefore appropriate to normalize the PTW Pinpoint response to a 5×5 cm field rather than the conventional 10×10 cm field. For a 10×10 cm field, 10 cm depth, the over response was reported to be 1.4%, whereas for a 1×1 cm field an under‐response of 12% was determined. Therefore the PTW Pinpoint chamber is of limited use for fields smaller than 2×2 cm. Here, we used a Wellhöfer CC01 chamber, which is smaller (0.01 cm3) than the PTW Pinpoint, but has a comparable geometry. Technique: We investigated Varian 6 MV beamlets generated with a 120 leaf MLC, with the chamber axis instead parallel to the direction of the beam and perpendicular to the water surface. Hence the primary radiation of the beam “sees” only a 2 mm diameter cross section of the chamber before reaching the chamber's stem. A special scanning technique was utilized to position the center of the chamber's 3.6 mm height effectively at the water surface. Oblique “fan‐line” beamlets were also examined. The gantry was rotated so the oblique beamlets would have nearly a perpendicular incidence on the water surface. An average of 18 individually radiated micro‐TLDs were used per beamlet to measure the dose for comparison purposes. Results: Superior accuracy (< 2%) of the CC01 response was determined for 1×1 cm beamlets along the central axis. This agreement is significantly better than the reported performance of the PTW Pinpoint and allows use of the CC01 for small IMRT beamlets.
SU‐FF‐T‐275: Invitro Dose and Dose Rate Feasibility Study for Effective Tomotherapy TMI Treatment
Authors: Terai K , Hui S , Griffin R , Song C
Published: July 2007

Purpose: We reported the successful use of Tomotherapy for the total marrow irradiation (TMI) treatment. However, Tomotherapy delivers treatment at a much higher dose rate (400–900 cGy/min) than current TBI treatment (10 – 25 cGy/min) delivered using a standard medical accelerator. Even with the more accurate dosimetry possible with Tomotherapy, the effect of high dose rates even at limited doses is unclear at best. We aim to study the effect of high dose and dose rate on cell survival. Method and Materials: #a549 human lung cells, BE(2)‐M17 human neuroblastoma cells and N2A mouse neuroblastoma cells were used. Cells were plated in T‐25 flasks with appropriate medium. After 8 hours attachment period, each flask of cells were irradiated with a different dose: 2, 4, 6, 9 and 12 Gy. As the dose rate, 112 cGy/min was used in #a549 and BE(2)‐M17 cells and 400 cGy/min was used in N2A cells. Cells were cultured for 10–15 days and survived cells were making colonies. Colonies were fixed with a mixture of methanol and acetic acid (10:1 v/v) and stained with 1% crystal violet. Colonies containing more than 30 cells were scored. Results: BE(2)‐M17 cells were more sensitive to radiation than #a549 and N2A cells. Mean lethal dose, D0, were 89, 136 and 188 rad in BE(2)‐M17, #a549 and N2A cells in each. Plating efficiency at 6Gy were 0.50, 6.23 and 6.97 % in BE(2)‐M17, #a549 and N2A cells in each. Extrapolation number were 8.17, 5.20 and 2.51 in BE(2)‐M17, #a549 and N2A cells in each. Conclusions: We show that different cell lines have different dose response invitro. Currently we are continuing the survival study for dose rate from 10 cGy/min to 800 cGy/min which will be reported in the conference.
SU‐FF‐T‐277: Is Jaw‐Only IMRT Feasible for Head and Neck Cancer Treatment?
Authors: Mu G , Xia P
Published: July 2007

Purpose: The Direct Aperture Optimization (DAO) has enabled direct machine parameters such as positions of MLC or conventional jaws use in optimization for IMRT treatment. With DAO, one can create IMRT plans using conventional jaws. Feasibility studies for simple tumor shapes such as the prostate and breast were reported. This study is to investigate the feasibility of using conventional jaws to deliver complicated IMRT plans for patients with nasopharyngeal cancer Method and Materials: Six patients with nasopharyngeal cancer previously received IMRT treatment at our institution were re‐planned with jaw only (JO) and MLC options using a commercial treatment planning system. The same beam angle arrangements (typically 9 beams) as in the original IMRT plans were used for both JO and MLC plans. The plan acceptance criteria were established according to the RTOG‐0225 protocol. Number of segments per beam from 5 to 9 was experimented with JO and MLC plans. Results: Of six these patients, the JO plans with the maximum 9 segments per beam met the plan acceptance criteria. Four MLC plans with 9 segments per beam met the plan acceptance criteria, and one MLC plan met the criteria with minor variation. Because the TP system uses the simulated annealing method for optimization, one MLC plan did not meet the acceptance criteria, potentially trapped in local minima. More specifically, we compared the plan conformity and uniformity for one patient, and found that the MLC plan has better plan conformity than the JO plans, 77.2% vs. 69.5%. The uniformity of the MLC and JO plans was similar, 87% vs. 86%. Conclusion: We concluded that JO IMRT plans are clinically feasible for patients with head and neck cancer, with estimated delivery time of 15 minutes without including patient setup time. Conflict of Interest: This study is partly supported by Prowess.
SU‐FF‐T‐280: LDR Brachytherapy for Low Risk Prostate Cancer with I‐125 — a Comparison of the Results with Loose Seeds From Two Different Producers
Authors: Baumann R , Machtens S , Warszawski A
Published: July 2007

Purpose: LDR brachytherapy is a well established treatment method for low risk prostate cancer. Different techniques and radioactive sources are used. In this study I‐125 seeds from two different manufactures were compared regarding dosimetric properties and clinical outcome. Method: From 08/2000 to 04/2005 nearly 500 patients have been implanted with I‐125 seeds. The first 201 patients were implanted with loose seeds from the first manufacturer the other patients have been implanted with loose seeds from a second manufacturer. Both kinds of seeds have the same geometric dimensions, the same application system and method can be used. To compare the clinical outcome two subgroups of 101 consecutive patients were defined. In all cases the same treatment method was performed always by the same operating team. In both groups clinical dosimetric parameters (intraoperative and postoperative) were investigated, morbidity of the patients was analyzed. Individual nomograms were calculated to compare physically differences. Results: Dosimetric and clinical data of 202 patients were analyzed. No significant differences could be found in the number of seeds, prostate volume or implanted activity comparing the two subgroups. The intraoperative and postoperative D90 of the prostate was the same in both groups as well as the D30 of the urethra and the V100 of the rectum. Our clinical data show a very low probability of side effects in both groups of patients. Looking to the correlation of implanted activity and intraoperative prostate volume for both groups we found a small difference for prostate size greater than 50cc. Conclusion: LDR brachytherapy with I‐125 seeds is a well established treatment method. The quality of the implantation depends on the treatment method and the experience of the operating team. Different kinds of sources with the same isotope do not influence the clinical outcome and the quality of life for the patient significantly.
SU‐FF‐T‐281: Leakage Radiation of Elekta Synergy‐S LINAC Machine
Authors: Yang C , Guo F , Liu T , Purdy J
Published: July 2007

Purpose: To evaluate leakage radiation through treatment head of Elekta Synergy‐S linear accelerator, which has a new “beam modulator” MLC design. Method and Materials: As part of the acceptance test of a newly installed Elekta Synergy‐S, Kodak XV films were used to encompass the entire treatment head to locate potential radiation hot spots. After evaluating film results, an ion chamber (with appropriate buildup caps) was used to measure the leakage radiation for 6MV and 15MV x‐ray beams with MLC leaves fully closed. The chamber was placed in a plane perpendicular to the beam axis and which contained the machine isocenter, at distances ranging from 13–100 cm away from the machine isocenter in three directions (inline, crossline and diagonal). A 4.5 cm lead block placed on the blocking tray was used to evaluate the scattered radiation contribution from closed MLC leaves to ion chamber measurements at selected locations near the secondary collimator edges. Results: Leakage radiation was greater than 0.1% of the dose at isocenter for the original beam modulator treatment head design. A Beam Modulator Head Shielding Kit was installed and measurements repeated. Leakage radiation was only slightly reduced. A second version of Beam Modulator Head Shielding Kit was installed, and subsequent measurements showed that head leakage radiation was finally reduced below 0.1%. Contributions to ion chamber readings from scattered radiation generated from closed MLC leaves can be as high as 60% at close proximity to the secondary collimator edge. Conclusions: Thorough measurement of head leakage is necessary during acceptance testing of a LINAC machine, especially for new type machines such as the Synergy‐S, to ensure patient and staff safety. It is recommended leakage radiation measurements be made by blocking the scattered radiation contribution from the closed MLC leaves.
SU‐FF‐T‐401: The Beam Performance and Dose Distribution of a Needle‐Based Miniature X‐Ray Generating System for Medical Application
Authors: Liu Y , Li S , Movsas B , Sozontov E , Gutman G , Strumban E
Published: July 2007

Purpose: To quantify the radiation and to assess the performance for a needle‐based miniature x‐ray generating system using GAFCHROMIC EBT films and ionization chambers on low photon energy. Method and Materials: A needle‐based miniature x‐ray generating system (Advanced X‐Ray Technology, Inc.) has been developed for intraoperative brachytherapy with a great dose fall off in certain directions. The system comprises a conventional x‐ray tube with an Ag anode and a 9.5 mm diameter needle device with an interchangeable Mo target. The secondary x‐rays from the Mo target are mono‐energy (Kα, 17.5KeV) beam with a ringtype dose distribution. Ionization chambers and GAFCHROMIC EBT films were used to measure the stability, linearity, uniformity, half‐value layer, and air dose rate of the primary beam, as well as the isodose distribution of the secondary x‐ray beam. Each irradiated film was calibrated with the average OD over the center area of 2×2cm2 on the film. An average OD over 5×5cm2 area on films in the same film batch was determined as the background. Results: The primary beam with a power setting of 58 kVp and 25 mA has ∼0.36Gy/min air dose rate at 91cm, <2% flatness for 3 × 3 cm field, ∼0.554 mm Al half value layer, and excellent stability and linearity (<1% deviation). For the secondary Mo mono‐energy beam (17.5KeV), reading from a micro‐chamber provided the radial dose function in air from 6.19 mm to 12.19 mm from the target center. The maximum dose (253cGy) measured by EBT film at 7.48mm from the target center agreed with the micro‐chamber measurement (260cGy). Conclusion: The measurements of radiation output and isodose distributions and the evaluation of the system performance provided us the characteristics of the two stage x‐ray generator and they are essential parameter for its medical application.
SU‐FF‐T‐404: The Effect of Detector Size On Measurement of Output Factor of 4 Mm Collimator in Gamma Knife
Authors: Lah J , Shin D , Hong J , Suh T
Published: July 2007

Purpose: We investigated the effect of detector size for output factor measurement of small field in Gamma knife. Measured and corrected output factors using deconvolution to remove the effect of detector size for the 4 mm collimator were compared. Method and Materials: The output factors were measured with a pinpoint ionization chamber, a glass rod detector (GRD) from the 14, 8 and 4 mm helmets relative to the 18 mm collimator. The measured output factors were corrected for spatial averaging effects by measuring dose profiles for the 4 mm collimator with the same detectors and deconvolving their response from the measured profiles. A Gaussian kernel was used to describe the detector response. The measuring volume of the pinpoint chamber was 0.015 cm3 and the size of the GRD was 1.5 mm in diameter and 8.5 mm in length. All measurements were performed in spherical polystyrene head phantom having a diameter of 160 mm. Results: The 4 mm collimator output factor measured by the pinpoint chamber was 0.699±0.009 and the GRD measurement was 0.861±0.015. The corrected output factor by deconvolving for the 4 mm collimator was 0.853 for the pinpoint chamber and 0.874 for the GRD. Conclusion: The output factors obtained with a GRD are within 1.1 % in good agreement with the values recommended by the manufacture. The difference for measured and corrected value of output factor based on GRD detector is within 1.5%. The GRD may be a useful detector for small field dosimetry. However, the measured value of the 4 mm collimator output factor from the pinpoint ion chamber is significantly lower than the results obtained using the GRD. Due to the large volume of pinpoint chamber compared to other detectors used, a significant deconvoltuion correction is noted for the for the smaller collimator size.
SU‐FF‐T‐405: The Effect of Magnetic Field On Relative Biological Effectiveness in Yeast Cells
Authors: Chen Y , Lau B , Becchetti F
Published: July 2007

Purpose: To investigate the effect of a magnetic field on the relative biological effectiveness (RBE) in yeast cells irradiated with γ‐rays. Method and Materials: A γ source was used to irradiate Saccharomyces cerevisiae (baker's yeast) with and without a modest magnetic field (0.78 T). A set of NdFeB permanent cylindrical magnets and another set of non‐magnetized pseudo‐magnets were used to obtain data with and without the magnetic field. The cells were located at the center between the pole faces and sandwiched between two layers of non‐nutrient agar to maintain lateral electron equilibrium. The variation of the magnetic field in the cell region was less than 1%. The Monte Carlo code PENELOPE was used to simulate the dose distribution in the presence and absence of the magnetic field to ensure the doses delivered were equal in both cases. Another group of cells without irradiation was used as a control for the two groups being irradiated. The cells were sampled and cultivated after irradiation, and colonies representing surviving cells were counted. Results: In five out of seven experiments the null hypothesis (no effect) could not be rejected with t‐test at p = 0.05. Nonetheless, the results still suggest that the survival rate may drop slightly in the presence of the magnetic field. The survival rate decreased by 3.9% ± 2.7% at about 50% survival rate. At two standard deviation level the effect was less than 10% at B = 0.78 T. Conclusion: The effect of a 0.78 T magnetic field on the RBE of Saccharomyces cerevisiae is smaller than 10%. Although the indication that a strong magnetic field may increase the RBE of the radiation is not conclusive at 0.78 T, further experiments at higher magnetic field (B >2 T) and larger sample size would reduce the uncertainties in the experiments.
SU‐FF‐T‐406: The Effect of User Definable CT Acquisition Parameters On CT Numbers for Materials of Various Electron Densities
Authors: Pevsner A , Hanley J
Published: July 2007

Purpose: To investigate the effect of all user definable CT acquisition parameters on CT numbers for materials of various electron densities, and therefore patient dosimetry. Methods & Materials: CT scanners come supplied with a large number of user definable parameters whose effects on CT numbers are not well documented. An electron density phantom (Gammex RMI 467), with 16 inserts, having 13 densities ranging from 0.28–1.69 g/cm3, was scanned using: helical and axial techniques; 3 kVp settings; 6 reconstruction filter types; 8 mAs settings; 4 different slice thicknesses; and 4 different field of view diameters. In our study we have utilize phantom with 16 inserts. A total of 77 scans were acquired. Results: The most significant variation in CT numbers resulted from the change in kVp settings. CT number variation progressively increases for inserts with density of 1.00 g/cm3 and higher. Standard deviation in CT numbers for 1.69 g/cm3 insert was 8%. However, in a case when kVp held constant adjusting other acquisition parameters (filters, mAs, slice thickness, field of view, technique) resulted in a standard deviation of no more then 3% for all 13 densities. Conclusions: The kVp settings has a most significant effect on CT numbers whereas change in mAs, reconstruction filter type, slice thickness and field of view diameter result in only minor variations. The dosimetric effect for pediatric cases, typically acquired at 90kVp, will be presented.
SU‐FF‐T‐407: The Effect of Weekend‐Off and Day of Treatment Initiation On Tumor Control Probabilities
Authors: Lee T , Rosen I , Hogstrom K
Published: July 2007

Purpose: Recently developed tumor control probability (TCP) models can incorporate temporal variations in the delivery of treatment fractions. This study investigated the effect on TCP of weekend treatment breaks and of the treatment initiation week day. Methods and Materials: Treatment plans were selected for three patient with tumors of different cell doubling times — squamous cell carcinoma of the head and neck (short doubling time), breast cancer (intermediate doubling time), and prostate adenocarcinoma (long doubline time). TCPs were calculated and compared for each treatment plan using fractionation schedules that included or omitted weekend breaks. We also computed TCPs for fractionation schedules that included weekend breaks but started on different week days — Monday, Wednesday, or Friday. We assumed the same numbers of fractions and total doses for all fractionation schedules. Results: For the primary PTV of the head and neck tumor, the TCP increased by 3.1% when weekend breaks were not given. For the breast cancer, the TCP was independent of weekend break. For the prostate tumor, the TCP increased by 1.4% when weekend breaks were omitted. When comparing different start days for treatment, TCPs were higher for Monday starts than for the other days for all cases. Conclusions: The calculations suggest that TCPs can be increased for the same total dose if treatment is delivered without weekend breaks. In addition, the overall treatment time is reduced, which is known to be an important factor in head and neck treatments. The comparison of TCPs calculated for different treatment start days suggests that Monday is the best day for treatment initiation. This is consistent with current clinical practice. However, we did not examine the effect on normal tissues of fractionation without weekend breaks.

Supported in part by a research agreement with TomoTherapy, Inc.
SU‐FF‐T‐409: The Importance of Accurate Dose Calculation for Treatment Planning of Patients with Small Lung Tumors
Authors: Fippel M , Wiezorek T , Salz H , Schwedas M , Wendt T
Published: July 2007

Purpose: To evaluate the accuracy of different dose calculation algorithms for treatment planning of patients with small lung tumors, film measurements in a heterogeneous phantom are performed. The measured dose distributions for two photon energies (6 MV and 15 MV) are compared to calculations with Pencil Beam (PB), Collapsed Cone (CC) and Monte Carlo (MC). Method and Materials: The phantom is made of solid water slices and styrodur. The center of the phantom contains a solid water cylinder modeling the planning target volume (PTV). Radiographic films are placed between the slices. Treatment plans consisting of 9 conformal beams with directions uniformly distributed around the PTV cylinder and two different photon energies (6 MV and 15MV) of a standard linear accelerator are created. Dose calculations are performed with the three algorithms. The calculated dose distributions are compared to dose profiles extracted from the film exposure after irradiating the phantom. Results: The best agreement (independent on the energy) can be observed between the film and MC profiles. The PB algorithm fails to reproduce the measured dose distribution. The CC results show good agreement for 6 MV and an acceptable agreement for 15 MV inside the PTV cylinder. The CC dose outside of the PTV in the styrodur material is overestimated for 15 MV. Both, MC and CC algorithms predict the absolute dose measured with ionization chamber at the isocenter with an accuracy of at least 3%. The absolute dose deviation of the PB algorithm is 4% for 6 MV and 10% for 15 MV. Conclusion: For treatment planning of patients with small lung tumors the use of PB algorithms should be avoided. CC algorithms provide good accuracy for low energies; MC algorithms provide good accuracy for low and high energies. Conflict of Interest: “Research sponsored by BrainLAB AG.”