bluuebunny/crossref_metadata_2025 huggingface dataset:
TH‐E‐330A‐07: Evaluation of Scatter Mitigation Strategies for X‐Ray Cone‐Beam CT
Authors: Lasio G , Williamson J
Published: July 2006

Purpose: Linac‐mounted cone‐beam CT (CBCT) imaging is a promising approach for improving the soft‐tissue targeting accuracy of external‐beam radiotherapy. However, the large proportion of signal due to scattered radiation results in large cupping artifacts and reduced contrast‐to‐noise ratio (CNR), hampering delineation of soft‐tissue structures. The goal of this investigation is evaluate the impact of three scatter mitigation strategies: scatter removal (SR), subtractive scatter corrections (SSC), and anti‐scatter grids (ASG) on CBCT image CNR.

Method and Materials: A one‐dimensional model was developed that to predict image noise, intensity, and contrast from the photon flux incident upon the flat‐panel detector. Using previously published scatter‐to‐primary ratio (SPR) data, the impact of Poisson signal statistics, Gaussian readout noise, reconstruction filter, and total air‐kerma on image uniformity, contrast, and CNR was evaluated. Using previously measured CBCT scatter and primary grid transmission; the impact of ASGs, SSR, and SR (e.g., via bowtie filters) was evaluated. In addition, synthetic polyenergetic CBCT projections of cylindrical low‐contrast threshold phantoms, including compound Poisson process noise, were used to evaluate the impact of scatter mitigation on filtered backprojection (FBP) images. Results: Relative to complete SR, typical CBCT scatter reduces CNR by 30%–70% depending on patient diameter. Both 1D analysis and simulated images demonstrate that SSC effectively reduces cupping artifacts but does not improve CNR. ASGs with high scatter selectivity (>5) or large primary transmission (>0.7) modestly increase (20–30%) CNR for thick subjects. However, ASGs diminish CNR for smaller body sections or for exposure levels where additive noise dominates sinogram signal statistics. Conclusion: Correcting CBCT sinograms for scatter radiation is effective in reducing structural image artifacts but does not improve CNR. Antiscatter grids effectively suppress scatter artifacts but improve CNR only in selected clinical settings. This project was supported in part by a grant from Varian Medical Systems.
TH‐E‐ValB‐02: Image Registration‐Based Tool for Correlation Studies of Radiation‐Induced Fibrosis and Local Dose‐Related Parameters in Conformal Non‐Small Cell Lung Cancer Radiation Therapy
Authors: Stroian G , Souhami L , Martens C , Collins DL , Seuntjens J
Published: July 2006

Purpose: Despite the clinical importance of radiation therapy (RT) induced pulmonary injury, methods to accurately predict the degree of RT‐induced dysfunction are still lacking. Many investigators are trying to develop methods to relate dose‐volumetric parameters to the risk of RT‐induced lung injury, but no consensus has been reached about which of these parameters should be used. Other investigators are attempting to develop a dose‐response curve for regional RT‐induced damage and several local parameters like computed tomography (CT) density and single photon emission computed tomography lung perfusion and ventilation have been measured to allow an estimate of local injury. Method and Materials: A software tool was developed for the evaluation of the correlation between RT‐induced fibrosis and local dose‐related parameters for a group of non‐small cell lung cancer (NSCLC) patients. Local dose‐related parameters were determined using both conventional and Monte Carlo (MC) dose calculations algorithms. The relation between dose, calculated on the planning CT scans and RT‐induced fibrosis, identified on follow‐up CT scans, was established through linear registration. Subsequently, tissues densities were determined and automatic segmentation methods were developed for lung and fibrotic tissues. Results: One patient participating in a phase I/II NSCLC multi‐center clinical trial was chosen for illustration. Patients' response to treatment was quantified by evaluating the variation of lung and fibrotic tissue volumes over the follow‐up period. MC and conventional dose‐ and normalized total dose‐response curves were generated for the RT‐induced fibrosis. Fibrosis probability was shown to increase with increasing conventional and MC dose as well as with increasing conventional and MC normalized total dose. Moreover, fibrosis probability was also correlated with MC predicted hot spots in high dose regions.Conclusion: The presented tool allows a systematic numerical study of the relations between RT‐induced fibrosis and dose, normalized total dose and MC predicted hot‐spots in high dose regions.
TH‐E‐ValB‐04: Dosimetric Impact of Changed FLT Uptake in AML Patients Treated with Chemotherapy
Authors: Vanderhoek M , Pyzalski R , Jaskowiak C , Jeraj R
Published: July 2006

Purpose: The PET radiotracer [F‐18]FLT (3′‐deoxy‐3′‐[F‐18]fluorothymidine), used to measure cellular proliferation, has the potential to validate the efficacy of chemotherapy. We investigate the effect of chemotherapy on the biological distribution and radiation dosimetry of FLT in patients with acute myeloid leukemia (AML). Method and Materials: Cellular proliferation was measured in adult AML patients injected with 5 mCi of FLT. Dynamic and whole body PET/CT scans were acquired one day prior to chemotherapy and one week after the completion of chemotherapy using a GE Discovery PET/CT Scanner. Organs were manually contoured in the PET images at multiple time points and time‐activity curves were generated for each contoured organ. Organ cumulative activities, organ radiotracer doses, and total body dose were determined using the standard adult male model and the RADAR method of dose calculation. Results: The biological distribution of FLT changed as a result of chemotherapy and this redistribution affected individual organ and total body radiation doses. The toxic effect of the chemotherapeutic drugs on the leukemia cells resulted in a five‐fold reduction of FLT activity in the bone marrow post‐chemotherapy. This reduction in the bone marrow uptake was accompanied by a three‐fold increase in FLT activities and radiation doses to the liver, kidneys, gallbladder, and adrenals while that of the spleen doubled. The total body radiation dose increased 30% post‐chemotherapy, given identical bladder voiding conditions.Conclusion: Systemic therapies such as chemotherapy can lead to significant changes in the biological distribution and dosimetry of radiotracers used in PET imaging for treatment assessment. Knowledge of these changes could impact the administered radiotracer dose to patients. Care should be taken in determining a suitable radiotracer dose for each specific case in order to avoid unnecessary dose yet maintain appropriate signal‐to‐noise ratios.
TH‐E‐ValB‐05: Analysis of Early Treatment Failure in Patients with Newly Diagnosed GBM Using Advanced MR Imaging
Authors: Choy R , Ozturk E , Crawford F W , Chang S M , Nelson S J , Pirzkall A
Published: July 2006

Purpose: To seek imaging characteristics predictive of early treatment failure (EF) following concurrent radiation/chemotherapy (RT/CHT) in patients with newly diagnosed GBM s/p surgical resection using advanced MRI techniques (3D1H spectroscopy (MRSI), diffusion weighted (DWI) and perfusion weighted (PWI) imaging). Methods and Materials: 26 patients were imaged at 1.5T prior to RT/CHT (pre‐RT) and immediately after RT (post‐RT). Analyzed imaging parameters included peak heights of choline (Cho), creatine, N‐acetyl‐aspartate (NAA), lactate and lipid; Cho‐to‐NAA (CNI), Cho‐to‐Cr (CCrI) indices and excess‐choline (Ex(Cho)); parametric maps of percent‐recovery and apparent diffusion coefficient (ADC) were calculated. Mutually exclusive morphologic abnormalities were contoured as contrast‐enhancement (CE), T2‐hyperintensity (T2), resection‐cavity, necrosis, and a reference for normal appearing white matter. Patients were categorized as EF if any new CE appeared or if the CE volume increased by >25% at post‐RT. Imaging parameters were subjected to a Wilcoxon Rank Sum to test statistical significance between EF and non‐EF. Results: 9/26 patients were classified as EF. Both patient groups did not differ statistically in terms of age, volume of CE or T2 at pre‐RT. There were trends to higher Cho, CNI, and Ex(Cho) for the EF group at pre‐RT, however, these did not reach statistical significance. Statistically significant findings within CE at post‐RT were mainly associated with Cho and related indices and included lower ADC and %recovery values suggesting higher cellularity and increased leakiness of vessels in the EF vs non‐EF group. Conclusion: Even though our preliminary data on 26 patients could not identify imaging parameters significantly different between EF and non‐EF patients at pre‐RT, the demonstrated trend encourages further evaluation of additional 31 additional patient data sets acquired at 3T in order to increase statistical power. In addition, the reported significant changes at post‐RT suggest that the parameters may be valuable in assessing treatment effects.
TH‐E‐224A‐01: Planning and Delivery of Dynamically Modulated Electron Radiotherapy
Authors: Klein E , Vicic M , Ma C , Low D , Li J
Published: July 2006

Purpose: Evaluate capabilities of modulating electrons using the installed multileaf collimators, computing modulated electron dose distributions using Monte Carlo (MC) and optimizing the dose distributions. Materials and Methods: The modulated electron radiotherapy (MERT) evaluation was conducted with a Varian 120leaf MLC for 6–20 MeV. To provide a sharper penumbra, measurements were conducted with short SSDs (70–85cm). Aperture sizes (AS) ranging from 7–100mm (surface) were configured for measurements and modeling, using BEAMnrc MC code with 109 particles incident on the exit window and DOSXYZnrc for phantom dose calculations. Parameter included: Voxel size 0.2×0.2×0.1cm3, photon and electron transport energy cutoffs 0.01MeV and 0.521MeV, respectively. Verification measurements were performed with film and micro‐ion‐chambers. Calculated and measured data were analyzed in MatLab. Once validation of static fields was successfully completed, modulated portals (segmented and dynamic) were configured for treatment and composed for calculation using DOSXYZ. Results: Beam penumbra sharpness degraded with: decreasing energy and AS; and increasing SSD. PDD decreased significantly with AS. Nearly identical profiles when fluences were delivered by segmental and dynamic MLC sequences. The exception was in the peripheral and bremmstrahlung dose, which was higher in the segmented and dynamic deliveries, respectively. Calculations agreed with measurement within a distance‐to‐agreement of <3mm. With segmented delivery, we found it necessary to introduce small (∼1.5mm) gaps between segments to retain ranges desired in the particular segment's path. The treatment time to deliver 5 segments of 3 energies, was ∼90s, including console reprogramming. Conclusions: This study shows that MERT is feasible and provides conformal electron‐beam dose distributions.

Acknowledgement: The primary author thanks Varian for technical support.
TH‐E‐224A‐04: IMRT Film QA in a Heterogeneous Anthropomorphic Phantom
Authors: Ranade M , Lynch B , Li J , Dempsey J
Published: July 2006

Purpose: To study the agreement of heterogeneous IMRT treatment planning dose calculations with radiochromic film (RCF) and radiographic film (RGF) measured in anthropomorphic heterogeneous phantoms. We intend to use heterogeneous‐phantom film dosimetry to improve IMRT quality assurance (QA). Method and Materials: A commercial heterogeneous anthropomorphic head‐and‐neck phantom, with equivalent tissue bone, soft tissue and air regions was employed for dosimetry verification (CIRS, inc.). High sensitivity EBT radiochromic film (ISP, Inc.) and EDR2 radiographic film (Kodak) were employed simultaneously for film dosimetry. The phantom had 12 axial sections, each 25mm thick, with alignment pins and a compression plate. We selected a plane containing bone, soft tissue and an air gap for IMRT verification. The RCF and RGF were cut to match the selected plane, inserted between the sections and light sealed. A 6MV IMRT head & neck treatment plan with 7 beams was delivered using a commercial linac (Varian Clinac 2100/CD). The RCF was scanned avoiding artifact described in a companion work using a CCD film scanner (Epson 1680 scanner). The dose distributions were compared with the treatment plan. Results: Both RCF and RGF disagree with the treatment plan dose near the air cavities by 15%. Away the air cavities 5% agreement was obtained with RCF. Pronounced artifacts were observed in the RGF that were not observed in the RCF. Conclusion: IMRT dosimetry and quality assurance can be improved by using anthropomorphic phantoms that incorporate realistic heterogeneities such as bone, soft tissue and air gaps, and corrections to dose calculations can be applied. We hypothesize that RGF suffers from Cerenkov in air cavities and in less opaque phantom materials, and this will be investigated in further studies.

This work was supported in part by NCI grants R01‐CA‐100636 and N43‐CM‐52214.
TH‐E‐224A‐06: Dosimetric Properties of 8 and 10 MV Photon Beams From a Flattening Filter Free Clinical Accelerator
Authors: Vassiliev O , Titt U , Gillin M , Mohan R
Published: July 2006

Purpose: In previous studies we indicated that removing the flattening filter from the beamline could improve some photon treatments. Optimal energies for treatments with unflattened beams may need to be higher than for the same treatments with flattened beams, in part owing to softer photon spectra. Since for flattened beams best results are often achieved at 6 MV, in this study of unflattened beams we investigated basic properties of 8 and 10 MV photon fields. Methods and Materials: This is a computational study based on the Monte Carlo method. We used the BEAMnrc program to model the Varian Clinac 2100 series accelerator. Results: Increasing the energy from 6 to 8 or 10 MV: (1) improves the efficiency of bremsstrahlung production. We calculated photon energy fluences (per incident electron) in air at 100 cm from the source for a 10×10 cm2 field. The ratios of energy fluences of unflattened beams to flattened 6 MV beam were 2.29, 4.14, and 8.23 at 6, 8, and 10 MV, respectively; (2) improves beam penetration. The percent depth doses (PDDs) for a 10×10 cm2 field at 10 cm depth were 63.6, 67.2, and 69.0 at 6, 8, and 10 MV; (3) reduces skin dose. PDDs for the above field at 0.3 cm depth were 76.5, 70.1, and 63.3 at 6, 8, and 10 MV, compared to 69.5 for a flattened 6 MV beam; (4) moderately increases nonflatness. The ratios of maximal to minimal doses within 80 % of the width of a 10×10 cm2 field at 10 cm depth were 1.14, 1.18, and 1.23 at 6, 8 and 10 MV. Conclusion: The main dosimetric parameters of unflattened 6 MV beams can be improved by increasing the energy to 8 or 10 MV. Conflict of Interest: Research is sponsored by Varian Medical Systems.
TH‐E‐224C‐02: Performance Assessment of a Deterministic Method Incorporating Coupled Photon‐Electron Transport for Photon Beam Dose Calculations On Acquired CT Data
Authors: Wareing T , Failla G , Vassiliev O , Barnett A , McGhee J , Titt U , Horton J , Mourtada F
Published: July 2006

Purpose: To evaluate the performance of a neutral and charged particle deterministic solution method for external photon beam dose calculations using acquired CT data. Methods and Materials: The Attila® radiation transport code, which solves the differential form of the linear Boltzmann transport equation for neutral particles and the Boltzmann‐Fokker‐Planck transport equation for charged particles, has been adapted for calculating dose distributions from acquired CT images. Comparisons were made with the Monte Carlo code EGSnrc (DOSXYZnrc) for a sample prostate treatment consisting of 8 10×10 cm2 open beams with a realistic 6 MV photon spectrum. The Attila computational mesh consisted of 125,000 arbitrary tetrahedral elements, of approximately uniform size, which encompassed an imaged torso region. CT numbers were mapped to the Attila tetrahedral elements using one of four materials: air, tissue, adipose tissue, and bone, each having four discrete densities. Attila dose was determined at the image resolution by extracting the energy dependent particle flux at each 1×1×2.5 mm3 pixel from Attila's calculated linear discontinuous finite element spatial representation, and multiplying by the energy dependent dose deposition response for that pixel material and density.

The DOSXYZnrc calculation used 2.5×2.5×2.5 mm3 voxels, and was run until an average statistical uncertainty of 0.4% was achieved for voxels in the target region. Results: Computational times Attila and DOSXYZnrc were 36 CPU minutes (2.4 GHz Opteron processor) and approximately 8,500 CPU minutes, respectively. Employing a deterministic electron cut‐off below 300 keV reduced the Attila computational time to 22 CPU minutes. Agreement between both codes was excellent in both high and low dose regions, with differences less than 2%/2mm for greater than 95% of points on a 2‐D plane through the entire torso. Conclusions: A general purpose deterministic solver was successfully applied for dose calculations using CT image data.

Research funded by NIH grant 1R43 CA105806‐01A1.
TH‐E‐224C‐03: Implementation of Monte Carlo Code VMC++ for Photon Beam Treatment Planning
Authors: Tillikainen L , Siljamäki S
Published: July 2006

Purpose: To implement the VMC++ Monte Carlo code for radiotherapy treatment planning of photon beams. The implementation includes the sampling of particles from a multiple‐source model, modeling of accessories, and commissioning of the model based on beam data measurements. Method and Materials: In this work, the radiation output from a linear accelerator was modeled using a multiple‐source model with separate sub‐sources for primary radiation, extra‐focal radiation and electron contamination. This approach enables the commissioning of an individual accelerator using a previously developed procedure, which determines the model parameters by minimizing deviations between measurements and superposition dose calculations. The same source model was used both for the superposition and for the Monte Carlo based dose calculation method. Physical wedges, MLCs, blocks and compensators were modeled by transporting particles sampled from the source model through a detailed geometrical model of the specific accessory. The MLC model accounts for tongue‐and‐groove, divergent leaf alignment, air cavities between adjacent leaves, and rounded leaf tips. Results: The accuracy of the developed algorithm was studied by comparing calculations to measurements for open fields, wedged fields, and irregular MLC apertures for 6 MV and 18 MV beam energies. The agreement between measurements and calculations was within statistical uncertainties for all of the studied cases. The calculation of a typical treatment plan takes from minutes to few hours, depending on the required statistical accuracy. Conclusion: It has been demonstrated that VMC++ with an optimized multiple‐source model and particle transport through accessories results in accurate dose distributions in a wide range of conditions. The developed algorithm has acceptable calculation speed, and has large potential for improving dose calculation accuracy in complex situations and heterogeneous cases compared to traditional dose calculation algorithms. Conflict of Interest: Research sponsored by Varian Medical Systems Inc.
TH‐E‐224C‐05: Verification of a Monte Carlo Based Technique to Correct for Intrafraction Organ Motion
Authors: Afghan M , Cao D , Earl M , Naqvi S , DSouza W , Shepard D
Published: July 2006

Purpose: The most common planning technique to account for intrafraction organ motion is the application of a treatment margin to the clinical target volume (CTV). A uniform dose distribution is then planned for the resulting internal target volume (ITV). We have developed an alternative Monte Carlo based approach where the pattern of organ motion is directly incorporated into both the dose calculation and the optimization of IMRT treatment plans. We have verified the accuracy of this approach through a series of measurements performed with a moving phantom. Method and Materials: For each patient, a 4DCT was used to determine the pattern of respiration‐induced anatomical displacement. The pattern of organ motion was incorporated into our Monte Carlo dose calculation by randomly sampling the isocenter location for each photon history. The resulting pencil beam dose distributions (incorporating motion) were used in the IMRT planning process. Treatment plans have been created for two phantom cases and three lung patients. Verifications were performed for two cases using a solid water phantom programmed for sinusoidal motion. Results: For the 3 lung patients in this study, the mean dose to the involved lung was reduced by 11.9% relative to the plans produced using traditional margin expansion. The verification measurements demonstrated a close agreement between the planned and delivered dose distributions. Incorporating organ motion into the IMRT planning process led to a 43.3% average reduction in the number of points that failed the gamma dose distribution comparison method. Conclusion: The use of Monte Carlo techniques to incorporate organ motion into IMRT optimization leads to significant improvements in normal tissue sparing and also results in improved agreement between planned and delivered dose distributions.
TH‐E‐224C‐06: The Effect of Dental Restorations and Fixed Prosthodontics On Radiation Therapy Dose Distribution: A Monte Carlo Study
Authors: Chin D , Friedland B , Treister N , Cormack R , Tishler R , Court L
Published: July 2006

Purpose: To investigate the effect of dental restorations, fixed prosthodontics, and implants on dose distributions in head and neck radiation therapy using Monte Carlo simulations. Specifically, we seek to understand how to prevent localized mucositis caused by backscatter dose. Method and Materials: Simplified models of a range of dental restorations, fixed prosthodontics, and implants were constructed using a representative sample of materials and configurations. These models were irradiated with a simulated 6MV lateral beam. The resulting dose distributions were compared against dose distributions on models without dental work. Results: Exposed dental alloy (Au‐Pd) caused the most significant amount of backscatter, and corresponding hot spots in the dose distribution. Dental alloy which was surrounded by porcelain also caused backscatter hot spots, although lower compared to exposed metal. These backscatter effects do not appear in pencil beam dose calculations. This work showed that backscatter from dental work caused a dose enhancement of up to 40% at a distance of 1mm in the upstream direction for exposed metal surfaces. The dose enhancement from porcelain‐veneered materials was up to 20% 1mm from the surface. The smaller enhancement was attributed to absorbtion within the ceramic veneer. Isodose lines for the backscatter formed a contour roughly conforming to the shape of the dental work. Beyond 3mm from the surface of the prosthodontic device, the dose enhancement had completely decayed. Conclusion: The metal content of dental restorations and fixed prosthodontics create significant enhanced dose to adjacent soft tissue. This is a major cause of morbidity. Since we have shown that the enhancement decays at a distance of about 3mm in tissue‐equivalent material, we may reduce the likelihood and intensity of mucositis by displacing the soft tissue from the teeth with 3mm of tissue‐equivalent material.
TH‐E‐230A‐02: Effects of IMRT Treatment Time Prolongation On Tumor Cell Survival
Authors: Mogili N , Joiner M , Burmeister J
Published: July 2006

Purpose: To quantify the loss in effective dose resulting from prolongation of treatment fraction delivery times associated with IMRT, and investigate the corresponding effects for neutron IMRT. Method and Materials: The effect of treatment fraction delivery time prolongation was investigated in vitro using human PC3 prostate and HGL21 and U373 glioblastoma tumor cell lines. Cells were maintained at 37 degrees Celsius and irradiated with photons from a conventional linac and with d(48.5)+Be fast neutrons. The delivery time for simulated, multiple‐port fractions was varied from acute to 60 minutes for photon irradiation, and acute to 120 minutes for neutron irradiation. Physical dose ranges for cell survival analysis were 0.5–6 Gy and 0.16–2 Gy for photons and neutrons, respectively. Results: Prolonging photon delivery time (from initiation to completion of irradiation) from 5 to 45 minutes resulted in a loss in effective dose of 6% and 11% in the PC3 and HGL21 cell lines, respectively. A loss of <1% in effective dose was observed for similar prolongation of neutron irradiation of PC3 and HGL21, and photon irradiation of U373 cells. More clinically common prolongations of 5 to 30 and 5 to 15 minutes resulted in effective dose reductions of 4% and 1.5% for PC3, and 6% and 2.5% for HGL21. Application of typical dose response gradients would result in even larger percentage reductions in calculated TCP. Conclusions: This work indicates that prolonged fraction delivery times may have a significant impact on treatment outcome for tumors with a low α/β ratio and short repair half‐time. These effects are significant at delivery times commonly associated with IMRT and are highly variable with cell type. Fraction delivery time should therefore be minimized to achieve the most predictable radiobiological effect. This work also demonstrates that the biological effect of neutron radiotherapy is independent of fraction delivery time.
TH‐E‐230A‐03: Equivalent Uniform Dose (EUD) Based Optimization for IMRT Treatment Planning
Authors: Mihailidis D
Published: July 2006

Purpose: To investigate whether IMRT optimization based on generalize equivalent uniform dose1 (gEUD) objectives for target volumes and organs at risk (OAR) alike can lead to superior plans as oppose to multiple dosevolume (D&V) based objectives plans, for head and neck (H&N) and postmastectomy chest wall (CW) treatment sites. Methods and Materials: We applied gEUD—based optimization to obtain IMRT plans for H&N and CW cancer patients and compared them with the corresponding plans a) optimized with stringent multiple D&V objectives and b) optimized with the standard in‐house physician requested (phys) D&V objective. The D&V optimized plans were created with objectives based on the resultant gEUD plans with the same weight of importance, in order to drive the optimization as close to the gEUD plan as possible. The plan comparison at this point was based on DVH analysis. Results: For all H&N and CW cancer patient in the study, we found that gEUD‐based optimization led to superior sparing of OARs, even beyond the specified requirements, with the same or better target coverage when compared to either the phys‐based or D&V‐based plans respectively. In order to avoid dose inhomogeneities in the target volumes created by the gEUD‐based optimization, use additional D&V objectives just for targets needed to be employed. Conclusions: The general conclusion drawn from our investigation is that the EUD objective function uses smaller number of parameters compared to the D&V and, allows a larger number of solutions with different DVHs but the same EUD. Thus, a better plan was delivered with EUD compared to multiple D&V objectives optimization for either the H&N or CW treatment sites. The use of EUD can allow the plan evaluation to be based on both DVHs and EUD. Details of the method will be discussed. 1Niemierko A. Med. Phys. 26 (abstract), 1100 (1999).
TH‐E‐230A‐05: Development of An Integrated Software Platform for Treatment Documentation and Outcome Analysis for IGRT
Authors: Chen G , Schultz C , Li X
Published: July 2006

Purpose: Information generated in IGRT is tremendous. The long‐term goal of this work is to develop a software package (named RAPID, Research Analysis Platform and IGRT Databases) that is capable of storing patient diagnostic, treatment and follow‐up data for IGRT, which documents treatment outcome, and allows dose‐response analysis based on biophysical models. Presented here are several key components of this development. Method and Materials: The RAPID consists of a database, software tools and auxiliary applications. The database, developed using FileMaker software, includes modules for storing demographics, diagnosis, treatment, and follow up data. Diagnostic and planning images of different modalities (e.g., CT, PET, MR, US) and treatment verification images (e.g., CT, US, radiography) can be stored in DICOM format. Using an integrated auxiliary application these images can be brought into the desktop. The database is integrated with another software package, CERR, developed at Washington University, allowing display of contours and dose distributions on planning images. Various software tools are developed to perform dose response analysis that is linked to documented treatment outcome. For example, treatment related toxicity definitions for a given anatomic site were incorporated into the database, allowing standardized documentation of toxicity which, in turn, facilitates dose‐response analysis. Calculations of EUD, TCP and NTCP are enabled based on 3D dose distributions. Results: The newly developed RAPID is found to be useful. Patient data collected in our clinic for two anatomic sites have been entered into the system. Analysis of treatment and follow‐up toxicity was effectively carried out using the RAPID. With a FileMaker server installed to host the database, users can access password‐protected information remotely.

Conclusion: We have developed a software platform, RAPID, to facilitate storage and analysis of IGRT clinical outcome data.
TH‐E‐ValA‐01: On the Dose Delivered to a Moving Target When Employing Different IMRT Delivery Mechanisms
Authors: Ehler E , Kim Y , Arvidson N , Nelms B , Tome W
Published: July 2006

Purpose: To investigate the influence of target motion on dose distributions generated using unmodulated open fields, solid intensity modulator (SIM), Step and Shoot MLC (SMLC) and dynamic MLC (DMLC). Method and Materials: For two lung cancer cases, four treatment plans were generated using Pinnacle3 7.9t consisting of an open field, SIM, SMLC and DMLC delivery on a Varian Clinac 600C/D equipped with a 120 leaf Millennium MLC. The coordinates (x, y, z, t) of the 4D motion trace for each of the tumors were determined using 4D‐CT from which a 4D motion kernel was generated. For each beam used in the experiment, the beams‐eye view tumor motion due to breathing was simulated using a computerized 2D tabletop apparatus. A MAPcheck diode array was incorporated into the apparatus for dose distribution analysis. Each of the four static treatment plans was delivered to the breathing MAPcheck ten times at various points of the breathing cycle. Results: The variation in diode dose readings within the tumor motion envelope was compared for the open field, solid, segmented, and dynamic IMRT deliveries. The open field provided the most uniform dose to the entire set of tumor mimicking diodes followed by SIM, SMLC, and DMLC IMRT, respectively. On an individual diode by diode basis over ten trials, the open field had the smallest average coefficient of variation of 0.122% followed by SIM (0.98%), SMLC (2.22%) and DMLC (3.88%) IMRT delivery, respectively. Conclusion: For the three IMRT delivery methods (SIM, SMLC, and DMLC), SIM consistently provided a more uniform dose to the tumor over many trials. SMLC performed as well as the solid modulators in many cases or was slightly out performed by SIM. DMLC consistently delivered the least uniform dose to the tumor over many trials.
TH‐E‐ValA‐03: Topographic Leaf‐Sequencing Using a Genetic Algorithm
Authors: Desai D , Breinig M , Ramsey C
Published: July 2006

Purpose: To develop a leaf‐sequencing algorithm for fixed‐gantry (nonrotational) treatment delivery on a commercial helical tomotherapy system (HI‐ART, TomoTherapy, Inc., Madison, WI). Method and Materials: A genetic algorithm was used to determine the multileaf collimator (MLC) leaf open times for a series of fluence test maps generated from a tomotherapy machine with a fixed gantry angle of 0 degrees (IEC scale). A series of wedge shapes (15, 30, 45, and 60‐degree) were mathematically created to test the algorithm's ability to produce simple modulations, similar to those which would be encountered in breast radiation therapy. Results: In general, the topographic treatment delivery yielded reasonable dose distributions. The agreement for the wedge cases was within ±2%, or 2‐mm distance‐to‐agreement (DTA) in the high dose gradient regions. The central axis measured dose was between 3.6 and 4.2 percent higher than the expected dose for the wedge cases. For double peaks, the agreement was within ±2%, or 2‐mm DTA across the entire measured film. For quadruple peaks, the agreement was within ±2%, or 2‐mm DTA in the high dose gradient regions. At the first peak, calculated and measured agreed to within ±0.5%. The dose gradient between the first peak and the first valley was 5 percent per centimeter. The dose in the first valley agreed to within ±1.6% of the prescribed dose (at the first peak). The maximum error in the quadruple peaks occurred at the second peak, where the measured dose was 3.8% low (relative to the prescribed dose at the first peak). Conclusions: The developed algorithm produced calculated deliverable distributions that agreed well with the artificially constructed distributions. This delivery technique could be used for treatment of a whole intact breast. Additional work is needed to optimize the algorithm to improve agreement between the calculated doses and deliverable dose distributions.
TH‐E‐ValA‐04: IMRT Delivery to a Moving Target by Dynamic MLC Tracking: Delivery for Targets Moving in Two Dimensions in the Beam's‐Eye View
Authors: McQuaid D , Webb S
Published: July 2006

Purpose: To outline a new modification of the dMLC delivery technique enabling the tracking of a target moving through rigid‐body translations in a 2D loop in the beam's eye view and the accuracy of the delivery versus that of deliveries with no tracking and of 1D tracking techniques with patient intensity‐modulated beams (IMB) is quantified.

Method and Materials: Leaf trajectories calculated in the target reference frame were iteratively synchronized assuming regular target motion. This allowed the leaves defined in the lab reference frame to simultaneously follow the target motion and to deliver the required IMB without violation of the leaf maximum‐velocity constraint. The leaves are synchronized until the gradient of the leaf position at every instant is less than a calculated maximum. The delivered fluence in the target reference frame was calculated with a simple primary‐fluence model. The new 2D tracking technique was compared with the delivered fluence produced by no‐tracking deliveries and by 1D tracking deliveries for 33 clinical IMBs. Results: The RMS difference between the desired and the delivered IMB was 15.4±3.3 MU for the case of a no‐tracking delivery; 10.9±2.3 MU for the case where one component of motion was corrected and 6.8±1.6 MU for the 2D tracking delivery. The residual error is due to interpolation and sampling effects. The 2D tracking delivery technique requires an increase in the delivery time evaluated as between 0 and 50% of the unsynchronized delivery time for each beam with a mean increase of 13% for the IMBs tested. Conclusion: The 2D tracking dMLC delivery technique allows optimized IMBs to be delivered to moving targets with increased accuracy and with acceptable increases in delivery time. When combined with realtime knowledge of the target motion at delivery time this technique facilitates improved target conformity relative to no‐tracking deliveries and allows margin reduction.
TH‐E‐ValA‐07: IMRT Dosimetry with An Active Matrix Flat Panel Dosimeter
Authors: Chen Y , Moran J , Roberts D , El‐Mohri Y , Antonuk L , Fraass B
Published: July 2006

Purpose: Dosimetric performance of a fully‐customized Active Matrix Flat Panel Dosimeter (AMFPD) is reported for IMRT measurements in a solidwater phantom. Method and Materials: The AMFPD consists of a‐Si:H photodiodes and thin film transistors deposited on a glass substrate. No scintillator screen or copper plate is present above the photodiodes. The device is operated in a continuous acquisition mode asynchronously with the radiation beam delivery at 0.8 fps. Prior to field delivery, a dark frame was acquired to take into account dark signal contributions to the radiation signal including any lag effects from previous irradiations. Dose was determined by summing the corresponding radiation frames (after subtracting from each frame a dark frame obtained prior to radiation delivery), correcting for defective pixels, applying a pixel‐to‐pixel gain correction, and then applying the measured dose response calibration curve. The response of the AMFPD was evaluated as a function of the applied bias voltage across the photodiodes, as this parameter affects dark signal, lag contributions, and sensitivity. In addition, the AMPFD response was evaluated as a function of dose, dose rate, and energy for static fields at 10 cm depth. For comparison, SMLC and DMLC IMRT fields were measured with the AMFPD and film, using standard methods for reliable film dosimetry. All comparisons were made in absolute dose values of cGy. Results: In continuous acquisition mode, the AMFPD maintained a linear dose response (correlation coefficient r2>0.9999) up to 1040 cGy over the period of study (six months). In order to obtain reliable integrated dose results for IMRT fields, effects of lag on the radiation signal were minimized. Compared to film, the AMFPD results were excellent, generally within 2% /±2 mm. Conclusion: We found that the AMFPD can be used as a dosimeter at multiple depths in phantom for static, SMLC and DMLC IMRT fields.
Sci‐Thur PM Therapy‐02: Accounting for the off‐axis detector response for improved a‐Si EPID dosimetry
Authors: McCurdy BMC , Rickey DW , Malkoske KE
Published: July 2006

In portal imaging applications using an a‐Si electronic portal imaging device (EPID), a calibration procedure must be performed before clinical images may be acquired. This calibration procedure incorporates a dark‐field image acquisition and a flood‐field image acquisition. Every clinical image is corrected by subtraction of the dark field image and then division by the flood‐field image. The flood‐field image exhibits two main features, the pixel‐to‐pixel sensitivity (present due to inherent limitations in the manufacturing process), and the beam profile shape as measured by the detector (a function of off‐axis distance, ie. energy spectra and incident fluence). While it is necessary to correct for the pixel‐to‐pixel sensitivity, the beam profile shape as measured by the detector is also removed by this process, which is undesirable for dosimetric applications. This work develops an approach which allows for the pixel‐to‐pixel sensitivity correction to be made, while still preserving the inherent response of the detector to the incident beam.

The approach consists of directly measuring the beam profile shape independently from the pixel‐to‐pixel sensitivity. This profile information may then be reintroduced into the clinical image. The beam profile measurement is achieved by building a small “point” detector using materials that are analogous to the clinical EPID. The point detector is scanned in two dimensions across the open radiation flood‐field at the same source‐to‐detector distance used for clinical flood‐field acquisition. Clinical images are then multiplied by this acquired 2D profile (normalized to central axis) to reintroduce the beam profile shape.
Sci‐Thur PM Therapy‐03: Automated Monte Carlo Simulation and Validation of an External Beam Radiotherapy Treatment Plan
Authors: Kirkby C , Field G , MacKenzie M , Fallone B
Published: July 2006

We present the implementation of an integrated EGSnrc‐based Monte Carlo (MC) system as a tool for the verification of Varian's Eclipse clinical external beam radiotherapy treatment planning system (TPS). Once a TPS plan is generated an automated and fully integrated MC simulation is performed based on the information contained in the DICOM RT Plan, RT Sset, RT Dose and CT image set. MC simulation parameters are determined from information extracted from the RT Plan. The CT image set is then used to generate a virtual phantom in a manner analogous to that of CTcreate (the relevant EGSnrc program), but with the added advantage of incorporating the structures in RT Sset for volume definition within the phantom. The MC simulation is broken into stages that progress from the target in the head of the accelerator, through the various stages of collimation, and into the phantom using both BEAMnrc and DOSXYZnrc on a dedicated cluster. Using RT Dose, we are then able to compare absolute dose differences between the approaches quantitatively in 3D using a χ metric. To illustrate the process, we compare dose distributions for an unblocked four field pelvic treatment generated both using Eclipse and the MC system. Significant differences (> 4%) between the MC and TPS dose matrixes are observed in the vicinity of the bony anatomy.
Sci‐Thur PM Therapy‐05: Evaluation of male pelvic phantom for megavoltage cone‐beam computed tomography
Authors: Schaly B , Varchena V , Au P , Pang G
Published: July 2006

Computed tomography (CT)‐based image guidance has the feature of including the tumour as well as normal tissue in the image localization process during radiation treatment. We propose to use a new anthropomorphic male pelvic phantom (CIRS Inc.) to test the feasibility of using a linear‐accelerator‐based megavoltage (MV) cone‐beam CT system for imaging the prostate. The objective of this work is to verify the tissue‐equivalency of the phantom to determine whether it is representative of prostate cancer patients. We evaluated the phantom in two ways. (1) The linear attenuation coefficient was determined by measuring the photon transmission through uniform samples of the phantom materials (e.g., prostate, muscle, bladder, etc.) using (2) The phantom material CT numbers (kVCT) were compared to CT number data acquired from randomly selected planning CT scans of prostate cancer patients at our institution. The measured linear attenuation coefficients agreed to ∼ 1% of the manufacturer's specifications, while the reproducibility of the measurement was ∼ 1%. The kVCT numbers in the phantom were also in good agreement with the manufacturer's specifications and the patient data. However, the bony structures are comprised of solid average bone equivalent material (cortical and trabecular bone), which caused artifacts within some regions of the kVCT images. However, MVCT imaging is expected to suppress these artifacts. These findings indicate that the phantom is representative of prostate cancer patients and will be a valuable tool in investigating the feasibility of MV cone‐beam CT.

This work was partially supported by Siemens Medical Solution USA, Inc.
Sci‐Thur PM Therapy‐06: Helical Tomotherapy for Adaptive Radiotherapy of Bladder Cancer: Treatment Planning Considerations
Authors: MacPherson M , Malone S , Eapen L , MacRae R , Montgomery L , Fox G , Carty K , Gerig L
Published: July 2006

We have developed a method that leverages the image‐guided radiotherapy capabilities of a tomotherapy unit to adapt daily treatments to changes in bladder shape and volume. Each patient receives three treatment‐planning CT scans: empty bladder; partially full bladder; and full bladder. For each scan, the radiation oncologist contours the initial and boost target volumes (PTV1 and PTV2) and organs at risk, and an initial treatment plan is generated to deliver 40 Gy in 20 fractions to PTV1. A second treatment plan is designed to deliver 20 Gy in 10 fractions to PTV2 (a total of six treatment plans per patient). Patients are asked to void immediately before each treatment. An MVCT of the pelvis is performed before each fraction to verify patient position and to assess the bladder volume. If the treating therapist concludes that the patient's bladder extends to within 10 mm of the PTV contour, then the corresponding treatment plan for a larger bladder volume is downloaded and the MVCT is repeated. Approximately 20% of fractions required a treatment change based on perceived bladder volume. Three of four patients treated to‐date have required the use of a larger volume treatment plan at least once, even though they are always instructed to empty their bladders just prior to treatment. Our early experience is that relying on patient compliance for treating “empty bladder” is insufficient to ensure proper target coverage, and that generous internal margins are required to ensure target coverage in the absence of adaptive IGRT capability.
Sci‐Thur PM Therapy‐07: Improving IMRT plans delivery for head and neck cases using aperture‐based MLC segments
Authors: Lavoie C , Beaulieu F , Beaulieu L , Tremblay D , Gingras L , Nadeau S
Published: July 2006

Purpose: To investigate the possibility of performing IMRT in head and neck treatment sites with less segments and monitor units (MU).

Materials and methods: Six pharyngeal cases (n = 6) were analysed and four cases (n = 4), in the sinonasal region. For each one, an IMRT plan was first realized using a commercial software (Pinnacle3 — IMFAST segmentation algorithm —). Then, an‐in‐house inverse planning system, called Ballista, based on predetermined segments, was used to realize comparable plans. Its segments are generated with the subtraction of the projection of the OARs with the PTV (planning target volume).

Results: For the pharyngeal Ballista plans, the average volume of the PTV that received at least 100% of the prescribed dose (V100) was 85.0±4.5% for the first prescription (PTV1) and the V100 for the second prescription (PTV2 — simultaneous integrated boost —) was 78.5±10.9%. With Pinnacle3, the V100 value was 86.6±4.8% and 81.5±12.4% respectively for PTV1 and PTV2 (see figure 2a and 2b). On average, Ballista plans have required 932±124 MUs and 52±10 segments compared to 1238±230 MU and 117±7 segments for Pinnacle3. For the sinonasal Ballista plans, the average V100 obtained was 80.0±3.1%. With Pinnacle3, the V100 gave 75.7±2.7%. Ballista plans have required an average of 406±54 MUs and 22±1 segments compared to 697±133 MUs and 99±14 segments for beamlet‐based IMRT.

Conclusion: In step‐and‐shoot head and neck IMRT, an anatomy‐based MLC optimization system can achieve similar dosimetric plans comparable to traditional beamlet‐based IMRT with less number of segments and MU.