bluuebunny/crossref_metadata_2025 huggingface dataset:
TH‐E‐224A‐05: Absorbed Radiation Dose Measurement with a μK‐Resolution Ultrasonic Thermometer
Authors: Malyarenko E , Heyman J , Guy S , Chen‐Mayer H , Tosh R
Published: July 2006

Purpose: To develop a μK‐resolution ultrasonic thermometer for non‐invasive measurements of absorbed radiation dose in water and to characterize the intensity profile of radiation beams used for medical treatment. Method and Materials: Subtle temperature changes in water were measured by monitoring the phase of an ultrasonic disturbance propagating in it. The current system includes a thermally insulated water tank, an ultrasonic transducer, a frequency counter, and a Pulsed Phase‐Locked Loop connected to a PC. The alpha‐prototype was initially tested and characterized experimentally with time‐controlled light pulses, and was subsequently evaluated with radiation heating from a therapy‐level Co‐60 source. The system was subjected to 30 one‐minute, 50% duty cycle radiation exposures; the temperature history was recorded and analyzed. Results: Preliminary Fourier analysis of the temperature changes caused by periodic radiation heating showed that the absorbed dose rate corresponds to 1.80 Gy/min, deduced from a 0.43 mK (±3% 1) per cycle temperature rise in water. The estimated nominal dose rate at 81.6 cm from the source and 3.2 cm below the water surface is estimated to be 1.65 Gy/min. The discrepancy can be attributed to the non‐standard water tank size and incomplete temperature calibration of the alpha prototype at test time. We expect to resolve these issues by equipping the system with a standard water tank and implementing a more advanced calibration procedure. Conclusions: The alpha prototype has been tested in Co‐60 radiation and produced reasonable results. The feedback from these tests has recently been incorporated into the design of a beta prototype. The new system routinely detects less than 10 μK temperature changes in water and shows great promise for precise dose measurements and beam profile characterization. The new calibration procedure does not require external sensors and makes the system more portable and fully self‐contained.

Research sponsored by NIST.
TH‐E‐224C‐01: Generic Source Models for Commonly Used Clinical Accelerator Beams for Monte Carlo Treatment Planning
Authors: Fan J , Li J , Chen L , Price R , Paskalev K , Chen Z , Stathakis S , Luo W , Ma C
Published: July 2006

Purpose: To develop measurement‐based generic source models for clinical beams from commonly used medical accelerators for Monte Carlo treatment planning. Method and Materials: Source modeling and beam commissioning are key elements in the clinical implementation of Monte Carlo techniques. Monte Carlo studies of dose distributions in patients for radiation therapy would benefit from generalized models of accelerator beams especially when the model could be generated by direct measured data. Several previous studies on Varian accelerators have shown that it is possible to derive source model parameters from measured beam data. In this research generic measurement‐based source model is developed where energy spectra are derived from depth dose distributions, fluence distributions are derived from measured profiles and the head scatter information is derived from in‐air output factor measurement. The new photon source model uses a small extended photon source to represent primary photons generated in the target, a large extended source to represent scattered photons from the primary collimator, flattening filter and other linac components, and an extended electron source to represent contaminant electrons emerging from the treatment head. A four‐source model will be used to reconstruct electron beams, which represent direct electrons, contaminant photons and electrons scattered from the first two scrappers of the electron applicator, respectively. Results: and Conclusions: An independent program is developed to generate source model parameters with measurement data automatically. The source parameters can be used by Monte Carlo dose calculation codes directly for phase space reconstruction. Comparisons have been done between dose calculated by Monte Carlo using this source model and measurement data for Varian and Siemens accelerators for different beams and energies. The results show excellent agreement (within 1%/1mm), which means that the measurement‐based source model is acceptable for clinical use.
TH‐E‐224C‐07: Monte Carlo Dose Calculation Procedure Using a 4D Motion Simulating Chest Model
Authors: Zhang J , George Xu X , Shi C , Fuss M
Published: July 2006

Purpose: To apply an anatomically detailed motion‐simulating virtual patient model to the study of external beam treatment planning. Method and Materials: Breathing induced organ motion modeling may be classified broadly into (a) geometry‐based and (b) physics‐based methods. We have been developing a 4D motion‐simulating chest model from a 3D tomographic model of the Visible Human images by varying the shape, size and location of the organs. Anatomical features where from the VIP‐Man model that contains 80 segmented organs and tissues. The Non‐Uniform Rational B‐Splines (NURBS) surfaces of the organs were reconstructed to deform the organs by changing the control points. Clinically measured motion patterns were used to guide the deformation and motion. Four‐field conformal photon beams were simulated for the treatment of a lung tumor case. The energy of the beam was assumed to be 6 MeV and the 4‐field irradiation geometry was assumed to be AP, PA, RLAT, and LLAT. The lesion was simulated in the left lung and the PTV is designed as a sphere of 5‐mm radius. The motion‐simulating model is implemented into the EGSnrc code to calculate the absorbed doses using various non‐registration registration methods. Results: The results showed that the dose to tumor could be up to 40% differences from phase to phase. DVH for this calculation showed less homogeneity for the whole breathing cycle. However, if the beam was gated to one phase, the result showed better homogeneity for target. Conclusion: A 4D chest motion‐simulating model has been developed using the segmented Visible Human images. This study summarized procedures to develop a 4D motion‐simulating chest model and demonstrated the usefulness of the model for Monte Carlo calculations. Possible ways to improve the motion simulation using physics‐based tissue properties available from surgical simulation community are discussed.
TH‐E‐230A‐01: Can Dose‐Volume Parameters Be Replaced with GEUD in the Treatment Planning Process?
Authors: Clark V , El Naqa I , Hope A , Suneja G , Bradley J , Deasy J
Published: July 2006

Purpose: Dose‐volume metrics have often been correlated with outcomes and are often used to evaluate treatment plans. Unfortunately, when used for IMRT treatment planning, dose‐volume metrics are computationally complex (non‐convex) and can warp DVHs near the constraint dose. We investigate whether the generalized equivalent uniform dose (gEUD) can be made to highly correlate with different parts of the DVH curve by tuning the exponential parameter. If so, gEUD may be a smooth and computationally attractive replacement for dose‐volume metrics in treatment planning and evaluation. Method and Materials: We correlated gEUD with various values of its parameter a and clinically applicable dose‐volume constraints. Three datasets were used: lung, esophagus, and prostate, with 219, 263, and 291 patient plans, respectively. We tested values of a between −10 and 10 by intervals of 0.2 and in some cases tested values as low as −40. The dose‐volume constraints tested include: V10, V20, and V30 for lung, V55 for esophagus, and D95 for prostate PTV and lung PTV. Results: For all cases tested, we found a Spearman correlation between 0.917 and 0.989 (mean correlation 0.956) with negligible (<1×10−6) p‐values. Values of a ranged from 0.4 to 3.2 for volume metrics and −7.8 to −27.2 for lung PTV and prostate PTV dose metrics (respectively). Conclusion: There is a significant and strong correlation between dose‐volume metrics and gEUD for the datasets tested. The practical application of this is that for a particular dose‐volume metric, we can find the value of a (the gEUD parameter) with the highest correlation and use the convex gEUD function in place of the non‐convex dose‐volume constraint in the IMRT optimization, thereby allowing optimization to be faster and more able to efficiently achieve a global optimum. Conflict of Interest: Partially supported by NIH grant R01 CA85181 and a grant from TomoTherapy, Inc.
TH‐E‐230A‐04: Total and Partial‐Body Zebrafish Irradiation
Authors: Jeraj R , Rodriguez M , Titz B , Mathias J , Huttenlocher A
Published: July 2006

Purpose: The zebrafish, Danio rerio has in recent years become a preferred model to study human disease. Our aim was to test the new micro‐irradiator for its capability to perform basic radiobiology experiments, in particular to investigate relationship between radiation, apoptosis and inflammatory response. Materials and Methods: A novel micro‐irradiator, which enables high dose radiation of biological samples below 1 mm, was used to irradiate zebrafish embryos at different age postfertilization. Two experiments were performed — total body irradiation and partial body irradiation. In total body irradiation, the embryos were irradiated up to 40 Gy and the amount of surviving neutrophils as a function of time was analyzed. In partial‐body irradiation, only zebrafish embryo tails were irradiated to 20 Gy and time‐dependent apoptotic and inflammatory response was assessed. FITS‐labeled neutrophil‐specific antibody protein myeloperoxidase was used for neutrophil labeling and TUNEL assays were used for apoptosis labeling. Results: In total body irradiation experiments, little effect was observed at earlier time points post irradiation, but a sharp decrease in the number of neutrophils was observed at 72 hours post irradiation (hpi) suggesting that inflammatory cells can be completely ablated. In partial body irradiation experiments the inflammatory response seems to follow apoptotic response. Significant apoptotic and inflammatory activity was observed at few hours after irradiation, which slowly decreased to almost no activity as early as 24 hpi. Conclusions: Our new micro‐irradiator can perform unique radiobiological experiments. Preliminary results investigating relation between irradiation, apoptosis and inflammation indicated severe posttreatment cell ablation effects for total body irradiation and early apoptotic and inflammatory response within 24 hours post irradiation.
TH‐E‐230A‐06: Comparison of Correction and Model Based Dose Algorithms in Lung Cancer Retrospective Dose Recalculation and Treatment Outcome Evaluation
Authors: Acosta E , McShan D , Kong F , Fraass B , Chetty I
Published: July 2006

Purpose: To perform a systematic comparison of the Monte Carlo (MC), convolution/superposition (CS), and equivalent path length (EPL)‐based dose calculation algorithms for the purposes of outcomes modeling in lung cancer treatment planning. Methods: Several treatment plans (originally planned using EPL) from a large database of patients treated on a lung dose escalation protocol were retrospectively recalculated using MC and CS. Doses were computed in the homogeneous (unit‐density) and heterogeneous geometries; homogeneous calculations were used to elicit differences in the beam models To evaluate algorithmic differences due to heterogeneity effects, beam model differences were minimized by adjusting beam weights in the homogeneous plans to achieve the same prescribed dose with each algorithm. These beam weights were then applied to the heterogeneous geometries. Absolute dose distributions were compared using: color‐wash dose difference displays, isodose lines, EUD (for the target) and mean lung dose (MLD) and NTCP (for the normal lungs). Results: For the target, MC and CS‐computed EUDs were in good agreement for both homogeneous and heterogeneous cases, with maximum dose differences of 1.2 Gy noted. Differences between EPL and MC (or CS) were generally much larger, in the heterogeneous plans extending up to 6 Gy. Differences in MLD computed with MC and CS ranged between 2% and 15% in the heterogeneous plans. These differences were similar in the corresponding homogeneous geometries, illustrating the importance of beam model disparities. For EPL, differences in the MLD and NTCP (relative to MC or CS) were much larger in the heterogeneous plans indicating systematic differences in the normal lung dose prediction. Conclusion: Evidence thus far is suggestive that discrepancies in dose computed with EPL and MC (or CS) will lead to differences in correlations of dose with outcome with respect to the target as well as normal tissue complications (radiation induced pneumonitis) and calculated NTCP.
TH‐E‐230A‐07: Estimate of Radiobiological Parameters From Clinical Data for Treatment Planning of Liver Irradiation
Authors: Tai A , Erickson B , Khater K , Li X
Published: July 2006

Purpose: Several different dose fractionation regimens are being developed in clinical trials for liver irradiation. For example, RTOG is initiating a new hypofractionation regimen (RTOG 0438) to treat liver cancer patients. To evaluate the radiobiological equivalence between different regimens, which is useful in the design of these trials, requires reliable radiobiological parameters. The purpose of this work is to estimate a plausible set of such parameters for liver tumor based on published clinical data. Method and Materials: A phenomenological expression inspired by the linear‐quadratic (LQ) formalism was developed to fit a series of clinical survival data for radiotherapy of hepatocellular carcinoma patients. The data are from different institutes using different fractionation (e.g., 1.5, 1.8 or 4.88 Gy). The phenomenological expression consists of 6 fitting parameters including radiosensitivity parameters α and α/β, potential doubling time Tb, and clonogenic cell number K. The expression considers the prescription dose, dose per fraction, overall treatment time and the elapsed time at which the survival data were collected. We have developed an algorithm to take into account the tumor cell repopulation during the elapsed time. Results: The newly developed phenomenological expression was found to fit well to the available clinical data. Based on the fitting, we have estimated a set of plausible radiobiological parameters for liver tumor: α/β = 12.8 ± 1.0 Gy, α 0.013 ± 0.002 Gy, the potential doubling time: 123 ± 9 days, and colonogenic cell number: 1302 ± 47. Using this set of parameters we have calculated a series of dose fractionation regimens that are biologically equivalent based on BED. Conclusion: A plausible set of radiobiological parameters have been obtained based on clinical data. These parameters may be used for radiation treatment planning of liver tumor, in particular, for the design of new treatment regimens aimed for dose escalation.
TH‐E‐ValA‐02: Simultaneous Multi‐Pencil Fan‐Beam‐Based Intensity‐Modulated Proton Therapy.
Authors: Mackie T , Kissick M , Flynn R , Westerly D , Hill P , DeLuca P , Jeraj R , Schreuder A , Farr J
Published: July 2006

Purpose: Intensity‐modulated proton therapy (IMPT) will improve the conformality of proton radiotherapy while preserving target homogeneity and low integral dose characteristics. IMPT is currently delivered using a single scanned pencil beam by placing Bragg peak spots at predetermined points in the patient. Very short pulse lengths and low repetition rates will characterize inexpensive compact proton beam systems of the future such as the dielectric wall accelerator. Their pulse structure is not amenable to scanning a single spot. The use of multiple intensity‐modulated pencil beams delivered in a fan beam is a simple approach to IMPT that can be used to upgrade current proton systems or with future accelerators. Methods and Materials: A fan beam is created with a pair of quadrupole magnets aligned so that the second magnet amplifies beam divergence initiated by the first magnet. A set of multiple range‐shifters simultaneously adjusts penetration of multiple pencil beams. The actuation for the range‐shifters is done out of the plane of the fan beam. The intensity is modulated for any pencil beam by placing a thickness, greater than the range in the protons, in that portion of the fan beam. This achieves binary (on‐off) modulation as is used in x‐ray tomotherapy. Results: Calculations show that a spot delivery rate increase of 8 times can be achieved with this system which would allow larger target volumes to be practically delivered with IMPT or used with pulsed systems with low repetition rates. The system could be used to deliver spot scanning with multiple pencil beams simultaneously or used to deliver proton tomotherapy. Conclusions: An intensity‐modulated proton therapy system, based on multiple pencil beams issuing from a fan beam geometry, would decrease the delivery time for either continuous or low repetition pulse systems.
TH‐E‐ValA‐05: Proton Lung Sterotactic Body Radiotherapy Treatment Method
Authors: Arjomandy B , Balter P , Zhang Xiaodong , Chang J
Published: July 2006

Purpose: Photon Sterotactic body radiotherapy has shown a great advantage over conventional treatment for stage I NSCC. Lung SRT has been limited by targets proximally to critical structures. Proton beam therapy can reduce the dose to the healthy tissue. Use of 4DCT introduces a great advantage in delineating the moving target and account for tissue motion in proton beam path. However, design of compensator is a critical factor for dose coverage in 4DCT. Method: We investigated the use of Free Breathing (FB), Maximum Intensity Projection (MIP) and Average CT (ACT) image data sets to obtain the best technique for proton beam treatment delivery. The target volume was the union of GTV on all phases contoured on MIP data set. Then MIP‐GTV volume density was set to the average HU of the tumor to account for density variation due to motion on all data sets. The ITV was drawn by expanding 8 mm around the MIPGTV. Apertures were designed by considering only beam penumbra and setup uncertainty. Distal and proximal distances and smearing were added to compensator design as described in reference (1). Compensators were designed on all three data using density corrected MIP‐GTV. Results: The plans were calculated on all data sets. We evaluated compensators that were designed in FB, MIP, and ACT. Dose comparison was done based on the ITV dose coverage. Plans with compensator designed in MIP or ACT produce insufficient dose coverage when calculated in FB. Conclusion: The best coverage is obtained when compensator is designed in the MIP data set and plan is evaluated on ACT data set. The dose coverage was also verified on the inspiration and expiration phases of 4D data set using MIP compensator.
TH‐E‐ValA‐06: 4D DMLC IMRT Delivery to Targets Moving in Two Dimensions (in BEV)
Authors: Rangaraj D , Papiez L , Yaddanapudi S
Published: July 2006

Purpose: To deliver proper IMRT to moving tumors using DMLC necessitate proper leaf sequencing technique to take care of the dynamic nature of the target. A complete compensation of motion can be achieved only if the compensation is in both the directions (direction along (x) and perpendicular (y) to the leaves motion). A technique to accomplish this is proposed. Method: The motion of the target is divided into two components. Then leaf trajectories to deliver the desired intensity modulated profile is calculated for all leaf pairs (LP) assuming the target is moving in one direction (x direction). Then leaf trajectories of all the leaf pairs are synchronized. Now the motion is compensated in x direction. As the MLC leaf can move only in one direction, the motion compensation in y direction is accomplished by switching the leaf trajectories of each pair appropriately i.e. say if the target is moving upwards in y direction, after a threshold value (value before which motion in y direction is neglected) the leaf trajectory of LPs are switched upwards meaning the leaf trajectory of LP1 is now followed by LP2 and trajectory of LP2 by LP3 and so on. The switching is in the other direction if the target is moving downward. Small dosimetric errors may occur while switching depending upon the time it takes to do the switch and also the threshold value after which switching happens. Results: An example of 4D‐IMRT delivery to an irregular shaped rigid target moving in an elliptical pattern is shown. Other related delivery issues are addressed (dealing with target motion exceeding maximum leaf speed). Conclusions: This method of compensating the two dimensional tumor motion in BEV with one dimensional moving MLC while delivering IMRT meeting all mechanical practical and constraints is possible and promising for 4D‐IGIMRT.
Sci‐Thur PM Therapy‐01: 6MV Photon dMLC IMRT: Near Surface Dose with Oblique Incidence
Authors: Popescu C , Beckham W , Bush K
Published: July 2006

The purpose of the study was to investigate absorbed dose in the build up region for oblique intensity modulated 6MV beams. For locally advanced breast carcinoma using multi‐field IMRT it is imperative to know with good precision the dose at the surface. This study intended to determine the effect of IMRT on the surface dose compared with open beams, to benchmark our Monte Carlo algorithm (BEAMnrc) in the build up region for clinical implementation and to characterize the dose calculation accuracy of our treatment planning system (Eclipse ™ 7.3) at the surface and depths up to dmax.

Measurements were performed in solid water using a parallel plate ionization chamber for a 10 × 10 cm2 open field, 1 cm slit and a typical breast IMRT treatment field, both delivered with sliding window technique at 00°, 350° and 700° angles of incidence. Compared with the open field the 1cm gap sliding window decreased the surface dose up to 10% for normal incidence and up to 5% for 70° incidence. The measured depth‐dose curves along the central axis are in excellent agreement within 2% of the Monte Carlo simulations. The TPS using the pencil beam algorithm overestimates the dose at the surface by a factor of 3 at normal incidence for both open and IMRT fields. The TPS does not take into account the increase in the surface dose due to oblique incidence. Monte Carlo can be used to calculate with accuracy the surface dose for a clinical multi‐ field IMRT plan.
Sci‐Thur PM Therapy‐08: Intra and Inter Observer Variability and Systematic Error in Prostate Delineation
Authors: Gerig L , Gao Z , Eapen L , Wilkin D , Morash C , Wassef Y
Published: July 2006

Individual sub‐processes of external beam radiation therapy can affect patient outcome. Specifically geometric uncertainties play a major role in tumor control and normal tissue complications. It is known that tumor definition varies among different observers but importantly, at least to our knowledge, few studies have reported their findings against a true gold standard, most relying on the population average as the reference value. This work uses the Visible Human Project® data for a set of cross‐indexed CT and anatomical photographic digital images of a human male cadaver. We use the anatomical images to define a gold standard for the prostate as defined by a group of experts. Six radiation oncologists then repeatedly (20 times over several weeks) defined the prostate alone on the corresponding CT images allowing us to quantify inter and intra observer random and importantly to measure systematic error.

Individual and population means were tested for systematic error against the gold standard. We found that the observers routinely over estimated the prostate volume, and that our intra and inter observer variability is not significantly different than that reported in the literature. Significantly we report a systematic error in target definition, with the physicians failing to include all of the posterior prostate volume near the rectum. In contrast the observers rarely missed true prostate volume in the left, right or anterior quadrants, but we observe a systematic error in that normal tissue anterior to the prostate was routinely included as target tissue.
Sci‐Thur PM Therapy‐11: Tomosynthesis with rotating and stationary radiotherapy cone beams
Authors: Yao W , Leszczynski K
Published: July 2006

We examine reconstruction of volumetric objects from megavoltage rotating and stationary beam projections. Quality of the reconstruction depends on the transverse and depth resolutions of the projections. In the rotating beam geometry, as the gantry angle varies, the depth resolution improves and thus more detailed anatomical structure of the object along the central axis of the radiation beam can be reconstructed, while the transverse resolution decreases. In the stationary beam geometry, where the source and detector remain fixed but the location of the object along the beam central ray is varied, the depth resolution is poor while the transverse resolution can be very high. We use the algebraic reconstruction technique (ART) and exact weighting factors to reconstruct 3‐D objects from 2‐D projections. When projections are noise‐free as those generated by a computer, the object is accurately reconstructed as long as the number of iterations is sufficiently high. For real world projections, in which noise can be significant, the reconstruction quality can be poor. Because both the transverse and depth resolutions are important in clinical applications, our work points to potential advantages of combining rotating‐ and stationary‐beam projections.
Po‐Thur Eve General‐01: Combination of IMRT Photon Beams with Electron Beams Shaped with MLC
Authors: Karnas S , Mosalaei H , Shah S , Martin J , Chen J
Published: July 2006

The standard technique for electron therapy has several inherent problems. First, the cutouts in electron cones for field shaping are labour intensive to produce, cumbersome to use and reduce the clearance between the treatment machine and the patient. Secondly, it is difficult to modulate treatment depth and match multiple fields with electron beams due to electron multiple scattering. We propose the use of photon MLC for electron field shaping to improve treatment efficiency and convenience. Unfortunately, using the photon MLC to shape an electron beam significantly increases the penumbra of the treatment field, which can be reduced using a subsequent photon IMRT boost field. Electron beams shaped with a Varian Millennium MLC were recently commissioned in Pinnacle V7.6 (Philips). Using forward or inverse intensity‐modulated radiotherapy (IMRT) planning techniques, MLC segments for a step‐and‐shoot IMRT field were determined for improving electron beam penumbra, depth modulation and electron‐photon field. The technique has been applied to a few clinical cases, such as whole CNS irradiation and cancer of parotid. Inverse planned IMRT will be used to optimize photon‐electron‐combined irradiation for cancer of the parotid. Generated plans are verified using film and ion chamber measurements. Using boost IMRT fields, we showed a similar 50%–95% penumbra for the MLC shaped electron field compared to the penumbra with an electron applicator (100cm SSD). In conclusion, combined photon IMRT and electron beam improves dose uniformity, beam junction, and depth modulation.
Po‐Thur Eve General‐03: The Use of Megavoltage Computed Tomography (MVCT) in Treatment Planning
Authors: Sasaki D , Field G , MacKenzie M , Rathee S , Fallone B
Published: July 2006

In modern radiotherapy treatment planning, the information in diagnostic CT images is used for two purposes: to delineate tumour and surrounding critical structures and to provide an electron density map of the patient that is used to calculate the dose distribution resulting from exposure to a certain beam arrangement. In pelvic cancer patients with hip prostheses, the metal implants produce artifacts in the diagnostic CT images such that both the location of the tumour and accurate electron densities are either difficult or impossible to obtain. We have used megavoltage CT (MVCT) images for treatment planning in an attempt to quantify the impact of metal artifacts and overcome the problems they introduce. This has been done in three different sets of experiments. The first was a calibration of the megavoltage CT number‐to‐electron density curve using a CIRS phantom. This also allowed for measurements of the impact of metal artifacts on apparent relative electron density in both kVCT and MVCT images. The second was the comparison of treatment plans generated for patients with metal implants using both diagnostic and megavoltage CT studies. This allowed for quantitative measurements of the calculated dosimetric effect of metal artifacts. The final set of experiments compared MVCT and kVCT treatment plans of a water tank containing a stainless steel 316L rod. Dose measurements were taken at various points and compared to the doses calculated using both MVCT and kVCT studies.
Po‐Thur Eve General‐04: Squeezing a balloon: the co‐dependences of calculated photon beam characteristics on model parameters
Authors: Rangel‐Baltazar A , Ploquin N , Kay I , Doll C , Dunscombe P , Baker Tom
Published: July 2006

Modeling photon beams in modern treatment planning systems is an iterative procedure as the model parameters generally influence the behaviour of more than one feature of the computed beam characteristics. In this work we explore the co‐dependence of the calculation of specified regions of beam profiles on the adjustable source parameters in Pinnacle®. The regions chosen are those used by Venselaar in recommending tolerances to be applied during the modeling process. A reference model, at 15MV, was modified in a controlled fashion so as to primarily influence one region of either the depth dose or cross beam profile at 10cm depth in a 20×20cm2 field. The effect on the five other profile regions was examined thus illuminating the degree of co‐dependence of the different regions on the model parameters. The regions of interest were: build‐up, descending depth dose, horns, tail, high and low penumbra. Our study suggests a sequence which could enhance efficiency in modeling measured photon beam data.
Po‐Thur Eve General‐07: Dosimetry of Small Lung Lesions with EGSnrc Monte Carlo and Treatment Planning Systems
Authors: Darko J , Joshi C , Osei E , Halsall T , Salomons G , Kerr A
Published: July 2006

Early stage lung cancer, presenting as a small solitary pulmonary mass, is treated with radiation when concurrent disease precludes a surgical option. These small lesions are usually surrounded by less dense normal lung, which affects the ability to deliver a homogenous dose to the target volume. In low‐density tissue, such as lung, there is increased transmission of photons. In addition, lateral scatter of electrons out of the beam can lead to increased penumbral width. The magnitude of these effects is known to be dependent on beam energy. Some of the commonly used commercial treatment planning systems have had limited success in predicting accurately dose distributions under these highly inhomogeneous conditions.

We present a quantitative comparison between Monte Carlo simulation and commercial planning systems for a select range of clinically relevant target geometries and beam parameters. Small water equivalent cylindrical lung tumors of diameter 3 and 5 cm were incorporated within a CT dataset at different locations. A Parallel Opposed Pair (POP) field arrangement with 6MV or 15MV photons and variable field‐edge margins were considered. These plans were calculated using BEAMnrc Monte Carlo code and on two planning systems; ADAC Pinnacle III Version 7.4 and MDS Nordion Theraplan Plus v3.8.

The analysis of dose profiles and DVH's show considerable and unique differences between Monte Carlo and the results from each TPS within the tumor and at the junction between tumor and lung. For both planning systems, the severity of these errors, increases with photon energy, and decreases with field size.
Po‐Thur Eve General‐08: Monte Carlo Simulation of Helical Tomotherapy
Authors: Zhao Y , MacKenzie M , Kirkby C , Thomas S , Drabik D , Fallone B
Published: July 2006

Helical tomotherapy represents the state of the art in intensity modulated radiation therapy (IMRT) and image guided adaptive radiotherapy (IGAR). This work is aimed at carrying out Monte Carlo (MC) dose calculations of tomotherapy deliveries to real phantom and virtual phantom/patient CT data. All MC calculations are performed with the EGSnrc‐based MC simulation codes, BEAMnrc and DOSXYZnrc. Various MC parameters and variance reduction techniques were investigated and optimized. Single projection simulations are carried out to get percent depth dose (PDD) and beam profiles at various depths and lateral field dimensions. Simulations are compared with measured results taken with an A1SL ionization chamber in a water tank. A complex delivery was simulated with the 64 leaf binary multileaf collimator (MLC) modulating at fixed radiation angle for a solid water phantom. Further, a rotational treatment plan to a cheese phantom CT data set was also simulated. Simulations are performed by taking the sinogram (leaf opening vs. time) of the treatment plan and decomposing it into different projections, each of which is segmented further into several opening configurations with different MLC settings and weights, which corresponded to leaf opening time. Then the projection is simulated with the result of sum of all of the opening configurations, and the overall rotational treatment is simulated with the result of sum of all of the projection simulations. Measured and simulated profiles and PDDs are found agree with each other well. Preliminary simulations of full treatment plans are also presented.
Po‐Thur Eve General‐13: Commissioning a gated kV imaging system
Authors: Smith W , Spencer D
Published: July 2006

The recent expansion in the use of breathing gating systems of various types necessitates the development of methods to efficiently evaluate them. An ideal commissioning procedure would not require extensive specialized equipment, would allow multiple measurements to be made quickly and easily, and be capable of detecting errors at levels which could conceivably be important in treatment situations. In this study, a simple method to evaluate timing accuracy and precision for gated kV imaging was developed, and used to evaluate the Brainlab ExactTracGating ™ kV imaging system.

The phantom used has a moving platform and a set of IR markers moved in a sinusoidal vertical motion by a cam milled with 0.001 inch accuracy. A 1mm thick lead sheet with a 3 mm diameter hole was placed under the moving platform. Kodak PP‐L film was placed on the moving platform and kV images were obtained at 5%, 50% and 95% gating levels with each of the two tubes. Ten measurements for each tube could be easily made on a single 11×17 inch film. The measurement was repeated for periods between 1.5 and 5 seconds, and with a platform moving close to a sine function with an amplitude of 2.1cm. This commissioning system was capable of detecting systematic timing errors of less than 40 ms, but none were observed. The standard deviation in timing accuracy was measured under difficult physiological conditions (3 second period, 2 cm amplitude of motion) and found to be about 0.03 cm, which corresponds to 30 ms.
Po‐Thur Eve General‐14: 3rd Generation Co‐60 based Megavoltage Computed Tomography
Authors: Peters C , Rogers M , Eyles D , Kerr A , Salomons G , Schreiner LJ
Published: July 2006

Patient set up verification is a necessary step for high precision radiation therapy. There has been considerable work to develop such verification using megavoltage (MV) imaging on Electronic Portal Imaging Devices (EPIDs). Recently, MV image guidance has been advanced with the extension to MV computed tomography (MVCT). At the CCSEO we have been investigating MVCT with a Cobalt‐60 unit, in the context of developing Cobalt‐60 tomotherapy. The system is comprised of an automated rotary stage added to a Theratronics T780C Co‐60 unit; recently our imaging has advanced with the development of a 3rd generation imaging system using a Varian PortalVision LC250 EPID. The EPID was calibrated for relative dose‐response using a parallel‐plate ion chamber; the calibration curve follows a quadratic form. The image reconstruction is based on a generalized Feldkamp technique; executed in MATLAB. Consistent with previous 1st generation studies, results on various phantoms show that Co‐60 based EPID — MVCT is free from beam hardening and metallic artifacts. Phantoms included a variety of orthopaedic hip implants, Rando head phantom with stainless steel and tungsten inserts, and a plug phantom containing materials of known atomic number. EPID determined MVCT numbers also correspond linearly with relative electron density. The work will summarize the differences between kVp and MV CT images, while highlighting the improvements of EPID based imaging over 1st generation Cobalt MVCT.
Po‐Thur Eve General‐15: Uncertainty of the parallel dose kernel assumption in portal dose prediction with a‐Si electronic portal imagers
Authors: Chytyk K , McCurdy BMC
Published: July 2006

The effect of beam divergence on dose calculation via Monte Carlo generated dose kernels was investigated in an amorphous‐silicon electronic portal imaging device (EPID). The complete detector was simulated in EGSnrc with a 3.0 cm water buildup. The model included details of the detector's imaging cassette, the front cover upstream of it and equivalent backscatter material to approximate the EPID's complex rear housing. Dose kernels were generated with an incident pencil beam of monoenergetic photons of energies 0.1, 2, 6 and 18 MeV. Dose was scored in the phosphor layer of the detector in both cylindrical (at 0° pencil beam orientation) and cartesian (at 0° – 14° in 2° increments) geometries. The parallel (0°) kernels were then convolved with a simple incident fluence map; a full superposition employing the tilted kernels (simulating a divergent beam) and the same incident fluence map was also calculated. Profiles of the dose kernels were observed to demonstrate increasing asymmetry with increasing angle and energy. Comparison of superposition to convolution dose calculation methods in worst‐case‐scenario geometries demonstrated an agreement between the two methods within 0.784 mm distance‐to‐agreement and up to a 1.5% dose difference. More clinically typical field sizes and source‐to‐detector distances were also tested, yielding at most a 1.0% dose difference and the same distance‐to‐agreement. The assumption of parallel dose kernels has less than a 1.5% dosimetric effect in extreme cases and less than a 1.0% dosimetric effect in most clinically relevant situations and should be suitable for most clinical dosimetric applications.
Po‐Thur Eve General‐16: Development and Application of a Visualization and Navigation System for Stereotactic Deep‐Brain Neurosurgeries
Authors: Guo T , Finnis K , Parrent A , Peters T
Published: July 2006

This work presents the development and application of a neurosurgical visualization and navigation system. Integrated with a digitized and segmented brain atlas, an electrophysiological database, and collections of final surgical targets of previous patients, this system provides valuable assistance in pre‐operative planning and intra‐operative guidance of stereotactic deep‐brain neurosurgical procedures for the treatment of Parkinson's disease, chronic pain, and essential tremor. The fusion of standardized functional and anatomical information, once registered to individual patient images, facilitates the delineation of surgical targets, surrounding structures, as well as functional borders. Three retrospective studies and one prospective study were conducted to assess the surgical target localization efficiency of this system, which is capable of non‐rigid registration, interactive manipulation and processing of clinical image data. The surgical target locations identified by an experienced stereotactic neurosurgeon using multiple electrophysiological exploratory trajectories were compared with those located by a non‐expert using this system on 36 thalamotomy, pallidotomy, thalamic deep brain stimulation (DBS), and subthalamic nucleus (STN) DBS procedures. The average displacement between the surgical target locations in both groups was 1.85±0.96mm, 1.87±0.82mm, 1.84±0.85mm, and 1.65±0.71mm for each category of surgeries respectively, indicating promising utilization of our system in stereotactic deep‐brain neurosurgical procedures, and demonstrating its capability of accurate surgical target initiation.
Po‐Thur Eve General‐17: Design and Characterization of a CT Scanner for Small Animal Imaging
Authors: Pekar J , Patterson M
Published: July 2006

A prototype cone beam computed tomography scanner has been designed for use in small animal imaging. The system consists of a small air‐cooled microfocus x‐ray source, a flat‐panel CMOS (complimentary metal‐oxide semiconductor) x‐ray detector, a motorized rotation stage and a personal computer. The scanner will ultimately be integrated into a planned quantitative optical molecular imaging system. Some initial performance characterization of the x‐ray detector has been performed. The detector response curve was found to be non‐linear at both low and high exposures. The response curve was “linearized” by using a polynomial gain correction technique involving fitting the response of the detector with a 3rd order polynomial function. The MTF of the detector was also measured using the angled edge technique. The MTF had a value of approximately 8.6 cycles/mm at 10% contrast. A transaxial slice of an acrylic phantom reconstructed using a Feldkamp back‐projection algorithm is also presented.
Po‐Thur Eve General‐18: Comparison of Imaging Performance: Cone Beam CT versus Conventional CT Simulators
Authors: Menon G , Tambasco M
Published: July 2006

Cone Beam Computed Tomography (CBCT) is emerging as an alternative modality for radiotherapy treatment simulation and verification. Evaluation of CBCT performance for radiotherapy applications has been confined to in‐house built or modified CBCT devices. In this work, we investigated the imaging performance of a commercial (Varian Acuity) simulator having CBCT capability. The objective of this study was to compare its performance to that of a conventional CT simulator (Picker PQ5000) used for acquiring 3D data for generating radiotherapy treatment plans. Specifically the tests conducted include: density resolution, temporal stability, scan uniformity and spatial linearity, effect of varying scan widths, resolution (high and low contrast), and noise measurements, performed with a Catphan phantom. The variation in density resolution that is representative of the linearity in CT numbers was negligible for both modalities except for high density materials. Operational stability monitored by routinely tracking the CT numbers showed variations <14%, which we predict based on other studies, will result in only <2% uncertainty in dose calculations. Spatial resolution measurements showed better resolution capability for CBCT unlike for low contrast resolution, which was better for conventional CT. These results indicate that CBCT is comparable to CT and demonstrates its potential for treatment planning applications. We will explore this possibility further in future work. Currently, neither AAPM nor CAPCA, have standards for evaluating CBCT performance tolerances. Hence, we also anticipate that this study will provide guidelines for these documents when establishing tolerances for using commercial CBCT simulators.
Po‐Thur Eve General‐28: Characterization and Real‐Time Measurements of Optical Density with GafChromic EBT Film
Authors: Rink A , Vitkin A , Jaffray D
Published: July 2006

To establish whether GafChromic EBT medium is suitable for real‐time fibre‐based in vivo dosimetry, we are investigating whether dose‐rate and temperature fluctuations introduce extraneous uncertainties to change in optical density (ΔOD) measurements. 1 cm × 1cm pieces of film were irradiated with a 6 MV beam within a 30 cm × 30 cm × 10 cm Solid Water phantom fitted with optical fibres for real‐time measurements, using a 10 cm × 10 cm field at SAD, 100 cm SAD, and 1.5 cm depth. Each exposure delivered a dose of 5, 10, 25, 50, 100, 250, 500 or 1000 cGy at a dose‐rate of 14, 32, 65, 130, 260 or 520 cGy/min. Change in OD obtained immediately at the end of each exposure were compared. The percent uncertainty in ΔOD for a given dose was calculated by using all the values obtained, regardless of the dose‐rate used. The percent uncertainty ranged between 1.8% and 3.6%, with an average of 2.8%. The expected variation in ΔOD within a single sheet of GafChromic EBT film for a given dose is 1.5% (two standard deviations). This suggests that there is an increase in uncertainty of real‐time ΔOD measurements when variations in dose‐rate are introduced for the doses in 5–1000 cGy range, and dose‐rates between 14 and 520 cGy/min. The average increase in uncertainty is 4%.
Po‐Thur Eve General‐32: Evaluation of calculated total scatter factor based on predicted collimator scatter factor and phantom scatter factor for kilovoltage x‐ray radiation fields
Authors: Karabrahimi V , Blais N
Published: July 2006

Kilovoltage x‐ray tube therapy beams are used for treatment of keloids and localized superficial malignancies. The purpose of this work is to calculate the total scatter factor for kilovoltage x‐ray radiation fields based on the predicted collimator scatter factor and the predicted phantom scatter factor. The radiation field is defined by a variable collimator with or without lead sheets on the patient's skin. Measurement and prediction have been done at tube potentials of 60, 100, 120 and 250 kVp. Our approach uses three different sets of field types: square fields defined by the collimator and square and circular fields defined by lead sheets. Two calculation methods are employed: the equivalent field method and Clarkson's method. Calculation and measurement are also done for rectangular and irregular shape fields. It is shown that while the equivalent field method may lead to errors up to about 6% for irregular fields, Clarkson's method is more accurate with errors up to 2.3%. The choice of the field type for the data base is not significant. Using the measured data for square fields defined by the collimator together with Clarkson's method is recommended. The implementation of this method requires a minimum number of measurements, which are acquired during the commissioning of the unit and can be applied in dose calculation for a variety of field shapes and sizes.
Po‐Thur Eve General‐37: Preliminary Analysis of a Cobalt‐60 Beam Under a MIMiC
Authors: Rogers M , Chng N , Salomons G , Kerr A , Schreiner LJ
Published: July 2006

Tomotherapy is a novel implementation of IMRT in which conformal dose is delivered by a fan beam of radiation that is modulated by an MLC as it revolves about a patient. The treatment geometry has the added advantage of enabling megavoltage CT (MVCT) imaging of the patient for setup verification immediately prior to treatment. Linac‐based tomotherapy units are now being rapidly adopted for clinical use. For a number of years we have been investigating the potential of using Cobalt‐60 (Co‐60) as the radiation source in tomotherapy. The ultimate goal of Co‐60 based tomotherapy is to provide the dosimetry benefits of tomotherapy while reducing the technical requirements needed to run and maintain a tomotherapy unit thus increasing the availability of tomotherapy worldwide. Previous studies with a benchtop Co‐60 unit have confirmed that conformal IMRT dose delivery is readily achievable with radiation from a Co‐60 source. Essentially, delivery concerns from beam characteristics such as wider penumbra and reduced penetration (compared to a linac beam) become insignificant in the rotational tomotherapy context. Recently we have extended our studies to a more clinically relevant implementation by adding a fan beam multi‐leaf collimator (the NOMOS MIMiC MLC) to our MDS Nordion T‐780C Co‐60 unit for serial tomotherapy. In this presentation we will discuss the challenges created by using Co‐60 for tomotherapy, present some early commissioning data and examine the subsequent steps needed to certify the unit for clinical use.