bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐F‐19A‐10: Recalculation and Reporting Clinical HDR 192‐Ir Head and Neck Dose Distributions Using Model Based Dose Calculation
Authors: Carlsson Tedgren A , Persson M , Nilsson J
Published: July 2014

Purpose:

To retrospectively re‐calculate dose distributions for selected head and neck cancer patients, earlier treated with HDR 192Ir brachytherapy, using Monte Carlo (MC) simulations and compare results to distributions from the planning system derived using TG43 formalism. To study differences between dose to medium (as obtained with the MC code) and dose to water in medium as obtained through (1) ratios of stopping powers and (2) ratios of mass energy absorption coefficients between water and medium.

Methods:

The MC code Algebra was used to calculate dose distributions according to earlier actual treatment plans using anonymized plan data and CT images in DICOM format. Ratios of stopping power and mass energy absorption coefficients for water with various media obtained from 192‐Ir spectra were used in toggling between dose to water and dose to media.

Results:

Differences between initial planned TG43 dose distributions and the doses to media calculated by MC are insignificant in the target volume. Differences are moderate (within 4–5 % at distances of 3–4 cm) but increase with distance and are most notable in bone and at the patient surface. Differences between dose to water and dose to medium are within 1‐2% when using mass energy absorption coefficients to toggle between the two quantities but increase to above 10% for bone using stopping power ratios.

Conclusion:

MC predicts target doses for head and neck cancer patients in close agreement with TG43. MC yields improved dose estimations outside the target where a larger fraction of dose is from scattered photons. It is important with awareness and a clear reporting of absorbed dose values in using model based algorithms. Differences in bone media can exceed 10% depending on how dose to water in medium is defined.
SU‐F‐19A‐11: Retrospective Evaluation of Thermal Coverage by Thermobrachytherapy Seed Arrangements of Clinical LDR Prostate Implants
Authors: Warrell G , Shvydka D , Chen C , Parsai E
Published: July 2014

Purpose:

The superiority of a properly‐administered combination of radiation therapy and hyperthermia over radiation alone in treatment of human cancers has been demonstrated in multiple studies examining radiobiology, local control, and survival. Unfortunately, hyperthermia is not yet a common modality in oncology practice, due in part to the technical difficulty of heating a deep‐seated target volume to sufficient temperature. To address this problem, our group has invented a thermobrachytherapy (TB) seed based on a commonly‐used low dose‐rate permanent brachytherapy seed for implant in solid tumors. Instead of the tungsten radiographic marker of the standard seed, the TB seed contains one of a self‐regulating ferromagnetic alloy. Placement of a patient implanted with such seeds in an oscillating magnetic field generates heat via induction of eddy currents. We present the results of studies of the capability of clinically‐realistic TB seed arrangements to adequately heat defined target volumes.

Methods:

Seed distributions for several past LDR prostate permanent implant brachytherapy patients were reproduced in the finite element analysis software package COMSOL Multiphysics 4.4, with the difference that TB seeds were modelled, rather than the radiation‐only seeds actually used for their treatments. The implant geometries were mainly of the modified peripheral loading type; a range of prostatic volumes and blood perfusion rates likely to be seen in a clinical setting were examined.

Results:

According to the simulations, when distributed to optimize radiation dose, TB seeds also produce sufficient heat to provide thermal coverage of the target given proper selection of the magnetic field strength. However, the thermal distributions may be improved by additional use of hyperthermia‐only seeds.

Conclusion:

A dual‐modality seed intended as an alternative to and using the same implantation apparatus and technique as the standard LDR permanent implant seed has been successfully evaluated for its ability to provide sufficient hyperthermia in clinically‐realistic implants.

This work was partially supported by the National Institutes of Health (NIH) STTR Grant No. R41 CA153631‐01A1.
SU‐F‐19A‐12: Split‐Ring Applicator with Interstitial Needle for Improved Volumetric Coverage in HDR Brachytherapy for Cervical Cancer
Authors: Sherertz T , Ellis R , Colussi V , Mislmani M , Traughber B , Herrmann K , Podder T
Published: July 2014

Purpose:

To evaluate volumetric coverage of a Mick Radionuclear titanium Split‐Ring applicator (SRA) with/without interstitial needle compared to an intracavitary Vienna applicator (VA), interstitial‐intracavitary VA, and intracavitary ring and tandem applicator (RTA).

Methods:

A 57 year‐old female with FIGO stage IIB cervical carcinoma was treated following chemoradiotherapy (45Gy pelvic and 5.4Gy parametrial boost) with highdose‐ rate (HDR) brachytherapy to 30Gy in 5 fractions using a SRA. A single interstitial needle was placed using the Ellis Interstitial Cap for the final three fractions to increase coverage of left‐sided gross residual disease identified on 3T‐MRI. High‐risk (HR) clinical target volume (CTV) and intermediate‐risk (IR) CTV were defined using axial T2‐weighted 2D and 3D MRI sequences (Philips PET/MRI unit). Organs‐at‐risks (OARs) were delineated on CT. Oncentra planning system was used for treatment optimization satisfying GEC‐ESTRO guidelines for target coverage and OAR constraints. Retrospectively, treatment plans (additional 20 plans) were simulated using intracavitary SRA (without needle), intracavitary VA (without needle), interstitial‐intracavitary VA, and intracavitary RTA with this same patient case. Plans were optimized for each fraction to maintain coverage to HR‐CTV.

Results:

Interstitial‐intracavitary SRA achieved the following combined coverage for external radiation and brachytherapy (EQD2): D90 HR‐CTV =94.6Gy; Bladder_2cc =88.9Gy; Rectum_2cc =65.1Gy; Sigmoid_2cc =48.9Gy; Left vaginal wall (VW) =103Gy, Right VW =99.2Gy. Interstitial‐intracavitary VA was able to achieve identical D90 HR‐CTV =94.6Gy, yet Bladder_2cc =91.9Gy (exceeding GEC‐ESTRO recommendations of 2cc<90Gy) and Left VW =120.8Gy and Right VW =115.5Gy. Neither the SRA nor VA without interstitial needle could cover HR‐CTV adequately without exceeding dose to Bladder_2cc. Conventional RTA was unable to achieve target coverage for the HR‐CTV >80Gy without severely overdosing OARs.

Conclusion:

The Ellis Interstitial Cap for the SRA offered superior dosimetric coverage as compared to the interstitialintracavitary VA. This represents the first reported use for this devise, and further investigation is warranted.
SU‐F‐BRE‐01: A Rapid Method to Determine An Upper Limit On a Radiation Detector's Correction Factor During the QA of IMRT Plans
Authors: Kamio Y , Bouchard H
Published: July 2014

Purpose:

Discrepancies in the verification of the absorbed dose to water from an IMRT plan using a radiation dosimeter can be wither caused by 1) detector specific nonstandard field correction factors as described by the formalism of Alfonso et al. 2) inaccurate delivery of the DQA plan. The aim of this work is to develop a simple/fast method to determine an upper limit on the contribution of composite field correction factors to these discrepancies.

Methods:

Indices that characterize the non‐flatness of the symmetrised collapsed delivery (VSC) of IMRT fields over detector‐specific regions of interest were shown to be correlated with IMRT field correction factors. The indices introduced are the uniformity index (UI) and the mean fluctuation index (MF). Each one of these correlation plots have 10 000 fields generated with a stochastic model. A total of eight radiation detectors were investigated in the radial orientation. An upper bound on the correction factors was evaluated by fitting values of high correction factors for a given index value.

Results:

These fitted curves can be used to compare the performance of radiation dosimeters in composite IMRT fields. Highly water‐equivalent dosimeters like the scintillating detector (Exradin W1) and a generic alanine detector have been found to have corrections under 1% over a broad range of field modulations (0 – 0.12 for MF and 0 – 0.5 for UI). Other detectors have been shown to have corrections of a few percent over this range. Finally, a full Monte Carlo simulations of 18 clinical and nonclinical IMRT field showed good agreement with the fitted curve for the A12 ionization chamber.

Conclusion:

This work proposes a rapid method to evaluate an upper bound on the contribution of correction factors to discrepancies found in the verification of DQA plans.
SU‐F‐BRE‐02: Characterization of a New Commercial Single Crystal Diamond Detector in Photon, Electron and Proton Beams
Authors: Akino Y , Das I
Published: July 2014

Purpose:

Diamond detectors even with superior characteristics have become obsolete due to poor design, selection of crystal and cost. Recently, microDiamond using synthetic single crystal diamond detector (SCDD) is commercially available which is characterized in various radiation beams in this study.

Methods:

The characteristics of a commercial SCDD model 60019 (PTW) to a 6‐ and 15‐MV photon beams, 6‐ and 20‐MeV electron beams, and 208 MeV proton beams were investigated and compared to the pre‐characterized detectors: TN31010 (0.125 cm3) and TN30006 (pinpoint) ionization chambers (PTW), EDGE detector (Sun Nuclear Corp), and SFD Stereotactic Dosimetry Diode Detector (IBA). The depth‐dose and profiles data were collected for various field sizes and depths. The dose linearity and dose rate dependency were also evaluated. To evaluate the effects of the preirradiation, the diamond detector which had not been irradiated on the day was set up in the water tank and the response to 100 MU was measured every 20 s. The temperature dependency was tested for the range of 4–60 °C. Angular dependency was evaluated in water phantom by rotating the SCDD.

Results:

For all radiation types and field sizes, the depth‐dose data of the diamond chamber showed identical curve to those of ionization chambers. The profile of the diamond detector was very similar to those of the Edge and SFD detectors, although the 0.125 cm3 and pinpoint chambers showed averaging effects in the penumbrae region. The temperature dependency was within 0.7% in the range of 4–41°C. A dose of 900 cGy and 1200 cGy were needed to stabilize the chamber to the level within 0.5% and 0.2%, respectively.

Conclusion:

The type 60019 SCDD detector showed suitable characteristics for depth‐dose and profile measurements for wide range of field sizes. However, at least 1000 cGy of pre‐irradiation is needed for accurate measurements.
SU‐F‐BRE‐03: Consideration of a Track‐Interaction Model for Radiochromic Film Response
Authors: Rosen B , DeWerd L
Published: July 2014

Purpose:

Conventional methods for characterizing the energy response of radiochromic film (RCF) typically involve assessing changes in response when exposed to various beam qualities and use Monte Carlo to determine absorbed dose. These methods represent RCF as a dose integrator of a homogeneous energy deposition volume. Apparent film saturation, nonlinearity, and intrinsic energy dependence are unpredicted with conventional methods. Recent work has shown significant RCF intrinsic energy dependence, which limits its use in absolute dosimetry. This work introduces a track‐interaction model (TIM) for RCF and assesses its ability to predict total energy response.

Methods:

A TIM based on Katz single‐hit theory was developed to accumulate energy flux along particle tracks within active crystals, represented as (1×1×20)um3 prisms about the Gafchromic™ EBT3 active volume using MCNP5 and Matlab. Energy flux contributed to film response only if near the threshold energy for polymerization in polydiacetylenes (2.5eV/monomer). Energy deposition in excess of maximum efficiency represented crystal saturation and did not contribute to film response. The TIM was applied to RCF exposed in air to various monoenergetic photon beams and Co‐60. Geometric distribution of energy flux was found for each beam quality in a (1×1)mm2 RCF area. RCF response relative to Co‐60 absorbed dose‐to‐water (S_TIM) was determined and compared to published values (S_PUB).

Results:

TIM successfully predicted that lower energy radiation is less effective at inducing polymerization, though the magnitude of the phenomenon was overpredicted. S_TIM was −29% and +20% for 20 and 40 keV, respectively. This agreed qualitatively with S_PUB of −27% and +16%. TIM‐generated sensitometric curves contained the non‐linearity and saturation apparent in RCF.

Conclusion:

This work indicates the possibility for TIMs to predict changes in RCF response to various energies. Future work will refine TIM by considering size distributions of active elements, other RCF formulations, and response in complex treatment fields.

Acknowledgements: The authors wish to thank Dr. Christopher Soares at NIST (Gaithersburg, MD) for performing film measurements and Dr. David Lewis of Ashland, Inc. (Wayne, NJ) for making the EBT3 film available to us.
SU‐F‐BRE‐04: Construction of 3D Printed Patient Specific Phantoms for Dosimetric Verification Measurements
Authors: Ehler E , Higgins P , Dusenbery K
Published: July 2014

Purpose:

To validate a method to create per patient phantoms for dosimetric verification measurements.

Methods:

Using a RANDO phantom as a substitute for an actual patient, a model of the external features of the head and neck region of the phantom was created. A phantom was used instead of a human for two reasons: to allow for dosimetric measurements that would not be possible in‐vivo and to avoid patient privacy issues. Using acrylonitrile butadiene styrene thermoplastic as the building material, a hollow replica was created using the 3D printer filled with a custom tissue equivalent mixture of paraffin wax, magnesium oxide, and calcium carbonate. A traditional parallel‐opposed head and neck plan was constructed. Measurements were performed with thermoluminescent dosimeters in both the RANDO phantom and in the 3D printed phantom. Calculated and measured dose was compared at 17 points phantoms including regions in high and low dose regions and at the field edges. On‐board cone beam CT was used to localize both phantoms within 1mm and 1° prior to radiation.

Results:

The maximum difference in calculated dose between phantoms was 1.8% of the planned dose (180 cGy). The mean difference between calculated and measured dose in the anthropomorphic phantom and the 3D printed phantom was 1.9% ± 2.8% and −0.1% ± 4.9%, respectively. The difference between measured and calculated dose was determined in the RANDO and 3D printed phantoms. The differences between measured and calculated dose in each respective phantom was within 2% for 12 of 17 points. The overlap of the RANDO and 3D printed phantom was 0.956 (Jaccard Index).

Conclusion:

A custom phantom was created using a 3D printer. Dosimetric calculations and measurements showed good agreement between the dose in the RANDO phantom (patient substitute) and the 3D printed phantom.
SU‐F‐BRE‐05: Development and Evaluation of a Real‐Time Robotic 6D Quality Assurance Phantom
Authors: Belcher AH , Liu X , Grelewicz Z , Wiersma RD
Published: July 2014

Purpose:

A 6 degree‐of‐freedom robotic phantom capable of reproducing dynamic tumor motion in 6D was designed to more effectively match solid tumor movements throughout pre‐treatment scanning and radiation therapy. With the abundance of optical and x‐ray 6D real‐time tumor tracking methodologies clinically available, and the substantial dosimetric consequences of failing to consider tumor rotation as well as translation, this work presents the development and evaluation of a 6D instrument with the facility to improve quality assurance.

Methods:

An in‐house designed and built 6D robotic motion phantom was constructed following the so‐called Stewart‐Gough parallel kinematics platform archetype. The device was then controlled using an inverse kinematics formulation, and precise movements in all six degrees of freedom (X, Y, Z, pitch, roll, and yaw) as well as previously obtained cranial motion, were effectively executed. The robotic phantom movements were verified using a 15 fps 6D infrared marker tracking system (Polaris, NDI), and quantitatively compared to the input trajectory. Thus, the accuracy and repeatability of 6D motion was investigated and the phantom performance was characterized.

Results:

Evaluation of the 6D platform demonstrated translational RMSE values of 0.196 mm, 0.260 mm, and 0.101 mm over 20 mm in X and Y and 10 mm in Z, respectively, and rotational RMSE values of 0.068 degrees, 0.0611 degrees, and 0.095 degrees over 10 degrees of pitch, roll, and yaw, respectively. The robotic stage also effectively performed controlled 6D motions, as well as reproduced cranial trajectories over 15 minutes, with a maximal RMSE of 0.044 mm translationally and 0.036 degrees rotationally.

Conclusion:

This 6D robotic phantom has proven to be accurate under clinical standards and capable of reproducing tumor motion in 6D. Consequently, such a robotics device has the potential to serve as a more effective system for IGRT QA that involves both translational and rotational dimensions.

Research was partially funded by NIH Grant T32 EB002103‐21 from NIBIB. Contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIBIB or NIH.
SU‐F‐BRE‐06: Evaluation of Patient CT Dose Reconstruction From 3D Diode Array Measurements Using Anthropomorphic Phantoms
Authors: Huang M , Faught A , Benhabib S , Cardan R , Brezovich I , Followill D , Popple R
Published: July 2014

Purpose:

To compare 3D reconstructed dose of IMRT plans from 3D diode array measurements with measurements in anthropomorphic phantoms.

Methods:

Six IMRT plans were created for the IROC Houston (RPC) head and neck (H&N) and lung phantoms following IROC Houston planning protocols. The plans included flattened and unflattened beam energies ranging from 6 MV to 15 MV and both static and dynamic MLC tecH&Niques. Each plan was delivered three times to the respective anthropomorphic phantom, each of which contained thermoluminescent dosimeters (TLDs) and radiochromic films (RCFs). The plans were also delivered to a Delta4 diode array (Scandidos, Uppsala, Sweden). Irradiations were done using a TrueBeam STx (Varian Medical Systems, Palo Alto, CA). The dose in the patient was calculated by the Delta4 software, which used the diode measurements to estimate incident energy fluence and a kernel‐based pencil beam algorithm to calculate dose. The 3D dose results were compared with the TLD and RCF measurements.

Results:

In the lung, the average difference between TLDs and Delta4 calculations was 5% (range 2%–7%). For the H&N, the average differences were 2.4% (range 0%–4.5%) and 1.1% (range 0%–2%) for the high‐ and low‐dose targets, respectively, and 12% (range 10%‐13%) for the organ‐at‐risk simulating the spinal cord. For the RCF and criteria of 7%/4mm, 5%/3mm, and 3%/3mm, the average gamma‐index pass rates were 95.4%, 85.7%, and 76.1%, respectively for the H&N and 76.2%, 57.8%, and 49.5% for the lung. The pass‐rate in the lung decreased with increasing beam energy, as expected for a pencil beam algorithm.

Conclusion:

The H&N phantom dose reconstruction met the IROC Houston acceptance criteria for clinical trials; however, the lung phantom dose did not, most likely due to the inaccuracy of the pencil beam algorithm in the presence of low‐density inhomogeneities.

Work supported by PHS grant CA10953 and CA81647 (NCI, DHHS).
SU‐F‐BRE‐07: Experimental Validation of a Lung SBRT Technique Using a Novel, True Volumetric Plenoptic‐Plastic‐Scintillator Detector
Authors: Goulet M , Rilling M , Gingras L , Beddar S , Beaulieu L , Archambault L
Published: July 2014

Purpose:

Lung SBRT is being used by an increasing number of clinics, including our center which recently treated its first patient. In order to validate this technique, the 3D dose distribution of the SBRT plan was measured using a previously developed 3D detector based on plenoptic camera and plastic scintillator technology. The excellent agreement between the detector measurement and the expected dose from the treatment planning system Pinnacle3 shows great promise and amply justify the development of the technique.

Methods:

The SBRT treatment comprised 8 non‐coplanar 6MV photon fields with a mean field size of 12 cm2 at isocentre and a total prescription dose of 12Gy per fraction for a total of 48Gy. The 3D detector was composed of a 10×10×10 cm2 EJ‐260 water‐equivalent plastic scintillator embedded inside a truncated cylindrical acrylic phantom of 10cm radius. The scintillation light was recorded using a static R5 light‐field camera and the 3D dose was reconstructed at a 2mm resolution in all 3 dimensions using an iterative backprojection algorithm.

Results:

The whole 3D dose distribution was recorded at a rate of one acquisition per second. The mean absolute dose difference between the detector and Pinnacle3 was 1.3% over the region with more than 10% of the maximum dose. 3D gamma tests performed over the same region yield passing rates of 98.8% and 96.6% with criteria of 3%/1mm and 2%/1mm, respectively.

Conclusion:

Experimental results showed that our beam modeling and treatment planning system calculation was adequate for the safe administration of small field/high dose techniques such as SBRT. Moreover, because of the real‐time capability of the detector, further validation of small field rotational, dynamic or gated technique can be monitored or verified by this system.
SU‐F‐BRE‐08: Feasibility of 3D Printed Patient Specific Phantoms for IMRT/IGRT QA
Authors: Ehler E , Higgins P , Dusenbery K
Published: July 2014

Purpose:

Test the feasibility of 3D printed, per‐patient phantoms for IMRT QA to analyze the treatment delivery quality within the patient geometry.

Methods:

Using the head and neck region of an anthropomorphic phantom as a substitute for an actual patient, a soft‐tissue equivalent model was constructed with the use of a 3D printer. A nine‐field IMRT plan was constructed and dose verification measurements were performed for the 3D printed phantom. During the delivery of the IMRT QA on to the 3D printed phantom, the same patient positioning indexing system was used on the phantom and image guidance (cone beam CT) was used to localize the phantom, serving as a test of the IGRT system as well. The 3D printed phantom was designed to accommodate four radiochromic film planes (two axial, one coronal and one sagittal) and an ionization chamber measurement. As a frame of comparison, the IMRT QA was also performed on traditional phantoms. Dosimetric tolerance levels such as 3mm / 3% Gamma Index as well as 3% and 5% dose difference were considered. All detector systems were calibrated against a NIST traceable ionization chamber.

Results:

Comparison of results 3D printed patient phantom with the standard IMRT QA systems showed similar passing rates for the 3D printed phantom and the standard phantoms. However, the locations of the failing regions did not necessarily correlate. The 3D printed phantom was localized within 1 mm and 1° using on‐board cone beam CT.

Conclusion:

A custom phantom was created using a 3D printer. It was determined that the use of patient specific phantoms to perform dosimetric verification and estimate the dose in the patient is feasible. In addition, end‐to‐end testing on a per‐patient basis was possible with the 3D printed phantom. Further refinement of the phantom construction process is needed for routine clinical use.
SU‐F‐BRE‐09: Linac Isocenter Quality Assurance: A Stereotactic Approach
Authors: McCabe B , Li J
Published: July 2014

Purpose:

A quantitative method was designed to independently determine the couch/collimator/gantry isocenters, as well as the overall LINAC mechanical isocenter in 3D.

Methods:

Performed on both a Varian TrueBeam™ STx and Trilogy with the gold standard front pointer positioned at 100 cm SAD, and a Radionics XKnife™ (RX) attached to the couch. At gantry and couch 0°, the RX laser alignment attachment (RXLAA) was centered to the front pointer using the micrometers (0.1‐mm precision) on the RX. The 3D coordinates of the micrometers were recorded. The collimator was rotated to 90° and 270°. At each collimator rotation, the RXLAA was re‐centered to the front pointer and the micrometer coordinates recorded. At collimator and gantry 0°, the process was repeated for couch angles 0°/90°/270°. Finally, at collimator and couch 0°, the steps were repeated for gantry rotations 0°/90°/180°/270°. The centers/radii of the smallest bounding spheres for the collimator, couch and gantry walkout were calculated (using MatLab™). The smallest bounding sphere containing the collimator, couch and gantry walkout spheres was then calculated. The center of this all‐encompassing sphere is the overall mechanical isocenter of the LINAC. This position was dialed in on the RX. LINAC mechanical and radiation isocenter coincidence was determined by performing Winston‐ Lutz test at four cardinal gantry angles.

Results:

TrueBeam and Trilogy mechanical isocenters had overall walkout radii of 0.8 mm and 1.5 mm, respectively. For the TrueBeam and Trilogy 6‐MV beams, the radii of radiation isocenter were 0.3 mm and 0.4 mm, respectively, with distances between LINAC mechanical and radiation isocenters of 0.5 mm and 0.9 mm, respectively.

Conclusion:

This efficient and simple method allows for an independent and reliable quantitative assessment of LINAC isocenter in 3D with equipment typically available in a radiation oncology clinic. It can easily be performed for LINAC commissioning and annual QA.
SU‐F‐BRE‐10: Methods to Simulate and Measure the Attenuation for Modeling a Couch Top with Rails for FFF Treatment Delivery On the Varian Edge Linac
Authors: Gulam M , Gardner S , Zhao B , Snyder K , Song K , Li H , Kearns W , Gordon J , Wen N , Chetty I
Published: July 2014

Purpose:

To measure attenuation for modelling of the KVue Couchtop for 6X and 10X FFF SRS/SBRT treatment

Methods:

Treatment planning simulation studies were done using 6X FFF beams to estimate the dosimetric impact of KVue couchtops (including the Q‐Fix IGRT [carbon fiber] and Calypso [nonconductive Kevlar material]) with a structure model obtained from a research workstation (Eclipse, advanced planning interface (API) v13). Prior to installation on the Varian Edge linac, the couchtop along with (Kevlar) rails were CT scanned with the rails at various positions. An additional scan with the couchtop 15cm above the CT table top was obtained with 20cm solid water to facilitate precised/indexed data acquisition. Measurements for attenuation were obtained for field sizes of 2, 4 and 10 cm2 at 42 gantry angles including 6 pairs of opposing fields and other angles for oblique delivery where the beams traversed the couchtop and or rails. The delivery was fully automated with xml scripts running in developer mode. The results were then used to determine an accurate structure model for AAA (Eclipse v11) planning of IMRT and RapidArc delivery.

Results:

The planning simulation relative dose attenuation for oblique entry was not significantly different than the Exact IGRT or BrainLab iBeam couch except that the rails added 6% additional attenuation. The relative attenuation measurements for PA, PA (rails: inner position), oblique, oblique (rails: outer position), oblique (rails: inner position) were: −2.0%, −2.5%, −15.6%, −2.5%, −5.0% for 6X FFF and −1.4%, −1.5%, −12.2%, − 2.5%, −5.0% for 10X FFF with slight decrease in attenuation versus field size. A Couch structure model (with HU values) was developed. Calculation compared to measurement showed good agreement except for oblique (rails: outer position) where differences approached a magnitude of 6%.

Conclusion:

A model of the couch structures has been developed accounting for attenuation for FFF beams.
SU‐F‐BRE‐11: Neutron Measurements Around the Varian TrueBeam Linac
Authors: Maglieri R , Liang L , Evans M , Licea A , Dubeau J , Witharana S , DeBlois F , Seuntjens J , Kildea J
Published: July 2014

Purpose:

With the emergence of flattening filter free (FFF) photon beams, several authors have noted many advantages to their use. One such advantage is the decrease in neutron production by photonuclear reactions in the linac head. In the present work we investigate the reduction in neutrons from a Varian TrueBeam linac using the Nested Neutron Spectrometer (NNS, Detec). The neutron spectrum, total fluence and source strength were measured and compared for 10 MV with and without flattening filter and the effect of moderation by the room and maze was studied for the 15 MV beam.

Methods:

The NNS, similar to traditional Bonner sphere detectors but operated in current mode, was used to measure the neutron fluence and spectrum. The NNS was validated for use in high dose rate environments using Monte Carlo simulations and calibrated at NIST and NRC Canada. Measurements were performed at several positions within the treatment room and maze with the linac jaws closed to maximize neutron production.

Results:

The measurements showed a total fluence reduction between 35‐40% in the room and maze when the flattening filter was removed. The neutron source strength Qn was calculated from in‐room fluence measurements and was found to be 0.042 × 102 n/Gy, 0.026 × 102 n/Gy and 0.59 × 1012 n/Gy for the 10 MV, the 10 MV FFF and 15 MV beams, respectively. We measured ambient equivalent doses of 11 mSv/hr, 7 mSv/hr and 218 mSv/hr for the 10 MV, 10 MV FFF and 15 MV by the head.

Conclusion:

Our measurements revealed a decrease in total fluence, neutron source strength and equivalent dose of approximately 35‐40% across the treatment room for the FFF compared to FF modes. This demonstrates, as expected, that the flattening filter is a major component of the neutron production for the TrueBeam.

The authors greatly acknowledge support form the Canadian Nuclear Commission and the Natural Sciences and Engineering Research Council of Canada through the CREATE program. Co‐authors Dubeau and Witharana are employees of Detec (Gatineau, Quebec), manufacturer of the Nested Neutron Spectrometer.
SU‐F‐BRE‐12: Optical Resonator Water Calorimeter
Authors: Abraham J , DeMarco J , Low D
Published: July 2014

Purpose:

Water calorimetry based on resistance thermometry has matured as a primary standard. Developing an optical technique hold the promise to push the boundaries of what is currently achievable with dosimetry. We will present a feasibility study and the current progress of construction of a Fabry‐Perot resonator for dose to water measurement. Additionally, estimations of the theoretical limits resonator sensitivity and potential sources of noise for the system are described.

Methods:

A temperature change from the dose to water would be measured by the change in the index of refraction from the water in the cavity. Calculations are presented of the expected signal from the resonator for dose to water. The Fabry‐Perot resonator constructed from optical quality narrowband mirrors is described. A water cell will be inserted into the cavity gap to provide the medium swept cavity length technique is explored as a specific implementation of this technique.

Results:

Calculations indicate that a dose to water on the order of a Gray is measureable with a reasonably implementable system. A resonator is currently under construction and progress towards a proof of principle measurement will be presented. The primary sources of noise, in order of importance, are expected to be; optical absorption by the medium, mechanical perturbations of the cavity length and thermal expansion of the optical mounts. Estimations of these noise sources and mitigation techniques will be discussed.

Conclusion:

A Fabry‐Perot resonator is a promising technique for measuring the absorbed dose to water from a radiotherapy beam. This technique has the potential to serve as a check on the current primary standard for dose to water measurements. As well, i0074 may be the foundation for a new class of optical property based dosimetry measurement.
SU‐F‐BRE‐13: Replacing Pre‐Treatment Phantom QA with 3D In‐Vivo Portal Dosimetry for IMRT Breast Cancer
Authors: Stroom J , Vieira S , Olaciregui‐Ruiz I , Rozendaal R , van Herk M , Moser E , Greco C
Published: July 2014

Purpose:

Pre‐treatment QA of individual treatment plans requires costly linac time and physics effort. Starting with IMRT breast treatments, we aim to replace pre‐treatment QA with in‐vivo portal dosimetry.

Methods:

Our IMRT breast cancer plans are routinely measured using the ArcCheck device (SunNuclear). 2D‐Gamma analysis is performed with 3%/3mm criteria and the percentage of points with gamma<1 (nG1) is calculated within the 50% isodose surface. Following AAPM recommendations, plans with nG1<90% are approved; others need further inspection and might be rejected. For this study, we used invivo portal dosimetry (IPD) to measure the 3D back‐projected dose of the first three fractions for IMRT breast plans. Patient setup was online corrected before for all measured fractions. To reduce patient related uncertainties, the three IPD results were averaged and 3D‐gamma analysis was applied with abovementioned criteria . For a subset of patients, phantom portal dosimetry (PPD) was also performed on a slab phantom.

Results:

Forty consecutive breast patients with plans that fitted the EPID were analysed. The average difference between planned and IPD dose in the reference point was −0.7+/−1.6% (1SD). Variation in nG1 between the 3 invivo fractions was about 6% (1SD). The average nG1 for IPD was 89+/−6%, worse than ArcCheck (95+/−3%). This can be explained by patient related factors such as changes in anatomy and/or model deficiencies due to e.g. inhomogeneities. For the 20 cases with PPD, mean nG1 was equal to ArcCheck values, which indicates that the two systems are equally accurate. These data therefore suggest that proper criteria for 3D invivo verification of breast treatments should be nG1>80% instead of nG1>90%, which, for our breast cases, would result in 5% (2/40) further inspections.

Conclusion:

First‐fraction in‐vivo portal dosimetry using new gamma‐evaluation criteria will replace phantom measurements in our institution, saving resources and yielding 3D dosimetry of the actual patient treatment.
SU‐F‐BRE‐14: Uncertainty Analysis for Dose Measurements Using OSLD NanoDots
Authors: Kry S , Alvarez P , Stingo F , Followill D
Published: July 2014

Purpose:

Optically stimulated luminescent dosimeters (OSLD) are an increasingly popular dosimeter for research and clinical applications. It is also used by the Radiological Physics Center for remote auditing of machine output. In this work we robustly calculated the reproducibility and uncertainty of the OSLD nanoDot.

Methods:

For the RPC dose calculation, raw readings are corrected for depletion, element sensitivity, fading, linearity, and energy. System calibration is determined for the experimental OSLD irradiated at different institutions by using OSLD irradiated by the RPC under reference conditions (i.e., standards): 1 Gy in a Cobalt beam. The intra‐dot and inter‐dot reproducibilities (coefficient of variation) were determined from the history of RPC readings of these standards. The standard deviation of the corrected OSLD signal was then calculated analytically using a recursive formalism that did not rely on the normality assumption of the underlying uncertainties, or on any type of mathematical approximation. This analytical uncertainty was compared to that empirically estimated from >45,000 RPC beam audits.

Results:

The intra‐dot variability was found to be 0.59%, with only a small variation between readers. Inter‐dot variability was found to be 0.85%. The uncertainty in each of the individual correction factors was empirically determined. When the raw counts from each OSLD were adjusted for the appropriate correction factors, the analytically determined coefficient of variation was 1.8% over a range of institutional irradiation conditions that are seen at the RPC. This is reasonably consistent with the empirical observations of the RPC, where the coefficient of variation of the measured beam outputs is 1.6% (photons) and 1.9% (electrons).

Conclusion:

OSLD nanoDots provide sufficiently good precision for a wide range of applications, including the RPC remote monitoring program for megavoltage beams.

This work was supported by PHS grant CA10953 awarded by the NIH (DHHS).
SU‐F‐BRE‐15: Verification of Energy Dependence of MAGAT Polymer Gel at Orthovoltage Energies
Authors: Roed Y , Tailor R , Pinsky L , Ibbott G
Published: July 2014

Purpose:

Investigation of dose response curves of methacrylic acid‐based “MAGAT” gel at different effective energies to verify an energy dependence of polymer‐gel dosimeters for orthovoltage energy x‐rays.

Methods:

Six small cylindrical MAGAT gel phantoms were exposed to different dose levels; one phantom was unirradiated for background subtraction. This experiment was repeated for three different effective beam energies.24 h post irradiation the spin‐spin relaxation times (T2) were measured with a 4.7 T Bruker MR scanner at 2 cm depth inside the gel. The T2 values were converted to relaxation rates (R2) and plotted against the respective dose levels corresponding to the different effective energies. The resulting dose response curves were compared for a 250 kVp beam, the 250 kVp beam filtered by 6 cm of water, and a 125 kVp beam.

Results:

The passage of the 250 kVp beam through water resulted in a half‐value‐layer (HVL) change from 1.05 mm Cu to 1.32 mm Cu at 6 cm depth with a change in effective energy from 81.3 keV to 89.5 keV, respectively. The dose response curves showed a shift to higher relaxation rates for the harder beam. The dose response measurements for the 125 kVp beam (HVL: 3.13 mm Al, effective energy: 33.9 keV) demonstrated even higher relaxation rates than for either of the other beams.

Conclusion:

The MAGAT dose response curves for three different effective energies demonstrate a complex energy dependence, with an apparent decrease in sensitivity at 89.5 keV effective energy. This energy dependence is consistent with observed discrepancies of depth dose data compared with ion‐chamber data. For future investigations of larger volumes, an energy‐dependent sensitivity function is needed to properly assess 3‐dimensional dose distributions.
SU‐F‐BRE‐16: VMAT Commissioning and Quality Assurance (QA) of An Elekta Synergy‐STM Linac Using ICOM Test HarnessTM
Authors: Nguyen A , Rajaguru P , Kaurin D , Paul T , He R , Plowman A , Yang C
Published: July 2014

Purpose:

To establish a set of tests based on the iCOM software that can be used to commission and perform periodic QA of VMAT delivery on the Elekta Synergy‐S, commonly known as the Beam Modulator (BM).

Methods:

iCOM is used to create and deliver customized treatment fields to characterize the system in terms of 1) MLC positioning accuracy under static and dynamic delivery with full gantry rotation, 2) MLC positioning with known errors, 3) Maximum dose rate, 4) Maximum MLC speed, 5) Maximum gantry speed, 6) Synchronization: gantry speed versus dose rate, and 7) Synchronization: MLC speed versus dose rate. The resulting images were captured on the iView GT and exported in DICOM format to Dosimetry Check™ system for visual and quantitative analysis. For the initial commissioning phase, the system tests described should be supplemented with extensive patient QAs covering all clinically relevant treatment sites.

Results:

The system performance test suite showed that on our Synergy‐S, MLC positioning was accurate under both static and dynamic deliveries. Intentional errors of 1 mm were also easily identified on both static & dynamic picket fence tests. Maximum dose rate was verified with stop watch to be consistently between 475‐480 MU/min. Maximum gantry speed and MLC speed were 5.5 degree/s and 2.5 cm/s respectively. After accounting for beam flatness, both synchronization tests, gantry versus dose rate and MLC speed versus dose rate, were successful as the fields were uniform across the strips and there were no obvious cold/hot spots.

Conclusion:

VMAT commissioning and quality assurance should include machine characterization tests in addition to patient QAs. Elekta iCOM is a valuable tool for the design of customized VMAT field with specific MU, MLC leaf positions, dose rate, and indirect control of MLC and gantry speed at each of its control points.
SU‐F‐BRD‐01: A Logistic Regression Model to Predict Objective Function Weights in Prostate Cancer IMRT
Authors: Boutilier J , Chan T , Craig T , Lee T , Sharpe M
Published: July 2014

Purpose:

To develop a statistical model that predicts optimization objective function weights from patient geometry for intensity‐modulation radiotherapy (IMRT) of prostate cancer.

Methods:

A previously developed inverse optimization method (IOM) is applied retrospectively to determine optimal weights for 51 treated patients. We use an overlap volume ratio (OVR) of bladder and rectum for different PTV expansions in order to quantify patient geometry in explanatory variables. Using the optimal weights as ground truth, we develop and train a logistic regression (LR) model to predict the rectum weight and thus the bladder weight. Post hoc, we fix the weights of the left femoral head, right femoral head, and an artificial structure that encourages conformity to the population average while normalizing the bladder and rectum weights accordingly. The population average of objective function weights is used for comparison.

Results:

The OVR at 0.7cm was found to be the most predictive of the rectum weights. The LR model performance is statistically significant when compared to the population average over a range of clinical metrics including bladder/rectum V53Gy, bladder/rectum V70Gy, and mean voxel dose to the bladder, rectum, CTV, and PTV. On average, the LR model predicted bladder and rectum weights that are both 63% closer to the optimal weights compared to the population average. The treatment plans resulting from the LR weights have, on average, a rectum V70Gy that is 35% closer to the clinical plan and a bladder V70Gy that is 43% closer. Similar results are seen for bladder V54Gy and rectum V54Gy.

Conclusion:

Statistical modelling from patient anatomy can be used to determine objective function weights in IMRT for prostate cancer. Our method allows the treatment planners to begin the personalization process from an informed starting point, which may lead to more consistent clinical plans and reduce overall planning time.
SU‐F‐BRD‐02: Application of ARCHERRT‐‐ A GPU‐Based Monte Carlo Dose Engine for Radiation Therapy ‐‐ to Tomotherapy and Patient‐Independent IMRT
Authors: Su L , Yang Y , Bednarz B , Sterpin E , Du X , Liu T , Xu X
Published: July 2014

Purpose:

As a module of ARCHER ‐‐ Accelerated Radiation‐transport Computations in Heterogeneous EnviRonments, ARCHERRT is designed for RadioTherapy (RT) dose calculation. This paper describes the application of ARCHERRT on patient‐dependent TomoTherapy and patient‐independent IMRT. It also conducts a “fair” comparison of different GPUs and multicore CPU.

Methods:

The source input used for patient‐dependent TomoTherapy is phase space file (PSF) generated from optimized plan. For patient‐independent IMRT, the open filed PSF is used for different cases. The intensity modulation is simulated by fluence map. The GEANT4 code is used as benchmark. DVH and gamma index test are employed to evaluate the accuracy of ARCHERRT code. Some previous studies reported misleading speedups by comparing GPU code with serial CPU code. To perform a fairer comparison, we write multi‐thread code with OpenMP to fully exploit computing potential of CPU. The hardware involved in this study are a 6‐core Intel E5‐2620 CPU and 6 NVIDIA M2090 GPUs, a K20 GPU and a K40 GPU.

Results:

Dosimetric results from ARCHERRT and GEANT4 show good agreement. The 2%/2mm gamma test pass rates for different clinical cases are 97.2% to 99.7%. A single M2090 GPU needs 50~79 seconds for the simulation to achieve a statistical error of 1% in the PTV. The K40 card is about 1.7∼1.8 times faster than M2090 card. Using 6 M2090 card, the simulation can be finished in about 10 seconds. For comparison, Intel E5‐2620 needs 507∼879 seconds for the same simulation.

Conclusion:

We successfully applied ARCHERRT to Tomotherapy and patient‐independent IMRT, and conducted a fair comparison between GPU and CPU performance. The ARCHERRT code is both accurate and efficient and may be used towards clinical applications.
SU‐F‐BRD‐03: Determination of Plan Robustness for Systematic Setup Errors Using Trilinear Interpolation
Authors: Fix MK , Volken W , Frei D , Terribilini D , Dal Pra A , Schmuecking M , Manser P
Published: July 2014

Purpose:

Treatment plan evaluations in radiotherapy are currently ignoring the dosimetric impact of setup uncertainties. The determination of the robustness for systematic errors is rather computational intensive. This work investigates interpolation schemes to quantify the robustness of treatment plans for systematic errors in terms of efficiency and accuracy.

Methods:

The impact of systematic errors on dose distributions for patient treatment plans is determined by using the Swiss Monte Carlo Plan (SMCP). Errors in all translational directions are considered, ranging from −3 to +3 mm in mm steps. For each systematic error a full MC dose calculation is performed leading to 343 dose calculations, used as benchmarks. The interpolation uses only a subset of the 343 calculations, namely 9, 15 or 27, and determines all dose distributions by trilinear interpolation. This procedure is applied for a prostate and a head and neck case using Volumetric Modulated Arc Therapy with 2 arcs. The relative differences of the dose volume histograms (DVHs) of the target and the organs at risks are compared. Finally, the interpolation schemes are used to compare robustness of 4‐ versus 2‐arcs in the head and neck treatment plan.

Results:

Relative local differences of the DVHs increase for decreasing number of dose calculations used in the interpolation. The mean deviations are <1%, 3.5% and 6.5% for a subset of 27, 15 and 9 used dose calculations, respectively. Thereby the dose computation times are reduced by factors of 13, 25 and 43, respectively. The comparison of the 4‐ versus 2‐arcs plan shows a decrease in robustness; however, this is outweighed by the dosimetric improvements.

Conclusion:

The results of this study suggest that the use of trilinear interpolation to determine the robustness of treatment plans can remarkably reduce the number of dose calculations. This work was supported by Varian Medical Systems.

This work was supported by Varian Medical Systems