bluuebunny/crossref_metadata_2025 huggingface dataset:
A New System to Improve Screw Fixation to Bones
Authors: Yánez A. , Garcés G. L. , Carta J. A. , Cuadrado A.
Published: November 2011

Plates and non-locked screws used in the treatment of osteoporotic bone fractures frequently become loose due to everyday mechanical demands. Currently, locking plates and screws are the gold standard treatment for these fractures. However, their use has several limitations and complications as they are technically demanding, and their cost is very expensive. To improve the fixation strength of traditional unlocked plate and screw constructs, we have developed a new fixation system based on a very old concept. The system consists of a screw locking element (SLE) manufactured from PEEK, which is attached to the end of the screw shaft once it has traversed both bone cortices. A specially designed tool is used to facilitate its attachment to the screw. This tool makes it possible for the screw to traverse an osteosynthesis plate or lockwasher as well as both bone cortices and to easily find the SLE, fixing it against the far cortex. We tested the pull-out strength of SLEs and compared the results with previously published data for human femoral cortex pull-out strength. Our laboratory tests demonstrate that the mean SLE pull-out strength was 3864 ± 47.61 N, while that observed for a human femoral diaphysis cortex was 4071.54 ± 1461.69 N. This difference was not significant (p > 0.05). This new system can easily be used with any type of osteosynthesis in osteoporotic or osteopenic bones, with the screws being placed on weakened areas of the bone (e.g., fissure lines, previous orifices, or thinned metaphyseal bone cortex), or to replace over-torqued screws. It is particularly suitable for veterinary trauma, where immediate weight-bearing protection after fracture treatment is nearly impossible.
SU‐C‐224‐07: Comparison of Two Detector Arrays for RapidArc Quality Assurance
Authors: Heilemann G , Poppe B , Laub W
Published: November 2011

Purpose: In our study DICOM files of clinical Rapidarc plans were modified with in‐house developed software to mimic leaf alignment errors and gravitation shifts. The Octavius 2D‐ARRAY (PTW‐Freiburg) in sagittal and coronal orientation and the Delta4 device (Scandidos) were used to investigate the effect of the simulated errors on the passing rate of quality assurance results. The manipulated Rapidarc plans were recalculated in Eclipse.Methods: Different errors were simulated and applied to five prostate (two arcs), three 2‐arc head&neck cases and three 3‐arc head&neck cases: (1) both MLC banks are opened by 0.25mm, 0.50mm and 1.00mm in opposing directions resulting in larger fields, (2) both MLC banks are closed by 0.10mm, 0.25mm and 0.50mm, (3) both MLC banks are shifted in the same direction for lateral gantry angles to simulate effects of gravitational forces onto the leaves by 1mm, 2mm and 3mm, (4) 25%, 50% 70% and 100% of all active leaves are shifted by 3mm as in (3). Evaluations were performed according to a gamma‐index criterion of 3mm/3% and 2mm/2%. Results: With the standard criteria (3mm, 3%) even the largest modifications would satisfy a >90% passing rate. This indicates that a global criterion for the passing rate may not be sufficient in all cases. All unmodified plans and the majority of the plans with the smallest modification pass the gamma‐index criterion of 2%/2mm with >90%. After that the passing rate drops below 90%. For the largest modifications passing rates were typically below 85%. The drop of the passing rate is slightly higher for the Delta4. Conclusions: Both devices are able to detect MLC positional errors of the investigated magnitude. A stricter gamma‐index (2mm, 2%) is necessary in order to detect MLC positional errors and a passing rate of >90% should be expected.

This research was supported by a Varian research grant.
SU‐C‐224‐09: Energy and Dose Calibration of a Synchrotron‐Produced Monochromatic X‐Ray Beam
Authors: Brown T , Matthews K , Ham K , Alvarez D , Hogstrom K
Published: November 2011

Purpose: This work describes planned enhancements to a synchrotron medical radiological beamline, which will improve its efficacy for therapeutic and diagnostic radiological research using monochromatic x‐rays. The primary focus of the present study was the beamlineˈs energy and dose calibrations prior to sample irradiation. This work is crucial for an ongoing study of cell survival of IUdR‐laden cells undergoing photoactivated Auger electron therapy using monochromatic x‐rays. Methods: Calibrations of the monochromatic x‐ray beam are essential for checking the beam energy as well as understanding the variability of the beam quality and dose output. Energy measurements have been performed using Bragg diffraction of a collimated beam from a crystalline silicon powder sample (Si640c). The resulting Debye‐Scherrer diffraction rings were recorded using a digital x‐ray detector. This detector has replaced the use of radiographic film utilized for earlier measurements. Dose measurements of the beam have been performed in a PMMA phantom, using a cylindrical air‐equivalent chamber to measure the dose to water for depths of 0.6 – 10.1 cm. Results: Current results suggest that the beam energy can be measured to within < 0.1 keV. Exposure times of 2 seconds were used to record the Debye‐Scherrer rings, and this has greatly improved upon the 30 – 60 minute exposure times required using radiographic film. The dose rate has been calibrated as a function of the decaying synchrotron ring current, and changes in the beam quality following synchrotron re‐injection have been demonstrated. Conclusions: The use of a digital x‐ray detector has improved the efficiency of the energy measurements. The dose calibration measurements provide a reliable test of the variability of the beam quality and dose output.

This research was supported by contract W81XWH‐10‐1‐0005 awarded by The U.S. Army Research Acquisition Activity, 820 Chandler Street, Fort Detrick, MD 21702‐5014. This report does not necessarily reflect the position or policy of the Government, and no official endorsement should be inferred.
SU‐C‐224‐10: Accurate Patient Dose Reconstruction from On‐Treatment EPID Images
Authors: Van Uytven E , McCurdy B , Chytyk K , Van Beek T
Published: November 2011

Purpose: This work presents an iterative algorithm that reconstructs delivered patient dose from the planning CT/CBCT and portal images obtained during treatment. This approach provides a mechanism for fully three‐dimensional in vivo patient dose verification. The strength of this dose reconstruction technique is that the patient doses are reconstructed with delivered beam fluences measured during patient treatment. Methods: Using an initial estimate of fluence upstream of the patient, a prediction model is used to predict fluences at the plane of the EPID, which are then converted to dose using EPID dose deposition kernels. The predicted dose is compared to the actual measured dose and a correction to the initial fluence estimate is applied. After convergence, a convolution/superposition algorithm is then employed to determine patient dose from the converged estimate of upstream fluence. An especially unique feature of this algorithm is the general physics‐based model used to accurately predict portal dose images over a variety of treatment conditions. Results: The method is demonstrated for a seven beam IMRT step‐and‐shoot prostate cancer treatment. The results of the patient dose reconstruction were compared to the patient dose obtained from a Pinnacle treatment planning system. Gamma comparison between the two dose distributions indicates 99.2% of voxels satisfying gamma< within the PTV, and 87.9% within the CT patient volume. Conclusions: Preliminary results indicate that the patient dose reconstruction model is successful in predicting dose to the PTV region. We are working to improve agreement for lower dose regions. With further improvements, this technique may become a routine tool for in‐vivo patient dose verification.

This work is funded in part by Varian Medical Systems.
SU‐C‐224‐12: Automated Quality Assurance System for Linear Accelerators
Authors: Letourneau D , Wang A , Homer P , Nurul A , Norrlinger B , Jaffray D
Published: November 2011

Purpose: The objective of this work is to develop an automated quality assurance (AQUA) system for linear accelerators, which will integrate multiple mechanical, dosimetric and imaging tests in a daily session of reasonable duration (30–45 min). Methods: AQUA is a web‐based application with a centralized database containing a library of quality assurance (QA) tests defined in a user‐scriptable language. AQUA communicates the test to the linear accelerator (and measuring devices) and analyzes the resulting data. The software supports concurrent testing of multiple linear accelerators. Five tests were implemented in the first AQUA prototype: Collimator and gantry rotation isocenter, radiation field‐size (multileaf collimator (MLC) and jaws), gantry angle readout and kV‐MV isocenter coincidence. Test results are obtained from the analysis of MV 2D images and cone‐beam CT datasets of a phantom setup on the treatment couch. Tests were performed on a nightly basis on one linear accelerator and elapsed time was recorded. The field‐size test reproducibility was extensively studied using simple multi‐segment beams and its accuracy was assessed using films. Results: For the first AQUA prototype which is semi‐automated, the elapsed time for the five AQUA tests was 45 min (setup, imaging and analysis). Results (n=17) for the different components tested were within institutional tolerances. The AQUA field size detection algorithm applied to 127 images of a static field showed a maximum variation in detected leaf position of ±0.15mm. AQUA was able to detect MLC positioning errors as small as 0.5mm. Field widths measured with AQUA were almost consistently smaller than with film with a maximum deviation of 0.34 mm (n=18). Conclusions: The AQUA system was developed and allows for semi‐automated and quantitative daily testing of multiple machine parameters. Work is on‐going to improve automation and efficiency and increase the number of available tests.

Supported in part by Elekta Inc
SU‐C‐BRA‐01: Efficient Generation of Beamlet Arrays with Hybrid Multileaf Collimator for Grid Therapy
Authors: Jordan K , Francis W , Dar A , Yu E , Yartsev S , Chen J
Published: November 2011

Purpose: To develop a simple method of generating megavoltage beamlet arrays for efficient grid therapy. Methods: Multileaf collimators (MLC) were constructed by mounting equally‐spaced, rectangular, lead sheets on Varian linac block trays, in a divergent geometry. The lead leaves were positioned orthogonal to the same alternating leaf pattern beneath a Varian MLC. Lead sheets with nominal thicknesses of 6.4, 3.2 and 1.6 mm resulted in projected beamlets at linac isocenter of 10×10, 5×5 and 5×2.5 mm respectively. Relative to an open beam, the symmetric grid beam area is 25% open, 50% shadow of single MLC leaf and 25% shadow of overlapping MLC leaves. Kodak EDR‐2 film, ISP Gafchromic EBT film and leuco crystal violet micelle gels with optical cone beam CT readout were used to characterize dose distributions. The impact of the x‐ray beam flattening filter was measured by operating the Varian 2100 series linac in 6 MV mode with and without the filter. Modulation ratios were recorded at water depths of 15 to 200 mm. Orthogonal 5×5 mm grid beams offset by 5 mm were delivered to a 95 mm diameter cylinder of gel in a cubic water phantom at 100 mm depths. Results: Dose rates from beamlet arrays and single beamlets, defined by linac jaws, were similar. Removing the flattening filter doubled the dose rate but had a minor effect on dose modulation. Optical density modulation ratio (5×5 mm beamlet / single leaf shadow) decreased from 7 at 15 mm to 4 at 200 mm depth in water. The gel 3D distribution demonstrated the anticipated features of intersecting diverging beamlet arrays with central dose modulations of 3.5. Conclusions: A hybrid system of orthogonal linac and block tray MLC leaves provides a simple method of generating arrays of megavoltage photon beamlets for efficient delivery of grid therapy.

W Francis was partially supported by a London Regional Cancer Program Small Grants for grid therapy development.
SU‐C‐BRA‐02: Modular Patient‐Specific Compensation for Co‐60 TBI Treatments Based on Monte Carlo Design
Authors: Serban M , Seuntjens J , Roussin E , Tremblay Jean‐Rene , Wierzbicki W
Published: November 2011

Purpose: Using Monte Carlo (MC) simulations and measurements, to develop a new treatment technique for the delivery of TBI at extended SSD using a custom modified Co‐60 unit equipped with flattening filter and modular, patient‐specific compensators. Methods: An existing Eldorado‐78 Co‐60 teletherapy unit was stripped from its original collimator and equipped with beam‐defining cerrobend blocks for extended SSD TBI treatments. An acrylic flattening filter was numerically designed based on detailed mapping of the dose distribution of the large open field at 10 cm depth in water using a primary radiation attenuation calculation. An EGSnrc MC model of the resulting unit was developed and experimentally validated. The model was used to calculate MC dose in whole‐body supine and prone CT images of a patient. The total dose, calculated by summing prone and supine dose after deformable registration (VelocityAI™) was used to design modular, patient‐specific compensators, based on the premise that dose in the patient mid‐plane ought to be uniform. Results: The designed flattening filter flattens the beam to within ±2% over a 200 cm × 70 cm area at 10 cm depth in water. Experimental validation of the calculated dose profiles in the open and flattened beams shows agreement of better than 2% and 1%, respectively. Patient MC dose calculations in the flattened, uncompensated beam showed dose deviations from prescription dose most notably in lung, neck, arm and leg areas ranging from −5% to +25%. Patient‐specific compensation reduced non‐homogeneities in the patient to within −5% to +10%. The clinical implementation involves a modular, Lego®‐style realization of the compensator using orthogonal parallelepiped blocks on a plate that slides in the treatment head tray. Conclusions: This work demonstrates that a Co‐60 TBI setup combined with patient‐specific compensators numerically designed using MC calculations is clinically feasible and highly improves the quality of the treatment.
SU‐C‐BRA‐03: Investigation of the Performance of a Simulated In‐Line 6 MV Linac in Longitudinal Magnetic Fields
Authors: St. Aubin J , Santos D , Steciw S , Fallone B
Published: November 2011

Purpose: To achieve real‐time image guided radiotherapy, a linac‐MR system is designed with the linac located on the symmetry axis of the MRI. Thus, an investigation was performed in order to determine the performance of an in‐line 6 MV linac in longitudinal magnetic fields. Method and Materials: Magnetic fringe fields from the PARAmed openMR™ were modeled and included in the simulation of an in‐line 6 MV linac. The electron gun simulation was modeled in OPERA‐3d/SCALA. The linac waveguide emulating the Varian 600C was modeled in COMSOL and the resulting dosimetry from the linac model was calculated using BEAMnrcMP 2007 and DOSXYZnrc. Results: With the linac on the symmetry axis of the openMR™ imager, the electron gun is expected to experience longitudinal magnetic fields ranging from 0.0022 to 0.011 T. At these field strengths, the rms emittance of the electron beam injected into the waveguide grew from 0.358 pi‐mm‐mrad at 0T to 3.255 pi‐mm‐mrad at 0.011 T. The increased emittance resulted from an increased beam diameter and an increasingly diverging beam. Upon injection, the larger emittance caused greater beam loss within the waveguide and a two orders of magnitude increase in the full‐width‐half‐maximum of the focal spot at the target. Despite the changes in the focal spot, the mean energy was altered by less than 0.5% and the calculated depth dose and profile distributions were identical to distributions at 0T. Magnetic fields up to 0.2T were also added to the linac simulation representing fringe fields from other MRIs. Both the injection current and target current reached a minimum at 0.06T, but increased slightly at higher field strengths due to increasing collimation in the electron gun anode. Conclusion: The effect of longitudinal magnetic fields on a simulated in‐ line 6 MV linac has been quantified enabling future development of a parallel linac‐MR system.
SU‐C‐BRA‐05: Commissioning of Rotational Total Skin Electron Irradiation on Trilogy Machine
Authors: Zhang Q , Chan R
Published: November 2011

Purpose: To commission 6MEV beam in HDTSe mode on a Varian Trilogy machine for rotational total skin treatment. Methods: HDTSe 6MEV beam with field size 40cm × 40cm at 380cm SSD was planned for total skin irradiation treatment. A custom designed flattening filter was added to improve beam uniformity. Rotational speed during treatment was to set at 3–5 revolution/minute. Beam calibration was at 100cm SSD following TG51 protocol. Beam parameters at extended distance were measured using GafChromic EBT2 films placed in solid water phantom. 30cm × 30cm square phantom and 33cm diameter round phantom were for stationary and rotational target respectively. Dose distribution during rotational treatment was verified by GafChromic films sandwiched inside a Rando phantom. Beam uniformity was verified using Kodak EDR2 ready pack films on a ½” thick, 4′×8′ plexiglas hanging perpendicular to the beam at distance. Results: At SSD of 380cm, with filter in place, stationary beam output was 89.6cGy/1000mu; dmax is at 0.9 cm; R50 is 1.82 cm and Rp is2.4cm. With rotational target, dmax is shifted to surface with R50 at 1.54 cm and output at surface was 38.8cGy/1000mu. Measured rotational factor was 0.433. X‐ray contamination is of the order of 3% for rotational target. Dose uniformity along the vertical axis from −90 cm to about 80cm is within +/− 5%, and within +/−10% from ‐95cm to 95cm. For the horizontal axis, the entire range is covered with the dose of better than 95%. Dose distribution using the rotational technique was shown to be uniform at different levels of a Rando phantom Conclusions: Dosimetric properties of the modified HDTSe 6MEV beam on a Varian Trilogy machine are verified to be optimal for rotational total skin irradiation treatment.
SU‐C‐BRA‐06: A Novel Device for Delivering Combined Partial Breast Hyperthermia and Partial Breast Irradiation
Authors: White T , Shvydka D , Parsai E
Published: November 2011

Purpose: The attraction of Partial Breast Irradiation (PBI) is related to minimizing the potential negative side effects of radiation therapy and the potential to reduce the total effective tumor dose. Heat has also proven to be a significant radiosensetizer. In this paper, we propose a novel technique for delivering combined partial breast irradiation and hyperthermia (PBHI) to gain the added benefits. Methods: Manufacturing of a prototype balloon applicator for PBHI delivery is currently underway at our institution. This new device will use a modified central lumen for heating of the surrounding fill solution by a removable miniaturized resistive heating element. The temperature of the fill solution will be controlled by a feedback loop system utilizing a thermocouple and proportional‐integral‐derivative (PID) industrial temperature controller unit. The feasibility of our system has been verified through mathematical modeling using the finite‐element partial differential equation solver package COMSOL Multiphysics (COMSOL Inc., Burlington, MA). Results: We modeled our balloon as a 6 cm diameter water filled sphere within a 20 cm diameter sphere of breast tissue. The temperature at the balloon surface was kept at 460 C for one hour. The results show a temperature at the 1 cm margin around the balloon of approximately 41 0C is maintained during 30 minutes of heating. The temperature gradient between the balloon wall and the 1 cm margin is very steep due to the low thermal conductivity of the modeled breast tissue. Conclusion: Hyperthermia, in combination with radiation, is a proven method of reducing local recurrence rates of breast cancer as an adjuvant treatment. Modeling of the proposed system confirms the viability for construction of a prototype system which is currently in development. The device will further expand the potential patient population qualifying for partial breast irradiation thus providing an enhanced treatment option with minimized side effects
SU‐C‐BRB‐03: Single Fraction Spine SBRT Utilizing Cone‐Beam CT Image‐Guidance and the HexaPOD Robotic Couch
Authors: Hyde D , Korol R , Davidson M , Lochray F , Ma L , Sahgal A
Published: November 2011

Purpose: Spinal SBRT was originally developed for the retreatment of patients with disease progression. However, the indication has evolved to the up‐front treatment of spinal metasteses with the intent to maximize the biological effective dose and improve local control, while sparing the adjacent spinal cord. We report on the stability of our technical approach of performing single‐fraction (18–24Gy) SBRT for spinal metastases using the Elekta Synergy‐S and BodyFIX immobilization systems. We evaluate the residual set‐up error and intra‐fraction motion using cone‐beam CT (CBCT). Methods: Analysis is based on 20 consecutive patients (93 image registrations) with spinal metastases treated with single‐fraction SBRT. Following initial set‐up, a CBCT was acquired to align the patient and a pre‐treatment CBCT was taken to verify the alignment and determine the residual set‐up error. Two intra‐fraction CBCT images were used to assess immobilization and correct any misalignment exceeding 1mm or 1°, in all six degrees‐of‐freedom using the HexaPOD robotic couch. A post‐treatment CBCT was used to verify immobilization for the entire treatment, but it was not acquired for 7 of the patients due to the time restriction of the BodyFIX immobilization. Results: Most positioning errors after the initial CBCT setup were relatively small, with 95% of the image registrations within 1mm and 99% within 1°. Although a higher frequency of misalignments was observed in the lateral direction (8 of the 13 translations exceeding 1mm; 2 of the 4 rotations exceeding 1°), there was no significant difference between the directions. The absolute intra‐fraction motion averaged over all patients and directions (±stdev.) was 0.4 (±0.5) mm for the translations and 0.3 (±0.4) degrees for the rotations. Conclusions: Near‐rigid immobilization with the acquisition of two intra‐fraction CBCT images and the correction of misalignments in all six degrees‐of‐freedom provides the necessary precision to safely perform single‐fraction SBRT of spinal metastases.
SU‐C‐BRB‐04: Impact of Residual Systematic Setup Error in the Treatment of Paraspinal Lesions with Single High Dose Stereotactic Body Radiotherapy
Authors: Hunt M , Hertanto A , Mageras G
Published: November 2011

Purpose: To use confidence limit dose volume histograms to evaluate the impact of residual systematic error during para‐spinal stereotactic body radiotherapy (SBRT). Methods: Retrospective analyses of the impact of residual systematic errors were performed for ten patients undergoing para‐spinal SBRT. Residual random errors were modeled by convolving the planned dose with a Gaussian distribution (Stdev 1.5 mm). Residual systematic setup errors were assumed to be normally distributed with standard deviations from 1 to 4 mm and were modeled using a Monte Carlo (MC) simulation. During the MC simulation, it was assumed that actual systematic setup error would be kept to institution‐defined thresholds which would equal one standard deviation. Confidence intervals were calculated by sorting the resulting MC DVHs according to a pre‐selected critical dose or volume (e.g. D95). Dose was calculated with an in‐house planning system and a grid spacing of 1mm. Results: Mean (Stdev) planned cord/cauda D05 was 46.1% (11%) of the prescribed dose. Mean 5% and 95% confidence intervals were 100.8% and 117.0% of the planned D05 for a 1 mm residual error and 94.1% and 155.1% for 4 mm residual error. The expected cord/cauda D05 (i.e., mean over all confidence levels) for tumors partially surrounding these structures was generally within 5% of the planned dose and the 5% and 95% confidence limits were approximately 5% to 15% lower/higher than planned dose for systematic errors no more than 2 mm. For tumors surrounding the cord/cauda, D05 equaled or exceeded the planned dose, regardless of confidence level or magnitude of residual error and the expected cord/cauda D05 could exceed planned D05 by 10% or more. Conclusions: Residual systematic error can result in clinically significant deviations between planned and actual dose particularly for patients with extensive disease. Such analyses can guide the level of treatment intervention required for individual patients.
SU‐C‐BRB‐06: High‐Precision Volume‐Staged Treatments with Stereotactic Radiosurgery
Authors: Ma L , Li K , Hwang A , Sahgal A , McDermott M , Sneed P , Larson D
Published: November 2011

Purpose: The purpose of the study was to develop a procedure using selective region mutual information image registration and anatomic landmarks transformation to improve the treatment accuracy of volume‐staged Gamma Knife (GK) radiosurgery for large arteriovenous malformations (AVMs). Methods: The procedure was implemented via Leksell Gamma Plan (LGP version 9.0) and an in‐house point registration program for multiple session staged GK treatments. A requirement of such treatments was to match the partial‐volume dose distribution of a previously delivered treatment with the current treatment. In our procedure, MR images of the current treatment plan were first imported and co‐registered with the initial reference MR studies via selective region mutual information algorithm in LGP. Then the current MR studies were re‐imported and defined for its stereotactic frame. Based on these two MR studies (one registered and one defined), four or more anatomic landmarks surrounding the target region were selected, and then a rigid‐body transformation matrix was calculated from these points to map the original treatment plan onto the current treatment plan for the existing frame coordinate system. We analyzed the accuracy of such procedures performed for a group of 12 patients with 14 staged treatments at our institution since 2007. Results: The average discrepancy in the landmark vector distance between the original treatment plan and the transformed treatment plan was 0.25±0.09 mm. This value was 3–4 times smaller than previously published results. Additional studies on the accuracy of mutual information registration of MR‐MR images of the same objects/patients on our LGP system yielded a mean standard deviation of 0.16±0.07 mm, where the largest deviation was generally observed along the image scanning direction Conclusions: Volume‐staged GK treatments can be delivered to the same level of accuracy as single fraction GK treatments.
SU‐C‐BRC‐01: RapidArc Planning Standardization for High Quality and Efficiency in Prostate and Post‐Prostatectomy Radiation Therapy
Authors: Zeng  , MacPherson M , Wang Y , Murphy J , Wang Y , Radwan J , McGowan T
Published: November 2011

Purpose: It is widely accepted that the quality of IMRT plans relies on individualsˈ planning experiences. RapidArc offers large degrees of freedom in dose shaping. While calculation and optimization take longer time, it is necessary to standardize planning in order to control the quality and efficiency. In this report, we present our progress in this regard in prostate and post‐prostatectomy (prostate bed) radiation therapy. Material and Methods: 12 prostate (78Gy/39) and 10 prostate bed (66Gy/33) pts were retrospectively planned using single arc and double arc respectively to test the standardization templates without interventional adjustment. The highest optimization priority is given to target, followed by sharp dose falloff (NTO in Eclipse) and OARs constrains with step‐down priorities. This priority setting aims to efficiently utilize arc features and produce optimal conformal dose. OARs within beam‐range were used as optimization structures to minimize the impact of patientsˈ anatomical variations on dose limits. The standardized plans were compared to the clinically treated plans which were previously optimized by individual dosimetrists using single arc for prostate and dIMRT for prostate bed. Results: Standardized plans show superiority over clinical plans in target dose, OARs sparing and dose conformity. For prostate, the average rectum D50 was reduced by ∼6Gy from 42Gy to 36Gy while D25 was decreased by 2Gy from 58Gy to 56Gy; target dose, bladder and femoral dose are equivalent. For prostate bed, the average rectum D50 was lowered by ∼5Gy from 37Gy to 32Gy; target dose is more conformal with Dmax being reduced by 1Gy; femoral dose is spared by ∼7Gy and bladder dose is equivalent. Conclusions: Standardizing Arc planning can significantly improve plan quality and planning efficiency. Our center has implemented the procedures. Clinical practice shows the standardized templates are robust and only ∼15% of the plans need individual improvement due to unusual anatomy.
SU‐C‐BRC‐02: Plan Quality and Delivery Efficiency with Varian TrueBeam FFF Mode at High Dose Rate
Authors: Zhao B , Yang Y , Heron D , Huq M
Published: November 2011

Purpose: The maximum dose rate at the regular filter mode of a Varian Linac is 600 MU/min. In comparison, the dose rate for flattering filter free (FFF) mode of Varian TrueBeam linac is up to 1400 MU/min at 6MV. The purpose of this study is to investigate the clinical use of IMRT treatment in FFF mode at high dose rate. Method: Ten prostate patientsˈ plans were generated using regular and FFF mode at 6 MV using Eclipse treatment planning system. PTV varies from 122–233 cc. Four types of plans were generated for each patient: multifield IMRT using regular filter mode, multifield IMRT using FFF mode, RapidArc(RA) using filter mode, RA using FFF mode. To facilitate comparison, same constraint table was used for each patient to optimize the fluence. Continuous high definition MLC sequences were generated at maximum dose rate of each mode. Results: With same optimization parameters, the plan quality is comparable for each plan. The maximum doses to target and organs are slightly higher for FFF mode with the same IMRT technique. Total MU for FFF mode and multifield IMRT technique is much higher than their counterparts. Nonetheless the beam‐on time is substantially lower for FFF mode due to higher dose rate. Conclusion: IMRT plans for the two filter modes are comparable. Although the dose rate for FFF mode is much higher than filter mode, FFF mode requires much more MUs to deliver the same amount of dose. Many MUs are used to move MLC leaves to compensate sloping region of beam profile in FFF mode. This reduces MU efficiency when irradiating large targets with irregular shapes. High dose rate treatment in FFF mode is more efficient to SRS treatment for small target. RapidArc technique may help reduce the delivery time for treatment of large target.
SU‐C‐BRC‐03: Three‐Dimensional Isodose Surface Manipulation for Multi‐Criteria Inverse Planning in Radiosurgery
Authors: Viulet T , Rzezovksi N , Schlaefer A
Published: November 2011

Purpose: Traditionally, the planning task for radiotherapy offers the human planner little direct spatial control of the dose distribution. Dose painting methods exist, but they typically suffer from side effects, such as uncontrolled spilling of dose. We developed a tool that allows for local three‐dimensional isodose surface manipulation which avoids this problem by employing constrained optimization and inverse planning. Methods: In our approach, the planner operates directly on the three‐dimensional dose distribution, e.g., selecting areas to be covered with a certain iso‐dose surface. The underlying mathematical model implementing constrained inverse planning prevents dose from shifting into areas that are not explicitly selected. To move towards a desired clinical goal, e.g., coverage of the target, the planner directly controls where to operate trade‐offs. First, a local objective is set graphically. Second, the potential trade‐offs are visualized. Third, the planner selects where to relax constraints. Notably, the relaxation steps imply quick re‐optimization in an interactive manner. We studied the use of the tool for a clinical case, where initially tight bounds on critical structures prevent sufficient target coverage. The dose bounds are then relaxed in specific areas, i.e., shaping the isodose surfaces in a controlled way. Results: Our experiments show that local isodose manipulation is possible, with little to no dose shifting. Outside the local target area, constraints stay in place and maintain the dose distribution. When the isodose surface is deliberately remodeled to relax constraints the some areas, the coverage of the target area is improved. Initial optimization times are below 40 seconds while re‐optimization is done in less than 5 seconds. Conclusions: The planning tool implements a novel approach to interactively shape the dose distribution, which would be of particular interest in radiosurgical planning with steep gradients. Our results illustrate that interactive multi‐criteria planning in the dose space is feasible.
SU‐C‐BRC‐04: Avoidance of Perfused Lung by Optimization of Functional Mean Dose in Direct Machine Parameter Optimization
Authors: St‐Hilaire J , Lavoie C , Dagnault A , Morin F , Beaulieu L , Tremblay D
Published: November 2011

Purpose: To implement and evaluate a functional mean dose‐based objective in a well‐established IMRT system, using lung perfusion images and direct machine parameter optimization (DMPO). Methods: Nineteen patients underwent SPECT perfusion imaging prior to treatment. In this retrospective study, plans were generated using Pinnacleˈ DMPO (Philips Healthcare, Andover, MA) with the aim of minimizing either the mean lung dose (MLD) or the lung functional mean dose (FMD). A seven equidistant beam configuration was used in all plans. Two levels of dose were prescribed: 50 Gy to the PTV1 (clinical target volume with margins) and 66 Gy to the PTV2 (gross tumor volume with margins). The MLD or FMD objective was decreased by steps of 1 Gy until dose to target volumes or organs at risk was deemed unacceptable. Plans were compared in terms of dose‐volume and dose‐function parameters. Statistical significance was assessed with a Wilcoxon matched pairs test. Results: While keeping PTV coverage similar (volumes receiving 93% of the prescribed dose were all over 98%), differences in MLD between both types of plans for a given patient ranged from –1.0 to +1.5 Gy (p = 0.2050), while FMD decreased significantly with a range of –2.1 to 0.0 Gy (p = 0.0003). The net improvement (FMD difference − MLD difference) ranged between −2.2 and 0.0 Gy. Dose to other organs at risk were similar and below widely‐ accepted tolerances. Conclusions: The use of SPECT perfusion images in conjunction with DMPO allowed a significant decrease of FMD while keeping dose to other structures at an acceptable level. Functionality‐aware dose redistribution could prove useful for dose escalation to improve tumor control with similar or lower lung complication probabilities. The approach can also be easily ported to arc therapy treatment planning using SmartArc.

This work is supported by the Natural Sciences and Engineering Research Council of Canada and by a research agreement with Philips Healthcare.
SU‐C‐BRC‐06: Adaptive and Robust IMRT Treatment Planning for Lung Cancer
Authors: Chan T C Y , Misic V V
Published: November 2011

Purpose: To improve IMRT treatments for lung cancer patients who exhibit breathing motion uncertainty; to demonstrate that combining adaptive radiation therapy with robust optimization can lead to simultaneous improvements in tumor coverage and healthy tissue sparing over non‐adaptive robust optimization methods. Methods: A model of breathing motion uncertainty was derived from previous robust optimization research. We then developed two algorithms ‐ exponential smoothing and running average ‐ to up‐date this model from fraction to fraction, using a sequence of retrospective motion probability mass functions (PMFs) from real patients. A robust optimization problem was solved for each fraction with an updated uncertainty model to generate a sequence of treatments. Delivery of the entire treatment was simulated and dose was accumulated according to the sequence of PMFs. The breathing motion model was created and updated in MATLAB, while CPLEX was used to solve the corresponding robust optimization problem as a linear program. The adaptive robust treatment was then compared with a robust treatment in which the uncertainty set was not updated over the treatment course. Results: Our adaptive robust optimization method exhibited a simultaneous improvement over non‐adaptive robust methods in both tumor coverage and healthy tissue sparing. The adaptive robust treatments were insensitive to the choice of the initial uncertainty model, and also closely comparable to idealized treatments that would be obtained with perfect foresight — i.e., treatments created with a priori knowledge of the entire sequence of future patient PMFs. Conclusions: By combining adaptive radiation therapy and robust optimization, our method combats both the instantaneous breathing motion uncertainty realized in each fraction and changes in the uncertainty that occur from fraction to fraction. Our method demonstrates that it is possible escalate dose to the tumor from the level possible with current robust methods without sacrificing healthy tissue sparing.
SU‐C‐214‐01: Design and Evaluation of a Low Megavoltage Imaging Beam from a Prototype Waveguide
Authors: Roberts D , Hansen V , Niven A , Poludniowski G , Seco J , Thompson G , Evans P
Published: November 2011

Purpose: To design and evaluate an optimised radiotherapy imaging beam, based on a prototype Elekta waveguide, with a novel coupling device (rotovane) allowing for a wide, continuously variable energy range. Methods: A waveguide test piece consisting of a buncher, rotovane and short relativistic section was used to investigate the performance of the novel continuously variable coupling device, and in particular investigate imaging at low megavoltage energies. An optimised imaging system utilising the lowest electron energy of the waveguide (1.4 MeV) was designed using BEAMnrc and implemented experimentally. Characterisation of the imaging beam was conducted experimentally and via Monte Carlo simulations. Cone beam computed tomography images (CBCT) were acquired using a Caesium Iodide imaging panel of humanoid anthropomorphic phantoms and a Catphan phantom. The contrast to noise ratio was assessed for CBCT and compared to kilo‐voltage and megavoltage imaging systems. Results: The optimised imaging beam target assembly consisted of an electron window, 5 mm carbon electron absorber and 2.5 mm of Aluminium filtration. The x‐ray beam had an average photon energy of 220 keV and a half value layer of 5.9 mm of Copper. Images with the same contrast to noise ratio as a 100 kVp CBCT system (XVI, Elekta) were obtained in doses less than 2 cGy; this is 11x higher than XVI but 150x lower than a megavoltage imaging system. Qualitatively, kilo‐voltage equivalent images of head and pelvis phantoms were obtained in doses between 1 and 8 cGy. Conclusions: The prototype waveguide section was capable of producing electron energies from 1.4 to 9 MeV, suitable for imaging and therapy. The waveguide technology has the potential for producing near kilo‐voltage equivalent images in acceptable dose from the therapy beam portal without the need for add‐on x‐ray systems. Additionally such a system could provide for enhanced target tracking during radiotherapy treatment.

This work is supported by Elekta Ltd and The Institute of Cancer Research. Work of the ICR radiotherapy physics group is partially supported by Cancer Research UK under programme grant C46/A3970
SU‐C‐214‐02: Reduced CBCT Imaging Dose Due to the New X‐Ray Source in TrueBeam™
Authors: Ding G , Munro P
Published: November 2011

Purpose: To report the x‐ray beam characteristics of the new x‐ray source and to describe the CBCT imaging dose to patients for the TrueBeam™ system. Methods: The Monte Carlo code, BEAMnrc, was used to simulate the new tube including its x‐ray defining systems and beam hardening filters. Two kV‐CBCT scan modes (100 kVp beam for Head scan and 125 kVp for Pelvis scan) available in the TrueBeam™ system were simulated. The simulated realistic incident beams were stored in phase‐space files, which were analyzed to obtain the beam characteristics. The kV‐CBCT beam output was determined by an ionization chamber in a phantom under specified reference conditions. The simulated realistic beams were then used to calculate dose to patients resulting from CBCT scans. The doses to the patients, as well as image quality using a Catphan‐504 phantom, for the TrueBeam and OBI imaging systems were compared. Results: The additional kV beam hardening filter in the new x‐ray source design removes lower energy photons in the energy spectrum, which in turn results in reduced radiation dose to patient. For a head scan the dose to eye, brain, brain stem, spinal cord are 0.14, 0.15, 0.13 and 0.13 cGy which are approximately 38% lower compared to Standard Head scan in OBI 1.4. For pelvic scan the dose to prostate, rectum, bladder, and femoral heads are 0.8, 0.86, 0.87 and 1.5 cGy, which are approximately 43% lower compared to Pelvis scan in OBI 1.4. In addition the dose to skin from new tube is only half of the doses from OBI 1.4. For the same CTDIw doses, the contrast sensitivity is superior in the TrueBeam images compared to OBI images. Conclusions: The newly designed x‐ray source is able to reduce CBCT image dose to the patient without compromising image quality.
SU‐C‐214‐03: Monte Carlo Simulation of Varian OBI Cone Beam CT (CBCT) and Dose Distribution in Head Phantom in 125kVp and 100kVp
Authors: Feng W , Guan H , Bakalyar D
Published: November 2011

Purpose: (1) To validate a Monte Carlo model for Varianˈs OBI CBCT in head scan mode and calculate the dose inside a Rando head phantom; (2) To compare the dose in head phantom for 125 kVp and 100 kVp scans. Methods: The BEAMnrcMP Monte Carlo package was employed and modified to simulate Varianˈs OBI CBCT system operated at 125kVp and 100kVp in full‐fan full‐bowtie mode for the same mAs and 200 deg scans only. Percentage depth dose (PDD) of a single KV beam was measured with a cc01 micro ion chamber in water tank. Dose profiles of the single KV beam at multiple depths including 0.5 cm were also measured with the same micro chamber. After the validation of the Monte‐Carlo model with the measured beam data, dose calculations were performed on a Rando head phantom. Results: The Monte Carlo simulated PDD curves agree very well with the measurements. Also, the simulated dose profiles matches well to the measured ones in both in‐plane and cross‐plane directions. Monte Carlo dose calculation made on a Rando head phantom indicated that the post right side of head get higher dose, due to the posterior KV scan from gantry 290 deg to 90 deg. The dose ratio between bone and soft tissue were ∼4.4 and ∼4.9 for 125kVp and 100kVp, respectively. The dose ratio between 125kVp and 100kVp is about 1.7, close to the theoretical value of 1.252=1.56. Conclusions: Our Monte Carlo model has been validated by measurements to simulate Varianˈs OBI CBCT system. It was shown that CBCT dose in bone is four times of that in soft tissue, such that the accumulated bone dose during a treatment course could be up to >100cGy in posterior area. Investigation on patient dose deposition due to kV CBCT is in progress.
SU‐C‐214‐06: Noise Power Properties of a Dual‐Detector Cone‐Beam CT System
Authors: Li H , Giles W , Roper J , Bowsher J , Yin F
Published: November 2011

Purpose: Very little information is available for noise properties using a dual‐detector cone‐beam CT (CBCT) system. Geometric misalignment, cross‐scatter, and inhomogeneous detector response are additional concerns in a dual‐detector system. This study characterizes the noise power spectrum of a dual CBCT system for image‐guided radiation therapy (IGRT). Methods: A bench‐top dual‐detector CBCT system with two orthogonal 40×30cm flat panels has been developed in our laboratory. The noise power spectrum (NPS) was evaluated by scanning two uniform phantoms with diameters of around 26cm and 20cm under different kVs, mAs, and acquisition modes. The three‐dimensional NPS was calculated using 3D Fourier transform for a region of interest of 10×10×10cm3 as the metric to evaluate the noise performance. The structural fluctuations were removed by subtraction between two identical scans. The radial averaging is then calculated every degree in the central slice of the 3D Fourier space to reduce the measurement fluctuation. The profile of the radial averaging is used for comparison between dual/single detectors, at different kVs, mAs, and acquisition modes. Results: The NPS amplitude of the dual CBCT system can be up to 50% lower than that of a single CBCT system in simultaneous acquisition mode. In alternating acquisition mode, the NPS of the dual CBCT system is comparable both in amplitude and shape with a single CBCT system. The cross‐scatter is identified as the major contributing factor for the discrepancy. The cross‐scatter and the amplitude of the discrepancy are correlated with the sizes of the scanned objects under various current and voltage parameters. Conclusions: The Cross scatter reduces the amplitude of NPS. The effect is more dominant for large objects. Quantitative results from this noise power analysis provide guidance for dual CBCT reconstruction, cross‐scatter reduction and acquisition optimization.

This work is supported in part by a research grant from Varian Medical Systems.
SU‐C‐110‐03: Utility of Maximum Intensity Projections of Gated PET Images in Determining Internal Target Volumes of Moving Lung Tumors: A Phantom Study
Authors: Jani S. , Lamb J.M. , Robinson C. , Laforest R. , Dehdashti F. , Bradley J. , White B.M. , Low D.A.
Published: November 2011

Purpose: To examine maximum intensity projections (MIPs) as a method for segmenting lung tumor internal target volumes (ITVs) in positron emission tomography (PET) images using two threshold‐based methods, and to assess various parameters that potentially affect the accuracy of tumor segmentation by this technique. Methods and Materials: An acrylic phantom setup, consisting of a 1.5cm diameter sphere filled with 11 ‐C solution inside a cylindrical bath of 18‐fluorodeoxyglucose solution, was used for a PET list‐mode acquisition. The phantom was attached to a robotic arm that moved according to four patient breathing trajectories via an abdominal breathing bellows device. List‐mode data was gated based on the breathing trajectories, and PET‐MIPs were created from the reconstructed gated images. ITVs were calculated and compared for both un‐gated PET images and PET‐MIPs by using varying thresholds of absolute standardized uptake value (SUV) and percentage of maximum SUV. In addition, a simulation of PET‐MIPs was performed to examine the effects of tumor size, image smoothing, and tumor trajectory on the accuracy of ITV measurement. Results: For any given threshold value, PET‐MIPs tended to produce larger ITVs than un‐gated PET. For optimal percentage thresholds across all trajectories, PET‐MIPs measured an ITV to within 5% accuracy in a larger SUV range with lower initial values than un‐gated PET for the same accuracy. For both PET‐MIP and un‐gated PET, a percentage threshold method produced a smaller deviation from true ITV in similar SUV ranges when compared to an absolute threshold method. The PET‐MIP simulation demonstrated greater accuracy of ITV measurement with increased image smoothing and sphere size. Breathing trajectories exhibiting baseline drift negatively impacted accurate ITV determination. Conclusion: Threshold‐based ITV segmentation is sensitive to lung tumor volume, trajectory, and SUV. PET‐MIPs appear to be a promising tool in the accurate delineation of lung tumor ITV.