bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of output factors for small photon beams
Authors: Sauer Otto A , Wilbert Jürgen
Published: July 2007

A variety of detectors and procedures for the measurement of small field output factors are discussed in the current literature. Different detectors with or without corrections are recommended. Correction factors are often derived by Monte Carlo methods, where the bias due to approximations in the model is difficult to judge. Over that, results appear to be contradictory in some cases. In this work, output factors were measured for field sizes from up to side length with different detectors. A simple linear correction for the energy response of solid state detectors is proposed. This led to identical values down to field size, as long as the size of the detector is small against the field size. The correction was of the order of a few percent. For a shielded silicon diode it was well below 1%. A physically meaningful function is proposed in order to calculate output factors for arbitrary field sizes with high accuracy.
Calculation of x‐ray spectra emerging from an x‐ray tube. Part I. Electron penetration characteristics in x‐ray targets
Authors: Poludniowski Gavin G. , Evans Philip M.
Published: July 2007

The penetration characteristics of electron beams into x‐ray targets are investigated for incident electron kinetic energies in the range . The frequency densities of electrons penetrating to a depth in a target, with a fraction of initial kinetic energy, , are calculated using Monte Carlo methods for beam energies of 50, 80, 100, 120 and in a tungsten target. The frequency densities for electrons in Al, Mo and Re targets are also calculated. A mixture of simple modeling with equations and interpolation from data is used to generalize the calculations in tungsten. Where possible, parameters derived from the Monte Carlo data are compared to experimental measurements. Previous electron transport approximations in the semiempirical models of other authors are discussed and related to this work. In particular, the crudity of the use of the Thomson‐Whiddington law to describe electron penetration and energy loss is highlighted. The results presented here may be used towards calculating the target self‐attenuation correction for bremsstrahlung photons emitted within a tungsten target.
Efficiency improvements of x‐ray simulations in EGSnrc user‐codes using bremsstrahlung cross‐section enhancement (BCSE)
Authors: Ali E. S. M. , Rogers D. W. O.
Published: July 2007

This paper presents the implementation of the bremsstrahlung cross‐section enhancement (BCSE) variance‐reduction technique into the EGSnrc/BEAMnrc system. BCSE makes the simulation of x‐ray production from bremsstrahlung targets more efficient; it does so by artificially making the rare event of bremsstrahlung emission more abundant, which increases the number of statistically‐independent photons that contribute to reducing the variance of the quantity of interest without increasing the CPU time appreciably. BCSE does not perturb the charged‐particle transport in EGSnrc and it is made compatible with all other variance‐reduction techniques already used in EGSnrc and BEAMnrc, including range rejection, uniform bremsstrahlung splitting, and directional bremsstrahlung splitting. When optimally combining BCSE with splitting to simulate typical situations of interest in medical physics research and in clinical practice, efficiencies can be up to five orders of magnitude larger than those obtained with analog simulations, and up to a full order of magnitude larger than those obtained with optimized splitting alone (which is the state‐of‐the‐art of the EGSnrc/BEAMnrc system before this study was carried out). This study recommends that BCSE be combined with the existing splitting techniques for all EGSnrc/BEAMnrc simulations that involve bremsstrahlung targets, both in the kilovoltage and megavoltage range. Optimum cross‐section enhancement factors for typical situations in diagnostic x‐ray imaging and in radiotherapy are recommended, along with an easy algorithm for simulation optimization.
Log transformation benefits parameter estimation in microwave tomographic imaging
Authors: Meaney Paul M. , Fang Qianqian , Rubaek Tonny , Demidenko Eugene , Paulsen Keith D.
Published: July 2007

Microwave tomographic imaging falls under a broad category of nonlinear parameter estimation methods when a Gauss‐Newton iterative reconstruction technique is used. A fundamental requirement in using these approaches is evaluating the appropriateness of the regression model. While there have been numerous investigations of regularization techniques to improve overall image quality, few, if any, studies have explored the underlying statistical properties of the model itself. The ordinary least squares (OLS) approach is used most often, but there are other options such as the weighted least squares (WLS), maximum likelihood (ML), and maximuma posteriori(MAP) that may be more appropriate. In addition, a number of variance stabilizing transformations can be applied to make the inversion intrinsically more linear. In this paper, a statistical analysis is performed of the properties of the residual errors from the reconstructed images utilizing actual measured data and it is demonstrated that the OLS algorithm with a log transformationis clearly advantageous relative to the more commonly used OLS approach by itself. In addition, several high contrast imaging experiments are performed, which demonstrate that different subsets of data are emphasized in each method and may contribute to the overall image quality differences.
SU‐BB‐BRB‐01: Close Encounters of the Media Kind
Authors: Fox M
Published: July 2007

Out of nowhere, I was contacted by The New York Times to talk about medical physics, and suddenly seemed to have lost my knack of being able to communicate with the public. Questions of statistical nature were being asked and where were my resources? The AAPM website had not yet been set up for public friendly information, especially for reporters that want information NOW.

The AAPM has made a commitment to raise media awareness of our profession and we need to be ready. The next two speakers will relate their experiences in this endeavor, and offer information on media communication and rapid response.
SU‐FF‐I‐127: Patient Specific Angiography Phantoms for Investigating New Endovascular Image‐Guided Interventional (EIGI) Devices
Authors: Sherman JR , Rangwalla HS , Dohatcu AC , Minsuok K , Ionita CN , Rudin S
Published: July 2007

Purpose: To provide patient‐specific phantoms for investigating new endovascular image‐guided interventional (EIGI) devices such as stents. Methods and Materials: A human cerebral/saccular aneurysm phantom was constructed from a patient's segmented CT‐scan data. The CT‐derived vascular lumen was converted to a standard stereolithography (STL) computer file which was used, through a rapid‐prototyping process, to make a plastic resin (Accura 25) mold of the vessel with the aneurysm. In this process, an ultraviolet laser is used to selectively cure a liquid plastic layer by layer resulting in a highly accurate and very detailed positive model. RTV silicone is placed around this plastic aneurysm, allowed to cure, and then cut in half to create a mold with a cavity in the shape of the aneurysm. This mold is used to make a wax replica of the human aneurysm. The wax replica is then immersed in a silicone elastomer solution, the bubbles removed via vacuum, and the elastomer allowed to cure. The wax is then dissolved using boiling water. Results: The finished product is a clear elastomer model with a patient‐specific aneurysm cavity that may be used in both x‐ray and optical flow experiments. Multiple identical elastomer phantoms can be generated from one mold so that new EIGI devices such as an asymmetric stent with a low porosity patch region to occlude aneurysmal flow can be variously deployed under fluoroscopic guidance and evaluated angiographically. Additionally, the effects of contrast media can be studied by comparisons with optical flow studies. Comparisons of angiographic and optical time‐density curves for an anterior cerebral artery with multi‐lobular aneurysm will be presented. Conclusions: We have developed a unique system for the creation of realistic patient‐specific phantoms which enable the evaluation of detailed vascular flow and experimental EIGI treatment techniques for clinically representative pathology.

(Support: NIH R01‐NS43924, R01‐EB002873, Toshiba Corp.).
SU‐FF‐T‐447: Verification of Bremsstrahlung Splitting in Geant4 for Radiotherapy Quality Beams
Authors: Tinslay J , Faddegon B A , Perl J , Asai M
Published: July 2007

Purpose: To evaluate the impact of applying the uniform bremsstrahlung splitting with charged secondary Russian Roulette variance reduction technique, on a Geant4 based thick target bremsstrahlung benchmark Monte Carlo simulation. Method and Materials: Thick target bremsstrahlung benchmark simulation runs were carried out using the Geant4 Simulation Toolkit (version 8.2). The primary electrons were generated at energies between 10–30MeV, with target materials of lead, aluminium and beryllium. To speed up the simulation, uniform bremsstrahlung with charged secondary Russian Roulette was applied. Various studies were carried out in order to get an idea of the Monte Carlo efficiency improvement with various bremsstrahlung splitting and Russian Roulette configurations. Verification studies were also carried out to check for unexpected bias introduced in various spectra. Results: Initial results indicate the photon fluence and energy distribution from thick targets of various materials are properly calculated when using a bremsstrahlung splitting factor of 100. The simulation is sped up by a factor of around 10. Conclusion: Reasonable speedup can be achieved through the use of uniform bremsstrahlung splitting with charged secondary Russian Roulette in Geant4. Further work is underway to evaluate the speedup compared with other codes, as well as continued verification studies. The results of this work will be reported at the meeting.
SU‐GG‐AUD‐06: Stray Radiation Exposure During Proton Radiotherapy of the Prostate: The Influence of the Patient On Scatter and Production
Authors: Fontenot JD , Zheng Y , Taddei P , Newhauser W
Published: July 2007

Purpose: To characterize the scatter, production, and attenuation of secondary radiation in patients receiving passively‐scattered proton radiotherapy for prostate cancer. Methods and Materials: A proton therapy treatment was simulated using a Monte Carlo model of a double scattering treatment machine. Whole body effective dose (E) from secondary radiation was estimated from a weighted sum of doses to the major organs in an anthropomorphic phantom. The effect of the patient on secondary dose was quantified by comparing E with ambient dose equivalent, H*(10), which was based on free‐in‐air spectral fluence calculations at isocenter. Various treatment parameters (proton beam energy, range modulation width, field size, and snout position) were varied in order to study their influence on E and H*(10). Results: The calculated E for the simulated treatment was 7.8 mSv/Gy, while the calculated H*(10) at isocenter was 16 mSv/Gy. Both E and H*(10) approximately doubled over the range of modulation widths and energies studied. As field size increased from 0×0 to 15×15, E doubled, while H*(10) decreased by 30%. When the snout position was changed from 30 cm to 48 cm, E decreased by less than 20%, while H*(10) decreased by 44% over the same interval. Simulations revealed that, while E is predominated by neutrons generated in the nozzle, neutrons produced in the patient contributed significantly (up to 40%) to dose equivalent in near‐field organs. In most cases, H*(10) provided a conservative estimate of E. However — because H*(10) does not account for neutrons created in the patient — it did not conservatively estimate E for large field sizes, where neutron production in the patient becomes significant. Conclusions: Neutrons generated in the patient contribute significantly to exposures to organs near the irradiated volume. When evaluating stray radiation exposure, production, scatter, and attenuation in the patient should be taken into consideration.
TU‐D‐AUD‐06: Cone Beam CT Number Correction Method for Accurate EBRT Dose Calculations
Authors: Liu B , Lerma F
Published: July 2007

Purpose: Benchmark a method for correcting CBCT Hounsfield numbers in support of accurate dose calculations. Method and Materials: Cone beam CT (CBCT) and Fan Beam CT (FBCT) scans of three phantoms: Head, Thorax and large solid water cylindrical ‘pelvic’ phantom are acquired under identical setup conditions. CBCT images are then modified by a step‐function correction method, descretizing the Hounsfield units to five discrete CT numbers for the thorax, three discrete CT numbers for HN and Pelvic anatomies. The groups include the three main ranges of 0 HU for gas/air, 1000 HU for water/muscle and 2000 HU for bone. The FBCT, CBCT and corrected CBCT scans for each body site surrogate (HN, lung and pelvis) are used to compute EBRT plans consisting of an IMRT HN, SBRT lung and a prostate IMRT plan, respectively. The CBCT and corrected CBCT plans are paired with the corresponding FBCT‐based plans to perform comparisons using dose difference analyses, DTA analyses and gamma‐analyses. The criteria used are 0.2cm for DTA, 2.0% of maximum dose for dose difference and 1.0 for gamma. Results: Compared with raw CBCT‐based plan, this correction method reduces gamma by about 0.2, reduces DTA by about 0.1 cm and reduces dose difference by about 1% for lung and pelvis. The passing rate of gamma, DTA and dose difference also improved significantly. For HN, with and without correction both give excellent results, which may be due to the fact that the HU numbers or raw CBCT is much closer to those of FBCT for HN than those for lung and pelvis. Conclusion: The HU correction method presented herein effectively improves the accuracy of dose calculation for CBCT‐based treatment plan, supporting CBCT as a viable imaging modality for accurate EBRT dose calculations.
TU‐D‐BRA‐01: Innovations in Particle Acceleration
Authors: Ma C
Published: July 2007

This presentation will start the symposium with a brief description of recent developments in particle acceleration techniques with a focus on the acceleration of proton and light ions and its impact on radiation therapy of cancer. Proton/ion therapy has great potential for improving local control and normal tissue sparing because of its superior dose distributions. However, the large cost of a proton/ion facility based on conventional accelerator technology has prevented its widespread use. Significant efforts have been made in recent years to develop compact particle accelerators in order to make proton or ion therapy a commonly available treatment modality. Compact particle accelerators based on dielectric wall accelerator, laser‐particle acceleration, and superconductor techniques will be discussed. The educational objectives of this presentation include (1) to describe the physical properties of proton and ion beams and their therapeutic advantages, (2) to analyze the cost‐effectiveness of conventional proton/ion therapy versus intensity modulated x‐ray therapy, and (3) to introduce recent innovations in particle acceleration and their potential for radiation oncology.
TU‐D‐BRA‐02: Compact Laser Acceleration of Paticles for Oncological Applications
Authors: Tajima T
Published: July 2007

The proton beam therapy of cancer has been practiced to yield good therapeutic results. However, it remains a costly treatment, primarily because of its huge accelerator facility needed to drive proton beams. The introduction of laser acceleration promises to greatly reduce the size and possible cost to accelerate and provide a medically needed system for therapy. Not only its compact acceleration section, but also its compactness of necessary radiation shield and beam handling section (a portion similar to the gantry) contribute to the compactness of the overall therapy machine size. A series of innovations such as the adiabatic acceleration, double‐layer target, the optimized target thickness, etc. constitutes to provide a new paradigm of laser‐driven compact therapy. We envision that the verification of dosage with the self‐auto‐activation by PET combined with pencil‐beam scanning characteristics amounts to a new feedback therapy of cancer.
TU‐D‐BRA‐05: Recent Developments in Proton Acelerators
Authors: Gall K
Published: July 2007

Purpose: Proton Beam Radiation Therapy has been clinically investigated for over 40 years. Despite obvious physical dose deposition advantages and compelling clinical results, the considerable financial cost of existing accelerator designs has hindered widespread use of this evidently superior treatment modality. Within the last few years however, materials have been developed that enable high concentrations of electromagnetic energy to be harnessed. These materials have opened the way for reducing the size and cost of accelerators for Proton Beam Radiation Therapy. Method and Materials: Two such materials, high current density superconducting wires and high field gradient dielectric elements, have led to the respective developments of compact superconducting cyclotrons and dielectric wall accelerators. Existing analytical tools for simulating the performance of circular accelerators and linear accelerators have been applied to guide the development of these designs in the new energy density regimes. These tools are essential for predicting the performance of accelerated proton beam dynamics in compact devices with high electromagnetic field gradients. Results: Each of these accelerators have been incorporated into single room proton therapy treatment system designs with a size and cost that are, or are projected to be, significantly below that of existing alternatives. Prototypes of these systems are now under construction and elemental prototype evaluation. Superconducting current density performance specifications have been met or exceeded in the development of the compact superconducting cyclotron. A working cyclotron has been prototyped and shown to accelerate an intense proton beam of more than 100 nA over the first stages of the cyclotron acceleration cycle. Likewise critical elements of the dielectric wall accelerator have met specifications for the final accelerator configuration, showing standoff fields in excess of 100 MeV / m. Conclusion: At least one of these new systems is expected to be completed and being used for patient treatment before the end of 2008. With the significant reduction in complexity and cost the successful demonstration of these systems will likely lead to a more widespread adoption of Proton Beam Radiation Therapy. Conflict of Interest: Kenneth Gall is a founder and Chief Technology Officer of Still River Systems Incorporated, a company involved in the design, production and clinical implementation of proton beam radiation therapy systems.
TU‐EE‐A1‐06: Model for Development of Tumor Vasculature Based On Monte Carlo Simulations
Authors: Adhikarla V , Jeraj R
Published: July 2007

Purpose: The work aims to simulate growth of tumor vascular structure, based on the anatomical and biological information obtained from a contrast‐enhanced CT and hypoxia PET images. The model would be beneficial for optimization in anti‐angiogenic therapy. Materials and Methods: The vascular structure was simulated as a two dimensional matrix. MATLAB environment was used in simulations. An upper limit on the vascular density was fixed and vessels were grown from a given root vessel into the space of a specific size. The vessel thickness was simulated variable, with vessels thinning as they grew. The vessel node selected for sprouting is the one which has the least density of oxygen around it. The direction in which the vessel sprouts is calculated by checking on the least dense direction around it using Monte Carlo simulation, and considering into account that vessels have a tendency to grow forwards. Results: The developed vasculature growth model was successfully implemented. The thicker vessels grown correspond to the thicker blood vessels and the thinnest ones to the capillaries. The density of the vasculature could be successfully controlled and modified to achieve tumor vasculature growth in the region required. A region of higher vasculature density corresponds to a less hypoxic region and vice‐versa. A known hypoxia map and an initial vasculature structure are the only requirements for the simulation. Conclusions: Micro‐PET/CT images acquired in vivo provide the basic information needed for tumor growth simulations. The model could be applied as an effective tool for optimizing anti‐angiogenic therapies, when fully benchmarked to the experimentally measured conditions. The vasculature growth in a three dimensional matrix is expected to put a much more severe restriction on the computational power needed.
WE‐C‐L100F‐01: The Promise of 256 Slice CT
Authors: Rifu T , Mather R
Published: July 2007

Advancements in CT technology invariably lead to new clinical applications. Helical scanning opened up anatomy outside of the brain. Multislice technology allowed fast volumetric scanning. Sixteen slice introduced vascular applications. Sixty‐four slice scanners made cardiac scanning a matter of routine practice. In recent years, each of the evolutionary steps in CT technology has sought to improve the way helical scanning is accomplished.

Toshiba Medical Systems has developed a Beta version of a 256 slice CT system to cover nearly 13 cm of anatomy in a single rotation with 0.5 mm slices to allow the complete organ coverage in a single rotation. This system can acquire images of an entire volume at a single, instantaneous time point which significantly reduces motion artifact and eliminates contrast phase differences within the volume. Since this system does not require helical acquisition for volumetric imaging, it will deliver significantly less dose for CT coronary angiography exams as well as reduced dose in most other applications.

With its wide volume coverage, the 256 slice system promises to revolutionize the way we approach acute stroke patients, the way we look at myocardial perfusion imaging, and the way we image other moving body parts such as the lung during respiration and peripheral joint motion.
WE‐C‐M100J‐01: Cancer Treatment with MRI‐Guided High Intensity Focused Ultrasound
Authors: Chen L
Published: July 2007

HIFU has long been known to offer the potential of very precise “Trackless lesioning” but has only recently with the current high quality methods of medical imaging, become a practical possibility for clinical treatment.

Focused ultrasound uses ultrasound energy for tissue ablation. When the sound waves are focused to a small volume in the body, the intensity is high and the temperature at the focal point rises to 70–95°C, high enough to ablate the tissue. Proper treatment design will ensure that the energy density will be high at the focal point but low at other locations, and thus avoiding damages to nearby normal tissues.

The volume of ablation (lesion) following a single HIFU exposure is small and will vary according to transducer characteristics, but it is typically cigar shaped with dimensions in the order of 1–3 mm (transverse) × 8–15 mm (along the beam axis). To ablate clinically relevant volumes these lesions must be placed side by side systematically to “paint out” the target tumor.

Simply calculating an optimal treatment plan is not enough to ensure optimal outcome. Patient anatomic variability and tissue inhomogeneities have been shown to produce vastly different responses to thermal energy deposition, especially deep in the body. High quality imaging techniques can provide precise visualization and localization of the tissue damage. MR images enable the physician to localize the tumor and plan the treatment in the full 3 dimensions. Real‐time MR thermometry can provide an indication of tissue damage if critical temperatures are known.

In several centers worldwide, HIFU is now being used clinically to treatment solid tumors (both malignant and benign), including those of the brain, breast, liver, kidney, prostate, bone matastases, pancreas and soft‐tissue sarcoma. The MRgFUS unit is currently available for clinical applications.

This lecture will provide an overview of MrgFU and introduce the equipment that is commercially available for clinical applications in cancer treatment.

Educational Objectives:

1. The principles of MRgFUS.

2. Advantages and limitations of MRgFUS for tissue ablation.

3. Potential clinical applications of MRgFUS for cancer treatment.
WE‐C‐AUD‐04: Monte Carlo Simulations of Air Cavities in Phantoms Submerged in Magnetic Fields
Authors: Yanez R , Dempsey J F
Published: July 2007

Purpose: We present studies in support of the development of a magnetic resonance imaging (MRI) guided intensity modulated radiation therapy (IMRT) device for the treatment of cancer patients. Fast and accurate computation of the absorbed ionizing radiation dose delivered in the presence of the MRI magnetic field are required for clinical implementation. Method and Materials: The fast Monte Carlo simulation code DPM, optimized for radiotherapy treatment planning, is modified to simulate electron transport in uniform, static magnetic fields. Results: Simulations of dose deposition in inhomogeneous phantoms in which a layer of air is sandwiched in water shows that a lower MRI field strength is to prefer in order to avoid dose build‐up, due to returning electrons, in tissue surfaces. For a magnetic field of 0.3 T, cavities of up to several millimeters in thickness are safe from build‐ups. Larger cavities require the use of opposing beams to cancel out the effect of returning electrons. Conclusions: The effect of a B=0.3 T magnetic field in the dose deposition by a Co‐60 source is only appreciable in large air cavities, where a dose build‐up due to returning electrons is formed near material boundaries. Cavities with dimensions smaller than the gyration radius are not severely affected by dose build‐ups. For B=0.3 T cavities of the order of several millimeters are safe from dose build‐ups. Research sponsored by ViewRay Incorporated.
WE‐C‐M100F‐04: Electron Density Mapping for MRI‐Based Treatment Planning
Authors: Wang C , Chao M , Xing L
Published: July 2007

Purpose: To develop an accurate MRI‐based treatment planning method by assigning MR images the desired electron density information through the use of diagnostic CT and a deformable registration model. Method: MR and CT images are registered using deformable or rigid registration method depending on the disease site or the patient positioning when acquiring the CT data. Based on the voxel‐to‐voxel correspondence established by the registration, the electron density distribution of the CT images (this can be a diagnostic CT with patient positioned arbitrarily) is mapped onto the MRI and plan is computed based on the modified MR images, which take advantage of the useful features of CT and simulation MRI. To evaluate the accuracy of the MRI‐based dose calculation, six clinical cases, including three brain and three head‐and‐neck cases are studied. For each case, three IMRT plans are computed with the same beam configuration but different image basis: CT, MRI with mapped electron density, and MRI with voxel values replaced by that of water. The dose distributions from the three types of calculations are compared. Results: In all cases, the MRI with mapped electron density yielded very close dose values as compared with the CT‐based calculation. The maximum discrepancy between the two is found to be less than 2.0% in the high dose region. Compared to the MRI with mapped CT electron density, MRI with uniform water voxel value yielded quite different dose distributions in all regions, and the dose discrepancy can be as large as 3–5% for both brain and H&N cases. Conclusion: The MR images with mapped CT electron density yield very similar treatment plan compared with the CT‐based calculations. The approach has potential to eliminate simulation CT by planning a patient treatment with only simulation MRI and diagnostic CT data.
WE‐C‐M100F‐05: Methodology and Guidelines in Treatment Planning of IMRT for Lung Cancers
Authors: Liu H , Wang X , Hu C , Yom S , Wang S , Tucker S , Liao Z , Chang J , Komaki R , Mohan R
Published: July 2007

Purposes: To review the current clinical outcome of lung cancer patients treated with IMRT; to develop a treatment planning methodology and guideline for using IMRT in treating lung cancers. Methods: Lung cancer patients treated with IMRT were reviewed with respect to clinical outcome and treatment plans used. Major toxicity including radiation pneumonitis and esophagitis were analyzed. In the process of IMRT inverse planning, methodologies of selecting appropriate beam angles and planning objectives were developed to limit the spread of beam angles and low‐dose radiation in protecting normal lung and other critical structures. Important IMRT planning parameters and their guidelines were derived, e.g. on the number of beams, beam angles, degree of intensity modulation, and DVH constraints for normal tissues. Results: We reviewed 68 locally advanced lung cancer cases treated with IMRT and with adequate clinical followup time. The rate of grade ⩾ 3 TRP at 12 months was reduced to 8% in IMRT patients in contrast to 32% in 3D‐CRT patients treated historically, despite larger GTV sizes in the IMRT cohort. Mean lung dose and the volume of low‐dose radiation to healthy lung may be important factors affecting the risk of radiation pneumonitis. In achieving quality IMRT plans, 6 beams were used in average, with the optimal beam angles being most frequently along the anterior‐posterior direction of the thorax; the number of MLC segments per beam was confined to be less than 15 with the total MU per fraction approximately 3 times of the prescribed tumor dose. Conclusions: With proper considerations given to treatment planning methodology and other technical issues involved, IMRT may be an effective and safe modality in radiation therapy of locally advanced lung cancers.
WE‐C‐M100F‐09: Dosimetric Comparison of Linac‐IMRT and Helical Tomotherapy (HT) for Head and Neck Cancer
Authors: Zhang X , Penagaricano J , Moros E , Corry P , Ivy A , Yan Y , Youssef E , Ratanatharathorn V
Published: July 2007

Purpose: To perform a dosimetric comparison between Linac and Helical Tomotherapy‐based IMRT on head and neck (H&N) patients treated similarly and to evaluate any potential clinical consequences of dosimetric differences. Method and Materials: This is a retrospective study of 23 H&N cancer patients treated on HT. All patients were planned both on the HT and the Pinnacle planning system. The prescribed dose was 66 Gy at 2.2 Gy per fraction to the PTV. The dosimetric parameters used for comparison were: R95 = the ratio of the average dose to 95% of the PTV to the prescribed dose; Rc = the ratio of the PTV coverage to PTV volume, where the former was defined as the volume enclosed by the 66 Gy isodose surface; Biologically equivalent doses (BED) to organs at risk (OAR) and PTV dose homogeneity were also studied. The tolerance range or TR (standard deviation/PTV average dose) was used as a surrogate for PTV dose homogeneity evaluation. Results: R95 results indicated that both IMRT techniques produced comparable conformal plans. Rc values showed that HT plans generally provided better tumor coverage. TR results suggest that PTV dose homogeneity was better for HT plans. Finally, the average OAR BEDs showed a trend of better normal tissue sparing with the HT plans. The exception was for the spinal cord, in which the maximum BED using HT was slightly lower than the maximum BED using linac‐IMRT. Conclusion: This study suggests that HT plans had in general better dosimetric characteristics, especially regarding tumor coverage, PTV dose homogeneity and normal tissue sparing in physician approved plans. Dose reductions to OAR may not yield any clinical differences in outcome in virtue of the delivered OAR doses which are well below normal tissue tolerance.
WE‐C‐M100F‐10: Evaluation of the Beam Segmentation Algorithm in the KonRad IMRT Treatment Planning System
Authors: Reitz B , Miften M
Published: July 2007

Purpose: Reducing the treatment time for IMRT patients is highly desirable. For step‐and‐shoot IMRT plans, the delivery time of a treatment fraction is determined by the number of beam segments, and to a lesser extent by the total number of monitor units (MUs). The objective of this work was to study the segmentation efficiency of the new clinical Siemens KonRad inverse treatment planning system (TPS) and compare it to the CMS XiO TPS. Method and Materials: For head and neck, liver and prostate cancer patients, step‐and‐shoot IMRT plans were designed using both CMS XiO and Siemens KonRad. Number, direction and energy of beams were the same in both systems. The plans were optimized to achieve the same clinical objectives concerning dose to the target volume and to the relevant organs‐at‐risk. The number of intensity levels were minimized until the clinical objectives could not be achieved anymore. DVHs, EUD, mean‐ and maximum‐doses were compared, as well as the number of beam segments and MUs. Finally the beams of each plan were delivered individually on a MapCheck device to verify the agreement between calculations and measurements to be less than 3%–3 mm distance‐to‐agreement. Results: Plans optimized with KonRad resulted in fewer segments and lower number of MUs and therefore reduced delivery time, while achieving similar dose distributions. CMS XiO plans exhibited a slightly steeper dose fall‐off outside the target volumes, however the difference was not clinically significant. DVHs to organs‐at‐risk were comparable. All calculated dose distributions passed the 3%–3 mm dose verification check. Conclusion: Both commercial systems produce clinical similar plans. The sequencer of the KonRad system is more efficient, reducing the number of beam segments and therefore delivery time on average by more than 20%.
WE‐C‐BRA‐05: Using Conformal Radiobiology to Find the Best Treatment Plan
Authors: Nahum A
Published: July 2007

How can we use TCP and NTCP models in optimising radiotherapy outcome? The most basic or Level‐I optimisation is to take an existing ‘standard’ treatment plan with its standard total dose and fraction size, compute the NTCP and then adjust the total dose until an acceptable NTCP value is obtained: isotoxic customised dose prescription. This will yield immediate benefits in, for example, lung‐tumour radiotherapy. Level‐II optimisation uses the TCP and NTCP functions ‘upfront’ in the optimisation process to determine the beam weights and even angles, or, in the case of IMRT, to perform ‘inverse planning’. A typical criteria might be ‘Maximise the TCP for NTCP = 2.5%’ or ‘Minimise the NTCP for TCP = 90%’. In this Level‐II mode no constraints need be set regarding uniform dose in the target volume — the TCP model will take of this. However, maximum doses may well need to be set outside the target volume.

Another exciting area is the connection between fractionation sensitivity and dose distributions in normal tissues. The ‘classical’ LQ‐based Withers isoeffect formula can be easily modified to reflect increasingly conformal dose distributions in organs at risk. The validity of this modification has been effectively demonstrated by the safe use of very large fractions in treating lung tumours with (highly conformal) body stereotaxy; the oversimplified BED concept would have forbidden such effective regimens.

We need to start using radiobiologically based optimisation, without waiting until the TCP and NTCP models are ‘perfect’. The clinician may still use conventional tools such as single‐CT slice isodoses and DVHs to approve a radiobiologically optimised plan but she/he is going to find a marked improvement in the quality of such a plan.

Educational Objectives:

1. Appreciate the potential of using TCP and NTCP models in treatment plan optimisation.

2. Understand what is meant by ‘isotoxic’ dose prescription.

3. Understand the limitations of the Withers' isoeffect formula and how it can be modified to approximately account for dose distributions in organs at risk.
WE‐D‐L100J‐06: Ultrasonic Scatterer Size Estimations in Liver Tumor Differentiation
Authors: Liu W , Zagzebski J , Kliewer M , Varghese T , Hall T
Published: July 2007

Purpose: To evaluate the utility of an ultrasound scatterer size estimation method (combined with B‐mode features) to differentiate liver hemangioma (benign) from malignant liver tumors in a clinical study. Method and Materials: Scatterer size is estimated from the frequency dependence of the backscatter coefficient, obtained using a reference phantom method. A tissue mimicking phantom containing glass beads randomly distributed in gel was scanned by a Siemens Antares scanner equipped with a linear array transducer to verify the scatterer size data reduction methodology prior to use in clinical determinations. Excised normal human liver tissues and in vivo liver hemangiomas and malignant tumors were evaluated using the methodology using either the Siemens Antares or GE Logic 9 scanners. The results were correlated with biopsy findings. Results: For power spectra estimation, Welch's average approach yields better results than a periodogram and an autoregressive approach. Averaging over 15 independent samples with a gating window length of around 10 wavelengths is desirable to obtain reasonable size estimates based on the trade off between spatial resolution and signal‐to‐noise ratio. Therefore, angular or elevational compounding was used for in vivo data acquisition. A liver hemanigomas exhibited larger effective scatterer sizes than surrounding normal liver tissues. Combining B‐mode features, such as a halo sign, with scatterer size estimates may be useful to confirm the benign or malignant classification. Conclusion: Ultrasound scatterer sizes estimated from frequency dependent backscatter properties may provide useful clinical information, not readily available in conventional ultrasound B‐mode images. Preliminary in vivo results suggest that it may be an effective processing method for distinguishing liver hemangiomas.
TH‐C‐M100F‐01: Chain Acceleration of Protons Using High‐Power Laser Pulses
Authors: Velchev I , Fourkal E , Ma C
Published: July 2007

Purpose: To study the temporal evolution of the target's charge and its effect on the acceleration of protons. To optimize the proton energy by designing new target geometry, in which the protons experience the full acceleration potential created by the laser pulse. Method and Materials: The process of proton acceleration is studied by means of fully relativistic two‐dimensional PIC simulations (2D3V PIC) with initial conditions similar to a realistic experimental situation, where a relativistically intense (I0=1.92×1021W/cm2, a=30) ultrashort (∼40fs) infrared (λ = 800nm) laser pulse interacts with a Cu target of thickness 400∼nm at normal incidence. Results: Protons at rest on the back surface of the target were initially studied and their dynamics was analyzed. It was determined, that in this geometry the protons experience acceleration in only 25% of the peak of the electric field. The conditions were subsequently optimized by assuming a moving proton bunch incident on the target. A novel two‐stage acceleration scheme was proposed (chain acceleration) and a total energy gain of 50% was estimated under optimum conditions. The implications for proton therapy are discussed. Conclusion: It is shown, that in a conventional proton acceleration geometry using lasers, the acceleration conditions are far from optimum due to the fact that the protons are expelled from the target before the maximum charge build up is reached and as a result experience reduced acceleration potential. We proposed and analyzed a new acceleration scheme employing a double target, in which the protons accelerated in the first stage are properly timed with the laser pulse that ionizes the second target. Under such conditions the energy gain is substantial, opening up the way for proton acceleration above 100MeV using the today's state of laser technology.
TH‐C‐M100F‐04: A Deformable Lung Tumor Tracking Method in Fluoroscopic Video Using Active Shape Models
Authors: Xu Q , Hamilton R , Schowengerdt R , Jiang S
Published: July 2007

Purpose: Elastic tumor deformation and different intra‐ and inter‐fractional tumor paths between inhalation and exhalation were observed for some lung patients. For high accuracy and possibly 100% duty cycle dose delivery, we propose and evaluate a noninvasive method to fluoroscopically track location and shape variations of lung tumors with different types of deformations. Method and Materials: During a fluoroscopic simulation, lung tumor contours in one complete respiratory period are manually drawn by an expert. Each contour is described by 65 landmarks. The respiratory period is divided into 9 phases and a Point Distribution Models (PDM) statistically describing typical tumor shape variations are built for each phase. When tracking starts, the breathing phase for an incoming frame is first determined by the respiratory signal generated simultaneously from diaphragm motion and the PDM for this frame is also found. Starting from an initial estimate of the tumor contour, the Active Shape Models algorithm searches the area near each landmark and finds a better location. Based on the shifts found for these landmarks, the initial estimate is deformed within a certain range of typical shape variations found in PDM and also rigidly transformed to match the shifts. The new generated contour iteratively updates the previous contour estimate until no significant difference appears between two consecutive iterations or a user defined number of iterations is reached. Results: Tumors demonstrating distinct types of deformations in fluoroscopic videos were well tracked. All the landmarks of the tracked objects were manually revised by an expert using a GUI tool. The average magnitude of the deviation between the tracked and revised results was within 2 mm for 95% the landmarks and within 3mm for all landmarks. Conclusion: This method affords precise tracking of lung tumor location and deformation and may be used for real‐time tracking or DMLC radiotherapy.
TH‐C‐M100F‐06: Therapeutic Uniform Scanning Proton Beam Development and Characterization: Longitudinal Results
Authors: Mascia A , Farr J , Hsi W , Allgower C , Nichiporov D
Published: July 2007

Purpose: To utilize a new depth‐dose detector and to construct, develop, characterize and quantify longitudinal beam profiles with an active uniform scanning proton beam delivery system. Method and Materials: The active uniform scanning beam delivery system is composed of two primary devices—wobbler magnet and binary range modulator. Through transverse scanning a large proton beam spot at about 10–15Hz, the wobbler magnet creates a flat integral transverse field. The binary range modulator constructs spread out Bragg peaks (SOBP) by incrementally stepping a pristine Bragg peak by 3mm or 6mm in depth. A depth‐dose detector was constructed to acquire all the longitudinal beam data. This detector, the Multi Ion Chamber Detector (MLIC), is composed of 122 parallel‐plate ion chambers in a 1D array. Results: Upon proper calibration and commissioning, the MLIC detector performed well for the acquisition of longitudinal beam data, providing adequate discrete data with 1.8mm spatial resolution. The full width half maximum of various energy pristine Bragg peaks was measured. This data necessitated changing the range modulator step size from 6mm to 3mm for all SOBPs below a certain proton energy (i.e. equivalent to 12.0cm range in water). The pristine peak and SOBP exhibit a range deficit that decreases as proton energy decreases. This effect is also correlated to the FWHM of the pristine peak. A library of various SOBP extents, from 14.5cm to 2.2cm, was constructed and shown to pass clinical specification. For a given SOBP, the skewness, defined as the slope of the 100% isodose or SOBP “flattop”, was correlated to proton range in water. Conclusion: The MLIC proved useful, efficient and satisfactory in measuring depth‐dose profiles for active scanning proton beams. The active scanning beam delivery system delivered therapeutically useable fields with increased modularity and flexibility compared to passive scattering techniques.
TH‐D‐L100J‐01: XCAT: A Dedicated, Portable, Head and Neck Volume CT Scanner
Authors: Stayman J , Alspaugh J , Goodsitt M , Christodoulou E
Published: July 2007

Purpose: Flat‐panel x‐ray detection technology has allowed for efficient data collection with simplified gantries. By using this technology and focusing on head and neck scanning, highly compact portable volume CT (VCT) scanners can be constructed. We present a portable unit designed with the intraoperative environment in mind. A number of novel approaches were used in this system design including a pivoting front axle for joint transportability and stability; a dual x‐ray source design for maximizing field‐of‐view and minimizing source‐to‐detector distance; an offset geometry for maximizing the transaxial field‐of‐view; and built‐in leaded acrylic to minimize scattered radiation. Method and Materials: We present the xCAT prototype, a portable flat‐panel VCT system. We present CT images from this system of a head phantom, line pair phantom, and initial clinical studies. We also measured patient dose using thermoluminescent dosimeters and measured the scattered radiation profile at several points around the scanner. All acquisitions were made at 120 kVp and approximately 80 mAs. Results: The xCAT system is highly mobile and weighs around 450 pounds and is 31″ wide, 40″ long and 66″ tall. The image volume has a 22 cm transaxial diameter and is 13 cm in axial extent. Line pairs up to 12 lp/cm are clearly visible in the reconstructed images. Head scans produce good images of high contrast features with a low patient effective dose (approximately 0.25 mSv). Measured scatter dose was 1.5 μGy at the position of the operator's thyroid. Conclusion: The xCAT is a highly portable VCT scanner that produces images acceptable for bone window imaging. As such, it should have significant utility for high contrast imaging situations such as intraoperative sinus imaging. For more general purpose scanning, significant improvements in low contrast imaging will be necessary. Conflict of Interest: Research sponsored by Xoran Technologies, Inc.