bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐J‐65: The Primary Study of Image Guided Radiotherapy in Liver Tumor
Authors: Yin Yong
Published: July 2007

Purpose To evaluate the interfraction and intrafraction reproducibility of intrahepatic tumor position using an active breathing control (ABC) and on‐line CBCT for direct tumor targeting in patients with unresectable liver cancer after transarterial chemoembolization(TACE). Methods Thirteen patients with liver cancer were treated with RT on study. Prior to radiotherapy, all patients underwent TACE which was performed with infusion of a mixture of iodized oil contrast medium and chemotherapeutic agents. Kv fluoroscopy was used to measure lipiodol spots position potential motion during ABC breath holds. ABC was used for planning CT scans and radiation delivery with the breath held at the same phase of the respiratory cycle (near end‐exhale). CBCT were taken using Varian IGRT system. They were compared online to previously computed CT using a 3D‐3D matching tool. Analysis relied on lipiodol spots on planning CT and CBCT by mannul. The treatment table was moved to produce acceptable setup, and began treatement delivery. Repeat CBCT images and another analysis were obtained after irradiation. Results No motion of the intrahepatic tumor was observed on fluoroscopy during ABC breath holds macroscopicly. From analyses of 130 sets of CBCT after RT, the average intrafraction craniocaudal(CC), anterior posterior(AP),and mediolateral (ML) reproducibility (σ) of the hepatic tumor position using ABC breath holds was 3.2 mm, 2.2mm and 1.8mm, respectively. However, based on 130 sets of CBCT prior to RT, the average interfraction CC, AP, and ML reproducibility (σ) of the hepatic tumor was 6.8 mm, 4.4 mm and 3.3mm, respectively. Conclusion Daily direct tumor targeting of liver cancers relying on lipiodol spots using CBCT and ABC is feasible. Our preliminary data demonstrate good intrafraction reproducibility of intrahepatic tumor position using ABC. However, the interfraction inhepatic tumor position is worse, suggesting the need for daily on‐line image guidance.
SU‐FF‐P‐02: Monitoring Hospital Entrances to Prevent Radioactive Contamination: Disaster Planning
Authors: Stanley T , Sherman J , McUmber A , Bednarek D , Rudin S
Published: July 2007

Purpose: To determine the optimal configuration of radiation detectors for monitoring a hospital entrance to prevent contamination by patients seeking treatment in the event of a disaster. Method and Materials: A digital area monitor (LUDLUM, model ♯375‐30) with two NaI‐scintillation detectors (LUDLUM, model ♯44‐99) was purchased for each entrance to the hospital. The counts of the detectors are summed and two radiation levels were set to trigger an audible and visual alarm. The sensitivity of the two‐detector combination was measured with a 10 μCi Cesium‐137 source as a function of detector configuration and source position. The configurations evaluated included placing the two detectors 1) with the end of the crystals facing each other horizontally and 2) over the entrance aiming downward. The detectors were then tested with the source moving at different speeds to determine system temporal response and the extent of traffic flow control needed. Results: Curves of iso‐sensitivity were measured that were bulbous in shape diverging from the face of the crystal. The sensitivity was sufficient to initiate the low alarm for the Cs‐137 source at a distance of about 175 cm in the collimated direction for a single detector. It was determined that for an opposed configuration separated by 10 feet or directly under an overhead mounted detector, an individual walking into the entrance at under 6.5 mph triggered the alarm if contaminated with 10 μCi of Cs‐137. Conclusions: Utilizing these results and the dimensions of a hospital entrance it was possible to determine an optimized configuration of radiation detectors. For the main hospital entrance, which consisted of two double‐door entryways having a combined width of 129in., the orientation of choice was when both detectors faced one another horizontally. This orientation allowed the detectors to monitor the entire entrance reducing the possibility of radioactive contamination.
SU‐FF‐T‐04: 3D CT Image‐Based Treatment Planning of Modulated Electron Radiotherapy for Breast Cancer
Authors: Jin L , Ma C , Fan J , Eldeeb A , Lin T , Li J
Published: July 2007

Purpose: To investigate optimizing 3D CT image‐based treatment plans for energy‐ and intensity‐modulated electron radiotherapy (MERT) using an in‐house developed Monte Carlo based inverse treatment planning system. Method and Materials: With the availability of an electron multileaf collimator (eMLC) that can be attached to a clinical accelerator, Varian Clinac 2100EX, a Monte Carlo based inverse treatment planning system is being developed, aiming at the clinic implementation of MERT. The planning procedure includes: 1. Calculating patient‐specific dose deposition coefficients (DDCs) for beamlets of selected energies by Monte Carlo simulation, 2. Optimizing beamlet weights (first optimization) and generating the intensity (beamlet weight) map and step‐and‐shoot leaf sequences (segments), 3. Calculating DDCs of those segments with scatter and transmission from eMLC, 4. Optimizing segment weights (second optimization), 5. Resetting weights in leaf sequences for each segment and calculating final dose. To investigate treatment planning in clinic scenarios for breast treatments, MERT plans were generated for a breast phantom, which mimics the geometry and densities of the breast cancer patient and for breast cancer patient previously treated with IMRT. Results: The results of MERT plan based on the breast phantom show that the differences in dose volume histograms (DVH) for the plans before and after the second optimization is significant due to eMLC scattering. The second optimization reduces the effect due to eMLC scatter and transmission, leading to an optimal, deliverable plan. For the patient case, dose distributions and DVHs of the MERT plan were compared with the photon IMRT and show significant advantages for sparing the organs at risk (OARs). Conclusions: Our Monte Carlo based inverse planning procedure with two‐time optimizations is capable of generating optimal and deliverable MERT plans using realistic 3D phantom/patient geometries, demonstrating the potential of MERT as an alternative treatment modality for shallow tumor treatments.
SU‐FF‐T‐06: 4D In‐Vivo Dosimetry in Radiotherapy
Authors: Cygler J , Saoudi A , Cherpak A , Ding W , Ashton R
Published: July 2007

Purpose: To present a novel concept of 4D in‐vivo dosimetry in radiotherapy. Method and Materials: A prototype of a new system capable of simultaneous measurement of dose and spatial position has been developed. The device, controlled by a computer, consists of a probe combining two technologies: MOSFET radiation detector coupled with a magnetic positioning device. Special software that allows sampling position and dose in user defined time intervals has been developed. The preliminary tests of the new device have been performed. The tests included measurements of spatial position accuracy as well as a dosimetric evaluation of the device in beam from a Theratron teletherapy unit. In addition, some tests were performed in a 6 MV beam from a Siemens linear accelerator to assess if the electromagnetic field from the linac interferes with the performance of the 4D dosimeter system. Results: Measurements of the angular response of the probe over 3600 range in a beam have been found isotropic within +/− 7%. Measurements of dose profiles in air in a beam have been performed. They agree very well with the measurements performed using RFA300 dosimeter system and a high quality photon diode. Tests conducted in a 6 MV beam indicate that there is no interference from the linac electromagnetic field on the performance of this new 4D dosimeter system. Conclusion: The results of the preliminary tests indicate that the device can be used for in‐vivo dosimetry in a and high energy beams from linear accelerators. Future work will involve technical improvements to the device, experiments in a 4D phantom and finally patient in‐vivo dosimetry.

This project has been supported by HTX‐OCE‐IRAP grant.
SU‐FF‐T‐08: A Comparison of EBT Radiochromic and EDR2 Radiographic Films for Tomotherapy Treatments Dose Verification
Authors: Avanzo M , Cappelletto C , Capra E , Dassie A , Drigo A , Kaiser S Ren , Sartor G
Published: July 2007

Purpose: To determine which film between EDR2 radiographic (Kodak, Rochester, NY) and EBT radiochromic (ISP, Wayne, NJ) is more accurate for dose verification in HI‐ART Tomotherapy System (Tomotherapy Incorporated, Madison, WI) treatments. Matherials and methods: We performed six Tomotherapy treatments (1 prostate, 2 head and neck, 1 lung, 1 brain, 1 spine cancer) on a “cheese” phantom loaded with EDR2 films, and repeated the procedure using EBT films. EDR2 films were scanned with a Vidar (Vidar Systems Corporation, Herndon, VA) scanner and elaborated with the Tomotherapy Film Analizer software. EBT films were read with an Epson 1680 Scanner using only the central 10×15cm2 scanner region. Net optical density was calculated and converted to dose with a home‐made MATLAB routine.

We compared measured 2‐dimensional dose distributions with dose calculated by the treatment planning software using the gamma function, with a tolerance of 3mm and 3% of the maximum calculated dose. We compared absolute dose readings in points inside regions in the phantom corresponding to patients' OARs and PTV. We repeated the gamma test after normalizing measured dose distributions to dose at the same point. Results: Radiochromic films show, in 5 over 6 treatments, better results of the gamma test for absolute dose. After normalization, EBT films still give better results in 4 patients over 6. Absolute dose readings in regions corresponding to OARs and PTV do not show a significant difference in films' behavour. Conclusions: 2‐dimensional dose distributions measured with EBT radiochromic films have a better agreement to calculated dose. We advise to perform treatment plan verifications in Helical Tomotherapy treatments using EBT films, especially when interested in verification of absolute dose.
SU‐FF‐T‐22: A Model to Evaluate the Spatial and Dosimetric Resolutions of IMRT for Desired High‐Gradient Dose Distributions
Authors: Havnen A , Munley M , Bourland J
Published: July 2007

Purpose: To create a 3D model to simulate IMRT treatment delivery. The model includes dose distribution resolution degradation from the MLC leaf width and dose deposition. The resulting deliverable distribution may be evaluated to determine conformity and the achievable spatial and dosimetric resolution. Method and Materials: Phantom image sets were created to represent head and neck anatomy with critical structures. Image sets contained 128 slices, each 128×128 pixels, with 1mm pixel size. Target volumes with radioresistant subvolumes of varying size and shape were added to the image sets. A nine‐field treatment technique was simulated. Slice‐by‐slice along the beam central axis, the image set was sampled in the direction perpendicular to leaf motion at each leaf width center. Each slice was convolved with a rect function representing MLC leaf width and convolved with a dose spread kernel appropriate for the depth along the central axis of the beam. The resulting 3D dose distributions for each field were summed to give the deliverable dose distribution for the nine‐field treatment. Results: The result of the model is a deliverable dose distribution in 3D. This deliverable dose distribution may be analyzed by comparison with the desired dose distribution. Information on the distribution's conformity may be attained by calculation of a distance‐to‐agreement map to provide information on spatial resolution, or by calculation of a map containing percent differences in dose to give insight into the dosimetric resolution. Dose intensity profiles may be extracted from the deliverable dose distribution as a means to determine achievable dosimetric gradients. Conclusion: This 3D model simulates the degradation of an ideal dose distribution due to the IMRT treatment delivery process. The resulting deliverable dose distribution may be used for evaluating the achievable spatial and dosimetric resolution for the current IMRT technique.
SU‐FF‐T‐26: A Monte‐Carlo Study of the Effects of Miniphantom‐Material On In‐Air Output Ratio
Authors: Li J , Zhu T
Published: July 2007

Purpose: To investigate the effects of miniphantom materials on in‐air output ratio measurement. Method and Materials: Monte‐Carlo (MC) simulations were performed to examine the effects of miniphantom materials on in‐air output ratio for photon energies 6 MV and 15 MV and for collimator settings ranging from 3×3 to 40×40 cm2. The miniphantom materials included water, PMMA, graphite, copper, and lead. EGSnrc and BEAMnrc (2007) software packages were used for the simulations. The miniphantoms in the simulations were of cylindrical shape and the lateral dimensions were 4 g/cm2, which were large enough to provide electron equilibrium. Attenuation coefficients and energy absorption coefficients for each collimator setting were calculated using energy spectra. Phantom scatter factors were obtained by dose simulations in miniphantoms. Correction factors were developed for attenuation coefficient, mass energy absorption coefficient, and phantom scatter factor. The product of these three correction factors gave a total correction factor for in‐air out ratio. Results: In‐air energy spectra for primary and headscatter beams were obtained for each energy setting and collimator setting using beam code. There is an energy spectrum shift between the headscattered and primary beams. In addition, the magnitude of headscatter energy fluence increases significantly as the collimator setting is changed from 3×3 to 20×20 cm2. Correction factors for in‐air output ratio, which were determined by MC simulations and measurements, respectively, were compared. Conclusions: MC simulations confirmed our experimental correction factors for miniphantoms made of different materials. The maximum total correction factors for miniphantom thicknesses up to 30 g/cm2 are less than 1.01. For the miniphantom thickness of 10 g/cm2, the maximum MC (measured) total correction factors for in‐air output ratio on the central‐axis are 1.002 (1.002), 1.004 (1.003), 1.005 (1.005), and 1.007 (1.007) for PMMA, graphite, copper, and lead, respectively. The correction factors are much larger at off‐axis locations.
SU‐FF‐T‐33: A Periodic Machine QA Program for Proton Therapy
Authors: Li Z , Slopsema R , Flampouri S , Yeung D , Lin L , Zhao Z , Palta J
Published: July 2007

Purpose: To systematically investigate the contents and frequencies of periodic proton therapy system (PTS) quality assurance tests Method and Materials: With the recent and planned opening of multiple new proton therapy centers in the US, experiences on a complete periodic QA program for PTS are urgently needed to ensure the safe and accurate delivery of this advanced radiation therapy technique. There has been no literature in this aspect for proton therapy. A PTS is a complex radiotherapy machine, with design and controls significantly different from conventional LINACs. We have established such a program, including daily, weekly, and monthly QA tests, based on our experiences of the past 6 months of proton therapy system QA and treatment delivery. This program incorporates fundamental concepts recommended by the AAPM TG40 report, and adapts them to our particular PTS. Results: Our periodic QA tests have demonstrated high consistency and reproducibility of PTS performance, in the areas of dosimetry, localization system, safety systems, and mechanical systems. These tests have identified two system component failures/degradations that may significantly impact treatment delivery accuracy. Conclusion: A program of periodic QA tests for PTS allows confidence in its highly accurate and reproducible performance, and serves to identify either system failures or performance degradation.
SU‐FF‐T‐37: A Primer On Plasma Wakefields: A New Method of Particle Acceleration
Authors: Obcemea C
Published: July 2007

Objective: We present in this paper a pedestrian, pedagogical introduction to the basic concepts, physical mechanisms and recent breakthroughs in the use of plasma wakefields as a completely new method of particle acceleration. This new method is attractive because it offers the possibility of really compact accelerators as electron, proton and ion beam sources; although here we focus on electrons. We describe the original ideas, basic equations and recent experimental results. Since the medical physics community is more familiar with RF microwave technology and concepts than those of plasma physics, we believe this primer would be useful and instructive. Summary: Finding new ways to accelerate particles has been a Holy Grail eversince it was realized 25 years ago that RF technology could only get to a theoretical limit of 100MeV/m field gradient. Past that, waveguide electrical breakdown and beam instability are insurmountable(1). The seminal work of Tajima‐Dawson (2–3) showed however that plasma, being fully ionized media can support much higher gradients, and when excited by short, high‐intensity laser pulse or similar electromagnetic pulse, can generate wakefields that can trap and accelerate the plasma electrons to fields as high as 10GeV/cm. With such high accelerating fields, particles can gain huge energy over very short distance. The 2‐mile SLAC accelerator can thus be shrunk into a few meters, a medical linac “waveguide” into about a millimeter.

We describe the basic equations characterizing a plasma, its parametric instabilities, and dispersion relations as response to any incoming disturbance. We derive the ponderomotive force(fig.1–3)(exerted by the incident laser/electromagnetic pulse on the plasma), which generates the wakefields (much like a moving boat would generate wakes along its trail) and show analytically how these wakefields trap and accelerate the plasma electrons. Finally, we describe several important breakthroughs(4–6,fig.4) as proofs‐of‐principle and advances towards a real, practical accelerator device.
SU‐FF‐T‐41: A Simple Algorithm to Account for Inter‐Seed Effects in the Dose Calculation of Prostate Implant Brachytherapy
Authors: Gao M , Sethi A , Rusu I , Xu M , Ke Y , Glasgow G
Published: July 2007

Purpose: For multi‐seed implant, dose calculation (TG‐43) is based on the superposition of doses from each source, i.e. attenuations of neighboring seeds are neglected. Monte Carlo (MC) and measurements showed that inter‐seed effects reduce dose coverage by up to 10%. However, MC is too time‐consuming for clinical practice. We developed a simple algorithm to include inter‐seed effects. Method: Seed is treated as a three‐segment line with different attenuations in the center and edge segment. Attenuation of an active source by a neighboring seed exists in the triangular shadow formed by the seed (line) and the source center (point). Effective lengths and attenuations for each segment (LC, LE, μC, μE) are obtained through best fit to published MC data.

I‐125 seeds (model 6702 and 6711) are investigated. For the fit, four parallel coplanar seeds are configured 0.75cm apart, one as active source and other three as neighboring seeds. Doses at forty‐eight grid points are calculated and fitted. To test the generality of fit results, another configuration of four seeds 0.5cm apart is calculated and compared. Results: For 6711, LC=0.38cm & LE=0.11cm and μC=0.47 & μE=0.81. Total effective length (0.60cm) is slightly longer than its physical size (0.46cm) due to perturbation at edge. Averaged over all points, calculated inter‐seed factor is 0.77 as compared with 0.80 from Monte Carlo.

For 6702, LC=0.12cm & LE=0.23cm and μC=0.83 & μE=0.71. In contrast to 6711, center segment is shorter and less attenuating because 6702 uses three small spheres while 6711 uses a 3mm wire. Averaged inter‐seed factor is 0.88 (algorithm) as compared with 0.87 (MC).

For 0.5cm configuration, fit results lead to inter‐seed factor of 0.64 (0.61 MC) and 0.80 (0.77 MC) for 6711 and 6702 respectively. Conclusion: Neglecting inter‐seed effects may significantly overestimate dose. A simple algorithm was developed and reasonable accuracy is achieved.
SU‐FF‐T‐169: Dosimetric Characteristics of An Optically Stimulated Luminescence Dosimeter, OSLD, Used for Clinical Measurements
Authors: Jursinic P
Published: July 2007

Purpose: to determine the dosimetric characteristics of OSLDs that are critical for their use in clinical measurements. Methods and Materials: OSLDs and a reader were obtained from Landauer, Inc., Glenwood, IL. The OSLDs are plastic discs infused with aluminum oxide doped with carbon . These discs are encased in a light tight plastic holder. Crystals of when exposed to ionizing radiation store energy that is released as luminescence (420 nm) when the OSLD is illuminated with stimulation light (520 nm). The intensity of the luminescence depends on the dose absorbed by the OSLD and the intensity of the stimulation light. OSLDs were irradiated with a Varian 2100 EX. Results: Repeated readings of an OSLD cause the signal to decrease by 0.05% per reading. Irradiated OSLDs could be totally discharged by illuminating them for >45s with a 150 W tungsten‐halogen light. After irradiation OSLDs had a transient signal that decayed with a 0.8 min halftime. Reading of OSLD was done after an 8 minute wait after irradiation. After the transient decay, the OSLD signal was stable for days. OSLDs could be irradiated and read repeatedly with a precision of ±1.8%. OSLD sensitivity was unchanged up to an accumulated dose of 15 Gy and thereafter decreased by 5% per 10 Gy of additional accumulated dose. For 6 and 15 MV x‐rays and 6 to 20 Mev electrons the OSLD sensitivity was unchanged ±1.8%. Conclusions: OSLDs exhibit high precision and accuracy in measuring dose, are small in size, have no energy dependence, and can be read long after irradiation. OSLD can substitute for TLD and diodes for in vivo dosimetry and routine clinical dose measurements.
SU‐FF‐T‐174: Dosimetric Comparison Under a Central Shielded Area Between Conventional and Dynamic MLC Blocking with Modified Intensity Fluence
Authors: Venencia C , Yañez R , Besa P
Published: July 2007

Introduction: Central blocking is needed for some radiotherapy treatments, like the mantle field. Typically central blocking is done with cerrobend. Dynamic MLC can be used to generate a central shielded area. When MLC are used dynamically the movements of the leaves can control the fluence to compensate the dose at certain depth and also it can generate a central shielded area by bringing the intensity to cero. The objective of this work is to make a dosimetric comparison between conventional and dynamic MLC (dMLC) fields with CAX shielding. Method and Material: A 6MV linac Clinac 21EX, equipped with a 120‐leaf MLC and a TPS Eclipse v6.5 (Varian) were used. Four plans were simulated on a square phantom, each with a single field (SSD=90cm), with different shielding shapes: square, circle, pyramid and rhomb. For each plan a surface compensator was used to compensate the dose at the isocenter plane and the resulting optimal fluence was modified to generate the same central shielding. All plans were normalized at the same point. Point dose, profiles and 2D dose measurements were done at the isocenter coronal plane using ionization chamber and film dosimetry. RIT software v4.2 was used for film analysis. Results: For each shielding shape the agreement of CAX dose was less than 1% between conventional and dMLC. For crossplane and inplane profiles the agreement was 2%, except for regions under the conventional block due to inhomogeneities or geometrical positional errors (<2mm). For non square shielding shapes the penumbra of dMLC fields increased due to MLC steps and optimal fluence beamlet size. Measured versus calculated dose for dMLC fields showed less than 5% of points with dose >1 (2%/2mm). Conclusion: Dynamic MLC can generate isolated central shielding to replace a conventional block. The dose under conventional central block and dMLC generated shielding is comparable.
SU‐FF‐T‐175: Dosimetric Effect of Rotational Variation On Helical Tomotherapy Treatment Plans
Authors: Staton R , Langen K , Shah A , Meeks S , Kupelian P
Published: July 2007

Purpose: Helical tomotherapy is subject to rotational output variation during treatment delivery. The purpose of this study is to evaluate the dosimetric effects of the rotational variation using recalculated treatment plans which include a rotational variation. Method and Materials: In order to assess the rotational output variation, the monitor chamber signal was extracted from delivered treatment data archives and analyzed. The rotational variation was then modeled and incorporated back into the treatment plan sinogram by adjusting the leaf opening times for each projection. The modified treatment plans were then calculated using a research version of the planned adaptive treatment planning software from TomoTherapy, Inc. Two treatment plans, one head and neck and one prostate, were evaluated in this study. Treatment plans were recalculated with an observed rotational variation of ±2 % and a hypothetical variation of ±10 % for comparison to the original calculated treatment plans. Results: The rotational variation from delivered treatments was found to follow a sinusoidal shape with a period equal to one gantry rotation and a magnitude of about ±2% on average. For a rotational variation of ±2%, the D95 for the target volumes and the D50 for critical structures were found to be different from the original plan by less than 0.26% for the recalculated treatment plans. For a hypothetical rotational variation of ±10%, the difference from the original was less than 0.5% for the D95 for target volumes and 1.4% for the D50 of critical structures. Conclusion: The rotational variation for delivered helical tomotherapy treatments was found to be on the order of ±2%. For this observed rotational variation, a negligible dosimetric effect on calculated helical tomotherapy treatment plans was found. Conflict of Interest: Our group holds a research grant from TomoTherapy, Inc.
SU‐FF‐T‐179: Dosimetric Evaluation of Radiophotoluminescent Glass Rod Detector; Measurement of Output Factors in Water for Cyberknife Stereotactic Radiosurgery System
Authors: Lah J , Shin D , Hong J , Suh T
Published: July 2007

Purpose: To evaluate the feasibility of the radiophotoluminescent glass rod detector (GRD) as a new dosimeter for measurement of the output factor in water for Cyberknife SRS system. Method and Materials: In this study, GRD‐301 glass rod dosimeter and FGD‐1000 automatic reader are used. The output factor measurements with the GRD were compared with those with a PTW 60008 p‐type silicon diode detector, PTW 31006 pinpoint chamber and a Gafchromic film (Type MD‐55). The output measured with GRD, pinpoint chamber and diode was performed at a depth of 1.5 cm in water phantom. The GRD was irradiated in a water phantom using a holder stand, which was custom designed for this study. The holder is composed of the PMMA tube with a hole for GRD at 1.5 cm from its top. The center of the GRD was set to be the center of the radiation field by aligning with a point laser built into the LINAC. The film was placed at the depth in the polystyrene phantom at which the maximum dose was achieved. Results: The measured output factors with four dosimeters show very similar results except for three smallest collimators (5, 7.5 and 10 mm). The mean value of the output factor for GRD in the 5 mm collimator is 0.705. The pinpoint chamber output is approximately 11% lower than the corresponding GRD values at the 5 mm collimator. The GRD output is 11% and 4.1% lower than pinpoint chamber and diode, respectively. The GRD results were in good agreement with those from the Gafchromic film for almost all the collimators. Conclusion: The GRD represented a high accuracy dosimeter for small field dosimetry. Future study will be devoted to investigate for possibility of using GRD for quality assurance audit program of stereotactic radiosurgery units.
SU‐FF‐T‐181: Dosimetric Uncertainties in IMRT QA in Plastic Phantoms Due to CT Calibration
Authors: Buettner F , Spezi E , Paganetti H , Seco J
Published: July 2007

Purpose: IMRT QA involves the use of heterogeneous plastic phantoms where measurements can be performed and compared to dose predictions by Monte Carlo (MC). The accuracy of these calculations is significantly influenced by the conversion scheme, used to assign an elemental composition to a voxel. Therefore the CT number scale is divided into a certain number of subsets. The purpose of this study is to find an optimal number of subsets so that uncertainties caused by miss‐assignment of media are minimized while accurately modeling the plastic composition. The energy dependence of this number is investigated. Method and Materials: A Gammex‐CT phantom with 17 different plastic inserts was scanned at 3 different scan energies. Based on these CT images different calibration curves were generated, using different numbers of subsets. The materials associated with these subsets were the materials of the inserts or materials whose weights were determined by interpolation. The dose delivered by 5 beams was calculated and compared insert‐wise to the dose delivered in the exact geometry of the Gammex phantom. The dependence on the scan energy was studied. Results: Results show that a division of the CT number scale into 14 dosimetrically equivalent subsets associated with materials whose elemental compositions were generated by interpolation, minimized dosimetric uncertainties. The maximum difference between the dose calculated with the CT based phantoms to the exact phantom was 3% for all scan energies. Conclusion: We conclude for dose calculations in IMRT QA that the materials used in the conversion scheme should not be the actual materials of the phantom inserts, but materials with elemental weights generated by interpolation. For the Gammex phantom, using 14 dosimetrically subsets leaded to minimal uncertainties caused by miss‐assignment of media. It turned out that the optimal number of subsets is independent of the scan energy.
SU‐FF‐T‐187: Dosimetry Limitations and a Dose Profile Correction Methodology for Sliding Window IMRT
Authors: Grigorov G , Barnett R , Chow J
Published: July 2007

Purpose: The aim of this work was to investigate dose uncertainties between planned and dynamically delivered IMRT beams using extreme dose rate and leaf speed. In‐house developed software, SWIMRT, was used to convert intensity‐modulated dose to dynamic MLC control points using the sliding window (SW) technique. In this study, the ability of SWIMRT to synchronize a Varian 21EX MLC to deliver dynamic beams was evaluated. Methods and Materials: Non‐clinical and clinical dose profiles were used to deliver from 2 to 100 cGy/beam of field sizes up to 14 cm × 14 cm with dose rates from 100 – 600 MU/min and leaf speeds from 1 – 5 cm/s. As the measurements in this investigation were based on the in‐house software, the relative output ratios for dynamic beams were measured using a Farmer‐type ionization chamber. The dose profile differences were measured using Kodak radiographic films. The differences between planned and delivered doses were determined point‐by‐point for every leaf. The correction methodology was examined on non‐clinical beams, which can be used to extrapolate the behavior of clinical beams. Results: Using point‐by‐point differences as correction factors, a close agreement between the planned and delivered dose profiles was achieved. Profile differences < 0.5 – 0.8% were measured for geometrically complicated and clinical beams irradiated with a dose rate = 600 MU/min and leaf speed = 5 cm/s. Conclusions: Our results suggest that it is possible to use DMLC with extreme DR and LS to achieve clinical SW beams of high quality for shorter delivery time and insignificant increase of leaf leakage outside the field. SWIMRT is found to be an extremely useful QA tool for SW IMRT. The correction methodology could be integrated in a commercial treatment planning system.
SU‐FF‐T‐192: DVS MOSFET Dosimeter Response to Kilovoltage Radiation
Authors: Fagerstrom J , Micka J , DeWerd L
Published: July 2007

Purpose: To determine the dose response of implantable DVS MOSFET dosimeters to kilovoltage x‐ray radiation compared to the response to irradiation from a source. Method and Materials: Sicel Technologies, Inc. has developed a Dose Verification System (DVS®), using implantable MOSFET dosimeters that are read telemetrically by an inductively coupled antenna. Three Virtual Water™ phantom cassettes, each containing four DVS dosimeters, were irradiated in a Virtual Water phantom with a source for five doses of 200 cGy. These were used as controls to compare to a fourth cassette that was similarly irradiated by the source before it was exposed to kV energy at NIST‐equivalent beam qualities of M80, L100, M100, M120, and M150. The dose delivered was determined following TG‐61 protocol (in‐phantom method at 2 cm depth), substituting Virtual Water for liquid water. Following the x‐ray exposures, the cassette was exposed again to to confirm device stability. Results: Before correcting for temperature, energy, and isotropy, the cassettes demonstrated a slight and consistent under‐response. After appropriate scaling, the overall ratio of corrected dose measured to dose delivered was 1.002 ± 0.004. The dosimeters irradiated with the kV beams, however, exhibited an over‐response. The overall ratio of corrected dose measured to dose delivered for the kV beams was 2.428 ± 0.043, whereas the same cassette demonstrated nearly the same response to the source as the other three cassettes, with a ratio of corrected dose measured to dose delivered of 0.9755 ± 0.0113. Conclusion: This data shows the DVS dose response for the examined kV beams at 2 cm depth is ∼2–3 times greater than the response at MeV energies from the source due to the shift in dominance from Compton to photoelectric events. Conflict of Interest: Funding for this research was provided by Sicel Technologies, Inc.
SU‐FF‐T‐193: Effect of Headscatter for IMRT Fields
Authors: Dimofte A , Zhu T , Both S , Lazarescu G
Published: July 2007

Purpose The purpose of this study is to determine the effect of headscatter for IMRT fields. Methods and material In air measurements were made for two types of fields. For the first type of fields, a series of offset fields with field sizes ranging between 10 and 21.5cm were used. The offset changed between 0 and 10cm depending on the field size. The detector was always placed on the central axis (CAX). For the second type of fields, headscatter factors were measured for a series of 10×10cm2 fields composed of slits 0.3, 0.4, 0.6, 0.8 and 1.0cm in width. In‐air output ratio, Sc, for a series of clinical IMRT fields was also measured. Sc is defined as dose per MU measured in a water‐equivalent miniphantom between IMRT field and a 10×10 cm2 open field. The measurements are compared with calculation using a two‐source headscatter model1. Results Sc on CAX for the same open beam with different offset changed by up to 4% for the Siemens accelerators. For stop‐and‐shoot method, Sc for 10×10cm2 fields composed of slit fields of different widths changes with the slit width to within 8% and 6.4% for 6 and 15MV, respectively. The 8% uncertainty is completely due to delivery error and does not seem to correlate with the slit width. The two‐source model predicts Sc for all cases including IMRT fields to within 1%. Conclusion In‐air output ratio changes with field shaping by up to 4%, even when the point of measurement is within the radiation field. Thus, it is important to model the headscatter in order to predict Sc for IMRT fields. Our two‐source model can accurately predict the headscatter for points within radiation field.
SU‐FF‐T‐200: Establishment of Japanese QA Booklet in Permanent Prostate Implant Brachytherapy
Authors: Takahashi Y , Sumida I , Sato T , Oita M , Hashimoto M , Nakamura Y , Mizuno H , Okumura M , Hoshina M , Inaba M , Ebihara H , Amano K , Yamashita T , Dokiya T
Published: July 2007

Purpose: I‐125 permanent implant brachytherapy for prostate cancer was approved in Japan in 2003, and is now offered by an increasing number of hospitals. Although existing QA guidelines (AAPM or ESTRO) in this therapy are well documented, many Japanese hospitals required more detail procedures, because of the shortage of medical physicists. In addition, it is important to supply reliability on clinical evidence in the future clinical trial in Japan. The purpose of this study is to establish new Japanese QA booklet which dedicates clinical practice. Materials and Methods: We first investigated several brachytherapy QA guidelines in western countries including AAPM reports or ESTRO booklet. Then we confirmed the procedures one by one by performing QA in our hospitals and made the resulting problems clear. After that, we determined QA items. To support QA, we developed the software for commissioning of brachytherapy treatment planning system (TPS). Results: The feature of this booklet is that (1) Much easier to understand by using many figures or examples. (2) For QA which requires expert knowledge including point dose calculation check in the commissioning of TPS, we developed software to support users. (3) The QA items were classified into two categories. Namely, category ‘Must’ indicates a recommendation that is necessary to be executed at least as a prevention of accidents, and category ‘Recommend’ indicates an advisory recommendation to keep quality of implant. (4) Items of acceptance testing and the detail procedures were described in detail to promote vendor acceptance test in the Japanese agencies. (5) Each sentence 2447_2was itemized so that users can easily read and understand. Conclusion: We established very practical Japanese QA booklet for permanent implant brachytherapy. This booklet can be helpful worldwide.

This study was supported by a Grant‐in‐Aid for Cancer Research (17‐10) of Ministry of Health, Labor and Welfare.
SU‐FF‐T‐207: Evaluation of a Dual‐Level Micro Multileaf Collimator in Reducing Dose Undulation and Leakage
Authors: Liu Y , Shi C , Lin L , Papanikolaou N
Published: July 2007

Purpose: To evaluate the reduction of dose undulation at the field edge, gap leakage and interleaf transmission by utilizing a dual‐level four‐bank micro multileaf collimator (mMLC) and compare with standard MLC. Method and Materials: A 6MV Varian 600C linear accelerator has been collimated with a dual‐level four‐bank mMLC. The dual mMLC consists of four banks of leaves with upper and lower levels. The two levels of leaves are perpendicular to the central axis and travel orthogonally to each other. It provides the capability of precise aperture shaping and high edge resolution by using micro leaves with widths of 2.1 mm and operating with two levels of leaves. The maximum field size of the dual mMLC is 97 mm × 108 mm formed by 96 leaves. To evaluate the reduction of dose undulation, dose leakage and transmission, radiographic films of a right triangle aperture generated by the two levels dual mMLC were compared with the aperture using only one level of leaves and the other level of leaves remained open. The parameters for comparison include 2D and 3D dose difference, isodose lines, gamma analysis, and dose profiles. Results: The dual mMLC smoothes the dose undulation at the field edge and reduces gap leakage and interleaf transmission. The maximum dose reduction at the field edge is 39%. The gap leakage was reduced from 12% to less than 3.5%. The mean interleaf transmission was reduced from 6% to 3%. Conclusion: This innovative dual mMLC has shown the advantage of reducing dose undulation, gap leakage and interleaf transmission. It offers the ability of better field‐edge shaping, reduced gap leakage and interleaf transmission which are considered important characteristics in IMRT and SRT for target conformity and high dose delivery. Conflict of Interest: Supported by Initia Medical Technology.
SU‐FF‐T‐209: Evaluation of An Implantable Dosimeter for Measuring Dose in CyberKnife Treatment: Experimental Methods and Preliminary Results
Authors: Pacheco A , Scalchi P , Kmety‐Stevenson C , Francescon P , Masterson‐McGary M
Published: July 2007

Purpose: Determine the performance of an implantable MOSFET dosimeter (Sicel Technologies, Morrisville, NC) for hypo‐fractionated SBRT regimens using CyberKnife (CK) (Accuray, Inc, Sunnyvale, CA). Evaluate response as a function of collimator size, dose fractionation, and delivery time utilized in CK. Determine dosimeter accuracy for CK treatment for various diseases, and validate it for clinical implementation as an in‐vivo dosimeter and as a fiducial for CK tracking (previously demonstrated). Method and Materials: A custom‐made solid water phantom at 22°C and a liquid water phantom at 37°C were used for initial studies. For the collimator study, dosimeters were calibrated at 200 cGy. Measurements were made in solid water and normalized to the 60 mm collimator. For the dose regimen study, doses of 700, 1000, and 1200 cGy/fx for 5 fractions were evaluated using high‐dose calibrated dosimeters. Dosimeters were irradiated in both solid water (room temperature) and liquid water (body temperature). Dosimeter response as a function of treatment time (ranging from 30–90 minutes) was analyzed using the solid water phantom. An anthropomorphic body phantom with water‐filled dosimeter holders at 37°C was irradiated using ten patient treatment plans. Dosimeter doses were compared with calculation, as well as with ion chamber and diode measurements. Results: Collimators in the range of 25–50 mm showed minimal variation (<3%) when normalized to the 60 mm collimator. The dose fractionation study showed an average deviation from predicted doses in the range of −0.9% to 4.4%. Conclusion: The dosimeters are accurate, on average, to within 5% in the phantom testing performed with CK. While results to date are very encouraging, further study is necessary prior to clinical implementation of this device as both a CK tracking fiducial and an in‐vivo dosimeter.
SU‐FF‐T‐213: Evaluation of Dose From TomoTherapy Irradiation of Superficial PTVs
Authors: Cheek D , Hogstrom K , Gibbons J , Rosen I
Published: July 2007

Purpose: To evaluate the accuracy of dose calculated by the TomoTherapy HI⋅ART treatment planning system for superficial planning target volumes (PTVs). Methods and Materials: TomoTherapy treatment plans were developed for three superficial PTVs (2−, 4−, and 6−cm deep radially by 90° azimuthally by 4−cm longitudinally) contoured on a 27−cm diameter cylindrical white opaque, high‐impact polystyrene phantom. The phantom included removable transverse and sagittal film cassettes. Kodak EDR2 films were cut using templates to match the phantom surface (±0.5mm). The phantom was aligned to the machine's isocenter (±0.5mm), and films were irradiated according to the treatment plans. Films were then scanned with a Vidar film digitizer and converted to dose using a calibration determined from a 6 MV perpendicular film exposure. Hence, axial film doses were scaled by 1.027 so that mid‐arc depth doses matched those measured in the sagittal plane. All film readings were scaled by 0.934 to correct for over response due to phantom Cerenkov light. Measured dose distributions were registered to calculated ones and compared. Results: For all PTVs, at depths less than 10mm calculated overestimated measured doses by as much as 8%. At depths greater than 10mm, the calculated and measured doses agreed to within 5%. In the dose falloff region, measured and calculated dose distributions agreed to within ±2mm. The calculated dose in the low dose region distal to the PTV was lower by as much as 2%. Each of these differences is related to an assumption in the dose model. Conclusion: Calculated dose distributions showed good agreement with measurement for depths greater than 10mm. For more superficial depths, the dose model should be carefully evaluated or one should use bolus of at least 10mm to ensure accurate calculated dose to superficial PTVs.

Supported in part by a research agreement with TomoTherapy, Inc.
SU‐FF‐T‐217: Evaluation of the Dosimetric Accuracy of a Commercial Adaptive Radiotherapy Process
Authors: Hesston R , Gibbons J , Cheek D
Published: July 2007

Purpose: To evaluate the dosimetric accuracy and reproducibility of the TomoTherapy Planned Adaptive system using MVCT datasets. Method and Materials: Comparisons were made between reference dose distributions computed with the TomoTherapy treatment planning system (TPS) and verification dose distributions computed on the TomoTherapy Planned Adaptive (PA) system. Both calculations were performed on TomoTherapy megavoltage CT (MVCT) datasets to eliminate differences associated with variations in CT‐density tables. Simulated PTVs drawn on an MVCT of an anthropomorphic radiosurgery head phantom (CIRS, Inc.) were used as input to calculate doses on the TomoTherapy TPS. After plan optimization, a post‐plan MVCT image was taken and used to calculate dose on the TomoTherapy PA system. Initially, calculations were made using the same MVCT dataset as input to both programs to investigate the agreement between the algorithms. Next, calculations were made using registered pre‐ and post‐plan MVCT images to investigate the variation with MVCT datasets. Comparisons were made using the tools available on the PA system and in‐house software to compare doses outside of contoured structures. Results: For calculations on identical datasets, isodose and DVH comparisons between the TomoTherapy TPS and PA systems showed insignificant dose differences (< 0.2%) to all contoured structures. Dose discrepancies within the phantom but outside of contoured structures showed slightly larger variations, although well within 0.5%. Similar results were found for dose calculations using different MVCT datasets. In both cases, the doses computed in air outside the phantom showed the largest differences (up to 21%). Conclusion: Comparisons between TomoTherapy's TPS and PA system MVCT dose calculations showed clinically acceptable agreement everywhere within the phantom. Differences between doses calculated in air are due to differences in the way the algorithms mask low density voxels.

Research sponsored by TomoTherapy, Inc.
SU‐FF‐T‐237: Gafchromic Film in Vivo Dosimetry for Oral Radiotherapy
Authors: Chuang K , Chen H , Lin J
Published: July 2007

Purpose: To verify the radiation dose delivered in intensity modulated radiotherapy by Gafchromic film Method and Materials: Three buccal cancer patients selected to evaluate the delivered tumor dose by EBT Gafchromic film in vivo dosimetry. Patients coached to put the film in the same position and five measurements performed for each patient during routine treatment. Films cut into 1.5 cm by 1cm size wrapped by thin plastic wrap for film protecting and cotton thread stuck. A measured dose point locating film stuck a tiny lead mark and a cotton thread inserted into the patient's month and closed to tumor during CT images acquired meanwhile pen mark labeled on the cotton thread to point the depth of film. Another five films stuck the same cotton thread inserted into the position the same as CT acquired to measure the treatment dose in following five treatment days. Epson 1680 flat‐bed scanner, MATLAB software used to scan the film and calculate the dose. Intensity modulated treatment plan dose performed on treatment planning system and compared to the measured dose. 6 MV photon beam used to treat patient, fraction dose are 200 cGy . Results: The deviation between planed dose and measured dose are 4.83%, 2.62%, 0.05% and the standard deviation of five measurements are 4.02cGy, 4.64cGy, 3.5cGy in three patients. In prospective work tongue cancer patients would be involved to measure the treatment dose in vivo. Conclusion: The EBT Gafchromic films cut to small size can be put into the position of radiation delivered. By appropriate locating procedure and patient selected Gafchromic film in‐vivo measurement verify the treatment dose in oral cavity radiotherapy.