bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐06: Investigation to Replace CT by MRI in Post‐Implant Dosimetry of Prostate Permanent Brachytherapy
Authors: Chen Q , Blair H , Ciezki J , Mahadevan A , Began N , Suh J
Published: July 2008

Purpose: To investigate the use of MRI alone in post‐implant dosimetry for prostate permanent brachytherapy. Method and Materials: A patient's pelvic MRI was ordered 30 days post implant in addition to the CT scan. The MR images were acquired using all standard pulse sequences. T2 weighted in‐phase images were used to delineate the prostate and out‐of‐phase images to localize the implanted seeds. A post‐implant plan was generated on the MR images. We compared the results of MRI based dosimetry to CT based as well as CT/MRI fused techniques. Results: Phantom studies were performed. A number of dummy seeds were implanted in a tissue‐equivalent phantom and scanned on a Siemens 1.5T MR scanner. The out‐of‐phase MR images manifested significant signal loss artifacts at the seed locations due to susceptibility effects, resulting in enhanced seed visualization. After IRB approval, we acquired the post‐implant CT and MRI for a seed implant patient. The dose coverage based on the CT images was 99.9%. A CT/MRI fusion modified the dose coverage to 90%. After seed localization from the out‐of‐phase MR images, a post‐implant plan was generated on the MR images. The dose coverage was 91.4%. Conclusion: Phantom and patient studies demonstrated the potential to use MRI alone in prostate post‐implant dosimetry. Further investigation will be required to validate this technology. Disclosure: The work was partially supported by a CCF Taussig Cancer Center research grant.
SU‐GG‐T‐08: Is a Volume‐Based HDR Brachytherapy Optimization Algorithm Comparable to a Classic Line‐Based One?: Toward Tumor‐Volume Adaptive Brachytherapy for Cervical Cancer By 3T MRI Guidance
Authors: Kim Y , Modrick J , Bayouth J , Pennington E , Bhatia S , Jacobson G
Published: July 2008

Purpose: Verify a volume‐based optimization algorithm against a classic line‐based one for intracavitary high‐dose‐rate (HDR) brachytherapy (BT) treatment planning for cervical cancer. Method and Materials: This retrospective study reviewed ten randomly selected tandem and ovoids plans. To investigate the dose‐behavior induced by different algorithms, no variations in T&O applicator geometry and imaging dataset were made. Three metrics were employed: 1) the total reference air kerma (TRAK), 2) the volume enclosed by the 100% prescription dose (VOL100%Rx) (both based on ICRU Report 38), and 3) the dose to Point H. Conventional HDR plans (BrachyVision™, Varian, version 6.1) were regenerated and reoptimized by the optimization algorithm capable of incorporating a volume‐based optimization (version 6.5). To optimize dwell times, both are based on Nelder‐Mead Simplex method. However, a volume‐based plan optimizes dwell times to give the desired dose to volumetric structures along with reference lines by utilizing a group of points in a structure, while a conventional algorithm affords solely reference lines. Results: A volume‐based algorithm was found to be comparable to a classic one in terms of three metrics described above. The ratio of TRAK values (i.e. a volume‐based algorithm over a classic), VOL100%Rx, and Point H doses showed on average 1.01 (std dev ± 0.01), 1.03 (std dev ± 0.07), and 0.99 (std dev ± 0.02), respectively. To obtain an identical HDR plan was limited. However, we found a volume‐based optimization algorithm generates a plan comparable to a classic line‐based one which is relatively simple and robust. The plan generated by a volume‐based optimization algorithm also affords the ABS recommendations for BT for cervical cancer. Conclusion: A high‐dose‐rate brachytherapy plan utilizing a volume‐based optimization algorithm was comparable to a classic one. Hence, a volume‐based optimization is available to be implemented for a tumor‐volume adaptive brachytherapy.
SU‐GG‐T‐127: Effect of Fluence Smoothing On Plan Quality and Delivery Accuracy in Intensity Modulated Radiotherapy
Authors: Pandya S , Burmeister J
Published: July 2008

Purpose: This study investigates the effect of fluence smoothing on plan quality using a commercial inverse planning system and how associated changes in complexity affect delivery accuracy. More complex plans may provide better plan quality but have larger delivery uncertainties, expose the patient to higher out‐of‐field doses, and result in longer delivery times. Method and Materials: IMRT plans were created using the Varian Eclipse treatment planning system for prostate and head/neck test cases established for AAPM TG‐119 “IMRT Quality Assurance.” Treatment plans were generated using varying smoothing parameters from XSmooth=YSmooth=0 (no smoothing) to XSmooth=YSmooth=100 (heavy smoothing). The number of monitor units required for delivery was recorded as a measure of complexity. Homogeneity Index (HI) and Conformity Index (CI) were used to evaluate plan quality as a function of smoothing. Delivery accuracy was investigated for default (XSmooth=40, YSmooth=30), heavy (XSmooth=YSmooth=100), and no smoothing (XSmooth=YSmooth=0), using ion chamber, film and MapCheck. Ion chamber measurements were compared to Eclipse calculated values. Percentage of points passing gamma analysis (3%/3mm) was used to evaluate film and MapCheck results. Results: The increase in MUs from simplest to most complex plan was 38% for prostate and 138% for head/neck test cases. HI increased 75% for prostate and 40% for head/neck from the simplest to most complex plan with CI staying relatively constant. Ion chamber measurements in OARs trended toward higher accuracy with lower plan complexity. Gamma pass rate decreased consistently with increasing plan complexity. This decrease for both film and MapCheck was <1% for prostate, but approximately 5% for head/neck plans. Conclusion: Reductions in plan quality and delivery complexity were quantified with increasing fluence smoothing. A measurable decrease in the accuracy of IMRT delivery was observed with increasing complexity, and the most complex head/neck plan actually failed departmental standards. Conflict of Interest: Partially supported by Varian Medical Systems.
SU‐GG‐T‐130: A Software Tool for IMRT and EPI Dosimetry Research
Authors: Vial P , Hunt P , Greer P , Oliver L , Baldock C
Published: July 2008

Purpose: To develop a software tool for intensity modulated radiotherapy (IMRT) and electronic portal imaging (EPI) dosimetry research. The software tool implements correction factors to a commercially available EPI dosimetry model to account for the change in EPI response to multileaf collimator (MLC) transmitted beam as compared to open beam in IMRT fields. Method and Materials: Software was designed to perform the following tasks: i) Read MLC files from IMRT treatment plans and calculate a matrix of open beam and MLC transmission components. ii) Read portal dose image prediction (PDIP) files exported from the Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA). iii) Interpolate correction factors from look‐up tables for each PDIP based on the MLC transmission components of the corresponding MLC file. iv) Calculate and write a corrected PDIP that can be imported back into the planning system. The software tool was developed using the Microsoft Visual Studio. NET framework with the C♯ compiler. The software tool was validated for functionality and accuracy with a series of test IMRT fields. Results: The software tool correctly calculated the open and MLC beam components for different MLC models and collimator rotations. The corrected PDIP pixel values agreed with manual calculations to within 1% in all cases. Artifacts in the corrected PDIP in regions of high dose gradients were avoided when the MLC transmission matrix was sampled with pixel size ⩽ PDIP pixel size. Additional functions available with the software tool include the ability to write the open beam matrix to file, perform arithmetic operations on images, display and save image files, and to plot profile comparisons across images and open beam matrices. Conclusion: A software tool was developed and validated for IMRT and EPI dosimetry investigations. The software tool is being developed further for EPI dosimetry using transit IMRT beams.
SU‐GG‐T‐158: Statistical Process Control and Independent MU Calculations as a Surrogate for IMRT QA Measurements
Authors: Shoales J , Fang B , Weinberg R , Montanaro P , Boyer A
Published: July 2008

Purpose: To evaluate the use of independent monitor unit (MU) calculation software and statistical process control (SPC) as a surrogate for physical measurements in intensity‐modulated radiotherapy (IMRT) quality assurance (QA). Method and Materials: Physical measurements were compared with the results from an independent commercial MU calculation software program (IMSURE v3.0.2, Standard Imaging, Middleton WI) for patients treated with IMRT. Treatment plans for delivering IMRT were computed for 25 patients using a commercial treatment planning system (TPS). A “phantom plan” was calculated for each patient by replacing the patient's virtual simulation CT data set with a CT scan of an ellipsoidal water equivalent phantom. Measurements of dose were made at two locations using a calibrated 0.2 cc Farmer‐type ionization chamber in the phantom. The dose was delivered using the same computer files and linear accelerator parameters that were to be used for treating the patient. The doses measured at two points in the phantom were compared with the doses computed by the TPS at the same points in the “phantom plan”. Independent calculations of dose were computed using IMSURE at the same points and compared with doses computed by the TPS. The dose differences were analyzed using the methods of SPC and linear regression correlation statistics. Results: The SPC parameters calculated for the dose measurements and the IMSURE dose calculations were 0.5% average, 3.3% upper limit, −2.4% lower limit and 1.2% average, 3.8% upper limit, −1.4% lower limit, respectively. IMSURE calculated doses and measured doses both demonstrated a significant correlation (P< 0.001) with TPS calculated doses. Conclusion: IMSURE dose calculations and physical dose measurements produced similarly stable SPC charts and both were significantly predictive of TPS dose calculations. IMSURE dose calculations can be used as a surrogate for physical dose measurements in IMRT QA.
SU‐GG‐T‐159: Has IMRT Delivery Improved in the Last 5 Years?
Authors: Molineu A , Lowenstein J , Hernandez N , Alvarez P , Followill D , Ibbott G
Published: July 2008

Purpose: To determine whether institutions that participate in clinical trials are delivering IMRT more accurately than they were five years ago. Method and Materials: Anthropomorphic head phantoms were mailed to institutions wishing to participate in national clinical trials using IMRT. The phantoms consisted of an imagable insert containing a primary PTV, a secondary PTV and an OAR. The insert also housed TLD and radiochromic film. The institutions imaged the phantom and planned and delivered an IMRT treatment as though it were a patient. The phantom was returned and the TLD and film were analyzed. The delivered dose distribution was compared with the institutions' treatment plans and the results judged against 7% dose and 4 mm DTA criteria developed by the RPC and the RTOG. Results: Between 2001 and 2007 the phantom was irradiated 475 times. The overall pass rate was 76%. The pass rate prior to 2005 was 70%. The pass rate for 2005 was 76% and the pass rates for 2006 and 2007 were 76% and 80% respectively. Conclusion: Institutions interested in participating in NCI sponsored IMRT protocols are delivering IMRT more accurately than they were 4 years ago. One of the reasons for this is improved modeling capabilities in treatment planning systems. One of the most common treatment machine/planning system combinations has shown an improvement in pass rates since a new version of software became available. Other reasons could include improvements in the tools for patient‐specific QA and improvements in the commissioning of IMRT systems. Though there has been improvement, there is still room for more. 20% of the irradiations are still failures. Adequate IMRT quality assurance and commissioning is essential now as always. Conflict of Interest: Work supported by PHS grant CA10953 and CA081647 from the NCI, DHHS.
SU‐GG‐T‐161: Commissioning IMSure 3.1 for IMRT Second Calculation Utility Program
Authors: Shahine B , Ramaseshan R
Published: July 2008

Purpose: A program for IMRT QA for dose point calculation (IMSure Ver 3.1 from Standard Imaging) was commissioned for applications at our clinic. The purpose of this study was to set out acceptability criteria of IMRT plans, and to determine the percentage discrepancy range of point dose calculation between IMSure QA software and Eclipse treatment planning system from Varian Medical Systems. Method and Materials: This program uses the patented 3‐source model algorithm for calculating a point dose in water for IMRT plans. The data for output factors and scatter factors were measured using a PinPoint ion chamber with an active volume of 0.01 cm3, and a brass cap was used for measurement in air. The 6 MV beam was commissioned with a value of 0.14 cm for the dosimetric leaf gap which gave the closest agreement with the treatment planning system. Results: Twenty patients planned on Eclipse with 7—field IMRT for head and neck were transferred using DICOM RT to IMSure. Dose calculation at isocentre was assessed with IMSure and compared with the treatment planning results. Percentage discrepancy in the isocentre dose calculation ranged from −2.0% to 2.0% with a standard deviation of 1%. In other words, the agreement in isocentre dose calculation for 13 patients between the treatment planning system and the IMRT QA software was within +/− 1%. Measurements with an ion chamber and a cylindrical phantom were also performed, and found to be consistent with the above results. Conclusion: Our investigation of the IMSure QA software for IMRT showed good agreement with Eclipse treatment planning system to within 1% for 68% of patients sampled in our study. Our results showed that IMSure software can be a reliable tool for IMRT QA with discrepancies with Eclipse not exceeding +/− 2%.
SU‐GG‐T‐164: Quality Assurance and Dosimetric Analysis of Intensity Modulated Radiotherapy Using Compensators for Head and Neck Cancers
Authors: Tyagi A , Nangia S , Chufal K , Mishra M , Ghosh D , Singh MP
Published: July 2008

Purpose: In this study we describe our experience in implementing compensator based Intensity Modulated Radiotherapy (IMRT) for head and neck cancer with regard to pre treatment quality assurance (QA), dosimetric parameters and other technical detail. Method and Materials: This study represents the analysis of initial 48 patients who underwent IMRT with compensators for head and neck cancers. All patients were treated with pre treatment QA in terms of point dose with ion chamber & spatial dose comparison with film dosimetery. Results: In our study for all 48 patients, compensators revealed a deviation in central axis dose of 2% ± 1.82% in terms of cumulative calculated versus measured dose. Target coverage for high dose volume (70Gy) was adequate in terms of volume receiving 93% and 95% of the prescription dose, which was 98.5% and 97.5% respectively. Parotid and other critical organs were spared adequately. Contralateral parotid (CLP) was spared. V30Gy and V35Gy was 55.9% and 36.8% for CLP and average dose was 31.7Gy. Median variation in cumulative measured dose versus cumulative calculated dose was 1.8% (SD + 1.8) and mean variation was 2.5% (95%CI 1.5, 2.6). Range was 0 to 7%.a. Conclusion: Compensator based IMRT is practically feasible. Our QA tests revealed high degree of concordance between cumulative measured doses versus cumulative calculated doses. All dosimetric parameters were within acceptable limits. The manufacturing of compensator is cumbersome but it is a one time job followed by easy treatment delivery and simple QA procedure, high MU efficiency & less treatment time. IMRT with compensator is easy to implement and now can be applied to larger number of patient with different type of tumor.
SU‐GG‐T‐169: Comparison of the Efficacy of MapCHECK and Portal Dosimetry in the Pretreatment QA of IMRT Treatment Plans
Authors: Liu J , Wang Z , Kumaraswamy L , Nazareth D , Podgorsak M , Malhotra H
Published: July 2008

Purpose: To compare the efficacy of MapCHECK and Portal Dosimetry in the pretreatment QA of IMRT treatment plans. Method and Materials: MapCHECK contains a 2D array of 445 diodes arranged in a grid of 22 cm × 22 cm to measure and compare pretreatment IMRT dose plans. An Isocentric MapCHECK Fixture holds the MapCHECK with 3 cm of solid water build up securely in the accessory tray and at precisely 100 cm SDD. Varian Portal Dosimetry compares an electronic relative dose image acquired from Varian's Portal Vision by IMRT irradiation to a predicted portal dose image calculated by Varian's Eclipse treatment planning system. 3 IMRT treatment fields were selected for this study. The IMRT plans were generated with Eclipse for sliding‐window IMRT dose delivery. For every field, the verification dose computations in Eclipse and the delivered dose measurements on Trilogy unit were made at 0° gantry and 0° collimator angle. The evaluation used gamma criteria of 3%/3mm. Results: The pass rate of each measurement was calculated and compared for both the MapCHECK and Portal Dosimetry. Results show that the field 3 has similar pass rate, but the fields 1 and 2 have differences with the maximum variation of up to 6.5%. Both systems give the pass rate within clinical acceptable limitation (>90%). Conclusion: This study demonstrates that both systems are effective in the accurate pretreatment QA of IMRT in a clinical environment. But the pass rates don't show the same trend between them.
SU‐GG‐T‐171: Accuracy in the Dose Delivery of Sliding Window IMRT as a Function of Gantry and Collimator Angles
Authors: Liu J , Wang Z , Podgorsak M , de Boer S , Kumaraswamy L , Malhotra H
Published: July 2008

Purpose: Because of their weight, MLC leaves may experience significant gravitational force particularly when moving against gravity. Traditionally patient specific IMRT QA is performed with the gantry directed downward not reflecting the actual beam and collimator angle used in patient. This study was done to evaluate the accuracy in dose delivery for IMRT treatments as a function of gantry and collimator angles. Method and Materials: We have taken 3 IMRT treatment fields optimized for sliding windows IMRT dose delivery. The dose plans were generated for a Varian Trilogy unit employing a millennium MLC. All the treatment fields had 320 control points for IMRT delivery. MapCHECK along with 3 cm buildup material, in its isocentric fixture, was mounted in the linac accessory tray and secured at 100 cm SAD. Fields were individually delivered with gantry and collimator angles in increment of 30°. This resulted in 72 combinations of gantry and collimator angles at which fluences were delivered. Plans were evaluated using standard 3%/3mm criteria. Results: The passing rates for the delivered planar distributions for field1, field2 & field3 at gantry=0° and collimator=0° were 99.6, 96.2 & 92.6%, respectively. The pass‐rate was found to vary from 100 to 97.9% in field1 as function of gantry and collimator angle. The respective value in field2 & field3 were 100 & 90.2 and 93.2% & 87.9%. The standard deviation of the passing rate for the three fields as a function of gantry and collimator angles was 0.4, 2.7 & 1.3, respectively. Conclusion: Our study has shown that the accuracy of sliding window IMRT dose delivery technique does not demonstrate significant gantry and collimator angle dependence. It is important that this type of study be carried out as a part of initial acceptance as well as periodically in a compressive QA program.
SU‐GG‐T‐174: Angular Dependence of Sensitivity of Surface Diodes
Authors: Jursinic P
Published: July 2008

Purpose: Commercially available surface diodes are found to have as great as ±12% change in sensitivity with the angle of incidence of radiation. This work is a study of the cause of angular dependence of diode sensitivity and how it can be decreased. Method and Materials: A number of different surface diodes were used in these measurements: a commercially available, CA, and two prototype diodes, D1 and D2. CA and D1 were mounted on a circuit board that had a plane of copper on its backside; D2 was not mounted on a circuit board but did have a metal contact‐pad on its back surface. Optically stimulated luminescent dosimeters, OSLDs, were used that were packaged in a plastic case. All detectors were mounted in a water equivalent cylindrical phantom that provided symmetric buildup. Irradiations were made with a 6 MV, 10×10 cm2 field of a linear accelerator. Results: CA, D1, and OSLD had angular dependence of sensitivity of ±12%, ±9%, and ±1%, respectively. It was hypothesized that the copper plane on the circuit board was the cause of the anisotropy in sensitivity of the diodes. When a copper disc was placed on the backside of the OSLD its angular sensitivity became similar to that of CA and D1. D2 was found to have an angular dependence of ±5%. Conclusion: The anisotropy in angular dependence of diode sensitivity is in part due to the mounting of the diode on a circuit board that has a plane of copper. Low energy electrons are back scattered at the high atomic number interface and this results in higher detector sensitivity to photons that enter from the directions of the front and back surfaces of the detector. Diodes that are modified and not mounted near a plane of copper have reduced angular dependence.
SU‐GG‐T‐178: End Effect Influence On Segment Precision
Authors: Lightfoot D , Hartzell S , Stevenson K
Published: July 2008

Purpose: To study the influence of end effect on segment precision as a guide to minimum MU per segment guidelines for IMRT. Method and Materials: Two methods of end effect measurement (linear curve fit and split exposure method) were correlated with measured variation in segment dose ranging from 0.5 to 128 MU per segment. End effect measurements were based on ionization chamber data. Segment dose variations were based on MapCheck results from a segmented field consisting of 4 equal segments, each exposing a quadrant of a 10×10 field. Results: There was considerable variation in the value of end effect determined by the split exposure technique depending on the value of the single exposure and the number of fractions into which it was split, i.e. α=((Rn‐R1)/(nR1−Rn))t was not consistent as t and n were varied. The segmented exposures tended to show overdose in the first and third quadrant and under dose in the second and fourth quadrant, again with some inconsistency. Variations as high as 40% dose difference in a segment compared to a non‐segmented field were noted. Maximum deviations were equivalent to 0.4 MU. Conclusion: The results suggest a reaction time equivalent to about 0.4 MU. This represents the time between reaching a desired dose and a circuit response. This is consistent with the variation in both segment precision and end effect measurements. To make 0.4 MU insignificant requires a high number of MU/segment and other considerations such as the percentage of dose delivered by a segment need to be included in developing final MU/segment guidelines.
SU‐GG‐T‐244: Commissioning of Flattening Filter Free Varian 600C Linear Accelerator
Authors: Stathakis S , Gutiérrez A , Rassiah‐Szegedi P , Mavroidis P , Papanikolaou N
Published: July 2008

Purpose: The purpose of this study is twofold: a)to determine the dosimetric features of an unflattened 6‐MV photon beam of a Varian 600C linac; and b)to compare the beam characteristics of the unflattened beam against the flattened one. Method and Materials: Comparison of measurements in water was performed for a Varian 600C linear accelerator with and without flattening filter. The measurements consisted of dose profiles (inplane and cross plane), percent depth doses, and photon beam output. The field sizes measured varied from 2cm×2cm to 40cm×40cm. All measurements were taken at 100cm SSD with an A1SL ion chamber.
Results: Our results showed a significant (about twofold) increase in the dose rate for all field sizes. Also, a steeper reduction in depth doses was observed when the flattening filter was removed. No decrease was observed in the out‐of‐field dose, but there was an increase of the dose in the build‐up region that can be attributed to the increase of contaminant electrons. Conclusion:. Our study revealed that, for recent radiotherapy techniques, removing the flattening filter could be beneficial since there is a an increased dose rate and higher output therefore less monitor are required for each fraction leading to reduced whole body dose equivalent. On the other hand, higher skin doses are observed per monitor unit delivered but the overall skin dose for a complete treatment is comparable to the one with flattening filter since less monitor unit are required per fraction.
SU‐GG‐T‐246: NIST Water Calorimeter Update: Measured Dose Rate as a Function of Exposure Time
Authors: Chen‐Mayer H , Tosh R
Published: July 2008

Purpose: To re‐establish the primary standard of absorbed dose to water for therapy level beams using the NIST room temperature water calorimeter, heat conduction corrections need to be addressed particularly for the radiation exposure times from 60 s to 120 s range. Method and Materials: Measurements have been performed using the calorimeter in a beam to determine the dose rate as a function of modulated shutter opening times. In previous studies, we show such a curve spanned across the irradiation time of up to an hour. In this study, we focus on the region of common standards practice of around 100 s, in order to reconnect with Domen's historical value obtained at 70 s, and to compare with other metrology institutes' (e.g. PTB's 120 s). Since our calorimeter operates with the water outside the core vessel being stirred constantly, the lateral temperature gradient can be great at the boundary defined by the vessel wall, and therefore we expect greater conduction effects. A preliminary 3D finite element model is used to study the gradient effect. Results: We have shown previously that the system response to the radiation exposure time strongly depends on the size of the core vessel and can be simulated qualitatively using a simplified 2D finite element model. We now examine closely the calorimeter's response at modulated shutter opening times between 60 s and 140 s range. The variation in this range is found to be about 2.5%. However, preliminary finite element simulation indicates only 0.5% variation is expected. Conclusion: The model study suggests that the temperature gradient alone does not account for the observed dose rate variation. Further investigations are required with a model including non‐water materials. The system stability reached after the repeated cycles of exposure will also be studied both experimentally and by simulation.
SU‐GG‐T‐258: A Fast and Simple Method for Improving Accuracy of Helmet Output Factor Measurements for Gamma Knife Perfexion Unit
Authors: Ma L , Nordstrom H , Kjall P , Novotny J , Verhey L
Published: July 2008

Purpose: To develop an efficient and an accurate method for measuring the helmet output factors for the new Gamma Knife Perfexion. The method aims to minimize the uncertainties caused by redundant equipment setup and the inter‐dosimeter variations associated with the conventional method. Method and Materials: The measurement was performed using a 16‐cm spherical phantom with the radiochromic film inserts. For each output factor measurement, we exposed a single film with two shots (e.g. either a pair of 16‐mm/8‐mm or a pair of 16‐mm/4‐mm shots) symmetrically placed along the x‐axis with respect to the central sagittal plane. The time of the exposure for each pair of shots was adjusted to deliver nearly equal amount of dose at the center of each shot. As a result, no calibration curve needed for the film analysis and the output factor was obtained directly from the ratio of the optical density readings for each pair of the shots. Results: The average values for the 8‐mm and 4‐mm helmet output factors were measured to be 0.928±0.009 and 0.817± 0.012, respectively. These values were in excellent agreement (<1.5%) with the manufacturer default values of 0.924 and 0.805 for the 8‐mm and the 4‐mm helmet respectively. In particular, the agreement with the 4‐mm helmet was the best reported in the literature showing the accuracy and the robustness of the method over the conventional measurement method. Conclusion: A simple and accurate technique for the helmet output factor measurements was demonstrated for Gamma Knife Perfexion unit. Excellent agreement was found between the measured values and the manufacturer default values. Given the simplicity of our method, we recommend validation of the output factors as part of initial commissioning or routine quality assurance checks for the new Gamma Knife Perfexion.
SU‐GG‐T‐261: Monte Carlo Simulation of Small Electron Fields for Small Animal Irradiation
Authors: Lee C , Chen A , Chao T , Tung C
Published: July 2008

Purpose: This work studies beam characteristics of small electron fields using Monte Carlo methods and evaluate its feasibility for small animal irradiation. Method and Materials: Small electron fields with diameters 4, 6, 14 and 30 mm were generated with 1.5‐cm‐thick Cerrobend inserts on a Varian 2100 C/D linear accelerator. Full simulations were performed using the BEAMnrc and DOSXYZ codes for geometries including the treatment head, electron cones, block inserts and water phantom. Input parameters of the initial energy, angular and spacial distrbutions at the beam exit window for 6 and 18 MeV beams were optimized to insure depth dose discrepancy with measurement to less than 2 % for a 6 × 6 cm field. Dependence of PDDs, lateral profiles and beam outputs at dmax on beam energy, beam diameter and photon jaw setting were studied. Results: Results revealed that photon jaw setting has minimum impact on lateral dose distributions except for, however, it can changes beam outputs dramatically. With photon jaws fixed at 3 × 3 cm, relative output for 4, 6, 14 and 30 mm inserts are 0.13, 0.26, 0.64, 0.88 for 6 MeV and 0.61, 0.78, 0.92, 0.97 for 18 MeV beams when normalized to the output for a open 6 × 6 cm cone. Two effects of reducing beam sizes on PDD were observed: (1) dmax shiftes toward the surface and (2) the beams become less penetrating. For beam energy selection, 18 MeV beams can generate broder area of uniform dose distribution than 6 MeV beams which makes them more favorable for small animal irradiation. Conclusion: Investigations were performed to study beam characteristics of small electron fields. We have concluded that 18 MeV beams are more favorable than 6 MeV beams for small animal irradiation.
SU‐GG‐T‐264: Correction Factor in Output Measurement for Small Fields
Authors: Srivastava S , Das I , Cheng C
Published: July 2008

Purpose: Small field dosimetry is challenging due to detector‐produced perturbation factor (wall, electrode, stem, gradient, etc) that becomes pronounced in the region of electronic disequilibrium. A correction factor (CF) is required in dosimetric measurements to account for these effects. In this study, the detector response and the effect of detector perturbations for various types of detectors are investigated. The CF for the output factors for different energies is also determined for the various detectors. Method and Materials: Output factors were measured in water for a range of field size 0.5×0.5 to 10×10 cm2 for 6 and 15 MV photons from a Varian accelerator. Three diode detectors (PFD, SFD, Mapcheck), an electronic portal imaging device (EPID), MOSFET, EDR2 film, a 0.125 cm3 ion chamber and a micro‐chamber were used. An empirical formula for the CF based on Monte Carlo derived from Francescon, MedPhys,35,504,2008: CF = α(β‐exp(‐δS)) is used to fit the measured data in the respective region of electronic disequilibrium for 6 and 15 MV beams. Here, α, β and δ are the fitting parameters and S is the field size. The CFs are applied to obtain the output factors in the region of lateral disequilibrium. Results: For 6 MV and 15 MV, the distance of lateral equilibrium is found to be 1 cm and 1.5 cm respectively for 6 and 15 MV beams. For both energies, the CF varies from 0.5–5% in the lateral disequilibrium region. The output factors obtained for all detectors agree to within ±3% for fields ⩾ 2×2 cm2 and 3×3 cm2 respectively for 6 and 15 MV beams. Conclusion: A field size dependent correction factor for various detectors is provided based on MC study. The lateral equilibrium distance is found to be 1 and 1.5 cm for 6 MV and 15 MV beams, respectively.
SU‐GG‐T‐268: Use of Normoxic Polymer Gel On Dose Verification of 3.1‐, 4‐, 6‐Mm Cones for Stereotactic Radiosurgery
Authors: Chao T , Wu J , Chu C , Lee C , Wey S , Liu H , Tung C
Published: July 2008

Purpose: This study demonstrates the use of normoxic polymer gel on dose verification of 3.1‐, 4‐, 6‐ mm cones for stereotactic radiosurgery which is lack of lateral electronic equilibrium. Method and Materials: Two different types of normoxic polymer gel — MAGAT and PAGAT are studied in both larger field (10cm×10cm) and small fields (3.1‐, 4‐, and 6‐mm cones) to obtain percentage depth doses, profiles, and cone factors. These results were than compared to same measurements using several different small‐volume detectors including a small volume ion chamber (PTW23323), a pinpoint chamber (PTW31014), a PTW diamond detector (faced parallel or perpendicular), KODAK XV‐film (scanned in 0.1 mm resolution), and even BEAMnrc06Monte Carlo (MC) simulation. Results: Preliminary results shows that the dose uncertainty of MAGAT and PAGAT are 7% and 4%, respectively in large field. The dose uncertainty can be reduced to 3.5 % for MAGAT gel by avoiding the inhomogeneous magnetic field and adding more stirring. However, the uncertainty of PAGAT cannot be reasonable enhanced due to its bubble problems. In small fields, the dose deviation between MAGAT gel and MC simulation is about 9%, however, the space deviation between MAGAT and MC is less than 0.4 mm mostly. These dose deviations can be reduced if avoiding the inhomogeneous magnetic field and adding more stirring Conclusion: Compared to MAGAT gel, PAGAT has lower sensitivity (about 1/10 of MAGAT) and often comes with some manufacture problems. MAGAT shows good agreement in spatial distribution but not so good agreement in dose distribution. This dose deviation can be reduced if avoiding the inhomogeneous magnetic field and adding more stirring.
SU‐GG‐T‐271: Design and Study of a Novel Dosimeter Based On Carbon Fiber Material
Authors: Ma J , Yeow J , Chow J , Barnett R
Published: July 2008

Purpose: To design the new generation of dosimeters that incorporate carbon fiber materials as sensing components for dosimetric measurements in radiotherapy. Method and Materials: A carbon fiber dosimeter mainly consists of a carbon fiber sheet as sensing material and two PMMA slices as holders. The carbon fiber sheet was sandwiched between the two PMMA holders, each having a hole with 1.8 × 1.8 cm2 dimension. Copper electrodes were made on one PMMA holder to make electrical contacts. The dosimeter was connected to a resistor array in serial. A Dose 1 digital electrometer was used to measure signals output from the dosimeter in real‐time. Both 6 and 15 MV photon beams generated from a Varian Clinac 21 EX medical linear accelerator were used to test the dosimeter. Radiation dosimetric measurements were carried out by varying the dose rates, total dose, and field sizes to characterize various properties of the dosimeter. Results: This carbon fiber dosimeter responded to different ionizing radiation beams with a change in current amplitude. For both 6 and 15 MV photon beams, when dose rate was varied from 100 to 600 MU/min, the current changes measured by the dosimeter increased. The similar results were observed in diamond dosimeters. When the dosimeter was irradiated with total dose range from 100 to 600 MUs for 6 MV photon beams, excellent linear responses were displayed. The dosimeter measured increase signals with field size increased from 0.5 × 0.5 cm2 to 1.8 × 1.8 cm2. Conclusion: The carbon fiber dosimeter displayed excellent linear responses to total dose and was able to provide real‐time information on dose and dose rate at the same time. The dosimeter is expected to offer a higher spatial resolution by miniaturizing the physical size of the carbon‐based dosimeter design.
SU‐GG‐T‐291: Comparison Studies of Thermistor‐Based and Ultrasonic Water Calorimeters
Authors: Tosh R , Chen‐Mayer H
Published: July 2008

Purpose: High‐precision ultrasonic thermometry offers a compelling technology for use in water calorimetry, since existing prototypes are able to register temperature changes corresponding to tens of mGy without the confounding difficulty of excess heat. In addition, its potential for providing three‐dimensional dose distributions in real time via CT may offer attractive options for reference dosimetry of nonstandard beams. The present study compares ultrasonic thermometry to thermistor‐based devices with respect to sensitivity to thermal signals, noise and susceptibility to systematic errors, with the objective of motivating further developments. Method and Materials: Single‐channel ultrasonic thermometer systems — one, a pulsed, phase‐locked loop; the other, a pulsed digital phase monitor — are compared to thermistor‐based water calorimeter cells monitored via a dual‐phase lock‐in amplifier. Radiation from incandescent sources, 60Co, and a Clinac 2100C are used to evaluate performance. Estimated signal‐detection limits, obtained from overall system resolution and characteristic noise, are compared with experimental output and analyzed with regard to frequency content and heat transport effects. Results: RMS noise in the ultrasonic systems exceeded both theoretical estimates and that observed with thermistor —based systems over comparable integration times; however, thermistor‐based systems are dominated by 1/f noise. Spatial averaging of temperature along the acoustic path reduces heat‐conduction effects in ultrasonic measurements, but the latter are relatively more susceptible to convection because of the open‐phantom design. At reduced exposure times, ultrasonic dose measurements are within a few percent of expected values. Conclusion: Noise reduction for both technologies is needed for rapid quantification of heat transport effects. For ultrasonic systems, improvements to low‐pass filter circuits and algorithms are recommended; for thermistor‐based systems, proper phasing of the shutter with respect to the bridge‐excitation signal may alleviate flicker noise.
SU‐GG‐T‐293: The Use of Field Centre Profiles in the Matching of Linear Accelerator Photon Beams
Authors: Ramalingam Ezhilalan , Hayes Eamonn , Bradley Fintan
Published: July 2008

Purpose: As part of commissioning, parameters such as PDD, output and boundary profiles were used to match four Siemens accelerators, two Primus and two Oncor units. The respective parameters agreed generally to within 1%. However, subsequent dose measurements in dual asymmetric fields on the four units showed a maximum difference of 3–4% between a Primus and an Oncor unit. Further measurements were performed to investigate the potential use of “Field Centre Profiles” (FCPs) in “matching” linear accelerator photon beams. Method and Materials: The Field Centre Profile (FCP) is the ratio of the dose at an off axis point for a given field size to the dose at the axis of collimator rotation for the same field size and depth. FCPs were generated by measuring dose at the geometric centre of a symmetric 10×10 cm field on the central axis in a solid water phantom and on a number of off axis points along a diagonal axis — whilst maintaining the same field size using independent jaws. In addition, the off axis dose measurements were normalised to the central axis dose. FCPs were measured for a range of depths from dmax to 20 cm and to a maximum diagonal distance of 21.2 cm (X, Y=15cm) on all machines. Results: (a) FCP values were higher in magnitude (1–2%) compared to the corresponding points on the 40×40 cm diagonal boundary profiles. (b) FCPs from a Primus and an Oncor unit were significantly different, 3–4% maximum, compared to a maximum difference of 2% from a comparison of boundary profiles. Conclusion: It is postulated that Field Centre Profiles are more sensitive to flattening filter shape than 40×40 cm boundary profiles. Field Centre Profiles are a potentially useful parameter in matching linear accelerators.
SU‐GG‐T‐307: Characterization of Thermoluminescent Dosimeter Reader Precision and Artifacts
Authors: Bartol L , DeWerd L
Published: July 2008

Purpose: To explore the response of different thermoluminescent (TL) readers to micro‐cube and chip LiF:Mg,Ti (TLD‐100) TL dosimeters. Method and Materials: The precision of Harshaw Company TL reader models 3500 and 5500 was evaluated for LiF:Mg,Ti micro‐cube and chip dosimeters. Sets of TLD‐100 1mm × 1mm × 1mm micro‐cubes and 3mm × 3mm × 1mm chips were mounted in PMMA holders and irradiated to a dose equivalent to 1.00 Gy to water using . After irradiation, TLDs were evaluated on one of the readers. The annealing procedure between irradiations consisted of one hour at 400 °C, quick cooling to room temperature, and 24 hours at 80 °C. The ratio of the output of each TL dosimeter to the median output was tracked for three irradiations for each reader. In addition, the response of the model 5500 reader PMT to various amounts of TL output was analyzed. Sets of micro‐cubes and chips were irradiated to doses ranging from 0.10 Gy to 10.00 Gy dose to water. The light output from the PMT was analyzed for linearity with dose. Results: The mean reproducibility (1 standard deviation) of TLD‐100 micro‐cubes was 2.27% when evaluated with the model 3500 reader and 1.00% for the model 5500 reader. The mean reproducibility (1 standard deviation) of TLD‐100 chips was 1.68% for the model 3500 and 0.55% for the model 5500. In addition, the PMT of the model 5500 reader was found to have an exponential response with dose. Conclusion: The reader used to evaluate TL dosimeters affects measurements. This work concludes that the model 5500 reader is more precise for evaluation of TLD‐100 micro‐cubes and chips than is the model 3500 reader. In addition, the overresponse of the PMT to TL must be accounted for when using the model 5500 reader.
SU‐GG‐T‐311: A Distributed‐Computing Platform for Monte Carlo Dose Calculations and Beam‐Weight Optimization
Authors: Bakhtiari M , Jones M , Malhotra H , Nazareth D
Published: July 2008

Purpose: Monte carlo (MC) algorithms remain the gold standard in dose calculation routines. However, their long calculation times generally make them infeasible for clinical implementation, especially for IMRT optimization which requires rapid pencil‐beam dose computation. In a preliminary study, we are investigating how high‐throughput computing may be employed to perform MC dose calculations and rudimentary optimization using non‐commercial software. Method and Materials: Two non‐commercial software packages were used for this project, VMC++ and CERR. VMC++ is an MC dose calculation package which is more efficient than general‐purpose MC routines due to its use of variance reduction techniques. A combination of particle splitting and Russian Roulette methods is employed in VMC++ for photon transport modeling, which enhances the efficiency and speed of the calculation, making it approximately 50 times faster than the BEAMnrc code. CERR is an open‐source environment for radiotherapy calculations. We used the data‐importation and beam‐shaping features of CERR in conjunction with the dose‐calculation routines of VMC++ to perform dose computation on a 4‐field 3D CRT prostate case. We then optimized the beam weights using DVH‐based constraints and the Nelder‐Mead simplex technique, a multidimensional unconstrained nonlinear minimization algorithm. Collaborating with members of an academic supercomputing facility enabled us to perform the calculations on a subset of a cluster of 1000 dual‐core Linux‐based computers. Results: The dose calculation and optimization, involving several million particle histories, required only a few minutes to complete. The resulting plan was optimal based on the constraints provided to the system, and indicated that additional constraints may be necessary for clinical feasibility. Conclusion: We have shown that, using a distributed‐computing platform, an MC routine may be employed for treatment planning in a reasonable amount of time. This work will be extended to IMRT and beam‐angle optimization by combining a variety of Techniques.
SU‐GG‐T‐313: The Contribution of Nuclear Products On the Lateral Profile of the Pencil Scanning Proton Beams From the MD Anderson Proton Therapy Center Houston Nozzle: The Halo Effect
Authors: Sawakuchi G , Titt U , Mohan R
Published: July 2008

Purpose: Secondary particles created by nuclear interactions produce a halo of low secondary dose that extends far beyond the center of primary pencil beams. This work investigates this “halo effect” in the MD Anderson Proton Therapy Center Houston (PTCH) pencil beam nozzle. Method and Materials: A model of the PTCH pencil beam nozzle was implemented using the Monte Carlo code MCNPX. Upstream of the nozzle a water phantom was placed to score: i) the energy deposited as a function of depth for cylindrical tallies with increasing radii; and ii) the lateral profile of the beam as a function of depth. Results: Lateral profiles at the Bragg peak depth for three beam energies (72.50 MeV 159.52 MeV, and 221.83 MeV) show that dose is deposited far beyond the center of the PTCH pencil beam. Although this halo dose is very low for a single pencil beam (0.01 % of the maximum at 15 cm from the center of the 221.83 MeV beam), for a large number of beams it accumulates, which causes the dose deposition to increase, a fact that is not properly accounted for in treatment planning dose models. Furthermore, the energy deposited as a function of depth shows differences of up to 8 % when comparing 4 cm with 10 cm tally radii for the 221.83 MeV beam. This result suggests that percent depth dose (PDD) measurements should be performed with ion chambers (ICs) large enough to cover not only the central part of the beam, but also the halo region of small doses. Conclusion: The halo dose can be deposited far beyond the center of the primary PTCH pencil beam and PDD measurements should be performed with ICs large enough to cover the halo region of small doses.