bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐178: Dosimetric Evaluation of Penumbra for Conformal and Intensity Modulated Radiotherapy
Authors: Subramani V , Rath GK , Chander S , Ganesh T , Joshi RC , Gopishankar N
Published: July 2007

Purpose: To evaluate the penumbral characteristics as function of collimation systems, conventional detector, depth, energy and MLC travel orientations using diode detectors. Methods and Materials: The experiment was carried out in a Clinac 2300C/D linear accelerator for 6 and 15 MV. For penumbral width determination, cross‐beam profiles were measured using Blue phantom radiation field analyzer, 0.13cm3 ionchamber and diode detectors, CU‐500E dual‐channel electrometer and OmniPro beam‐data acquisition system. The penumbra was measured for both Jaw and MLC‐defined field sizes of 4×4 to 30×30cm2 depths at dmax, 5, 10, 15 and 20cm and also for cross‐plane and in‐plane MLC travel orientations at 100cm SSD. The penumbra width was calculated as the distance between 80%–20% intensity in the profiles. Results: The MLC penumbra was in the range of 3.6mm to 4mm for 6MV and 5.4mm to 7.1mm for 15MV at dmax depth for field sizes of 4×4 to 30×30cm2 as measured by diode detector. The mean differences between Jaw and MLC‐defined field's penumbra were 0.8mm ± 0.2, and 1mm ± 0.2 for 6MV and 1.2mm ±0.2 and 1.8mm ±0.5 for 15MV at dmax and 10cm depths. The maximum difference of 1.9mm ±0.3 for 6MV and 2.2mm ±0.2 for 15MV between diode and 0.13cm3 detectors were found. Use of diode detectors, maximum differences of 3.4mm ±3 at 20cm depths between energies and the differences of 3.3mm ± 2.2 for 6 MV and 1.2mm ± 0.3 for 15MV between cross‐plane and in‐plane MLC penumbra were found. Conclusions: The rounded‐end MLC penumbra was larger as compared to Jaw penumbra. The penumbra measured by diode was increasing with energy upto a certain depths and field sizes and then it increases more with low‐energy at larger depths and field sizes as compared to high‐energy. The adequate accounting of penumbra is important in intensity modulated beam dose calculation.
SU‐FF‐T‐182: Dosimetric Uncertainty of Small Cutouts in Electron Radiotherapy: Monte Carlo Dosimetry Evaluation
Authors: Chow J , Grigorov G
Published: July 2007

Purpose: To evaluate the dosimetric uncertainty when using small electron cutouts in electron radiotherapy. Monte Carlo simulation verified by measurement was used to predict the variations of dm, R90, R80 and relative output factor (ROF) for small electron cutouts with field centers shifted away from the beam central beam axis (CAX). Methods and Materials: EGSnrc‐based codes (BEAMnrc and DOSXYZnrc) were used to predict the beam profiles and percent depth doses (PPDs) of a 4 cm diameter cutout with field center shifted 0, 2, 4, 6, 8 and 10 mm from the CAX. Electron beams with energies of 4, 9 and 16 MeV produced by a Varian 21 EX linac were used. The Monte Carlo simulation was verified by measurement using a big scanning water tank and diode. The measured beam profiles and PDDs agree well with simulation results within ± 2%. The dependences of dm, R90, R80 and ROF on the shifted distance from the CAX were calculated and measured by the Monte Carlo simulation and diode, respectively. Results: dm, R90 and R80 do not change with the shifted off‐axis distance for the 4 MeV electron beams. However, these parameters decrease with increase of the shifted distance for the 9 and 16 MeV beams. This may be due to the relatively shorter electron path length of the 4 MeV beam compared to the dimension of the circular field. The ROFs decrease with increase of the shifted distance for all electron beam energies. Both measured and Monte Carlo simulated results agree well within ± 2%. Conclusions: Monte Carlo simulation is proved to be useful to predict the variations of dosimetric parameters such as dm and ROF when the small cutout center is shifted away from the CAX in the electron radiotherapy. This avoids time‐consuming measurement in the dosimetric QA.
SU‐FF‐T‐185: Dosimetry Along Edges of An Irregular MLC Field with Different Stepping Angles
Authors: Chow J , Grigorov G , Jiang R
Published: July 2007

Purpose: To measure dose profiles along edges of an irregular MLC field with different leaf stepping angles. So that dependences of dosimetric parameters for the profile on the stepping angle can be measured and investigated. Methods and Materials: Seven diamond‐shaped MLC fields enclosed by a 10 × 10 cm2 square with different stepping angles namely 21.8°, 31.0°, 38.7°, 45.0°, 51.3°, 59.0° and 68.2° were generated using the Varian SHAPER program. The fields were input to the console of a Varian 21 EX linac with a MLC of 120 leaves. Radiographic films were used to measure dose profiles parallel to the field edges. The film was placed at SAD = 100 cm, with 5 cm of solid water slab on top. Photon beam of 6 MV was used for the irradiation. The irradiated film was then developed and digitized using the Vidar VXR‐16 film scanner. Film dosimetric analysis was carried out using the RIT 113 system. Results: Width of profile measured at the 50% relative dose increases with the leaf stepping angle from 21.8° to 45° and then decreases when angle > 45°. Both width of ripple and its relative peak height decrease with increase of the stepping angle. However, profiles having different tongue‐and‐groove arrangements measured at the positive and negative in‐line axis (between the Y1 and Y2 jaw) have similar results. Conclusions: This study provides measured results to understand the dosimetry along the edge of an irregular MLC field when the leaf stepping angle is varying. The data are expected to benefit the physics modeling of the MLC dosimetry, particularly at the penumbra region for the treatment planning system, and the Monte Carlo simulation related to the irregular segmental MLC field for IMRT.
SU‐FF‐T‐189: Dosimetry of Small Pulmonary Nodules: A Comparison Between Treatment Planning Calculations and In‐Phantom Measurements
Authors: Ott J , Lamba M , Elson H
Published: July 2007

Purpose: To evaluate the accuracy of three commercially available treatment planning systems (Philips Pinnacle, Brainlab Brainscan, and Radionics Xknife) in their ability to calculate dose distributions delivered to small pulmonary nodules. Method and Materials: A radiological equivalent lung phantom consisting of cork (0.26 g/cm3) and acrylic spheres (∼1, 2, and 4 cm diameters) was constructed to simulate the presence of solitary pulmonary nodules. Single beam, 3D conformal, and IMRT treatment plans were generated for the lung phantom on each planning system. Evaluation was accomplished by comparing the dose distributions predicted by each planning system to the in‐phantom dose distributions measured with radiochromic film. Evaluations were made based on dose profiles and gamma analysis. Results: For single beams, the location of the largest disagreement between the measured and predicted dose distributions occurred at the distal side of the embedded nodule. The differences between measured and predicted doses in the distal edge of the medium sphere were 3%, 8%, and 23% for Pinnacle, Brainscan, and Xknife respectively. For 3D conformal medium sphere plans the largest differences between measured and predicted doses were 2%, 11%, and 13% for Pinnacle, Brainscan, and Xknife respectively. Gamma analysis showed the largest errors at the nodule/lung interface. Similar results were found for the IMRT plans. The largest errors were found for the smallest nodules in all three systems. Conclusion: As expected, the collapsed cone convolution algorithm of Pinnacle yielded the best agreement between predicted and measured doses in solitary lung nodules, with the largest differences at 3–5% for single beams, 3D conformal and IMRT plans. Brainscan and Xknife differed at points from 4% to 23% for the single beams, 3D conformal and IMRT plans. A systematic review of the results will be presented.
SU‐FF‐T‐190: Dosimetry Verification for Pencil Beam, Clarkson and Equivalent Square Dose Calculation Algorithms
Authors: ElDib A , Emam I , Hassan W , Habash A , ElLGohary M , Ma C
Published: July 2007

Purpose: To study the accuracy of Brainlab pencil beam and Clarkson algorithms in homogeneous phantoms and heterogeneous geometries. Materials and method: In this study PMMA was used as the homogenous phantom. Different heterogeneous geometries were designed by introducing different materials between the PMMA slabs. In each situation one material is used with different thicknesses: (a) thickness of 2.7 cm and 8.1cm low‐density material and (b) thickness of 3.8 cm and 7.6 cm of high‐density material. The low‐ and high‐density material is placed beneath 6 cm of PMMA. All the phantoms were CT scanned. Six different irregularly shaped beams were designed using a Brainlab multileaf collimator. Calculations were performed for these beams in the homogenous phantom using the Brainlab pencil beam, Clarkson, Modified Clarkson and equivalent square dose calculation algorithms. The percentage depth doses based on different models were compared to measurements. A dose of 150 cGy was specified to a point beneath the inhomogeneous material for the heterogeneous cases. Modified Clarkson, pencil beam and equivalent square algorithms were used to calculate the dose and then compared to measurements. Results: All the models agreed well with measurements; the equivalent square method showed the highest average deviations of 0.55% from measured dose values. In heterogeneous geometries the equivalent square method gave very large deviations (up to 17 %). Clarkson and pencil beam algorithms disagreed with measurements by 4.8 % and 3.8 % for the low‐density material of 2.7 cm thickness, and by 8 % and 6.9 % for the low‐density material of 8.7 cm thickness, respectively. Conclusion: All Brainlab dose calculation models are reliable for homogeneous phantom but some may differ significantly from measurements in heterogeneous geometries, especially in low‐density issues.
SU‐FF‐T‐194: Effect of Processor Temperature On Film Dosimetry:
Authors: Srivastava S , Das I
Published: July 2007

Radiographic films have proven to be an important dosimetric tool for dose verification in advance treatment techniques such as IMRT, IGRT and SRT etc. Optical density (OD) is related to dose, and depends on various parameters including beam energy, depth, field size, film batch, dose, dose rate, air film interface, post exposure processing time and temperature of the processor. Most of these parameters have been studied for the XV and EDR films used in radiation oncology, however; there is no information on the functional form of OD with processor temperature, which is investigated in this study. Films were exposed with 6 MV photon beam and were processed in a Kodak X‐OMAT 5000 RA processor. For sensitometric evaluation, films were processed at clinically set 95° F. A batch of films with same dose was processed at different temperatures in the range of 85°–105° F by changing the processor temperature in service mode with proper password and instructions. At each setup of temperature, enough time was given to change the display status from “WAIT” to “READY” sign. Additional 10 minutes were given after ready sign to stabilize the temperature further. One set of film from XV and EDR was processed at each temperature setting. It is observed that for a given dose, OD is linearly dependent on the processor temperature within narrow range of the temperature. It is also known that OD versus dose curve is a quadratic function. Combining these two functions provides OD in a quadratic function of the processor temperature. It is concluded that estimation of dose with film is strongly dependent on the processor temperature that should be maintained constant to reduce dosimetric uncertainty for XV and EDR films. Processor temperature should also be studied for each institution to estimate the correct OD and errors in dosimetry.
SU‐FF‐T‐197: Effect of the Added Buildup On P‐Type Silicon Diodes' Response in Daily in Vivo Dosimetry Measurements
Authors: Alaei P , Higgins P , Gerbi B
Published: July 2007

The silicon semiconductor diodes used for routine in vivo dosimetry in radiation therapy are commonly constructed with added buildup to place the effective point of measurement (location of the silicon chip) at the depth of maximum dose (dmax) or beyond and to shield against low energy scattered photons. This added buildup, commonly made of high‐density metals, could however introduce increased uncertainty to the accuracy of diode measurements. Previous work in this area has shown that buildup diodes show certain imperfections when applied in small photon fields.1

We have studied the effect of this buildup on in vivo dosimetry measurements, especially for small‐field dosimetry associated with IMRT treatments. We compared the response of the buildup diodes to that of no‐buildup ones, commonly referred to as skin diodes. We have used identical commercially available p‐type diodes, with and without the additional buildup, to measure the dose delivered by small static fields. The measurements with the no‐buildup diode were performed by placing tissue‐equivalent bolus on top of the diode to bring the diode measurement point to the depth of Dmax. In addition, we measured the dose to sample IMRT phantom plans as well as patient IMRT plans delivered to phantom, using both types of diodes.

Our results indicate that the buildup diodes, in general, produce higher readings than no‐buildup ones for small field dose deliveries, adding to the uncertainty of diode measurements. We have also observed that using the skin (no‐buildup) diodes with added tissue‐equivalent buildup decreases the measured‐to‐expected percentage differences by about 2%.
SU‐FF‐T‐199: Electronic Tissue Compensation Achieved with Both Dynamic and Static Multileaf Collimator in Eclipse Treatment Planning System for Clinac 6EX and 2100CD Varian Linear Accelerators: Feasibility and Dosimetric Study
Authors: Kinhikar R , Sharma P , Patkar S , Deshpande D , Shrivastava S , Dinshaw K
Published: July 2007

Purpose: Dynamic multileaf collimator (DMLC) and static multileaf collimator (SMLC) along with three dimensional treatment planning system (3DTPS) opens the possibility of tissue compensation. A method using electronic tissue compensator (ETC) has been implemented in Eclipse 3DTPS at our centre. The ETC was tested for head and neck conformal RT planning. The purpose of this study was to verify the feasibility of DMLC and SMLC in head and neck field irradiation, simultaneously delivering homogeneous dose at depth. In addition, the emphasis was given on the dosimetric aspects in commissioning ETC in Eclipse. Methods and Materials: A Head and Neck Phantom (The Phantom Laboratory, USA) was used for the dosimetric verification. Planning was carried out for both DMLC and SMLC ETC plans. The point dose calculated at central axis by Eclipse with DMLC and SMLC was noted. This was compared with the measured dose on machine with ion chamber and TLD. The calculated idsodoses and profiles were also compared with the measured one. The line dose profiles from Eclipse were also compared with the profile obtained from Amorphous Silicon (AS500) Electronic portal imaging device (EPID) on Clinac 6EX machine. Results: In uniform dose regions, measured dose values resulted in agreements with the calculated doses within 3%. Distance to agreement between calculated and measured isodoses in dose gradient zone was within 3 mm. Both the comparison of isodoses and the profile were in good agreement and acceptable. The measured and the calculated line dose profiles were flat for both DMLC and SMLC. Conclusion: The dosimetric verification of ETC for both the linacs demonstrated the feasibility and the accuracy of the ETC treatment modality for achieving uniform dose distributions. Therefore ETC can be used as a new tool in head and neck treatment planning optimization for improved dose uniformity.
SU‐FF‐T‐202: EUD‐Based Indication for Re‐Planning During Image‐Guided Adaptive Radiotherapy
Authors: Qi S , Li X
Published: July 2007

Purpose: A quantitative real‐time indicator for re‐planning due to the inter‐fractional anatomic changes (e.g., volume and shape changes) during the course of radiation treatment is not available for image‐guided adaptive radiotherapy (IG‐ART). In this work, we propose a EUD‐based indicator to address this issue.

Methods and Materials: A EUD‐based quantity
urn:x-wiley:00942405:media:mp0863:mp0863-math-0001 is introduced to assess the complicated EUD changes due to the inter‐fractional changes collected with daily CT images. Both target(s) and normal structures are considered. IEUD is calculated using the DVHs based on the original planning CT and the daily CT images. To demonstrate feasibility, we have calculated IEUD for selected representative cases in three tumor types: lung, head‐and‐neck and sarcoma, based on daily CT images acquired with a tomotherapy (HiArt, Tomotherapy) and a CT‐on‐Rail (Primatom, Siemens). The clinical target volumes (CTVs) and various normal structures were contoured on a series of daily CTs. The verification DVHs were generated by applying the original (or previous re‐planning) beam setups to the CT set of the day using either the XiO (CMS) planning system (for the patients treated with the Primatom) or the tomotherapy Planned Adaptive software (for those treated with Tomotherapy). Results: Significant inter‐fractional anatomic changes were observed on all selected cases, and these changes can be quantified by the values of IEUD. For a soft‐tissue sarcoma at chest, for example, the CTV in the middle of the treatment was found approximately doubled from that on the planning day. Such dramatic volume changes resulted in significant decreases in the CTV coverage (95 reduced to 30%), the IEUD value dropped by 15.6% (compared to that of the original plan), indicating the necessity for re‐planning. Conclusions: Dramatic anatomic changes may be observed during the treatment. The presently proposed EUD‐based indicator, capable of quantifying such changes, can be used to trigger re‐planning for IG‐ART.
SU‐FF‐T‐203: Evaluation and Performance Characteristics of An Implantable Dosimeter with the TomoTherapy Hi‐Art System
Authors: Esquivel C , Rassiah P , Beyer G , Childress C , Papanikolaou N
Published: July 2007

Purpose: To evaluate the performance of an implantable dosimeter for IMRT treatment delivery and daily dose verification with the TomoTherapy Hi‐Art System. Method and Materials: The DVS® dosimeter from Sicel Technologies was used to measure the dose delivered with TomoTherapy and was compared against the treatment planning calculation and ion chamber measurements. Two DVS® devices were used. One of the devices tracked the dose for a static tomotherapy beam. The other device tracked the dose for an IMRT prostate treatment plan. Comparable measurements were obtained with a NIST‐traceable calibrated ion chamber. All three detectors were placed at isocenter within a uniform cylindrical phantom. Before delivery, MVCT scans were made for treatment position verification followed by measurements. Results: Day‐to‐day position verification using MVCT scans for both types of dosimeters was reproducible. When comparing the results for the static beam delivery, the cumulative dose error was 0.4% between the ion chamber and the DVS® dosimeter. The results for the first 10 fractions resulted in an average percent difference of 0.70% from the planned dose with a standard deviation of 0.92%. When comparing the results for the prostate plan, the cumulative dose error was 3.2% between the ion chamber and the DVS® dosimeter. The results for the first 10 fractions resulted in an average percent difference of 0.68% from the planned dose with a standard deviation of 1.0%. Conclusions: The measurements made by the DVS® dosimeter falls within the device's specified accuracy of the dosimeter. The DVS® device is easily identifiable with the Hi‐Art TomoTherapy System and is an effective fiducial marker. MVCT verification scans did not contribute any significant dose nor did they affect the accuracy of the DVS® device. Conflict of Intertest: Sicel Technologies, Inc provided the equipment and materials used in the study.
SU‐FF‐T‐204: Evaluation of 3‐D Cluster Formation in Inhomogeneous Dose Distributions
Authors: Jiang H
Published: July 2007

Purpose: One shortcoming of using dose volume histogram (DVH) to evaluate a radiotherapy treatment plan is that the spatial distribution of the high dose voxels is not accounted. Cluster models, on the other hand, address not only the volumes but also their spatial location. This study is to evaluate cluster formation in clinical volume or organ at risk (OAR) receiving an inhomogeneous dose distribution. Method and Materials: In this study, upper five‐centimeter esophagus in patients with squamous cell carcinoma of the head and neck was used as the OAR. The patients were treated using IMRT. An in‐house program has been developed to analyze the Pinnacle dose grid, and to perform cluster searching. We studied one‐ and two‐connectivity of cluster formation. Results: The characteristics of cluster formation are very different under different fractional density. The size of the largest cluster shows a distribution with a peak and also some extent of spread. The cluster size can vary in a large range and cannot be accurately predicted. Therefore, the clinical outcome of the same DVH could be potentially different. The relationship between the mean size of the largest cluster (MSLC) and fractional density is non‐linear. Above a specific fractional density there is a dramatic increase in the MSLC, which possibly also represents an increase of the chance of percolation. At a certain high fractional density, MSLC increases linearly with fractional density, and additional voxels just increase the size of the existing largest cluster instead of forming new independent short clusters. Comparing one‐connectivity with two‐connectivity, two‐connectivity presents a steeper slope in the exponential increase area on the MSLC v.s. fractional density curve. Conclusion:Our investigation on the superior section of esophagus disclosed some interesting charasteristics. In the future, it may also be possible to incorporate these new NTCP models in the dose optimization algorithm.
SU‐FF‐T‐210: Evaluation of CT Extended Field of View Imaging Impact On Radiation Therapy Treatment Planning
Authors: Nixon E , Shukla H , Siochi R , Bayouth J
Published: July 2007

Purpose: CT imaging of patients for radiotherapy treatment planning frequently includes anatomy that extends beyond the 50cm nominal field‐of‐view (nFOV): 28.35% of the 575 CT scans we acquired from July–December 2006. The purpose of this study was to evaluate 1) the degradation of Hounsfield units (HU) in the extended field of view (eFOV), and 2) the dosimetric impact of ignoring or correcting this degradation within the treatment planning system (TPS). Method and Materials: CT images were acquired at maximum FOV (82cm) on a diamond‐shaped 30×30×13cm solid‐water phantom, challenging the reconstruction algorithm's ability to model the truncated projection data adequately. A unique dataset was acquired for 19 phantom locations (1cm intervals outside of the nFOV) and each imported into a TPS. Each phantom was contoured using a threshold of −200HU. A single treatment beam with isocenter placed at the center of the phantom was planned to deliver 100cGy to isocenter for each CT data set; dose map comparisons with and without homogenous correction for density (HU>−700 =1gm/cc) were performed relative to the control phantom within the nFOV. Results: Significant variation of HU was observed as a function of phantom displacement outside the nFOV (range +513 to −873HU); most dramatically ∼5cm beyond the nFOV border. If uncorrected, these changes in HU produced significant dose errors. Plans were compared to control plans and dose difference maps were generated; uncorrected images 3cm outside the nFOV demonstrated >5% difference, where overriding HU values above −700HU maintained <5% error for phantom positions ∼10cm beyond nFOV. Conclusion: HU values differ significantly for anatomy 2cm outside the nFOV, can be visualized and should be corrected for dose calculations. These results show <5% dose error can be accomplished for anatomy extending ∼10cm beyond the nFOV (which accounts for 98% of eFOV patients here).
SU‐FF‐T‐211: Evaluation of Diagnostic Radiation Exposure Risks From CT Scans for a Radiosensitive Patient
Authors: Pollard J , Angel E , McNitt‐Gray M , Gatti R
Published: July 2007

Purpose: To estimate the effective radiation dose a radiosensitive patient received from a series of computed tomography (CT) scans and to make estimates for the excessive cancer risk due to that exposure. Method and Materials: The Imaging Performance Assessment of Computed Tomography Scanners/NRPB (ImPACT, St. George's Hospital, Tooting, London) dose estimation spreadsheet was used to estimate effective dose from 14 CT image series obtained from an Ataxia‐telangiectasia (A‐T) patient. The scans were all taken within 4 months. For each exam, scanner parameters (i.e. kVp, mAs, etc) were retrieved from the dicom header. Excessive leukemia and cancer risks were calculated based upon the findings of a 15‐country long‐term study of nuclear power plant workers (Cardis et al. 2005). Results: The estimated effective dose for the CT scans was a combined 124 mSv and an average dose of 8.9 mSv per exam. The excessive cancer risk was 12% and the excessive leukemia risk was 24%, using estimates for normal subjects. Conclusions: This case indicates that diagnostic radiation exposure can pose an unintentional risk to radiosensitive patients. Given the clinical hypersensitivity to radiation and cancer predisposition for A‐T patients, most have been protected from receiving standard radiation therapy to treat their cancers. On the other hand, they readily undergo diagnostic radiological procedures which incur smaller radiation exposure. The effective dose and cancer risk estimates from this study show a need for monitoring the radiation exposure from all medical sources for radiosensitive patients. Other diagnostic modalities can be suggested for radiosensitive patients in order to monitor medical responses and progress, such as magnetic resonance imaging or ultrasound.
SU‐FF‐T‐212: Evaluation of Different Simulation and Dosimetry Techniques in Small Field Relative Measurements
Authors: Feng Y , Lamba M , Elson H , Kassing W , Spitz H
Published: July 2007

Purpose: The purpose of this study is to compare and evaluate several dosimetry and simulation techniques that can be used to quantify the beam profiles in small radiation fields, especially in the penumbral regions. Method & Materials: Several different radiation dosimetry techniques, including ion chamber dosimetry (RK & PinPoint chamber), diode dosimetry (Scanditronix stereotactic diode), and radiographic (EDR) and Gafchromic (HS) film dosimetry were utilized to measure the beam profiles of 5 mm and 45 mm radiosurgery fields at 95 SSD and depth of 5 cm. The profiles from the ion chambers were further analyzed using a detector response function previously determined by Monte Carlo simulations using deconvolution methods to reduce the beam broadening effect produced by the finite detector size. The same radiosurgery fields were also modeled using Monte Carlo (MCNP5) simulation, including modeling chambers and their respe ctive responses. The beam penumbras from the various dosimetry and simulation techniques were then compared. Results: For the 5 mm cone, the Monte Carlo and diode measurements were in agreement with a 20%–80% penumbra at 1.1 mm. Gafchromic film and the deconvolved RK measurements were larger at 1.8 mm. EDR film and the raw RK chamber penumbras were 2.7 mm. For the 45 mm cone the penumbra results were as follows: Monte Carlo‐2.7 mm; diode‐2.0 mm; Gafchromic‐3.2 mm; deconvolved RK‐3.6 mm; EDR‐3.7 mm; and raw RK‐4.1 mm. Conclusions: Monte Carlo simulations, stereotactic diode measurements and Gafchromic film have a range of penumbras that are within ∼ 1 mm for stereotactic cones. For ion chambers, such as the RK chamber (radius 2 mm), deconvolution of previously determined detector response functions can reduce penumbral broadening due to the larger detector size.
SU‐FF‐T‐220: Evaluation of the Performance of An Implantable Dosimeter Used in 4D‐Gated Radiotherapy
Authors: Esquivel C , Eller C , Rassiah‐Szegedi P , Papanikolaou N
Published: July 2007

Purpose: To evaluate the performance of an implantable dosimeter for treatment delivery and daily dose verification for 4D‐gated radiotherapy Method and Materials: The DVS® dosimeter from Sicel Technologies, Inc. measured doses delivered to a volume in a Quasar™ Respiratory Motion Phantom. DVS® measurements were compared to the Pinnacle's treatment planning results and to those made by a NIST‐traceable calibrated ion chamber. Each dosimeter measured dose within the phantom's lung equivalent moving insert. The phantom was scanned on a GE Lightspeed RT‐CT scanner using the 4D respiratory gating option. From the scans, a composite CT data set was built and an average GTV was defined within the lung equivalent moving insert. A treatment plan was created using the composite scans. Both the ion chamber and DVS® were used to track GTV dose. A second plan was created that uses a gated‐beam able to deliver radiation to the GTV during the expiration breathing phases only. Results: Both plans were created to deliver 200 cGy per fraction for 5 fractions to the GTV. The first plan included an average GTV that includes all the breathing phases. The DVS® and the ion chamber differed up to 3% from the Pinnacle average GTV plan. Measurements between the two dosimeters differed by 2.5%. Measurements made by the DVS® and the ion chamber for the gated‐ treatment are also comparable to those made using the treatment plan with all breathing phases. Conclusions: The measurements made by the DVS® dosimeter falls within the device's specified accuracy of 3%. The Dose Verification System provides the ability to verify that the prescribed dose is delivered during 4D‐gated treatments on a daily basis. Conflicts of interest: Sicel Technologies, Inc provided the equipment and materials used in the study.
SU‐FF‐T‐221: Evaluation of Titanium Metal Imaging Artifacts On Proton Dose Calculation
Authors: Wu R , Zhang X , Amos R , Prado K , Lii M , Sahoo N , Gillin M , Zhu X
Published: July 2007

Purpose: Metal implants can cause CT image artifacts that result in inaccurate CT number determination local to the metal implant. For proton therapy treatment planning, CT numbers are converted to stopping powers to calculate dose and range, which affects the treatment compensator design. We evaluated the effects of metal artifacts from titanium surgical implants on proton dose calculation accuracy and on range uncertainty. Method and Material: Surgical Titanium metal screws and discs are placed in a small water tube that was inserted into a plastic Head Phantom. The phantom was imaged on a GE 16 slice scanner using a standard Head protocol. Each image slice was then carefully reviewed and the Titanium metal and its artifacts in adjacent areas were contoured using Varian Eclipse TPS. Proton treatment plans were generated with and without overwriting the CT numbers within the metal artifacts contours on the planning image sets. Then, treatment plans were created and compared to each other for dose and range differences. Result: There appears to be less than 2% overall dose differences in the treatment target between the plans with and without artifact overwrite. There was an increase in proton range of 3 mm for the overwritten image set. Conclusion: For the commonly used Titanium screws and discs in brain surgery, the image artifacts appear to cause minimal (<2%) effect on dose delivered to the treatment target. The range uncertainty is of the order of 3mm, which is within the distal margin for passive scattering proton planning.
SU‐FF‐T‐222: Expected Clinical Impact of the Differences Between Planned and Delivered IMRT Dose Distributions
Authors: Mavroidis P , Ferreira B , Papanikolaou N , Svensson R , Lind B , Brahme A
Published: July 2007

Purpose: Due to the highly conformal distributions that can be obtained with intensity modulated radiation therapy (IMRT), any discrepancy between the intended and delivered distributions would likely affect the clinical outcome. Consequently, there is a need for a measure that would quantify those differences in terms of a change in the expected clinical outcome. Material and Methods: To evaluate such a measure, the case of a cervix cancer was used where the bladder and rectum, are proximal and partially overlapping with the internal target volume. A solid phantom simulating the pelvic anatomy was fabricated and a treatment plan was developed to deliver the prescribed dose to the phantom. The phantom was then irradiated with films positioned in several transverse planes. The racetrack microtron at 50MV was used in the treatment planning and delivery processes. The dose distribution delivered was analyzed based on the film measurements and compared against the treatment plan. The differences in the measurements were evaluated using both physical and biological criteria. Results: For the computerized treatment plan, the maximum value of P+ was 84.1%, for a mean dose to the ITV of D̄ = 93.3Gy, associated relative standard deviation and biologically effective uniform dose, D̿ITV of 89.2 Gy. The delivered dose distribution from all the beams produced a P+ value of 77.0% for and D̿ITV of 83.5 Gy.

Discussion and Conclusions: Whereas the physical comparison of dose distributions can assess the geometric accuracy of delivery, it does not reflect the clinical impact of any measured dose discrepancies. With highly conformal IMRT, the accuracy of the patient setup and treatment delivery, are critical for the success of the treatment. A method is proposed to evaluate the precision of the delivered plan based on changes in complication and control rates as they relate to uncertainties in dose delivery.
SU‐FF‐T‐223: Experience with Thoratec Left Ventricular Assist Device (LVAD) During Radiotherapy Treatment
Authors: Lasher D , Wojcicka J , Malcom R , Shears L
Published: July 2007

Purpose: This poster describes the response of a Thoratec left ventricular assist device (LVAD) to the scatter, leakage, and RF environment in a linear accelerator vault. The device contains a mechanical pump attached to the abdominal wall that the supports left ventricle in circulating blood. Cannulae connect the paracorporeal pump to the ventricle. An electric lead relays pump filling information to the TLC‐II computer‐controlled driver, and a pneumatic lead transfers positive air pressure to the pump and ejects the blood. Method and Materials: The treatment plan was 4500 cGy to the rectum over 25 fractions with three 15MV photon beams on a Varian 2100EX. All beams avoided the pump and leads. The response to EMI was evaluated by observing a duplicate driver in the treatment configuration as the patient's fields were delivered to a 30×18×60cm3 solid water equivalent phantom. Pre‐treatment dose assessment included calculations with Pinnacle treatment planning system, AAPM TG36 data analysis, and MOSFET measurements with high sensitivity bias on the surface of the driver during the phantom irradiation. During the first patient treatment, MOSFETs were placed on the pump and leads under 1cm of bolus, approximately 1cm from the left lateral treatment portal. No additional shielding was applied to the LVAD. Results: EMI interference was absent and the LVAD operated normally during the pre‐treatment test and throughout the treatment course. Radiation to the driver was too low to be detected by the MOSFETS. Cumulative dose estimates to the pump were 425cGy to 0.1cc (DVH), 262.8cGy (TG36), and 158.5cGy (MOSFET). MOSFET readings to the leads were 70.5cGy. Conclusion: External beam radiation treatment was safely delivered to a LVAD dependent patient. The Thoratec TLC‐II exhibited no adverse response to EMI and doses up to 425 cGy. Our results are based on one case and further study is encouraged.
SU‐FF‐T‐226: Extensive Patient Specific IMRT QA for a Head & Neck Patient with Pacemaker
Authors: Cao F , Ramaseshan R , Kohli K , Nuraney N , Kristensen S , Wong F , Karvat A
Published: July 2007

Purpose: We have used Intensity Modulated radiation therapy (IMRT) technique to reduce the dose to the pace maker, while providing adequate coverage to the target. However there is a greater uncertainty in the delivered dose due to many factors. In this study we have attempted to verify the dose delivered to the pace maker by performing extensive patient specific quality assurance (QA) and as well with in‐vivo dosimetry. Methods: The physical location of the pacemaker made it impossible to eliminate the pacemaker outside of the jaw setting from the field, however we were able to shied the pace maker with MLC. We have used Eclipse treatment planning system inverse planning to optimize the dose delivery. The option of using a “hard” constrain to the pacemaker during the plan optimization was engaged. The QA carried included ion‐chamber measurement; EPIDose based on Electronic Portal Imager [1]. Mont Carlo simulations were carried out. Results and Discussion: The Monte Carlo calculated maximum and mean dose of 342.3cGy and 140.7cGy to the pace maker agreed well with Eclipse planning system data of 348cGy and 117.1cGy. The ion‐chamber measured dose at the location of the pacemaker was 11.4% higher then the planned dose. The EPIDose showed about 9.7% higher then the calculated dose. The patient QA plan was created using Radon phantom and the MOSFET measurement is carried out. The results indicated that the planning system underestimated the dose to the pace maker by 20%. The dose to the pacemaker reported included all the uncertainties. In consultation with the manufacturer of the pace maker, we have determined that it is safe to proceed with the treatment, with the advice to have more frequent cardiology appointment to determine the proper functionality of the pacemaker.
SU‐FF‐T‐228: Factors Affecting IMRT QA
Authors: Sethi A , Gao M , Rusu I , Xu M , Ke Y , Glasgow G
Published: July 2007

Purpose: We evaluate the impact of plan parameters and setup errors on IMRT QA results. Methods and Materials: Prostate (23MV, 5 field) and H& N (6 MV, 7 field) IMRT plans were exported to a cubic QA phantom. First, the magnitude of random errors in IMRT QA was estimated by repeating each measurement ten‐times. Next, the following parameters were independently adjusted: intensity levels (5 to 20), minimum MLC segments (0.5 to 1.5cm), phantom setup error (0.5cm), and their impact on IMRT QA was evaluated via ionization chamber and EDR film. Coronal plane film results were analyzed using γ‐index, dose difference, and distance to agreement. Results: Based on repeat QA measurements, the central axis ion chamber dose was found to be highly reproducible and in good agreement with planned dose (mean deviation, prostate: 1.79±0.10%; and H&N: 2.62±0.25%). Measured isodose distributions displayed similar consistency (mean γ‐index, prostate: 0.72±0.04; H&N: 0.60±0.09). Varying the number of intensity levels and MLC segment size had a small effect on the central axis ion chamber dose (<1.2%). However, film results showed that mean γ‐index changed by 10% when minimum MLC segment size was increased from 0.5 to1.5cm. The number of intensity levels had a smaller effect on γ‐index. For a 0.5cm setup error in H&N plan, 3.8% deviation was noted in the measured dose, however this coincidentally improved QA results. No corresponding dose discrepancy was seen in a fiducial‐less film. Conclusions: For an accurately modeled linac, IMRT QA results show remarkable consistency and agreement with planned dose. Plan complexity tended to worsen QA results for H&N patients. Sub‐optimal IMRT plans with incorrect plan parameters and setup errors can have detrimental effect on patient treatment. However, IMRT QA does not always flag these errors, as their impact on QA results may not be large enough to be noticed.
SU‐FF‐T‐231: Feasibility of Use of Anatomical Geometry and Volume for Prostate IMRT Planning
Authors: Kassaee A , James P , Vapiwala N
Published: July 2007

Purpose: To study the feasibility of using simple geometrical information such as volumes of prostate, bladder, and rectum, and volumes of bladder wall and rectal wall to streamline the optimization process in intensity modulation radiotherapy (IMRT) of prostate. Method and Materials: Class solutions (standard dose constraints and standard beam arrangements) for prostate IMRT planning have been developed in many treatment centers. While it is possible to achieve similar solutions and desired dose constraints among most patients, there are some cases in which it is difficult, even after using different beam arrangements. Prostatic size and geometrical shape relative to normal tissue (rectum and bladder) and their overlapped regions attribute to sub‐optimal solutions. In this study, we analyze the volumetric information of prostate, bladder, bladder wall, rectum, and rectal wall to understand if some definitive conclusions can be drawn. We planned ten prostate patients with CT scans using IMRT. Each plan consists of five coplanar beams using 15 MV photon beams and similar dose constraints. Each plan was optimized to according to our protocols. Total dose for each plan is 79.2 Gy for 44 fractions. Results: We calculate dose volume histogram (DVH) for each organ for each optimized plan. We compare the V40, and V60 of rectum, rectal wall, bladder and bladder wall between each patient's plans using the ratio of the volumes of above organs to that of prostate. Conclusions: There is a complex relationship between the ratio of normal tissue organs and prostate. However, an overall trends exist that could be to used to reduce the treatment planning time based on what is achievable relative to standardized dose constraints. Among the more significant finding is the reduction of V60 for rectum and bladder as function of the volumetric ratios.
SU‐FF‐T‐235: Fluence Determination in a Polarized Synchrotron‐Generated Monochromatic X‐Ray Beam
Authors: Dugas J , Oves S , Matthews K , Sajo E , Hogstrom K
Published: July 2007

Purpose: K‐edge capture radiotherapy using monochromatic x‐ray beams necessitates development of corresponding dosimetry techniques, one being utilization of Monte Carlo methods. Since Monte Carlo simulations yield dose per particle, a methodology to measure the beam fluence of a monochromatic synchrotron‐generated x‐ray beam was developed using Compton scattering techniques. Method and Materials: X‐rays were produced at the LSU CAMD synchrotron by passing a 1.3‐GeV electron beam (≈200 mA) through a 7‐T superconducting wiggler. The resulting polychromatic beam passed through a double multilayer monochromator to generate a ∼0.1×2.8‐cm2 beam. Since high fluence rates make direct measurement of beam fluence impossible, photons Compton scattered by thin polyethylene targets into a 1‐mm thick × 2.54‐cm diameter NaI(Tl) scintillation detector were counted. Incident beam fluence was calculated from net peak counts via application of the Klein‐Nishina collisional cross section for polarized radiation at multiple scattering angles. Simultaneously, percent polarization of the beam was determined by separating the Klein‐Nishina cross section into its two orthogonally‐polarized components. Measurements were taken at 4 scattering angles (15°–60°) with the fraction of photons (f), polarized such that the angle between the direction of incident polarization and the scattering plane is 0°, determined by minimizing the standard deviation of the calculated fluences. Results: Incident beam fluence rates for 100 mA of synchrotron ring current were determined with ∼1% precision and ranged from to . Measurement showed that the 35‐keV beam was highly polarized in the plane of the synchrotron, e.g. f =7%. Conclusion: We have demonstrated a feasible method based on Compton scattering to determine incident synchrotron‐generated x‐ray beam fluence and polarization. Fluences determined via this method can provide input data that will allow currently underway Monte Carlo simulations to predict dose distributions in homogeneous phantoms for comparison with more traditional dosimetry methods (ion chamber and radiochromic film).
SU‐FF‐T‐236: Functional Representation of Tissue Phantom Ratios for Small Photon Fields
Authors: Sauer O A , Wilbert J
Published: July 2007

Purpose: For small photon fields tabulated TPR data are not readily available. Aim of this work was to find a functional representation valid at all depths d and for fields as small as 4 mm side length s. Such functions generate high quality beam data as input to a treatment planning system, or for calculating monitor units and dose to any point in water for the full range of s and d required clinically. Method and Materials: For beams of 6 and 10 MV collimated with the Elekta Beam Modulator, TPR was measured in water with a shielded silicon detector. Field sizes were from 4×4mm2 to 160×210mm2. For each field size the TPR was fitted to . The first factor describes the build up, with Ds being the surface dose and β a build up gradient. α is a normalization factor, introduced to normalize TPRs to 1 at a depth of 10 cm. The exponential, describes the declining part of the curve, where μ is a pseudo attenuation coefficient and η a beam hardening coefficient. The field size dependence of the parameters was analyzed separately and appropriate functions were fitted to these data. The surface dose Ds was set to the respective mean values, while β(s) was fitted to a power function and μ(s) and η(s) to exponentials. Results: The measured TPR data could be fitted very well with the proposed function. Owing to the increasing amount of scattered photons μ decreases with field size. The beam hardening η turned out to be negative except for field sizes below 1 cm. Conclusion: The proposed model predicted TPRs generally better than with 1 % accuracy at all field sizes and depths. Its implementation improved the quality of monitor unit calculations.
SU‐FF‐T‐238: GafChromic XR‐QA Film in Testing Panoramic Dental Radiography
Authors: Chu R , Lam T , Liang Y
Published: July 2007

Purpose: The location and the field size of the incident X‐ray beam in panoramic dental radiography most often cannot be ascertain visually. These parameters are needed for quality control testing and dosimetry determination. To alleviate this problem, GafChromic XR‐QA film was tested on two panoramic systems. Method and Materials: For each system, the dose‐area product was computed with the length of a cross section image of the incident beam and the exposure measurement with a pencil ion chamber. The result was confirmed by direct analysis of a dose distribution on a film. Placement of the ion chamber was determined by these images. The dose‐area products were thus determined to be 713 cGy.mm2 and 721 cGy.mm2 . From these values, the effective doses were computed to be 5.2 μSv and 5.3 μSv respectively. Results and Conclusion: This version of radiochromic film has thus been demonstrated to be a useful complement to a pencil ion chamber in the testing of a panoramic radiography system.