bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐31: A New Table‐Top System for Extended Characterization of Brachytherapy Radioactive Seed Batches
Authors: Douysset G , Roy H
Published: July 2007

Purpose: To improve the quality control of brachytherapy radioactive seeds. Method and Materials: It is estimated that almost 10 million brachytherapy sources for implants (125‐I, 103‐Pd, 131‐Cs) are produced every year all over the world. Source manufacturing is a complex process due to the combination of the small size of the sources and their sophisticated design. Quality control of the seeds is therefore an important issue. The control should first include calibration but several other tests such as a check of the actual internal structure of the seed is relevant.

An innovative system (patents pending) has been developed in order to fully characterize a batch of sources. It is a compact table‐top device which is able to perform the following tests for each source: visual inspection, source calibration (through low energy x‐ray spectrometry), monitoring of radiochemical impurities (e.g. 126‐I or 137‐Cs) by γ spectrometry, control of the internal structure of the seed (dimensions, presence of high‐Z markers) by micro‐radiography, control of the distribution of radioactive material by autoradiography and possible control of materials purity by XRF analysis. Results: A prototype instrument has been built. A source can be fully characterized within 30–40 s and sorted according to the results delivered by the various instruments. All tests are performed without any user's radiation exposure. Conclusion: This automated system allows a better characterization of large batches of sources and can possibly lead to improvement of the manufacturing processes. A simplified version of the instrument could be implemented at the end‐user's level allowing therefore a fast automated 100% assay of sources prior to implantation.
SU‐FF‐T‐35: A Practical Monte Carlo Method for Modeling Dose Deposition in Intensity Modulated Radiation Treatment Planning
Authors: Aleman D , Karakayalı I , Dempsey J
Published: July 2007

Purpose: To determine dose deposition coefficients for a beamlet with improved accuracy using a limited Monte Carlo method that will be practical for use in a clinical setting, as well as in IMRT optimization techniques, which require smooth dose models. Method and Materials: To model the depth dose curve and lateral penumbra, smoothing functions are applied to the data from Monte Carlo experiments with histories ranging from one million to one billion. The quality of each approximation is determined by comparison to the approximation obtained from the one‐billion history experiment. The depth dose curve is approximated by the most accurate polynomial fit from polynomials with degrees ranging from 10 to 45. Such high‐degree polynomials are able to capture the behavior of the depth dose curve, even in the presence of heterogeneous tissues. The lateral penumbra can be modeled as the average of several error function pairs. A Levenberg‐Marquardt quasi‐Newton minimization method is employed to determine the parameters of each error function, as well as the number of error functions required. This method is applicable to both homogeneous and heterogeneous mediums. Results: Examination of the models obtained for both the depth dose curve and the lateral penumbra show that for both homogeneous and heterogeneous mediums, the models obtained provide satisfactorily accurate approximations using as few as 100 million Monte Carlo histories. Conclusion: This work indicates that it is possible to obtain very accurate dose deposition coefficients using Monte Carlo techniques in a reasonable amount of time using the smoothing functions methods presented. Because the resulting dose model is smooth, in contrast to the inherently noisy Monte Carlo data, the dose model is suitable for optimization techniques. Further testing and refinement could lead to even fewer required Monte Carlo histories.
SU‐FF‐T‐36: A Preliminary Comparison of a New Fluence Benchmark for Clinical Electron Beams with Fluence Calculated with a Commercial Planning System
Authors: Faddegon B , Traneus E , Chen J
Published: July 2007

Objective: To compare a newly developed benchmark of electron fluence in clinical electron beams with fluence calculated with a commercially available treatment planning system. Materials and Method: An accurate and detailed benchmark of fluence for clinical electron beams is under development. The benchmark is based on detailed measurements of the source, geometry, relative output factors and dose distributions of clinical electron beams of a single clinical linear accelerator, including the largest available field with no applicator, along with a precision Monte Carlo simulation of the treatment head. Fluence is divided into direct electrons and those scattered from collimators, including the applicator and insert. This benchmark is used to determine the accuracy of fluence calculated with a beam model in a commercial treatment planning system commissioned with measurements done on the same accelerator used to develop the clinical benchmark. Results: Energy and spatial distributions of fluence for direct and indirect electrons are generally in good agreement between the benchmark data sets and the commercial beam model. Differences include lower resolution in the beam model than in the actual beam in the penumbra and differences in the relative contribution of direct and indirect electrons to the total fluence. Conclusion: The new benchmark is very useful in determining the accuracy of models of clinical electron beams. The benchmark is not only an excellent benchmark for the large, open field, which is seldom used for treatment. It is useful for any applicator and SSD, assuming Monte Carlo can accurately account for the jaws, applicator and insert in the somewhat simplified geometry used in the simulation. The work, currently limited to the linac from a single vendor, is to be expanded to included linacs from other vendors.

Support from NIH R01 CA104777‐01A2 and Nucletron.
SU‐FF‐T‐39: A Robust Method for Megavoltage X‐Ray Spectra Reconstruction Using Attenuation Measurements
Authors: Manciu M , Vulcan T , Manciu F , Nes E , Waggener R
Published: July 2007

Purpose: To propose a novel reconstruction algorithm which is accurate even for transmitted data affected by the usual experimental error, and therefore can be used in a typical clinical environment. Method and Materials: Ionization chamber and radiologic films. Results: Because the attenuation coefficients in various materials depend, in general, on the wavelength of the incident radiation, it was suggested long ago (Silberstein 1932) that attenuation measurements at various depths could be used for determination of the spectral distribution of the X‐ray sources. While mathematically straightforward, the inverse problem is strongly ill‐conditioned and many efforts have been since devoted to circumvent the inherent difficulties of the spectra reconstruction. The numerical instabilities of the inversion techniques are usually extremely large when the transmission measurements are affected by experimental error, of magnitude typical for a clinical environment (e.g., of the order of a few percents). For example, a solution based on Singular Value Decomposition (the Moore‐Penrose Inverse) is reasonable accurate (within 5%) if computer simulated transmission data are used; however the estimated spectra are wrong by orders of magnitude when 1% random Gaussian noise are affecting the transmission data. It is shown in what follows that a novel reconstruction method, based on a Simulated Annealing algorithm, coupled with a special requirement for the continuity of the reconstructed function, generates excellent results even when relatively large levels of noise are affecting the transmission data. Conclusion: The robustness and the simplicity of the method make it an ideal candidate to accurately estimate the X‐ray spectra of the therapeutic sources in a clinical environment, using ionization chamber and radiologic films for attenuation measurements.
SU‐FF‐T‐47: A Study of Homogeneity Parameters in Low Dose Rate Interstitial Brachytherapy
Authors: Chaswal V , Henderson DL , Thomadsen BR
Published: July 2007

Purpose: In this work we investigate four homogeneity parameters to determine a suitable parameter for dose homogeneity optimization. Method and Materials: A new version of the Adjoint‐based Greedy Heuristic (GH) 3D optimization algorithm is used to generate treatment plans for low dose rate prostate brachytherapy. Treatment plans are generated for seed‐strengths varying from range 0.2mCi to 1mCi for I‐125 BEST 2301 seed model. Each plan is designed for 98% target coverage. Four main homogeneity parameters‐Conformation number (CN), Dose non‐uniformity ratio (DNR), Uniformity index (UI) and Homogeneity index (HI) are quantified and analyzed against varying seed strengths. The local homogeneity parameter is defined as the value of a homogeneity parameter at any instant as the seeds are added during the treatment plan optimization process. Results: An analysis of the final values of the homogeneity parameters using varying seed strengths reveal a strong relation between the CN and the seed strength used in an implant. The HI, UI and DNR parameters fluctuate greatly. Using a fixed source strength, a study of the local CN reveals that, as seeds are added, the CN increases to a maximum value and then decreases as more seeds are added. Conclusion: Based on our study, homogeneity parameters such as HI, UI or DNR can be useful in a dose homogeneity‐optimization routine using a pre‐decided source‐activity. An optimization routine can also aim at searching for optimum activity of the seeds to be used in an implant. For such an optimization routine CN based parameter (viz. CN/DNR) can be useful.
SU‐FF‐T‐48: A Study of the Impact of X‐Ray Energy and Angular Distributions Produced From 6‐ and 18‐MV Medical Linear Accelerators On Non‐Target Exposures
Authors: Hanlon J , Bednarz B , Xu X
Published: July 2007

Purpose: Recently two AAPM task groups have identified the need for more detailed data on the energy and angular distribution of bremsstrahlung X‐ray photons produced from the target of medical linear accelerators. Previous studies had focused on primary photon beam toward the patient. On the other hand, X‐ray photons produced at large angles are responsible for neutron activations in the surrounding of the accelerator head and for the secondary radiation dose to the patient outside the treatment volume. This paper presents our effort to create a database on angular X‐ray energy and fluence distributions from a medical linear accelerator outside the primary beam direction. Method and Materials: Parameters for a Varian Clinac 2100C operated in 6‐ and 18‐MV were modeled using the MCNPX code. Energy and 360° angular distributions of photons and electrons emanating from radiotherapy targets were determined using MCNPX. A tally sphere was placed around the target in order to characterize the full 4π emittance of these particles from the target. The tallies were binned according to energy and the directional cosine from a fixed vector pointing perpendicular to the target face. Results: Both the 6‐ and 18‐MV photon beams are mostly emitted in forward direction and are predominately in low energies. A low energy peak in the backward direction is also seen in the 6‐MV distribution. The electron distribution emanating from the target for the 6‐MV platform is highly backward directed, while the electron distribution from the 18‐MV platform is mostly low energy and has a forward and backward directed contribution. Conclusion: The results in these studies are consistent with data from previous studies. A detailed database on angular X‐ray energy and fluence distributions from medical linear accelerator outside the primary beam direction will allow the sources of the secondary photonuclear interactions to be located and mapped.
SU‐FF‐T‐49: A Study of the Transit Dose for the Leksell Gamma Knife Model 4C Using Diode and GafChromic Film in Air and Phantom
Authors: Mota H , Austerlitz C , Gay H , Allison R , Sibata C
Published: July 2007

Purpose: To evaluate the undesirable dose to the patients due to uncollimated beam occurring in the Gamma Knife Model 4C before and after helmet docking, using diode and GafChromic film. Material and Method. The Leksell Gamma Knife model 4C was used for this study. The transit dose was measured along the Z axis (Leksell coordinate system) in air and inside the Elekta plastic phantom using diode and EBT GafChromic film. The detectors were calibrated against an Exradin‐A19 0.007 cm3 ionization chamber calibrated for AAPM‐TG‐21 protocol. The electrometer was the Max4000 (Standard Imaging). A Microtek scanner 1800f was used to scan the films. For the measurement in air, a 30 cm long plastic tube was mounted in air along the Z axis inside the helmet allowing the diode to be moved in all its extension, including the cap. The film were sandwiched in two half plastic cylinder mounted inside the helmet. A special cassette made of Mix‐D (density 0.99 g cm−3) was constructed to allow the diode slide inside the phantom along the z axis. The diode measurements were done for different exposure times, to obtain the dose for time zero by extrapolation. Results. The highest transit dose was observed in the region inside the helmet, with distribution approximately constant of 5.70 ± 0.12 cGy/shot. Inside the helmet the dose reaches another plateau of 4.72 ± 0.02 cGy/shot (10 to 15 cm from the isocenter). When the diode was positioned at the inner surface of the phantom, no significant difference from the in‐air measurements was obtained. The film results presented higher values since they were obtained with several shots and includes scatter radiation. Conclusion. The results show that a significant dose may result for several shots and confirm the values published for the model C, obtained with different methodology.
SU‐FF‐T‐50: A Systematic Approach to Practical Multi‐Criteria IMRT Planning
Authors: Craft D , Halabi T , Shih H , Bortfeld T
Published: July 2007

Purpose: To describe and demonstrate a new practical multi‐objective treatment planning procedure for intensity modulated radiation therapy (IMRT) planning. The approach developed addresses the question of how to generate a useful database of Pareto optimal treatment plans without time consuming human iterations. Method and Materials: The creation of a database of Pareto optimal treatment plans proceeds in two steps. The first step solves an optimization problem which finds a single treatment plan which is close to a set of clinical aspirations. The results of this step are then used to determine mutually satisfiable hard constraints for the subsequent generation of the plan database. Healthy structures are modeled with piecewise linear approximations of EUDs, and targets are modeled with a linear underdose penalty term. Optimizations are done using linear programming. Results: The method is successfully applied to a variety of clinical cases, including a brain, prostate, and lung case. It is shown that the databases generated are both diverse and filled with predominantly clinically interesting plans. Conclusion: The proposed method brings Pareto surface‐based multicriteria IMRT planning closer to clinical usability. With this method, we begin to fulfill the aims of multicriteria IMRT planning: reduce the human‐iteration time common in IMRT treatment planning and make the trade‐offs involved in treatment planning explicitly known.
SU‐FF‐T‐51: A Technique for Verification of Isocenter Position in Tangential Field Breast Radiotherapy
Authors: Prabhakar R , Rath GK , Ganesh T , Joshi RC , Julka PK
Published: July 2007

Breast irradiation is one of the most challenging problems in radiotherapy due to its complex shape of the target volume and its proximity to the surrounding normal structures. Proper implementation of the planned parameter during treatment is very essential, especially, verification of the planned isocenter position. Small deviations in the positioning of the patient with regard to the beam set up could have a relatively important impact on the treatment volume. Hence, treatment verification and reproducibility of the breast treatment portals is an important step in breast radiotherapy. A simple technique has been proposed in this study to verify the planned isocenter during treatment using electronic portal imaging device. Ten patients were recruited in this study and CT based planning was performed with conventional tangential field technique. For verification purpose, in addition to the standard medial (F1) and lateral (F2) tangential fields, a field (F3) perpendicular to the medial field was used for verification of the treatment portals. Lead markers were placed along the central axis of the two defined fields (F1 & F3) and the separations between the markers were measured on the portal images and verified with the marker separation on the digitally reconstructed radiographs (DRRs). Any deviation will identify the shift in the planned isocenter position during treatment. The average deviation observed between the markers measured from the DRR and portal image was 1.6 mm and 2.1 mm with a standard deviation of 0.4 mm and 0.9 mm for field F1 and F3 respectively. The maximum deviation observed was 3 mm for field F3. This technique will be very useful in patient set‐up for tangential breast radiotherapy.
SU‐FF‐T‐52: A Test for Water Equivalence of Solid Phantoms in the Kilovoltage Photon Energy Range
Authors: Hill R , Brown S , Baldock C
Published: July 2007

Purpose: To test water equivalence of solid phantoms in the kilovoltage photon energy range Method and Materials: Three solid water‐type phantoms and perspex were compared with water. Technetium‐99m radionuclide was confined within and collimated by a lead container to produce a narrow beam of photons of 5 mm diameter. The transmitted gamma rays were measured as a function of phantom thickness using a Phillips Skylight gamma camera. An energy window of ±10% was set to exclude low energy scatter. This setup was modeled within the EGSnrc Monte Carlo code V4.2.2 and the FLURZnrc user code. The total fluence was scored after passing through the different phantom thickness. The linear attenuation coefficient was calculated from the resultant transmission curves and compared with published NIST data using the XCOM program. Results: The measured transmission values for the Plastic Water, RMI‐457 Solid Water and RW3 phantoms were in good agreement with those for water, with a maximum difference of 1.5%. However the transmission values through perspex differed by up to 4% as compared to water. The agreement between EGSnrc and measured transmission values was good, with a maximum difference of 1.3% for all phantom materials. The linear attenuation coefficients derived from measurements and EGSnrc calculations agreed to within 2.4%. For all phantom materials, the NIST linear attenuation coefficient was greater than the measured and EGSnrc calculated coefficients. Differences between experimental values and published NIST data are attributed to the energy resolution of the scintillator crystal within the gamma camera and limitations in the source description for the Monte Carlo calculations. Conclusion: This work has demonstrated a simple method for testing water equivalency of solid phantoms at low photon energies using gamma ray transmission values determined from measurements and Monte Carlo calculations.
SU‐FF‐T‐55: A Treatment Planning Study Comparing Active Scanning Beam Delivery and Passively Scattered Beam Delivery in Proton Therapy
Authors: Wang N , Coutrakon G , Patyal B , Miller D
Published: July 2007

Purpose: Treatment planning study for proton passive, active beam with and without bolus and aperture. Method and materials: Most beam delivery systems in proton therapy use passive beam spreading techniques to conform the dose to the target. Chordoma, meningioma, GBM and other head and neck cases are routinely treated with passively scattered beams at many proton centers. Patched fields are often used in sparing normal tissues surrounding the targets. This is achieved by matching the distal edge of one beam with the lateral edge of another beam to get good target coverage and reduce normal tissue doses. Boluses are used to compensate for the distal contour of the target and tissue density variations upstream of the target. This modality offers distinct advantages over conventional (X‐ray) radiation therapy, viz., no dose to the normal structures distal to the tumor and easy avoidance of normal structures surrounding the tumor; resulting in low integral dose to the patient. Passively scattered proton beams offer limited treatment field size, necessitating the use of matching fields to cover larger targets, e.g., medulloblastoma, and cranio‐spinal irradiation. Also there is increased risk of neutron contamination for passively scattered proton beams. Active beam scanning eliminates these limitations. This technique can, in principle, be used without the use of boluses and apertures. However, comparing with passively scattered beam, the increased penumbra of the active scanning beam for a shallower target may compromise treatment delivery by giving increased dose to the healthy tissues. Results: In this study, we compare the passively scattered beam and active scanning beam plans for the extra‐medullary plasmacytoma of the right maxilla, a head and neck tumor. Conclusions: A significant reduction in dose to healthy tissues can be achieved by using the scanning beam technique in conjunction with a limited use of apertures and boluses.
SU‐FF‐T‐56: A Virtual Frame System for SRS Planning
Authors: Tryggestad E , Christodouleas J , McNutt T , Kleinberg L , Purger D , Daniele R , Wong J , Ford E
Published: July 2007

Purpose: Stereotactic radiosurgury with the Gamma Knife (GK) (Elekta, Stockholm, Sweden) is oftentimes unnecessarily complicated by sub‐optimal placement of the stereotactic frame; in some cases, treatments must be aborted. The GK treatments are frequently prolonged by the time required for Leksell Gamma Plan (LGP) RTP. We are developing a virtual frame tool for GK SRS which will 1) provide the neurosurgeon with visual and quantitative guidance for optimal frame placement and 2) facilitate the creation of preliminary treatment plans within LGP using pre‐treatment or non‐stereotactic MR studies. Method and Materials: A Gamma Knife virtual frame (GKVF) placement system exploits the framework of the Pinnacle3 treatment planning system (Philips Radiation Oncology Systems, Fitchburg, WI) which provides the capability to load and subsequently manipulate externally‐created triangular surface mesh objects (in VTK format) within its internal (image‐referenced) coordinate system; natively‐created or manipulated meshes can be exported in these internal coordinates. We have created, e.g, mesh representations of the Leksell frame, including the posts and fiducial indicator box. We have developed interactive Pinnacle scripts and external C++ codes to aid the user in optimizing frame placement. Results: Given a set of lesions to be treated, the GKVF software currently provides the ability to 1) interactively place a 3D mesh representation of the Leksell frame on a patient's MR study and test for clinical tenability using a fast collision detection algorithm — accessible regions for various GK modalities are indicated with computed surface meshes — and 2) “paint” virtual fiducials onto DICOM MR copies of the patient's original study based on the current virtual frame location, which, together with the GKVF system's ability to generate virtual “skull and frame” measurements, facilitates LGP dose planning on the painted image sets. Outlook: Currently under development is a quantitative mechanism to guide the neurosurgeon in actual frame placement.
SU‐FF‐T‐57: Accelerating IMRT Optimization by Voxel Sampling
Authors: Martin B , Bortfeld T , Castanon D
Published: July 2007

Purpose: To develop a new algorithm to accelerate IMRT optimization by voxel sampling. Methods and Materials: We note that standard gradient descent is fairly slow due to the time spent calculating the dose to the patient. We realized that a reasonable estimate of the objective function and gradient could be obtained by calculating the dose to only a fraction of the voxels in the patient. To avoid adding a systematic error, we randomly sample the voxels at each step to create an unbiased estimate of the gradient. Supposing sufficient samples are chosen, the errors in the gradient will tend to cancel each other out as the algorithm progresses.

Uniform sampling is inefficient, however, since small, important organs tend to need a higher sampling rate while large, less critical organs could use a lower rate. We developed an algorithm to tune the sampling rates for each objective by minimizing the variance of the estimate of the objective for a fixed overall sampling rate.

With the errors in the estimated gradient, we could not use techniques like conjugate gradient to further accelerate performance. However, we found that the delta‐bar‐delta algorithm provided an additional speed boost. Results: For our example case of a lung patient with 384,979 voxels and 1460 beamlets we were able to achieve a speed improvement of approximately three times using uniform sampling, 14 times using automatically tuned sampling, and 20 times using delta‐bar‐delta with automatically tuned sampling. While the algorithms are randomized, the results were very reliable and stable in our experience. Conclusions: Voxel sampling proves to be a viable way to dramatically improve the speed of IMRT optimization. It could be particularly helpful for applications such as temporo‐spatial optimization and multi‐criteria optimization, where a great many variants of a problem are optimized.
SU‐FF‐T‐61: Adaptive Planning for Accelerated Partial Breast Brachytherapy Using the Elliptical MammoSite Balloon
Authors: Esthappan J
Published: July 2007

Purpose: To demonstrate that accelerated partial breast brachytherapy using the elliptical MammoSite balloon requires a thorough evaluation of balloon geometry by use of volumetric imaging during the course of treatment, and may require adaptive planning. Methods and Materials: Patient was implanted with 4cm×6cm elliptical MammoSite. CT scans were acquired and sent to treatment planning system (TPS). Balloon and CTV (balloon plus 1cm margin, excluding 0.5cm from skin) were contoured. Dwell positions were activated along the catheter. Dwell times were optimized to deliver at least 90% of prescription dose, 340cGy, to greater than 90% of the CTV. Dwell times and positions were used for treatment of first fraction. Treatment was to be delivered in ten fractions, twice daily. The patient was re‐imaged using CT prior to subsequent fractions. We evaluated the effect of using the original treatment plan for subsequent fractions. The repeat scans were sent to TPS for contouring and catheter entry. Dwell times and positions from fraction one were used to recalculate dose distributions on repeat scans. The resulting plans were compared to that from fraction one in terms of target volume coverage, skin dose (<125% of prescription), volume of tissue receiving 150% and 200% of prescription dose (V150<50cc and V200<20cc). Results: Use of the first fraction plan for treatment of subsequent fractions led to acceptable target volume coverage, V150, and V200. However, skin dose increased due to gradual changes in balloon shape over the course of treatment. Subsequent fractions did require adjustments in dwell time to decrease skin dose. Conclusion: When using the elliptical MammoSite, the appropriateness of using the original plan for treatment of subsequent fractions depends on changes in balloon shape as well as volume, necessitating the acquisition of volumetric images prior to each treatment. Furthermore, volumetric imaging facilitates adaptive planning to account for these changes.
SU‐FF‐T‐63: Amorphous Silicon Based Electronic Portal Imaging Device (aSi‐EPID) in Routine QA Procedures for Dynamic IMRT Technique
Authors: Cordero‐Ramirez A
Published: July 2007

Purpose: To improve the methods applied for evaluating mechanical parameters involved in routine QA for dIMRT, using a series of in‐house developed tests which apply the aSi‐EPID as main tool for verification. Methods and Materials: All measurements were performed using a Linac with energy of 6 MV, equipped with an amorphous silicon imager and a collimator of 120 leaves. The tests were produced using leaf sequence files created in a MLC SHAPER software and transferred to the machine by connection to a USB port. In the comparison procedures, the dosimetric measurements were done with a Farmer type Ionisation Chamber (IC), connected to an Electrometer used in charge mode. For film measurements, films were placed in a phantom for irradiation, and optical density values to define profiles were obtained using a digital transmission Densitometer. All the tests were performed using the EPID positioned at 180 cm from the source, in order to avoid the effect of saturation in the imager. Results: In this work is shown that the main advantage of applying the EPID for routine QA is the reduction of time required for data acquisition and analysis. The data acquired using the EPID provides information equivalent to that acquired with more laborious conventional methods. Furthermore, it produces digital portal images which have some of the important characteristics of other methods. The results obtained using the aSi‐EPID show good agreement with film and I.C, while giving better consistency and reproducibility. Conclusions: Using the EPID of the Linac, it is possible to evaluate the mechanical parameters of the MLC quickly and accurately. From the results presented, although its detection system in is not water equivalent, the aSi‐EPID is an efficient tool for routine IMRT QA, being a faster method than conventional QA used in dIMRT.
SU‐FF‐T‐64: An Alternative IMRT SBRT Treatment Technique Utilizing Solid IMRT
Authors: Cashon K
Published: July 2007

Purpose: To show an alternative method for providing IMRT for SBRT in a more efficient manner. Method and Materials: Hypofractionated stereotactic body radiosurgery has shown very promising results in radiation oncology. At times it is difficult to limit the doses to normal structures using 3DCRT planning only. IMRT can offer a better advantage for obtaining a more optimal plan. With MLC based IMRT the number of monitor units and the limited dose rate created very high treatment times. If using any gating techniques treatment times increase to unmanageable times. Solid IMRT has shown to be a viable alternative for IMRT (Chang, 2004). Solid IMRT has shown that it can provide a more uniform dose for a moving tumor (Ehler, 2006). An investigation in IMRT with brass modulators was investigated as an alternative. Results: Using 3DCRT with a typical 9‐field treatment plan on average necessitated ∼3000MU. If one needed to use IMRT to limit normal structure doses, with MLC based IMRT, approximately 8000MU was necessary. Using a dose rate of 300MU/min and a gating cycle with a duty cycle of 0.4, the amount of treatment time to deliver was close to 60 minutes. Using a compensator based IMRT delivery the number of monitor units needed was around 5000, but since a higher dose rate could be used, treatment times decreased to 21 minutes and 12.4 minutes for dose rates of 600MU/min and 1000MU/min respectively with the same gating technique. Conclusion: The use of solid IMRT for treating SBRT lung lesions with hypofractionated doses clearly showed a lower treatment time for patients, allowing more time for image guides processes and increased comfort for patients.
SU‐FF‐T‐65: An Analytic Investigation of the Effect of Inter‐Patient Heterogeneity On Alpha/beta Ratio Estimates for Tumors
Authors: Schinkel C , Carlone M , Warkentin B , Fallone B G
Published: July 2007

Purpose: To analytically determine the relationship between the α/β ratio that would be obtained by fitting the individual (non‐averaged) tumor control probability (TCP) model to clinical data and the α/β estimate from a fit of the population‐averaged TCP model to the same clinical dose‐response dataset. Method and Materials: Recently, Carlone et al. (Med. Phys., 2006. 33(6): p. 1634–42) published fundamental forms of the population TCP model for the limits of dominant heterogeneity in radiosensitivity, and in clonogen number. In each case, the model is parameterized by γ50 and D50. The individual Poisson TCP model has also been expressed in terms of these geometric parameters. Since the functional forms of these TCP models are similar, approximately the same γ50 and D50 values would be obtained for each model if they were fit to the same clinical dataset. This fact allows us to determine mapping relationships between parameter ratio estimates obtained from fits using the averaged or non‐averaged TCP model. Mapping relationships are determined for the case of dominant heterogeneity in clonogen number, and in radiosensitivity. Results: When heterogeneity in clonogen number dominates a clinical dataset, the individual and population‐averaged α/β estimates are virtually identical. However, for the case where heterogeneity in radiosensitivity dominates, the individual α/β estimate will not be the same as the corresponding population α/β estimate. Conclusion: Heterogeneity in radiosensitivity is believed to be the dominant form among clinical datasets. Hence, our analytic expressions suggest the individual α/β ratio estimate should be different from the population estimate. Because of this ambiguity, we suggest that modelling has limited value in α/β determination; only the clinical hypofractionation trials will have the ability to validate the hypothesis originally put forth by Brenner and Hall (IJROBP, 1999. 43(5): p. 1095–1101) that prostate cancer responds to fractionation as does a late responding tissue.
SU‐FF‐T‐67: An Estimation of Relative Output Factors of 6 MV Photon Beam Using Monte Carlo Simulation
Authors: Saprangsi W , Phaisangittisakul N
Published: July 2007

Purpose: To estimate the relative output factors of 6 MV photon beam using Monte Carlo simulation from a Varian Clinac 2100C accelerator. Method and materials: The model of linear accelerator head was obtained using the Monte Carlo simulation. Each physical component of the accelerator head was modeled by the EGSnrc/BEAMnrc code according to the manufacturer's specifications. The absorbed dose in water phantom was calculated by the EGSnrc/DOSXYZnrc code. In the simulation, the free parameters were energy and radial Gaussian width (FWHM) of an intensity distribution for an incident electron beam on x‐ray target. Optimal values of the beam parameters were determined by a comparison of local doses between the measurement and the calculation. The absorbed dose comparison was done on central axis depth dose and profiles at depth of 10 cm of 20×20 cm2 photon field at 100 cm SSD. The beam parameters were adjusted systematically so that the absolute differences of the local dose were within 1.0 %. The investigation of the relative output factors at 5.0 cm in depth were determined in the simulation for five field sizes; 5 × 5, 10 × 10, 15 × 15, 20 × 20, and 30 × 30 cm2. Results: Our optimal parameters of the incident electron beam for 6 MV photon beam were 6.2 MeV in energy and 1.0 mm in FWHM. The calculated relative output factors, in which the backscatter effect was included, were in good agreement within 1.0 % with the measurement values. Conclusions: In conclusion, Monte Carlo simulation was demonstrated to be an alternative tool for the relative output factors, accompanying the measurement.
SU‐FF‐T‐68: An Evaluation of the Use Factor (U) for Instantaneous Dose Rates for Cyberknife Shielding Design
Authors: Yang J , Wu X , Hasson B
Published: July 2007

Introduction: A small use factor (U = 0.05) has been widely accepted and used in Cyberknife shielding design. Since the use factor is small and the cross sectional area of the beam is small, the overall workload (W) for a given barrier wall to be shielded in any one direction is small. The small U and small W tends to reduce the required shielding when considering the integrated dose. Methods: Given the distance from 1 foot outside of the primary wall to the isocenter is (d) 4.84 m. Primary workload is 250 Gy/week at 0.8 m. The leakage workload is 250 *15 = 3750 Gy/week at 0.8 m with a head attenuation rate of 0.001. For a controlled area the occupy factor (T) is 1, and permissible dose limit for the control area is 10−4 Gy/week, which is 10% of the allowed. From the Equation B = Pd2/ WUT, the number of TVL's required for the barrier is 3.64. However, when this value for the number of TVL's is used and the instantaneous dose rate is determined at the barrier calculation point, the instantaneous dose rate is determined to be 63.5 mr/hr for the 600 MU/min output rate. It was determined from the room survey that the maximum dose rate in the control area was 160mrem/h, and the area of the barrier that has a dose rate of 60mrem/h or higher is around 50cm × 50 cm. Conclusion & Discussion: Using the small Use factor of 0.05 as recommended in NCRP report 151 in the barrier shielding equation for a Cyberknife facility reduces the integral dose to below 10% of the regulatory limits. When considering the instantaneous dose rate in the control area of the unit, the dose rate will be considerably higher than the customary limit.
SU‐FF‐T‐69: An Independent Delivery Quality Assurance Calculation for the Tomotherapy Linac
Authors: Woo M , Ramsey C , Seibert R
Published: July 2007

Purpose: To develop an independent dose calculation method as a QA tool for Helical treatment delivery. Method and Materials: The Tomotherapy linac delivers radiation using a helical delivery method where the gantry rotates around the patient while the couch advances continuously. The MLC consists of 64 leaf pairs, with each opening for a specified duration for each of 51 gantry positions, or ‘projections’. The specification for the MLC leaf patterns for the whole treatment constitutes a ‘sinogram’. The Tomotherapy system provides a Delivery QA (DQA) method whereby the sinogram for the approved treatment plan is used to deliver radiation to a phantom. An ion chamber can be inserted and the measured dose is compared to the calculated value in the DQA plan. The current project is aimed at developing an independent dose calculation method to verify the ion chamber calculated dose shown on the DQA plan, as well as providing a second check of the dose calculation algorithm. The proposed method extracts the DQA plan sinogram as well as other necessary plan information to calculate an expected ion chamber dose. Results: Parameters for a Tomotherapy plan have been extracted and preliminary calculation using a homogeneous cylindrical phantom has been performed, using only the primary beam. Further comparison with the Tomo DQA plan will be presented. Conclusion: Delivery QA is a crucial step for the Tomotherapy linac, where, unlike conventional step‐and‐shoot IMRT plans, MLC field shapes of individual segments cannot be verified relatively easily. The standard DQA procedure requires access to the machine, and for rush plans often poses a problem. The current project provides an independent check as well as an alternate means to verify the integrity of the sinogram before treatment begins, possibly allowing the standard DQA measurement to be scheduled more efficiently.
SU‐FF‐T‐70: An Independent Verification of Dosimetry for the Leksell Gamma Knife® Treatment Planning System
Authors: Weldon M , Gupta N , Kanellitsas C
Published: July 2007

Purpose: To develop an independent and reproducible verification of the dosimetry for the Leksell Gamma Knife® treatment planning system (Gamma Plan). Method and Materials: We have developed a simple method to independently compute point doses for a Gamma Knife® treatment. This was motivated by AAPM Report 46 (Task Group 40), which recommends independent calculation of the dose at one point in a treatment plan. Several authors have suggested methods to independently compute doses for Gamma Knife® treatments. Our method takes a different approach by utilizing measurements of the dose‐rate for an array of points within a polystyrene anthropomorphic phantom while keeping each of the points at the center of the Gamma Knife® focus, and normalizing the measurements to the total source activity. Multiple measurements of the dose‐rates were taken along the Z and Y axes throughout the phantom. We have created a program using the computer code MATLAB which calculates the dose delivered to any point within the patient's skull. The program uses patient and treatment specific information from the treatment plan, as well as measured dose rates in the anthropomorphic phantom, to calculate delivered dose at selected points. Results: Validation of the program was initially performed using a CT of our anthropomorphic phantom that was used to create treatment plans. A maximum error of 2% was observed between point doses computed with our program to those by Gamma Plan. Patient treatment plans were also evaluated with the program, with an error no more than 4%. Conclusion: Our independent dose calculation method for the Gamma Knife® can be easily implemented into a PC based application and used to quickly verify treatment plans before treating patients.
SU‐FF‐T‐71: An Interactive Software Tool for Modifying Outcome Models and Predicting Patient Specific Outcomes in Treatment Planning
Authors: Apte A , El Naqa I , Deasy J
Published: July 2007

Purpose: For patient‐specific radiotherapy outcomes predictions (i.e., NTCP predictions), the choice of model and its parameters is of utmost importance. Our goal is to develop a software tool which will let clinicians conveniently select the model, understand the effects of the model parameter values, and thereafter predict plan specific outcomes. Method: This module has been developed within CERR (Computational Environment for Radiotherapy Research), an open source Matlab‐based software system. Two commonly used NTCP analytical models implemented are the Lyman‐Kutcher‐Burman (LKB) and the Critical volume (CV). The parameters of LKB model include: the exponent for calculating EUD, D50, and the slope parameter m; whereas the parameters of the CV model include the mean relative damaged volume, critical fraction of FSUs, and interpatient variability. Our interactive outcomes estimation module lets the user change the model and parameters and observe the change in predicted outcome simultaneously. We have also implemented a new model to predict radiation lung injury, which is a logistic regression model with two predictors — mean dose to lung (MDL) and the GTV location (GTV‐superior‐inferior position). We developed an interactive nomogram to let users look at the variation in outcome with respect to MDL and GTV‐SI position. Results: The interactive outcomes‐prediction module within CERR was tested on sample plans. The user is presented with slider bars to change the parameter values, or the user can input their own values, and the estimated outcome is simultaneously updated. The interface is simple, user‐friendly, and easily extensible to include other outcomes models. Conclusion: This outcomes prediction module, along with other graphical visualization and analysis capabilities within CERR, may aid clinicians in understanding and using NTCP models for radiation treatment planning.

This research was partially supported by NIH grant R01CA85181 as well as a grant from TomoTherapy, Inc.
SU‐FF‐T‐72: An Intercomparison of RPC Verified IMRT Plans with Ferrous Xylenol Orange Gels and Optical CT Scanners
Authors: Babic S , Battista J , Jordan K
Published: July 2007

Purpose: To verify the dose distributions in an IMRT head and neck treatment in accordance with the RPC protocol by using ferrous xylenol orange (FX) gels and optical CT scanners. Method and Materials: The RPC head phantom was x‐ray CT imaged and an IMRT treatment plan was generated according to RPC prescribed doses. The phantom was irradiated using a Varian 2100EX linac. The RPC dosimetry insert was removed from the head phantom and replaced with an FX cylindrical gel dosimeter. The FX gel was positioned so that its isocentre coincided with the head phantom isocentre, and the residual volume was filled with water and sealed. The irradiation protocol was repeated with two independent FX gels in order to enable dose readout measurement with either Laser or cone beam (Vista™, Modus Medical Inc.) optical CT scanners, respectively. An FX gel from the same batch was irradiated with a 12 MeV electron beam (Varian Clinac 2100C) and absolute depth‐dose curves were used to establish independent dose calibrations for the two gels and scanners. Results: Computed and measured doses at equivalent locations within the RPC dosimetry insert and FX gels (scanned optically with Laser and Vista™ cone beam, respectively) are in agreement and meet the 7% RPC dose difference criteria. In comparable slices, the computed and measured FX gel dose distributions obtained with Laser and cone beam CT scans, respectively, pass the gamma function criteria (5% dose difference, 4 mm distance‐to‐agreement). Conclusion: The RPC head phantom together with FX gels and optical CT can be used to verify dose efficiently in three‐dimensional space to ensure that IMRT treatments are being safely and accurately delivered in line with established protocols. Conflict of Interest: Two of the authors (KJ, JB) have a licensing agreement with Modus Medical Devices Inc. concerning the commercialization of Vista™.
SU‐FF‐T‐75: An Open‐Source Software Tool to Support Film‐Based IMRT Quality Assurance
Authors: Khullar D , Molineu A , Followill D , Bosch W , Willcut V , Simpson T , Ju T , Deasy J
Published: July 2007

Purpose: IMRT QA using phantom measurements requires registration of a phantom CT scan with the original treatment plan, registration of measured dose planes to extracted plan data, and quantitative comparisons. To facilitate this, we have developed a new open‐source film QA tool, the IMRT Quality Assurance Software tool (IQAS, pronounced, I‐case). Method: IQAS was built in MATLAB with the addition of 2D and 3D gamma calculations which were written in C++ and integrated with MATLAB. Results: To perform QA, the phantom film is scanned into a Flexible Image Transport System (FITS) file format. That file is read into the film QA tool and displayed using image information stored in the file header. The film is registered to the known pin prick coordinates. The coordinates are stored in a phantom profile file and the registration is done by manually selecting points on the film. Users can select between custom optical‐density‐to‐dose conversion profiles to compute dose in the film plane. Additional tools provide options to generate plots for dose, changing the color map, or export the film to a DICOM dose file. The integration of film with the 3D treatment plan is done via CERR (Computation Environment for Radiotherapy Research) which is open‐source MATLAB based software. The user registers the 3D dose to coordinates by selecting fiducial points on the CT scan of phantom within CERR. A rigid transformation, which is purely translations and/or rotations, is applied to the 3D data. Once 3D data is transformed, tools available for evaluation include distance‐to‐agreement, and 2D gamma and 3D gamma functions based on new mesh algorithms for adequate speed. The results can be stored in CERR and can later be retrieved. Conclusions: IQAS is a flexible open‐source tool which provides broad functionality supporting 3D dose comparisons for film QA.