bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐197: Dosimetry in a Lung Moving Phantom
Authors: Zhu T , Finlay J , Das I , Li J
Published: July 2006

Purpose: Respiratory organ motion is known to be one of the largest intrafractional organ motions. In the lung phantom, secondary electron transport in addition to the photon transport tends to extend the dose disequilibrium region, resulting in a larger penumbra region and thus potentially different motion effect on dose distribution for IMRT fields. This study quantifies the effect of motion on dose distribution in lung medium. Method and Materials: Measurements were performed in a water‐equivalent made of solid water and a lung phantom composed of 15‐ cm thick 0.25 g/cm2 cork slabs at several depths a computerized motion phantom. Dose distributions for 6 MV and 15 MV photon beams are measured for a 10×10 cm2 open field and for IMRT fields. The motion is created using a ±1‐cm motion amplitude and 6 second per breathing cycle. EDR2 films were used to determine the dose distribution with or without motion at various depths in the solid and lung phantom. Results: At 10 cm depth, the penumbra of dose profile in the lung phantom is much broader than the solid phantom. The penumbra width increases with increasing photon energy. The effect of motion can be modeled with a motion kernel determined by the probability function of the breathing cycle and is independent of the phantom (solid or lung) types used. At 2.5 cm depth, the penumbra width between the lung and solid phantom is similar since the same solid water material was used above the point of measurement. Conclusion: We have shown that dose distribution in a moving lung phantom can be predicted from the measured dose distribution in the static lung phantom and a motion kernel. The introduction of secondary electron transport actually reduces the effect of motion on the dose distribution because the penumbra width is wider in lung material.
SU‐FF‐T‐273: Improved Calculation of Energy Spectra From Electron Depth Dose Curves
Authors: Lehmann J , Schreiber E , Chen J , Faddegon B
Published: July 2006

Purpose: Improve the precision of response functions, account for scatter in air, and significantly reduce the scatter component of the beam. Methods and Materials: (1) The electron depth dose measurements that are used as the reference are an open field (40cm × 40cm) without any applicator. (2) The vacuum in the space between source and phantom has been replaced with air, to account fully for in‐air scatter. (3) The point source simulation with a set of small scoring regions around the central axis has been replaced by a 0.001 cm radius pencil beam simulation on a single 100 cm radius voxel. This is equivalent to a broad beam normally incident on the phantom with small scoring area. The resulting depth dose curves are corrected to a point source with an inverse square law correction. Ds(d) is the simulated dose at depth d in a 100 cm SSD setup. The corrected dose Dc(d) can be found with: Dc(d)=Ds(d)*10,000/(100+d)∧2. Results: The electron depth dose curves simulated with a small pencil beam and large planar scoring regions agree well with those simulated with a point source and small regions after the inverse square law correction has been applied. The simulation times for the pencil beam calculations were shorter by orders of magnitude and the precision is much better. Use of the large field with very low scatter from peripheral components and including the air scatter in the response functions provides for a more realistic estimate of the true spectrum. Otherwise the low energy component is overestimated. Conclusions: The three improvements of the unfolding procedure improve the accuracy of the electron spectra and require less simulation time.
SU‐FF‐T‐274: Improvements of a Commercial Treatment Planning System On the MLC Field
Authors: Chow J , Grigorov G , Jiang R
Published: July 2006

Purpose: A recently released Pinnacle treatment planning system software, v7.4f includes some new physics features such as modeling of the rounded multi‐leaf collimator (MLC) leaf ends and the tongue‐and‐groove structure between leaves. In this study, the above physics modeling improvements were verified by comparing the peripheral dose profiles for the small MLC fields calculated by the new Pinnacle v7.4f and the old Pinnacle v6.2b with those obtained from measurements experimentally. Method and Materials: Three test MLC fields with different jaw sizes were prepared, and specific dose profiles (along cross‐line, in‐line and diagonal axis) at different depths were measured using a Varian 21EX linear accelerator with 120‐leaf Millennium MLC, big scanning water tank and photon diode. Estimated dose profiles for the test fields were calculated using Pinnacle v6.2b and v7.4f. Results: By comparing the measured and calculated results, both v6.2b and v7.4f performed well in calculating the cross‐line (along the gap between the longitudinal lengths of two leaves) and diagonal axis dose profiles at different depths. However, v7.4f gave calculated doses closer to the measured field for in‐line (gap between junctions of two rounded leaf ends) axis dose profiles at different depths. For the shape of profile along the in‐line axis, v7.4f calculated a flat “platform” dose profile of about 34.3% (inter‐bank leakage) at depth dmax beyond the MLC field edge using a clinical dose grid size of 0.4 × 0.4 × 0.4 cm3, compared to the “zigzag” dose profile varying between 35.4% and 42.1% measured using water tank and diode. However, both versions calculated the percentage depth dose for the test fields well compared to the measurements. Conclusion: The physics improvements of the rounded leaf end, tongue‐and‐groove structure, inter‐leaf and inter‐bank leakage corrections as new features of v7.4f were verified.
SU‐FF‐T‐275: Improving IMRT Plans Delivery for Head and Neck Cases Using Aperture‐Based MLC Segments
Authors: Lavoie C , Beaulieu F , Gingras L , Nadeau S , Sévigny C , Tremblay D , Beaulieu L
Published: July 2006

Purpose: To investigate the possibility of performing IMRT in head and neck treatment sites with less segments and monitor units (MU). Materials and Methods: Six pharyngeal cases (n = 6) were analysed and four cases (n = 4), in the sinonasal region. For each one, an IMRT plan was first realized using a commercial software (P3 IMRT, Pinnacle3 — IMFAST segmentation algorithm). These patients had to receive 32 fractions of simultaneous integrated boost external beam radiotherapy at 1.8 and 2.15 Gy/fraction, respectively to the low and high risk planning target volumes (PTV1 and PTV2). Then, an‐in‐house inverse planning system, called Ballista, based on predetermined segments, was used to realize comparable plans. Its segments are generated with the subtraction of the projection of the OARs with the PTV (planning target volume). Results: For the pharyngeal Ballista plans, the average volume of the PTV that received at least 100% of the prescribed dose (V100) was 85.0±4.5% for the first prescription (PTV1) and the V100 for the second prescription (PTV2 — simultaneous integrated boost —) was 78.5±10.9%. With Pinnacle3,the V100 value was 86.6±4.8% and 81.5±12.4% respectively for PTV1 and PTV2 (see figure 2a and 2b). On average, Ballista plans have required 932±124 MU and 52±10 segments compared to 1238±230 MU and 117±7 segments for Pinnacle3. For the sinonasal Ballista plans, the average V100 obtained was 80.0±3.1%. With Pinnacle3, the V100 gave 75.7±2.7%. Ballista plans have required an average of 406±54 MU and 22±1 segments compared to 697±133 MU and 99±14 segments for beamlet‐based IMRT. Conclusion: In step‐and‐shoot head and neck IMRT, an anatomy‐based MLC optimization system can achieve similar dosimetric plans comparable to traditional beamlet‐based IMRT with less number of segments and MU.
SU‐FF‐T‐277: IMRT Composite Plan ‐ A Must Tool for Final Dose Analysis with IMRT Boost Plan
Authors: Yang C , Peng L , Chen Y , Weiss M
Published: July 2006

Purpose: To observe the IMRT composite planning results from two different dose object functions and to suggest the adequate methodology for plan evaluation. Method and Materials: IMRT treatment planning often presents a two‐steps process; initial plan to cover the gross tumor disease and potential nodal areas; then the Cone Down (CD) plan for gross tumor boost. IMRT CD plan usually is a great challenge to generate the composite dose distribution due to its dose non‐uniformity nature for IMRT planning. Quality of the composite IMRT plan can be greatly influenced by the initial treatment planning due to its non‐uniform dose distribution nature to add on the CD IMRT plan. Since there are still a few planning systems not even provide the composite dose options for summarizing the initial IMRT plan with IMRT CD plan. We are currently using the Pinnacle system to analyze composite dose effects by using the total dose constraints as well as the separate dose constraints. Results: Clinical findings are summarized as 1) Initial IMRT and CD IMRT plans are highly correlated and the dose constraints have to be considered in a composite way (see fig. 1 (composite dose constraints) and fig. 2 (CD dose constraints)); 2) A sub‐optimal initial IMRT plan can disturb the IMRT CD plan for composite dosimetry, creating a unacceptable MUS due to the non‐uniform intensity map summarization (figs. 3 and 4 shows high MUS to compensate the PTV coverage compared to figs. 5 and 6); 3) Scaled CD IMRT dose constraints may not generate an acceptable composite dose distribution since summation of the two acceptable individual intensity maps may not create an optimum composite intensity to meet the final dose criteria. Conclusion: From planning comparison, composite IMRT plan has to be analyzed in order to create the true dose distribution in IMRT.
SU‐FF‐T‐283: In‐Vivo Diode Dosimetry for IMRT Fields
Authors: Circiumaru O , Zhu T
Published: July 2006

Purpose: In‐vivo dosimetry can be a useful tool to verify overall patient treatment quality with respect to calculation, setup and delivery. We have conducted a study in our department using the Sun Nuclear QED diode detectors to verify the accuracy of dose delivery in patients undergoing IMRT treatments. The expected diode reading calculation is based on an in‐house developed MU calculation program. Method and Materials: The diodes were placed on the surface of a 4 cm thick solid water phantom and the IMRT fields were delivered to the assigned SSD under treatment conditions. We compared the diode dose readings during the IMRT treatment delivery with the calculated entrance dose in the Penn MU program. The results were also compared with the Dose/MU ratio between PennMU and Oncentra treatment planning system (OTP). Results: A large number of cases were selected and 73 IMRT fields were checked. The calculated values were in 8% agreement with the diode readings. The magnitude of error is dependent on the diode position in the delivered field, whether it is located in a high or low dose region. Shifting the diode position to a high dose region lead to improved accuracy. Conclusion: Penn MU calculation program and the diode measurements are within 8% agreement and we expect our MU calculation program to be clinically used as a reliable tool for verification, keeping in mind that the diode point of measurement should be in a high dose region to avoid large errors.
SU‐FF‐T‐284: Independent Calculation of Dose for a Helical TomoTherapy Treatment Plan
Authors: Gibbons J , Smith K , Cheek D , Rosen I
Published: July 2006

Purpose: To develop a new calculation algorithm for independently verifying doses from helical TomoTherapy treatment plans. Method and Materials: The calculation algorithm confirms dose to a point in a high‐dose, low‐gradient region where modulation is expected to be minimal. Inputs to the algorithm are the coordinates of the point, the radiological depth for each projection angle in the axial plane of the point, and the treatment sinogram. The algorithm uses common dosimetric functions (e.g., TPR, Scp), which were measured on the TomoTherapy treatment unit. Measured lateral and longitudinal profile data are used to quantify the off‐axis dose dependence. Test comparisons were made using a 7‐cm diameter PTV centered in a 20‐cm diameter cylindrical phantom. The phantom was positioned in the center of the treatment bore for one test and 10 cm off‐axis for another. Comparisons were also made for five prostate cancer treatment plans. In all cases, the point of calculation was the geometric center of the PTV. Results: Measurements of both TPR and Scp on the TomoTherapy unit demonstrated small variation with jaw width (field length) and the number of open leaves (field width). Therefore, average values of TPR(d) and Scp were used in the algorithm. Comparisons with the cylindrical phantom treatment plans demonstrated good agreement, with the calculated doses agreeing within 2% at the PTV center for both the on‐axis and off‐axis treatment plans. Results for the prostate patients also showed good agreement within 4%. Conclusion: A new calculation algorithm that uses standard dosimetric functions has been developed for verifying doses from helical TomoTherapy treatment plans. Supported in part by a research agreement with TomoTherapy, Inc.
SU‐FF‐T‐287: Inhomogeneity Effect On Gamma Knife Stereotactic RadioSurgery
Authors: Xiong W , Huang Y , Lee L , Feng J , Morris K , Calugaru E , Burman C , Yeh K , Li J , Ma C
Published: July 2006

Purpose: To study the inhomogeneity effect from skull and air cavity on Gamma Knife Stereotactic Radiosurgery. Method and Materials: Patient CT data from Gamma Knife procedure was used in a Monte Carlo simulation. In the Monte Carlo simulation, the 201 Cobalt‐60 sources were considered to have the same activity. Each Cobalt‐60 source was contained in a cylindric stainless steel capsule. The beam data was stored in four beam phase‐space files which were generated in the inner side for each of 4 treatment helmets, after the Cobalt beam passed through primary collimator and secondary collimator. The dose was calculated using Monte Carlo simulation in both homogenous and inhomogeneous geometries rebuilt from patient CT data with identical beam parameters. A small volume was created around the iso‐center for DVH comparison. The doses in a 16cm diameter spherical QA phantom were also measured and calculated with and without 1.5mm Lead‐covering. Results: For QA phantom, the dose ratios with and without 1.5mm Lead covering are 89.8% from measurement and 89.2% from Monte Carlo. For patient's CT phantoms, Monte Calor results show that although the isodose lines remain almost the same with and without inhomogeneity correction, the difference in absolute dose has been observed. The dose in CT phantom is about 4.1% lower than the dose in water replaced CT phantom. With various skull densities, the inhomogeneity effect could reach as high as 8.0%. Conclusion: Monte Carlo has been applied to dose comparison for CT image with and without inhomogeneity correction. Monte Carlo simulation matches very well with measurement for the spherical QA phantom. It shows that the implementation of Monte Carlo simulation for Gamma Knife is accurate. It shows that the inhomogeneity effect should be considered for gamma knife treatment planning.
SU‐FF‐T‐288: Initial Experience with a Commercial Monte Carlo Electron Treatment Planning System
Authors: Mah D , Hong L , Yaparpalvi R , Engler M
Published: July 2006

Introduction: Monte Carlo modeling of clinical electron beams has the potential to substantially improve accuracy and quality of treatment planning, as excessive compute time and lack of commercial availability has hindered its application. Here we present a preliminary evaluation of a commercial electron Monte Carlo algorithm. Methods and Materials: Percent depth dose and profiles of 6‐20 MeV electrons and 6×6 – 25×25 cm cones were measured in a water tank at 100 cm SSD using a Farmer chamber for electrons. Absolute output was measured at 110 and 100 cm SSDs. Outputs and distributions of two extreme test cases were measured: a 2.1 cm × 3 cm insert in a 6×6 cone and a 2.8 cm × 15.7 cm long slit on a 25×25 cone. The algorithm's ability to accurately model relative and absolute dose of an obliquely (30°) oriented beam was evaluated ionization chamber measurements. Clinical cases with were checked using Mosfet dosimeters in vivo. Monte Carlo calculations were performed with a 2 mm grid, and smoothing filters provided with the algorithm were applied to minimize noise in the data. Results: Agreement of 2% of measured and modeled doses was found over the evaluated range of energies, cones, obliquities and SSDs. Compute times of 1–5 minutes were a function of increasing field size. Visual comparison of the shapes of the profiles was in agreement with measurement. Profiles of the eccentric geometry test cases appeared to be to be physically unrealistic (e.g. an inverted V) in the smaller dimension suggesting that the beam model was not valid. Conclusions: The Monte Carlo electron algorithm provides accurate distributions for most clinical cases. For extreme cases, measurements should be made to test the accuracy of the system, and further development of the algorithm is recommended.
SU‐FF‐T‐289: Initial Study On the Dose Evaluation of Boron Neutron Capture Synovectomy Using THOR Epithermal Neutron Beam
Authors: Yeh Kuan Chuan , Chuang Keh Shih , Hsueh Yen Wan , Su Chi Chuang , Huang Ying Chiang
Published: July 2006

Purpose: To evaluate the feasibility of Boron Neutron Capture Synovectomy by Tsing Hua Opening‐pool Reactor (THOR) in Taiwan and to determine the optimal treatment parameters with epithermal neutron beam. Method and Materials: MCNP5 was used to model the THOR epithermal neutron beam interactions with knee and finger phantom. The phantom was established according to the structure of human joints with different boron concentration. The treatment parameters were used to model the optimum treatment assembly, such as different thickness of reflectors and beam orientations. The Figure of merits (FOMs) such as total treatment time, total maximum skin dose and synovium to bone treatment ratio were used to evaluate the effect of the treatment parameters. Results: Monte Carlo calculations predict a total therapy time of BNCS between 5 and 15 min for the human knee by optimum THOR beam assembly. The treatment parameters of BNCS vary with joint sizes. The optimum treatment condition for different joint size can be achieved by using the opposed parallel beam, placing the inflamed joint near the source, and adding 10cm side and rear graphite reflectors. To compare with BNCS using the neutron beam produced by accelerator, the THOR epithermal beam will reduce the total skin dose from 205 RBEcGy to 130.24 RBEcGy and increase the TRbone from 72 to 74.28. Conclusion: This study predicts the optimum THOR beam assembly for BNCS. The result shows the quality and overall clinical efficacy of THOR epithermal neutron beam for BNCS is more suitable than the beam produced by accelerator. It provides the potential application of BNCS by epithermal neutron beam.
SU‐FF‐T‐291: Intensity Map Verification for IMRT QA Using An A‐Si EPID
Authors: Lometti M , Morin O , Bellerose J , Malfatti C , Aubin M , Pouliot J
Published: July 2006

Purpose: To use a flat panel electronic portal imaging device (EPID) for intensity map verification as part of an intensity modulated radiation therapy (IMRT) QA program. Method and Materials: An a‐Si EPID was used to image each field shape segment of a head and neck IMRT treatment. The DICOM images produced from each gantry angle were summed together and weighted according to the number of MU measured. Open field portal images were acquired at the end of the treatment, for each gantry angle, and subtracted from each composite image. A GUI was created in the MATLAB environment to read and sort each image according to gantry angle and MU measured, perform the image summations/subtractions, produce and display a final composite image. Results: Subtraction of the open field portal image from the field shape composite image removed the anatomy from the treatment area and produced a true intensity map. Outside of the treatment area the anatomy was visible, thus providing verification of correct radiation delivery location. The composite images were compared to other composite images produced using the same treatment plan delivered with and without a phantom and were found to be in excellent agreement. The images were also compared to the intensity map images created in the treatment planning system. Imaging of each segment of an IMRT treatment required no additional radiation be delivered to the patient. The open field portal images require additional radiation, however each image needs only to be acquired once during a course of treatment. Conclusion: This work indicates that flat panel EPIDs may be used to image an IMRT treatment and accurately and efficiently verify the intensity and location of the delivered radiation. Future investigations will use the 2D images to produce a 3D intensity “cloud” image.
SU‐FF‐T‐292: Inter‐Institution Comparison of Patient Quality Assurance Analysis for Tomotherapy
Authors: Shi C , Yan Y , Wu C , Papanikolaou N
Published: July 2006

Purpose: HiArt Tomotherapy unit has been used in the clinics for a couple of years. However, the patient‐specific intensity modulated radiotherapy (IMRT) quality assurance (QA) methods vary from center to center. The purpose of this investigation is to analyze and compare the patient specific QA results for the HiArt TomoTherapy between two different institutions. Method and Materials: A patient‐specific IMRT QA procedure was developed and implemented in two different institutions. Once a QA plan is calculated, the dose to a phantom is measured using ion chamber and film. The ion chamber is placed 5 mm below the film which in turn is placed in the equator of a cylindrical solid water phantom. After the plan is delivered to the phantom, the point dose is recorded and the film is processed. Dosimetric analysis is performed after the film and planar dose are co‐registered in the TomoTherapy planning station. In total, sixty‐three patient QA from one center and fifty‐four patients QA from the other were analyzed. Results: For 52.99% of QA measurements done, the difference between measured and calculated doses was less than 1%. In both centers, the patient specific QA tolerance is set to 3%. In total, 92.31% of patient specific QA can pass the QA. Failure to pass the QA can be attributed to: (1) setup uncertainty (2) resolution differences in scanned QA phantom (3) the machine output fluctuation. Conclusions: A comprehensive patient QA program has been developed and the results of 117 patients from two different centers are analyzed in this paper. Given the novel approach of TomoTherapy towards IMRT, it is important to see that for the majority of the patients the deviation between planned and delivered doses is less than 3%.
SU‐FF‐T‐296: Inverse Treatment Planning Using Volume Sampling with Monte Carlo Dose Calculations
Authors: Craig J , Wong E , Gladwish A , Oliver M , Chen J
Published: July 2006

Purpose: We investigated three methods of random sampling of voxels within regions of interest (ROI), using Monte Carlo dose calculations for inverse treatment planning. We studied their effects on file size, accuracy of dose volume histograms, computation time and accuracy in the objective function and determined the impact of number of simulation histories on the objective function. Method and Materials: A dose distribution, stored as double precision, of a clinical lung cancer plan was calculated using Monte Carlo simulation (NXEGS NumeriX, LLC). Only the dose in the ROI (excluding the external) is required, and is stored as integers. Three equations were tested to determine the number of sampling points within each ROI. The first was to keep the same relative percentage of volume for all ROIs. The second was proportional to the hyperbolic tangent of each ROI volume, while the third was proportional to the cube root of the volume. A least square objective function was calculated on all resulting sampling methods. Results: By saving dose values as integers instead of doubles, a 75% reduction is seen in file size, while keeping accuracy to 0.001%. The objective function computational efficiency improvement is directly proportional to the data storage reduction. A further reduction of 94% and 73% occurred when using the cube root and hyperbolic tan of the volume respectively, while producing a 0.03% and 1.8% difference in objective functions compared to that calculated with full ROI volumes. Conclusion: By sampling ROIs where the number of points is proportional to the hyperbolic tangent of the volume there was a savings of approximately 75% of data stored which directly translated into reduction in objective function computational time, with a 0.03% difference in objective function compared to that with full volume calculations.
SU‐FF‐T‐302: Investigation of the Effects of Approximations in Gamma Knife Stereotactic Radiosurgery Planning
Authors: Cernica G , de Boer S , Malhotra H , Podgorsak M
Published: July 2006

Purpose: GammaPlan, the planning software for Gamma Knife stereotactic radiosurgery, approximates many parameters to increase calculation speeds. These approximations can introduce dosimetric errors. We investigate the magnitude of these errors. Method and Materials: A planning algorithm was written to avoid the use of approximations. Skull scaling is accomplished through a simple edge detection algorithm on the imported images. Dose calculations are accomplished through the use of TMR measurements and beam profiles taken at various depths, and of all collimators. To measure the TMR of one beamlet, 200 of the 201 sources were plugged. A tungsten sphere was designed to provide additional attenuation of unwanted beamlets when taking film and ion chamber measurements in its center. Ion chamber measurements are obtained for various amounts of water‐equivalent ABS plastic for the 14 and 18 mm collimator. Additionally, Gafchromic film is placed between the pieces of plastic to obtain both PDD and TMR measurements of all collimators. The Gamma Knife calibration sphere was then modeled in both GammaPlan and our algorithm, and treatment times for isocenters placed throughout the sphere were calculated using both algorithms. Results: The 18 mm collimator TMR showed small deviations past 40 mm from the 0×0 curve used by GammaPlan, and differences up to 2% are seen at shallow depths. The 14 mm collimator showed similar behavior. Discrepancies of 3.8% on average (maximum 8.9%) were calculated for shots placed in various locations throughout the calibration sphere. Differences increased with the distance from the middle of the sphere and increasing gamma angle. Conclusion: We developed a robust Gamma Knife planning software, effectively reducing errors due to approximations. Our results agree with GammaPlan's calculated times. Skull scaling differences are the major source of the discrepancies between the two calculations.
SU‐FF‐T‐305: Is It Necessary to Adjust the Prescription Dose to Compensate for Cobalt Source Decay of Gamma Knife Radiosurgery?
Authors: Zhang P , Brisman R , Isaacson S , Watanabe Y , Shah J , Kutcher G
Published: July 2006

Purpose: To investigate whether it is necessary to adjust the prescription dose to compensate for Cobalt source decay of Gamma Knife radiosurgery (GKRS) for trigeminal neuralgia. Methods and Materials: A radiobiological model for GKRS was utilized to estimate the radiation response of the brain tissue to the radiosurgery dose distributions expressed in the form of dose volume histogram (DVH). For each bin of the DVH, a linear‐quadratic equation was constructed to calculate the Extrapolated Response Dose (ERD) given the specific dose rate and dose. The relationship between ERD and clinical outcome, i.e., pain relief of trigeminal neuralgia, was evaluated using the data from 343 patients who received trigeminal neuralgia GKRS during a period of 6 years. This model is validated by published clinical data. Necessary dose adjustment to compensate for source decay was obtained during a period of 10 years (two half‐lives). Results: Between the first (5/98–5/99) and last (11/01–11/03) quartile of the patients, complete or nearly complete (⩾90%) pain relief rate was 54.2% verse 50.6%, respectively, at the 12 month follow‐up. Dose rate was not a factor influencing pain relief (p=0.755). Meanwhile, our model predicts that pain relief rate drops 3%, and the necessary dose adjustment for the trigeminal neuralgia GKRS should increase 3–5% for the typical prescription dose of 70–90Gy at the end of two half‐lives. The model also suggests that when the radiosurgery prescription dose is low (e.g. cases such as acoustic neuroma or brain metastases) dose adjustment is not necessary because the change of ERD is negligible. Conclusions: Prolonged treatment due to radiation source decay has little effect on clinical outcome for trigeminal neuralgia GKRS with up to 10 years of service.
SU‐FF‐T‐306: Is It Safe to Switch Patients Between Different Linacs
Authors: Lin T , Fan J , Luo W , Li J , Chen Z , Ma C
Published: July 2006

Purpose: Due to the limited machine availability, a clinical decision has to be made as whether a patient can be treated on a different machine of the same nominal energies. This work investigates the feasibility of switching patients between machines for IMRT. Method and Materials: We have performed Monte Carlo simulations of photon beam from different models and different vendors' clinical accelerators. Treatment plans of Siemens Primus, Primart and Varian‐21Ex accelerators and tumor sizes are studied including prostate and head and neck tumors. We have compared the maximum, minimum and mean dose to the target and to the critical structures, and the tumor‐control‐probability to determine the possibility to switch a patient or cancel the treatment. Results: In FCCC, three attuned Primus linacs can be used one set of beam data for commissioning the treatment planning systems, while Primart and two Varain‐21Ex linacs have different beam data. Our results show that the two Siemens machine models share similar features. The dose difference at the minimum dose point for Primus is 2% higher than Primart. Owing to a larger beam penumbra for Varian machines, it appeared a cold spot at the target, but more volume at higher dosages to the surrounding structures. To reduce the cold spot, a factor of 1.042 can be used to increase the MUs. The TCP results show that treating a patient with slightly less accurate dose is generally better than canceling the treatment. Conclusions: Patients can be safely treated using same nominal energy photon beams on linacs of the same model and vendor; small differences are found between different vendors, which may be compensated by adjusting MUs to ensure target coverage. It is generally more therapeutically beneficial to switch a patient to a different machine than to cancel a treatment.
SU‐FF‐T‐313: Low Energy Experimental Elastic Cross Sections for Medical Physics Application
Authors: Epps M
Published: July 2006

Purpose: Elastic cross sections for electron energies below 10 MeV are fundamental quantities needed in treatment planning systems used at hospitals and health facilities. To date, there is very little if not no data within that energy regime. Method and Materials: In collaboration with the high current, high energy resolution continuous electron beam of the Department of Energy's Jefferson Lab accelerator, we have performed a first stage of dedicated experiments with energies of 100–150 keV to collect data for this type of reactions. The targets used were gold, copper and silver. A Mott scattering chamber was used to detect the outgoing electrons. Results: We will present the first results of this program that aims at performing a wide range of measurements including the use of polarization data for spin studies. Conclusion: This research will provide much needed electron cross section data in the 100–150 keV range for treatment planning, dose calculation, and development of mono energetic Brachytherapy sources. The effects of polarization on cancer cell response will also be studied.
SU‐FF‐T‐314: Macro Monte Carlo Simulation of Visible Light Transport in Heterogeneous Media
Authors: Finlay J , Zhu T
Published: July 2006

Purpose: Although Monte Carlo (MC) simulation has become the gold standard in the transport of visible and near infra‐red photons in turbid media such as tissue, its computational intensity limits its practical application. To increase the computational efficiency of visible MC, we have adapted a macro‐Monte Carlo (MMC) method (Neuenschwander, et al. 1995, Phys. Med. Biol. 40, 543–574) to the modeling of light transport in heterogeneous media. Method and Materials: Traditional MC routines trace individual photons step‐by‐step through the tissue. Instead, the MMC approach relies on a data set consisting of spheres or ‘kugels’ in which the light absorbed in each voxel is pre‐calculated using a traditional MC routine. At each MMC step, the pre‐calculated absorbed light dose in the appropriate sphere, aligned to the current position and direction of the sphere, is recorded in the dose matrix. The position and direction of the photon exiting the sphere are chosen from the exit distribution of the pre‐calculated sphere, and the process is repeated. By choosing the size of the pre‐calculated sphere appropriately, arbitrarily complex boundary geometries can be simulated. To allow the simulation to remain accurate arbitrarily close to boundaries, we have it automatically switch to photon‐by‐photon MC for points less than one kugel radius. Results: We compare the accuracy and calculation time of the MMC method with a traditional MC algorithm for a variety of tissue optical properties and geometries. We find that the MMC algorithm can increase the speed of calculation by as much as two orders of magnitude, depending on the optical properties and geometry being simulated, without a significant loss in accuracy. Conclusions: The drastic improvement in efficiency accomplished by implementing the Macro Monte Carlo algorithm for visible photons makes it fast enough to be potentially useful in photodynamic therapy treatment planning and optical measurement analysis.
SU‐FF‐T‐317: Measured and Simulated Non‐Target Whole‐Body Doses for Selected IMRT and 3D‐CRT Treatment Plans
Authors: Bednarz B , Xu X George , Wang B
Published: July 2006

Purpose: A symposium at this year's APPM Summer Meeting calls for the attention to the non‐target exposures during emerging modalities. The change from 3D‐CRT to IMRT has been accompanied by many new challenges including a potential for an increase in second malignancies due to more fields and longer exposure times. The goal of this study is to establish a robust method for comparing in‐phantom measurements and Monte Carlo simulations of whole body doses resulting from 3D‐CRT and IMRT treatment plans. Method and Materials: Measurements using a RANDO phantom and MOSFET dosimeters were re‐constructed to determine organ doses from three typical treatment plans, 4‐field 3D‐CRT, 6‐Field 3D‐CRT, and 7‐Field IMRT for the prostate. Dose tally estimates using a segmented RANDO computational phantom were generated by MCNP5 and compared with experimental data. Results: The dosimeter readings show that the doses decrease as distances increase for all treatment plans. At 40 cm from the target, doses are reduced nearly 100%. At this location, the IMRT plan resulted in a dose that is a factor of 3–5 times higher than the two 3D‐CRT plans. This is due to increase scattered radiation from the extended exposure time for IMRT treatment. Comparisons of organ doses will be done with ongoing studies of MCNP5 simulations for the above treatment plans. Conclusion: A method has been developed and tested to use MOSFET dosimeters to measure whole‐body doses resulting from IMRT and 3D‐CRT treatment plans. The case study for prostate shows that IMRT delivers higher out‐of‐field photon doses than 3D‐CRT. By combining the Monte Carlo simulation of various accelerators and patient anatomy, the potential non‐target doses can be better assessed than what is currently done, thus offering a practical way for the management of secondary exposures from emerging modalities.
SU‐FF‐T‐319: Measurement of Neutron Background in Electron Beams From a Dedicated IORT Linear Accelerator and a Conventional Linear Accelerator
Authors: Jaradat A K , Biggs P
Published: July 2006

Purpose: To measure the neutron background for an electron‐only (IORT) and a conventional linear accelerator. Methods & Materials: A Siemens Mevatron ME [6 (not used), 9, 12, 15 and 18 MeV] and several Varian Clinacs [6 (not used), 9, 12, 16 and 20 MeV] were used in this study. Bubble (Type BD‐PND; BTI Bubble Technology Industries) and track‐etch detectors (TE) (Luxel+, Ja; Landauer) were used in these experiments. The detectors were placed at 1m from the target at azimuthal angles of 0°, 45°, 90°, 135° and 180°. Results: For conventional electrons at 0° the neutron leakages (Sv/Gy) are: 2×10−5 at 12 MeV, 1.0×10−4 at 16 MeV and 4×10−4 at 20 MeV. For angles >0°, the leakage is almost angle independent [2×10−6 at 12 MeV; (0.5–2)×10−5 at 16 MeV and (2–4)×10−5 at 20 MeV]. For the ME, the neutron leakage was lower than for the conventional linac and also independent of angle for angles >0° {[0°: 6×10−6 at 12 MeV; 2×10−5 at 15 MeV; 5×10−5 at 18 MeV]; [other angles: (2–5×10−7 at 12 MeV; (0.7–1.0)×10−6 at 15 MeV; (2–4)×10−6 at 18 MeV]}. Using the upper limit of 5×10−7 Sv/Gy at 12 MeV for angles >0° and assuming a workload of 200Gy/wk and an inverse square factor of 10, the neutron dose is 0.01 mSv/wk. For the primary beam at 12 MeV, the 10x higher dose is compensated by the attenuation in the primary beamstopper†. Conclusions: Measurements have been made of the neutron leakage from an IORT machine and a conventional linear accelerator. The results show that the IORT has a leakage well below that of the conventional machine and that at 12 MeV for the IORT machine, the leakage is sufficiently low at all angles as not to be a regulatory problem.
SU‐FF‐T‐320: Measurement of Surface and Exit Dose in Megavoltage X‐Ray Beams Using Micro‐MOSFET Detectors
Authors: Deeley M , Ding G , Coffey C
Published: July 2006

Purpose: Knowledge of entrance and exit dose, specifically in breast cancers, is of significant clinical importance. New micro‐MOSFET (Thomson & Nielsen Electronics Ltd., Ottawa, Canada) detectors offer an efficient means to accomplish this task. In this study we investigate the use of MOSFETs to measure surface and exit dose in external photon beams. Method and Materials: Ratios of measurements at the surface and a depth of dmax in a solid water phantom were correlated with Monte Carlo (BEAM) generated percentage depth dose curves to determine the water‐equivalent thickness of the micro‐MOSFET detectors. This was done for 6 and 18 MV x‐rays in a 10×10 cm2 field, both normally and obliquely incident. Exit dose was measured similarly and equivalent thickness determined. Results: Correlation of the predicted depth dose and measured ratios indicates a water‐equivalent thickness of 0.8–1.0 mm for the micro‐MOSFET at the surface. All results indicate that the equivalent thickness is independent of angle of incidence and energy. The same detectors show an equivalent thickness that is approximately 0.4 mm and energy independent when measuring exit dose. We anticipate final results to include additional measurements at 10 MV and a field size of 40×40 cm2. Conclusions: This work indicates micro‐MOSFET detectors are a reliable (reproducible within 3%) detector of surface dose and exit dose as they exhibit a water‐equivalent thickness that is independent of energy and angle of incidence. We believe they offer a unique opportunity in their application to in vivo surface dose measurement.
SU‐FF‐T‐322: Miniature TLDs for Use in Beta Dosimetry
Authors: Holmes S , DeWerd L , Micka J
Published: July 2006

Purpose: To demonstrate the production and utility of new miniature thermoluminescent dosimeters (TLDs) for use in electron fields such as those generated by intravascular brachytherapy (IVBT) sources Method and Materials: Square lithium fluoride Harshaw TLD rods were cut into 1×1×0.5mm3 “half‐microcubes” for measurements of dose near a Novoste Beta‐Cath IVBT source pellet. Calibration exposures were performed with a Tracerlab RA‐1 ophthalmic applicator directly traceable to the NIST absorbed dose to water standard. All beta exposures were made in liquid water since it is the reference medium of interest. TLDs were read 24 hours post‐exposure using a Harshaw 3500 planchet reader. TLD mass was tracked throughout the investigation, and individual TLD response characterization factors were determined from exposures before and after each exposure. Results: Radial depth dose measurement (and consequently radial dose function) results show slightly decreased volume averaging effects and increased precision of half‐microcubes compared to standard 1×1×1mm3 microcubes. Half‐microcube measurements also correspond within uncertainty estimates to radiochromic film data and to data published by Soares et al (Med Phys, 25(3), 1998), which is the current NIST standard for IVBT beta sources. Monte Carlo statistical modeling with the MCNP5 transport code served to corroborate the measured radial dose function. Conclusion: Particularly for investigation of beta emitting sources, decreasing the volume over which TLDs average dose provides slightly increased precision in the measurement of relative dose rate at the 2mm reference depth. Precision changes from 12.2% for microcubes to less than 11% for half‐microcubes at the 95% confidence interval. Both sets of measurements also avoid the experimental uncertainty inherent in most current published work by eliminating the conversion to water from data taken in water‐mimicking media. TLDs were used to take advantage of their relatively well‐known response to changes in energy, dose, and dose rate.
SU‐FF‐T‐327: Monte Carlo Dose Calculations in Homogeneous and Heterogeneous Media: A Comparison Between the PMCEPT Code and the MCNP5, EGS4, DPM Codes and Measurements
Authors: Kum O
Published: July 2006

The Monte Carlo method for high‐energy photon and charged particle transport is the most accurate means for predicting dose distributions in radiation treatment of patients. Owing to rapid development of computer hardware and network technologies the use of this method is not restricted only to big research centers any more. A new parallel Monte Carlo electron and photon transport (PMCEPT) code [Kum and Lee, J. Kor. Phys. Soc. 47 (2005) 716] for calculating electron and photon beam doses has been developed based on a three dimensional geometry defined by computerized tomography (CT) images and implemented on the Beowulf PC cluster. The PMCEPT code was validated on the homogeneous and multi‐layered targets for megavoltage electron beams by comparing with the results of experiments and calculations from conventional Monte Carlo codes of the MCNP5, EGS4, and DPM. The computing time of the PMCEPT code was approximately twenty times faster than that of MCNP5 on the IBM ThinkPad X40 (laptop) with 1.2‐GHz CPU and 512‐MB ram memory operated by RedHat Linux 9. The PMCEPT results in general agreed well with others in homogeneous and heterogeneous media within a maximum of 2–3 % error. At the conference, we will show various benchmark results for the PMCEPT. This work was supported, in part, by the SRC/ERC program of MOST/KOSEF (grant number: R11‐1999‐054).
SU‐FF‐T‐328: Monte Carlo Dosimetric Evaluation of Patient Intra‐Fraction Motion
Authors: Garnica‐Garza H M , Mitsoura E , Alvarez‐Romero J T
Published: July 2006

Purpose: to include the patient intra‐fraction motion into a Monte Carlo‐based treatment planning system. Materials and Methods: the PENELOPE Monte Carlo code was used along with a mathematical phantom consisting of a target shape (water) that surrounds a critical structure (bone). Both structures are embedded in a cylindrical water phantom. Two types of motion were modeled: shifting and deformation both periodic in time. It is assumed that deformation does not alter the density of the structure and that no movement occurs during the tracking of a single particle history. A PENELOPE subroutine was designed that performs the simulation in the following way: at the beginning of a history a configuration of patient geometry is sampled from a probability distribution with equi‐probable bins. Each bin describes a particular state during the evolution of the geometric configuration. Once the configuration is selected, a history is started and followed under the geometry selected until it is terminated; the process is repeated for another geometry configuration. The simulations were carried out using a point source emitting a published Varian 15MeV x‐ray spectrum at an SSD of 100cm, and for probability distributions with 5, 10 and 20 bins, with amplitude of movement or deformation in the order of 1cm. At the end of the simulation, the calculated dose arrays for each configuration are mapped onto a static reference configuration for display. Results: when a comparison is made at selected points between the simulation for the reference configuration and the composite simulation, discrepancies on the order of 2% to 10% are found, with the larger differences seen at the edges of both the target and the critical structure. Conclusions: the work here presented represents a first step towards fully modeling the movement of organs during the treatment planning process. Experimental verification of the results is underway.
SU‐FF‐T‐334: Moving From Measurements Toward Independent Computer Calculations for Patient‐Specific IMRT QA
Authors: Pawlicki T , Yoo S , Russell J , Boyer A
Published: July 2006

Purpose: A medical physicist's time and resources are always balanced between many clinical obligations. It would be advantageous for physicists to forgo measurements of IMRT plans in lieu of independent computer calculations. In this work, we investigate the appropriateness of independent computer calculation monitor unit checks of IMRT treatment plans in place of physical measurements before treatment delivery. Method and Materials: This study consists of one academic and one community institution with active IMRT programs. Each institution followed their in‐house IMRT procedures for treatment planning and quality assurance. The independent computer calculation software was common between the institutions (IMSure QA program; Standard Imaging, Middleton, WI). The treatment sites considered were prostate and head and neck. Process behavior charts were used as the quality assurance tool for investigation. A correspondence between the measurements and IMSure calculations was established by using time‐ordered data to demonstrate that IMRT measurements and IMSure calculations were predictable and independently stable processes. Quantitative justification for discontinuing measurements in place of computer calculations was achieved once process stability was established. Results: Natural process limits were determined to be ±1.8% for prostate case measurements and ±1.5% for the corresponding IMSure calculations. Limits for the head and neck cases were larger at ±7.1% for measurements and ±12.1% for the corresponding IMSure calculations. We found that that stable process behavior limits could be calculated in as few as 15 cases for prostate but head and neck cases required at least 25 data points before the two processes showed concomitant stability. Conclusion: Process behavior charts as a part of an IMRT quality assurance program can provide a quantitative justification for when it is appropriate to discontinue measurements in lieu of independent computer calculations. The number of cases that need to be measured depends on treatment site and the individual quality assurance program.