bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐391: Simplified Monte Carlo Simulation for Absolute Dose Distribution of IMRT
Authors: Lee C , Lin M , Chao T , Tung C
Published: July 2006

Purpose: A simple method was proposed to incorporate heterogeneous fluence distribution due to time‐dependent leaf motion of IMRT delivery into Monte Carlo based static dose calculation to predict the dose distribution inside a patient and the transmission dose distribution at the EPID. Method and Materials: Weighting factors stored in an efficiency map was incorporated into an open photon field of the Monte Carlo simulation (BEAMnrc, 2005) to mimic the heterogeneous fluence distribution of an IMRT field. The efficiency map was obtained by dividing the measured in‐air IMRT absolute dose distribution to that of the same open field using an aS500 EPID (Varian, Palo Alto, CA) which was carefully calibrated for absolute dose measurement. The EPID was setup at 140 cm SDD and 3.84 mm polystyrene phantom was used for the 6 MV photon beam to ensure full electronic buildup. Absolute dose calibration of Monte Carlo simulation was performed under a standard setup where absolute dose at a reference point was verified by ion chamber measurement. Transmission dose comparison at the EPID for two dynamic wedge fields and two IMRT fields as well as absolute dose distributions at the midplane (SAD=100cm) of a 24.8 cm polystyrene phantom among film dosimetry, EPID measurement and Monte Carlo simulation will be presented. Results: Max discrepancy of 3.3% was observed along the central axis for F1 IMRT field. In addition, penumbra along the field edge of the Monte Carlo simulation appeared to be 1–3 mm wider than EPID and film dosimetry. Conclusion: A simplified method to incoporate heterogeneous fluence distribution of IMRT fields into static Monte Carlo simulation for absolute dose verification was proven feasible. The system can also serve as an independent absolute dose check system to commercial TPS calculation.
SU‐FF‐T‐392: Simulating Absorbed Dose: Finite Element Modeling of a Stirred Water Calorimeter
Authors: Chen‐Mayer H , Tosh R
Published: July 2006

Purpose: To conduct detailed, finite‐element studies of heat transfer mechanisms — in particular, heat conduction and forced convection due to stirring — in a room‐temperature water calorimeter intended as a new primary standard for high‐energy photon dosimetry. Method and Materials: The direct realization of absorbed dose is achieved by measurement of water temperature change of order 0.24 mK per Gy of radiation, posing experimental difficulties due to thermal and electrical noise. Prolonged irradiations can improve the signal‐to‐noise ratio, but the heat‐transfer processes inherent to the calorimeter design can cause systematic errors in the results. We model the calorimeter components and the experimental conditions, and compare the simulation with data obtained experimentally. Results: We find that the simulated temperature history wave form agrees with experimental data if we include the forced convection treatment of the water external to the detection vessel. This is done by artificially raising the thermal conductivity of the water in the stirred region, simulating the actual magnetic stirrer deployed in the phantom. We also investigate artificially constructed scenarios that are difficult to achieve experimentally but offer closed‐form solutions. We decompose both the spatial and temporal components and find that the simulated results indeed agree with the analytical calculation. We proceed to add up all the components such that they represent the results under realistic conditions. Conclusions: Finite‐element modeling of a water calorimeter provides insights into the effects of heat transfer on the dose determination. The systematic response observed experimentally over a large time scale can be reproduced in the model. When broken up into components, the simulated response can be calculated analytically.
SU‐FF‐T‐393: Source Measurement for Electron Monte Carlo Calculations
Authors: Antolak J , Weinberg R
Published: July 2006

Purpose: Monte Carlo calculations can reproduce most measured data for electron beams with very good accuracy. It should be possible, and arguably desirable, to be able to accurately reproduce all measured data with only knowledge of the linear accelerator geometry and the initial electron source; however, information regarding the latter is not as readily available. The purpose of this study is to study the properties of the electron beam close to the electron source. Method and Materials: Radiochromic film was inserted into a film holder consisting of two thin pieces of polystyrene, and attached to the distal side of the linear accelerator scattering foil assembly. For each electron energy, films were exposed to 50 and 100 MU. The films were scanned with a resolution of 89 microns, and images were converted to dose using film strips irradiated to known doses. For Monte Carlo calculations of the same geometry, the initial source was assumed to be Gaussian with a FWHM of 1.5 mm, and a phase space was collected between the polystyrene slabs. It was assumed that electron fluence was proportional to the film dose. Results: Agreement between measurement and calculation was generally very good, but better for higher energies. Varying the initial electron source in the Monte Carlo calculations showed that these profiles are relatively insensitive to the initial source size; profiles are dominated by scattering in the foil assembly. Conclusion: Because of the insensitivity of the profiles to exact details of the initial electron source, it may not be possible for these measurements to resolve the details of the initial source parameters. The results may have also been affected by the assumption that the electron fluence was proportional to film dose; therefore further calculations of the dose to the film (rather than fluence) need to be done.
SU‐FF‐T‐395: Stereotactic Body Frame Apparatus
Authors: Papiez L , Timmerman R , Chien S
Published: July 2006

Purpose: To improve immobilization and setup accuracy of patients undergoing extracranial radiosurgery (SBRT therapy). Make possible SBRT treatments of oversized patients. Method and Materials: Advantages of frame based extracranial therapy are: (i) Reduction of the imprecision of positioning. (ii) Reduction of the geometrical treatment error in case frameless delivery tools malfunction. (iii) Efficient and reliable method of abdomen pressure for decreasing tissue motion effects. (iv) Comfort positioning that diminishes body movements caused by fatigue or pain manifest for an extended period of treatment. To utilize advantages of frame based approach to extracranial radiotherapy, and to also address some of the shortcomings of the existing immobilization devices, the new stereotactic body frame (SBF) has been constructed. The frame has been designed to be light, strong and rigid. The frame allows minimizing the disturbance of the penetrating radiation and facilitates its easy handling by therapists/physicists/physicians. The frame displays necessary rigidity to minimize errors associated with potential deformation. For these reasons the frame, as well as all accessories, have been all built from carbon fiber. The system is additionally equipped with the multimodal marker sources allowing synchronizing images of frame and patient body from a variety of imaging platforms. In summary, the new SBF frame allows dealing effectively with: (i) patient size limitations, (ii) abdominal pressure adjustments and pressure monitoring, (iii) stability and positioning accuracy of the measuring arc, (iv) fixation mechanism connecting frame to the couch (to utilize automatic couch motion capability), (v) facilitating extended distance SAD treatments. Results: The device satisfying considerations discussed above have been constructed. The device's parameters and accessories have been evaluated and its clinical applications defined and verified. Conclusion: The new carbon fiber immobilization frame for SBRT treatments is ready for clinical evaluation. Conflict of Interest: Research supported by a grant from Varian Medical Systems.
SU‐FF‐T‐397: Stereotactic Radiotherapy: Varian 120‐Leaf MLC Verses Radionics' MMLC
Authors: Jaywant S , Kagan E
Published: July 2006

Purpose: With the continuing rapid development of technology, more options are becoming available for the clinician. However, this is bringing forth technical challenges to the Physicist, especially in the integration of these technologies. The most commonly used Varian linear accelerator that has the 120 leaf MLC attached to its head prohibits the use of an additional Radionics' MMLC. Hence the 120 leaf MLC was integrated into the Radionics' XKnifeRT system to be used in both cranial and extracranial applications. This presentation provides technical information and clinical experience using this combination. Methods and Materials: The stereotactic technology utilizes the Radionics' XKnifeRT system consisting of a) treatment planning software capable of both 3D conformal as well as step and shoot IMRT; b) stereotactic frames for cranial and extracranial applications. The Varian 120 leaf MLC with an inner leaf width of 5 mm was integrated into the XKnifeRT system so that either the 120MLC or the MMLC could be used clinically. The limitations in the use of the MMLC in this configuration include reduced clearance, especially for extracranial applications, and a reduced maximum field size of 9 × 11 cm. Thus far nearly 40 patients have been treated, mostly using the IMRT component, and include GTC/TLC for cranial sites, and, Head & Neck Localizer(HNL)/Body Localizer(BL) for extracranial sites. Results: A comparison of dose distributions/DVHs of the 120 MLC and the MMLC indicate only marginal improvement if the MMLC were to be used. Not only is the setup time at the linac reduced, the actual IMRT delivery is only a few seconds more than a corresponding dynamic delivery. A cranial and a C‐spine application of the technology validates this integration. Conclusions: We continue to utilize the 120 leaf MLC successfully for all stereotactic applications with XKnifeRT.
SU‐FF‐T‐399: Study the Difference Between IMRT and Forward Planning in Designing the Flash Required in Breast Treatemnet
Authors: Abdulhay I , Boulay L , Kedroske E , Khalifeh A , Fitian I
Published: July 2006

Purpose: Study the difference between IMRT and forward planning in designing the flash required in breast treatment Method and Materials: Using the Pinnacle3 radiation treatment planning system to deliver a uniform dose to the Planning Target Volume (PTV) by using both forward planning techniques and inverse planning techniques (IMRT). The most important issue when it comes to IMRT breast treatment planning is to control the flash opening between the breast and the MLC in order to consider the breathing parameter. In forward planning 80% of the dose is delivered by open segmentation, and similarly, in IMRT the percentage of open segmentation can be controlled by drawing a flash target volume by extension of the PTV by 2.5 cm to 3.0 cm depending on the amount of flash desired. This flash target volume is included in the objective list of the IMRT parameters as “Uniform Dose” assigned to a minimum of 80% of the total prescribed dose and a minimal weight contribution such as 1E‐15. In comparing IMRT and forward planning techniques, IMRT not only allows a percentage of flash similar to the forward plan but it also achieves a more uniform dose coverage of the breast both at superior and inferior borders with reduced dose to the surrounding structures such as heart and lung. Results: Using a flash target volume with minimal weighting as a objective parameters allows a higher percentage of each beam to be treated open in the modulated segmentation. Conclusion: For IMRT breast treatment planning, a flash target volume can be used as an objective parameter during optimization to help increase the percentage of segmentation to be treated as open.
SU‐FF‐T‐402: Synchronized Dynamic Dose Reconstruction
Authors: Litzenberg D , Hadley S , Tyagi N , Chetty I
Published: July 2006

Purpose: To present a dose accumulation method that explicitly incorporates real‐time target volume motion, and real‐time machine configuration, MLC motion and fluence state. Method and Materials: Effects of inter‐ and intra‐fraction motion on dose distributions delivered to a target volume are typically estimated by statistical methods that rely on idealized assumptions such as constant random and systematic errors, and constant offsets, periods and amplitudes of motion over the entire course of therapy. In addition, implementation and delivery related issues such as leaf sequencing limitations, spatial interplay between MLC leaf and target volume motion, and temporal interplay of the fluence state in IMRT, are assumed to average out during treatment. To include these effects, a dose accumulation technique is proposed which explicitly incorporates real‐time target volume motion data, and real‐time treatment machine data. Several technologies are becoming available to continuously monitor (10–30 Hz) the patient position, organs‐at‐risk, and the target volume during therapy. Likewise, real‐time (20 Hz) machine configuration, leaf position and fluence state data, is currently available in the Varian DynaLog files. These datasets are synchronized at the beginning of each beam; and the Monte Carlo method, which inherently accounts for time dependence, was used for dose calculations. Results: By synchronizing target volume position, machine configuration, leaf positions and fluence state, with sub‐second resolution, the dose delivered by each beam, may be accumulated. Such accumulated dose distributions reflect the interplay between target volume motion, MLC leaf motion and other machine‐related delivery effects based on real‐time, patient specific, measured data. The delivered dose distribution to date may then be compared to the planned dose distribution to provide input for dynamic refinement of treatment planning and delivery. Conclusion: Delivered target volume dose may be reconstructed using real‐time motion and machine data, and serve as a basis for dynamic refinement of treatments.
SU‐FF‐T‐403: Target Failure and Beam‐On Load in Helical Tomotherapy
Authors: Moros E , Treble C , Yan Y , Jiang H , Weng X , Novak P
Published: July 2006

Purpose: To predict target failure for a Helical Tomotherapy unit based on cumulative beam‐on time. Materials and Methods: Helical Tomotherapy is a novel external beam clinical technology and as such it presents new quality assurance challenges. Our unit started treating patients in April 2004. By February 2006, the x‐ray target had been changed three times (03/05, 09/05 and 01/06). On visual inspection, a damaged disk‐target shows an annulus of severe wear. In one case, the annulus cut through the entire thickness so that the target was split into two parts. Another target had pieces of the outer rim flaked off. Near the failure of a target, machine output changes significantly (>±3%), which requires additional output checks and tuning, and it can also result in treatment interruptions. Although the output seems to decrease most of the time nearing target failure, increases have also occurred. Output variations can be explained by the nature of the target disintegration and its impact on the water‐driven target rotation and cooling. We hypothesized that target failure frequency is related to cumulative beam‐on time. Since a target replacement almost always entails clinical downtime, such a relation would be useful in predicting when a target change should be scheduled. Results: We collected beam‐on data—including daily and patient specific QA procedures but excluding MVCT beam‐on and engineering time—for periods between target changes. The first, second and third periods of 12, 5.75 and 3.75 months had total beam‐on times of 369, 212 and 136 hours, respectively. The average daily times for the same periods were 1.4, 1.68 and 1.65 hours. Conclusion: These data are not yet conclusive but do suggest that lower average daily load impacts on target life stronger than cumulative load. An extended analysis of target wear and beam‐on statistics, including MVCT times, will be presented.
SU‐FF‐T‐405: Temporal Correlation in Monte Carlo Sampling to Evaluate Interplay Between Tumor and MLC Motion in IMRT
Authors: Naqvi S , D'Souza W
Published: July 2006

In radiation therapy, tumor motion due to normal breathing displacements can smear out dose distributions. An additional problem in MLC‐based IMRT is that the phase relationship between the patient's breathing and each MLC segment differs between fractions. For courses lasting only a few fractions, it is necessary to also evaluate the MLC‐tumor interplay effects. However, calculating such effects requires precise values of quantities, such as, the starting breathing phase (τ0), machine's dose rate (DR), interbeam delay (τg) and intersegment delay (τs). Though it may be infeasible to perform time‐dependent calculations for specific deliveries, we can estimate the interplay effect by assuming typical timings involved. In this work, we extend the Monte Carlo Superposition dose engine to analyze interplay effects. The algorithm starts by randomly sampling a beam and field segment. For the segment sampled, the time elapsed until that point is calculated by using typical values of DR, τg, and τs. This time is then correlated with the tumor trajectory, and the isocenter is shifted accordingly. Photons are them sampled from the head and propagated through the collimator and the phantom. Energy is deposited by kernels issued from the photon interaction sites. Different fractions were simulated by using different τ0. We analyzed the interplay effect by comparing correlated simulations with uncorrelated simulations (infinite fraction average). It was found that the interplay effect varies strongly with the dose rate, number of fractions, type of leaf sequencing (close‐in vs. sliding window) and the direction of motion with respect to MLC's. More complex dependencies occur with MU efficiency and segment numbers. The correlated time Monte Carlo sampling method provides a way to ensure that the dose received by the patient will not deviate significantly from the more predictable ensemble average for the chosen treatment dose rate and the number of fractions.
SU‐FF‐T‐411: The Delivery of DMLC IMRT to Stationary and Moving Targets with Variable Intensity Rate Beams
Authors: Papiez L , McMahon R
Published: July 2006

Purpose: Time dependent intensity rate delivery for DMLC IMRT therapy increases the efficiency of IMRT treatments for stationary and moving targets. Method and Materials: The method of time dependent intensity rate DMLC IMRT relies on algorithms relating leading and following leaf velocities over their subsequent positions over the target with the intensity rate (dose rate) of the linear accelerator. Allowing the choice of leading leaf velocity over subsequent points over the target and the choice of beam intensity rate parameters, provided special mutual relationships between these quantities are preserved, allows to deliver treatment plan derived intensity maps to targets while adjusting the amount of radiation to which organs at risk are exposed. This ability to chose, within limitations that assure delivery of given intensity profile over the target, the leading leaf speed and the beam intensity rate, creates a situation when optimized delivery, optimal in the sense of minimal dose being delivered to organs at risk, can be achieved. Considerable technical simplification of the described above optimized DMLC IMRT delivery problem may be implemented if adjustment of intensity rate is treated independently from other parameters and treated as straightforward scaling of the time axis. This approach separates two parameter (functional) optimization problem into two single parameter optimization problems. Results: Two simple examples of variable intensity rate DMLC IMRT delivery are calculated. One concentrates on optimization of the time of delivery in case of stationary target. The other one shows the optimization of the time of delivery for moving target. Conclusion: The method of time dependent intensity rate DMLC IMRT delivery shows considerable potential for improving the efficiency and the quality of IMRT treatments.
SU‐FF‐T‐412: The Development of Multiple Heterogeneous Head and Neck Phantoms for Quality Assurance of Inhomogeneity Correction Algorithm in the Intensity Modulated Radiotherapy and Conformal Dynamic Arc Radiotherapy
Authors: Shin D , Yoon M , Park S , Lee S , Lee S , Lee S , Huh H , Cho K , Shin D
Published: July 2006

Purpose:To quantify the differences between heterogeneous dose estimates from the calculation algorithm of 3‐D treatment planning systems and dosimetry measurements, radiation was delivered in accordance with IMRT and conformal dynamic arc radiotherapy (CDART) QC procedures using multiple heterogeneous head & neck phantoms. Method and Materials: The multiple inhomogeneous head and neck phantoms were constructed by various materials such as water equivalent homogeneous (acryl), air equivalent (cavity), bone equivalent (teflon) materials. The absolute and relative dosimetry were done using pinpoint ionization chamber, film and TLD (Harshaw 100). With custom‐written software modules, the measured and calculated dose distributions were superimposed and compared. Results: The point dose measured at the interface between water and air material region shows about 7 % mismatch with the treatment planning system in IMRT. This dose difference is even increased at the interface between bone and water material region reaches up to 18 % under‐dose comparative to planned value. However, the point dose error at the interface between water and bone material region is relatively small showing about 3.2% over‐dose. In CDART, the distribution verification result shows that there is a mismatch of about 1.5 % of whole points revealing more than 5% of the dose difference between measured values and planning value when the inhomoheneious phantoms are used. Our results suggest that the effectiveness of the imhomogeneity corrections used in IMRT and CDART planning should be evaluated in order to ensure meaningful quality assurance and delivery. Conclusion: The robustness of inhomogeneity correction used in IMRT and CDART treatment planning may be a significant factor in assuring predictable, accurate delivery of IMRT.
SU‐FF‐T‐413: The Deviation of Leaf Position in DIMRT
Authors: Lo C , Huang Y , Huang H , Hong J
Published: July 2006

Purpose: To analysis the deviation of leaf position on fraction size and on dose rate in dynamic IMRT (DIMRT). Methods and materials: The shape of target, which is similar to 60° wedge profile, was drawn at 11.5cm depth in phantom on the Eclipse system. The field size is set as 11.3*16.0 cm2 and energy used is 6 MV. The constraint of optimization of target is kept the same, using single portal, for different fraction sizes which varied from 500cGy/fx to 5cGy/fx. The treatment file is transferred to LINAC for beam‐on at Varian 21EX. The MLC log file is analyzed using MLC dynalogfile viewer 6.2.0. We varied the dose rate during beam‐on for the same treatment file to take the leaf deviation data. Results: Dose rate 300MU/min is using in clinical treatment. It is the least error for dynamic leaf for the biggest fraction size with single portal. The quantity, field width multiplied dose rate then divided by MU, is used to evaluate the average speed of dynamic leaf. The evaluated average speed is 89.2 (cm/min) and 121.1 (cm/min) for 10cGy/fx and 5cGy/fx respectively and the larger speed caused the large average deviation around 0.4mm and 0.6mm respectively. The maximum deviation is 1.0mm and has 37.7% in deviation histogram for the smallest fraction size, 5cGy/fx.

For the same fraction size, 5cGy/fx, the dose rate affected the leaf deviation too. A larger dose rate has larger average leaf speed and a large deviation too. Our data showed the average speed is 242.2 (cm/min) and the average deviation is 0.9mm for dose rate 600MU/min. With this dose rate, the maximum deviation is 2.5mm and has 14.3% in deviation histogram. Conclusion: The positions accuracy is better for lower dose rate even with a large dose fraction.
SU‐FF‐T‐414: The Dose Area Product in Radiation Therapy‐ a New Concept for the Parameterisation of Small Fields
Authors: Poppe B , Djouguela A , Rubach A , Harder D , Willborn K
Published: July 2006

Purpose: To establish a new parameterisation of small fields.

Material and Methods: The traditional parameterisation of a narrow photon field via the central axis dose and the relative transversal dose profile has met considerable methodical complications. These difficulties are to find a sufficiently small detector, to adjust the detector accurately on the axis of the narrow beam and to find a detector material not responding to lateral changes of the electron spectrum within the small field. These obstacles can be avoided by reconsidering the parameterisation of the narrow‐field dose distribution. The new parameter recommended for characterising the absolute dose values in a plane perpendicular to the beam axis is the dose‐area product DAP (the area integral of the dose in this plane). It can be measured with a flat ionisation chamber of large cross section of the sensitive volume. Results and Conclusions: The radial adjustment of the large area chamber is by no means critical. The dose‐area product provides a simple normalisation of the relative transversal dose distribution which can be measured with radiochromic film. We have investigated the abilities of a large‐area flat ionisation chamber of PTW Freiburg (PTW TM 34070‐2,5) of 8,1 cm diameter and 2 mm thickness of the sensitive volume to measure the DAP of narrow photon beams with side lengths up to 5 cm. A modified output factor has been defined as the quotient of the DAP, measured at 5 cm phantom depth for SSD 100 cm distance of the phantom, and the monitor reading. Besides the useful feature of the DAP is its direct measurability during patient treatment by means of the DAVID chamber, an on‐line monitor arranged in the accessory holder, so that non‐negligible deviations of the actual from the chosen field size of narrow photon fields can be immediately detected.
SU‐FF‐T‐416: The Effect of Skin Flash On Skin Dose for Breast IMRT Treatment
Authors: Xu Z , Yang G
Published: July 2006

Purpose: Treatments using tangential IMRT fields for Breast treatment are prone to fluctuation in the body outline area. Deviations may arise from inaccuracies in patient positioning, patient movement, or respiratory motion. The skin flash tool from Eclipse, which visualizes the fluence of the selected field in the BEV, allows the planner to graphically extend the fluence outside the body surface with a paintbrush. The painted extension is then filled with transmission values. The purpose of this work is to investigate the effect of skin flash tool on skin dose for breast IMRT treatment. Method and Materials: A breast phantom filled with tissue equivalent material was constructed to investigate the skin dose. The IMRT tangent fields, which reproduces the two fields traditional setup for breast irradiation, was used to generate IMRT plan based on the CT images. The skin flash tool was used to extend fluence 2 cm outside the breast skin to accommodate the respiratory motion. The skin flash was then filled with different transmission values ranging from 0 to 1 to generate different plans. Skin dose was then measured on these plans using diode. The respiratory‐gated treatment was also measured. Results: Compare to the skin dose through conventional parallel opposing tangential beams with wedges, the variation of skin dose is plusmn;15% with transmission values ranging from 0–1. The transmission value of 0.5–0.6 produced the plan with similar skin dose to the conventional plan. If measurement was done with the respiratory motion, the skin dose variation could be ±25%. However this variation could be reduced to +‐20% if gated treatment was used. Conclusions: The skin flash transmission factor should be kept at 0.5–0.6 in order to keep the skin dose similar to those from the conventional plan. Breast gating treatment minimizes the skin dose variation.
SU‐FF‐T‐418: The Effect of Total MU, Number of Segments, and Field Size On IMRT Point Dose QA Results
Authors: Lasher D , Wojcicka J , Bialkowski M
Published: July 2006

Purpose:To investigate the relationship between Corvus IMRT treatment plan parameters (total MU, number of segments, and average field size) and point dose QA results (ion chamber measurements and RadCalc independent MU calculation software). Method and Materials: The Corvus treatment planing system (TPS) employs a “calibration factor” in its dose calculations. We determined this factor using a set of test plans with an average of 555 MU, 220 segments, and 7.3×7.3cm2 equivalent square field size. The TPS was then used to calculate 95 patient plans. Hybrid plans were created by transferring patient plans to a 30×30×18 cm3 solid phantom. A 0.3cc ion chamber and Kodak EDR2 film were placed inside the phantom. The treatment fields were delivered on a Varian 21EX via the QA Mode of Impac Multi‐Access R&V system. For each plan, dose to the isocenter was calculated with RadCalc and compared to the plan's prediction. Ion chamber and RadCalc percent error values versus total MU, number of segments, and field size are plotted and analyzed with the Pearson's product moment correlation coefficient. Results: The data indicate that for this equipment configuration, both ion chamber measurement and independent calculation percent error results are directly proportional to total MU, number of segments, and average field size. In general, RadCalc predicts a smaller percent error than ion chamber measurements for all three variables. For ion chamber measurement, field size produces the strongest positive correlation with percent error and MU the weakest. For RadCalc percent error, number of segments produces the strongest correlation and field size the weakest. Conclusion: For the Corvus TPS, the measured dose percent error increases as the total MU, number of segments, and field size increases beyond the average value used to determine the calibration factor. RadCalc independent MU calculation software predicts the same trend.
SU‐FF‐T‐420: The Impact of Random and Systematic Errors of MLC Leaves On Head‐And‐Neck IMRT Plans
Authors: Mu G , Xia P
Published: July 2006

Purpose: To investigate dosimetric effect of the random and systematic errors of MLC leaf positions on the IMRT plans for patients with head and neck cancer. Method and Materials: For six clinical IMRT plans, random errors (from −2 to 2 mm) and systematic errors (±0.5 mm and ±1 mm) MLC positioning errors were introduced into the each segment for these plans, simulating the mechanical uncertainties and potential miscalibration of the MLC. The altered MLC segments for each plan were imported to a commercial treatment planning system to evaluate dosimetric changes of the plan. The altered plans were compared to the original plans, based on DVHs and defined endpoint doses. Results: With the up‐to‐2 mm random errors in MLC positions, the dose changes to the 95% of the tumor volumes were (−1.247± 1.153)%. For serial structures, the dose changes to the 0.1 cc of the brainstem and spinal cord were (0.230±0.794)% and (−0.340±1.254)%, respectively. The dose changes to the 50% of the parotids varied from patient to patient, from 0.536% to 11.333%, (1.641±4.274)% and (4.910±5.322)% overall. With systematic errors in MLC positions up to 1 mm, the dose changes to the 95% of the tumor volumes were (−0.078±1.048)%. The dose changes to the 0.1 cc of the brainstem and spinal cord for all patients except one were (0.591±1.058)%. The dose changes to the 50% of the parotids overall were (7.782±3.111)%, beyond 5% limit. Conclusion: The dosimetric changes introduced by the random errors for each leaf within 2mm were not significant compared to the original plans. The systematic error up to 1mm for each leaf did not significantly changed the target dose and the maximum doses to the serial structures while the doses changes to the parotid could be significant. Conflict of Interest: This project is partly funded by SIEMENS.
SU‐FF‐T‐421: The Influence of Field Length, Pitch, and Modulation Factor On the Quality of Helical Tomotherapy Plans
Authors: Bhavin C , Langen K , Meeks S , Zeidan O , Jeswani S
Published: July 2006

Purpose: The planning process for helical tomotherapy plans is governed by three basic planning parameters: field length, pitch ratio, and modulation factor. The purpose of this work is to evaluate the influence of each parameter on the achievable plan. Method and Materials: Using commercial treatment planning software (TomoTherapy, Inc.) the same head and neck case was re‐planned by the same person using different planning parameters. The standard settings (field length of 25 mm, pitch of 0.287 and modulation factor of 2.5) were used to generate a baseline plan. Using the standard settings for two parameters the third parameter was varied for different plans. Three field lengths (10, 25, 50 mm), four pitch ratios (0.2, 0.287, 0.5, 1) and seven modulation factors were used (1.5, 2, 2.5, 4, 6, 8, 10). The plans were evaluated in terms of treatment time, target coverage, conformity, and sensitive structure sparing. Results: Increasing the pitch ratio leads to an increase in the volume of unspecified tissues that receives prescription dose (less conformal plans) and an increase in the dose to sensitive structures. The treatment time did not significantly reduce with increasing pitch (there are fewer gantry rotation but the gantry rotation period increases).

Similarly, an increased modulation reduces the dose to unspecified tissue and sensitive structures. However the cost is an increased treatment time. Using a larger field length increases the dose to unspecified tissue and sensitive structures while reducing the treatment time. All plans achieved acceptable target coverage and homogeneity. Conclusions: Changing the planning parameters mainly affects plan conformity. It appears that with respect to our standard settings a slight reduction of the pitch or a slight increase in the modulation factor should improve plan quality with a modest increase in treatment time. Conflict of Interest: One co‐author is employed by TomoTherapy, Inc.
SU‐FF‐T‐422: The Influences of Detector Energy Dependence and Perturbation On the Determination of Small Field Output Factors
Authors: Yue N , Yang Y , Ding C , Li F , Saw C , Heron D , Huq M
Published: July 2006

Purpose: In radiotherapy, accurate dose determination requires accurate measurements of output factors. The employment of small field size cones in radiosurgery imposes significant challenges for such accurate measurements, since the detector perturbation and its energy dependence, along with the detector size, may introduce significant errors in the output factor determination. This study investigated the impacts on the output factors introduced by the detector perturbation and energy dependence. Method and Materials: Output factors were measured on a Varian Trilogy™ linear accelerator for various collimator defined small square fields and twelve radiosurgery cones that were shipped with the machine. The measurements were performed by using four different detectors, namely, a pinpoint ion chamber, Koda EDR2 films, a stereotactic diode detector, and a pinpoint diode detector. The energy dependence was also evaluated for the ion chamber and diode detectors. Results: Both perturbation and energy dependence present non‐trivial effects in the determination of the output factors. The impact of detector perturbation was more pronounced for the small fields, while that of energy dependence was evident for all the field sizes investigated. For a cone of 5 mm diameter, the perturbation introduced by using a PinPoint ionization chamber could result in almost 30% reduction in the output factor value. The deviations caused by the energy dependence varied from 3% to 6% depending on field sizes. Conclusions: The output factor measurements of small fields must not only account for the size of detector but also its perturbation and energy dependence to ensure the measurement accuracy for stereotactic radiosurgery.
SU‐FF‐T‐423: The Limitation of Patient‐Specific QA in IMRT with Detector Arrays
Authors: Huh S , Cho W , Park Y
Published: July 2006

Purpose: To investigate the limitations of patient‐specific QA (quality assurance) procedures in IMRT plans for H&N and prostate cancer patients. Method and Materials: The detector array with 445 solid states diode detectors has been used to verify dose distributions obtained by a commercial RTP. The IMRT plans were generated using the Helios optimization algorithms with sliding window technique. It was calibrated according to the manufacturer‐recommended calibration procedure. Then, it has been used to investigate (1) the accuracy of dose distributions with rectangular collimator settings, (2) a single field IMRT plan was generated to deliver uniform dose to a thin (2mm thick) square target imbedded in the center of the 30cm*30cm*30cm phantom, and (3) the patient‐specific QA procedures for patients with prostate and H&N cancers. Results: (1)The differences between the plan and the measurements that are larger than “+/− 3%” for conventional rectangular collimator settings were observed along the X‐axis. (2) The penumbras of a IMRT plan with a single field as a part of annual RTP QA routine procedure were investigated and the results were the same as the first case. These dose differences showed similar statistics as the case one. (3) With routine IMRT plan with 8 fields setup, the dose differences (the same definitions as above) shows the similar statistics as those of the second case, but observed to 94% –97% passing rate (with +/− 3mm DTA and +/−3% criteria). Conclusion: The limitations of the detector arrays are investigated and its effects upon the overall passing rates with the same criteria are found to be 3%–7%. But, the dose at the central axis of the routine treatment plans are within “+/− 2%” except in very special cases where the CA is very close to the high dose gradient regions.
SU‐FF‐T‐424: The Measurement of Moving Tissue Maximum Ratio for Dynamic MLC Based Total Body Irradiation
Authors: Ando S , Yi B , Sun M , Sarfaraz M , Hasegawa T , Yu C
Published: July 2006

Purpose: Total body irradiation (TBI) with moving couch technique has been used in our clinic for years. Moving couch TMR for a field size has been acquired by multiplying TMR for a stationary beam and the ‘m factor’ defined as the ratio between moving phantom dose to stationary beam dose for same setup. The ‘m factor’ has been known to be field size dependent and not depth dependent. In order to replace lung and kidney blocks and achieve better dose uniformity using dynamic MLC, comprehensive understanding on the TMR for variable field sizes for an MLC leaf sequence is required. Method and Materials: A thorough measurement on the moving TMR has been performed. TMR's for field sizes of 5cm×40cm, 10cm×40cm, 15cm×40cm, 20cm×40cm and 30cm×40cm were measured both for the stationary and for the moving phantoms. Field sizes are defined at SAD 100cm and TMR's were measured at SCD 170cm for depths dmax to 34cm in 50cm×50cm×50cm water phantom with 6MV photon (21EX, Varian, Palo Alto, CA). Results: TMR's in moving phantom for different field sizes showed not more than 1% difference, while those of stationary phantom showed up to 34.8% difference for different sizes. The m factor which has been known to be a constant for a given field size rapidly increases after 15cm depth. Analysis of the data allowed us to understand the phenomena of measuring a point dose in a moving phantom. The key to the understanding was that both the phantom scatter and primary dose seen at the measurement position does not vary with field size when normalized to the reference condition. Conclusion: TMR's are independent to field size when couch is moving. This suggests that TMR variation does not need to be considered when designing the dose compensator with dynamic MLC for moving couch TBI.
SU‐FF‐T‐425: The Potential for Dose Dumping in Normal Tissues with IMRT
Authors: Reddy N , Mazur A , Sampath S , Osian A , Sood B , Ravi A , Nori D
Published: July 2006

Purpose: To understand the potential for dose dumping in normal tissues (>85% of prescription dose) and to analyze effectiveness of techniques used in reducing dose dumping during IMRT planning. Method and Materials: We reviewed 249 IMRT plans for 53 patients with H&N, cervix and prostate cancers (6MV X‐rays, 5‐7F), to analyze in how many cases dose dumping occurred and types of techniques used to reduce dose dumping. Gantry angles, depth of beams, duration of optimization, severity of dose‐volume‐constraints (DVC) on normal structures, and spatial location of PTV were reviewed in relation to dose dumping. In addition, the effect of not contouring part of rectum in beam's path on dose dumping was studied. Results: Dose dumping occurred at Dmax depth in 16 pelvic cases (85%–129%). This was related to: 1) depth of beams, 2) narrow PTV and 3) reduced presence of rectum and bladder in beam's path. Dose dumping could be reduced to 85% by changing beam angles and/or DVC for normal structures in 4 cases and by creating phantom structures in 12 cases. Decreasing iterations also reduced dose dumping and MUs. Part of uncontoured rectum, if present in the field, received a higher dose than the contoured rectum with DVC, indicating that complete delineation of normal structures and DVC is necessary to prevent dose dumping. In H&N, when PTV extends inadvertently into air beyond the body even by a few millimeters, dose dumping occurred in beam's path (170%, 7F plan). Keeping PTV margins within body contour reduced this type of dose dumping. Conclusions: Beamlet optimization, duration of optimization, spatial location of PTV and DVC on PTV and normal structures, with the potential to cause dose dumping, are an integral part of IMRT inverse planning. Use of appropriate technique/s would reduce the dose dumping and time needed to obtain optimum plan.
SU‐FF‐T‐426: The Response of RadioChromic EBT Film in High‐Energy Electron Beams
Authors: Gerbi BJ , Han EY
Published: July 2006

Purpose: To describe the response of a new radiochromic film to highenergy clinical electron beams. GAFChromic® EBT film was designed primarily for verification of high energy photon IMRT treatments. It has been shown to be a very accurate and convenient photon detector. However, there is no literature on this film's response to high‐energy electron beams. We initiated a study to characterize the response and utility of this film for routine electron beam dosimetry. Method and Materials: The linac was first calibrated to ensure that the delivered dose was known accurately. We exposed a series of EBT films at dmax in polystyrene to 6, 9, 12, 16, and 20 MeV electrons to develop standard characteristic curves. All films were from the same batch. We also exposed the EBT films in a solid water phantom to produce central axis depth dose curves. These data were compared against percent depth dose curves measured with either Kodak XV2 film in a solid water cassette, or in a water phantom using an IC‐10 ion chamber or a PTW electron diode. Results: Results showed that the characteristic curve for this film was the same for electron energies from 6–20 MeV. Also, the response of the EBT film along the central axis of the beam matched the central axis percentage depth dose characteristics (R50, Rp) determined using Kodak XV2 film, electron diodes, and corrected ionization chamber measurements. Conclusion: GAFChromic® EBT film is shown to be an accurate detector for clinical electron beam applications. It is convenient to use since the solid water film cassette can be loaded in the linac room and requires no post‐irradiation processing. It gives the same results for electron dosimetry as silver halide films, diodes, and ionization chambers. Research sponsored by International Specialty Products corporation.
SU‐FF‐T‐427: The Significance of Higher Dose Rates for Stereotactic Body Radiation Therapy
Authors: Hamidian H , Thames H , Story M , Timmerman R , Forster K
Published: July 2006

The dose rate effect (DRE) has been much studied in the past, but has been less investigated in the simultaneously high fractional dose (HFD) and high dose rate (HDR) regime. These are circumstances akin to stereotactic body radiation therapy (SBRT) treatments. We have studied the possible significance of higher dose rates for SBRT fractionation regimens by analyzing model predictions based on the extensions of the linear quadratic (LQ) model and the lethal‐potentially lethal (LPL) model. Previously reported data for seven human tumor cell lines have been used in this analysis. We have also analyzed possible modifications due to oxygenation for mixed populations of oxic and hypoxic cells. We have found that considerably higher cell kill levels are predicted to occur in the HFD and HDR regime for most of the cell lines that we have studied for both uniformly oxic and mixed cell populations. These model predictions suggest that some human tumor cells may exhibit significantly higher radiosensitivity in the HFD and HDR regime. This is pertinent to the implementation of SBRT and its fractionation regimens and, if shown to persist clinically, could lead to significantly higher tumor control.
SU‐FF‐T‐432: Time‐Delay Measurement of a Varian Real‐Time Position Management System
Authors: Hanna G , Wang Z , Yin F
Published: July 2006

Purpose: Respiratory gating systems have been developed to reduce treatment uncertainty caused by organ motions related to respiration. The time‐delay between when the gating signal is triggered and when the beam is turned on may affect the actual radiation dose delivered. In this study, we investigated the time delay for a Varian real‐time position management (RPM) respiratory gating system. Method and Materials: A motion phantom with infrared markers was used to simulate a respiration signal. A small metal target ball was also attached to the phantom. An infrared camera was used to observe the motion of the phantom and to plot out a true motion curve. A narrow 2% phase gating window was set at various phases of 0%, 10%, 20%… and 90%. Electronic Portal Vision images were taken with a Varian Linear Accelerator at various phases of the simulated breathing cycle, and the location of the target ball was then plotted verses time to give a measured motion curve. The shift in time between the RPM gating signal and motion curve from portal imager was then measured to determine the time delay in the gating system. Results: The time‐delay in Varian's gating system was found to be 0.0173 seconds. Conclusion: This work shows that there was a short time delay between the gating signal and the time when the radiation beam is turned on. The effect of time‐delay on patient dosimetry needs to be further investigated.

Partially supported by Varian research grant.