bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐545: MLC Distance Travelled as a Predictor for Motor Failure
Authors: Stathakis S , Defoor D , Linden P , Kirby N , Mavroidis P , Papanikolaou N
Published: June 2015

Purpose:

To study the frequency of Multi‐Leaf Collimator (MLC) leaf failures, investigate methods to predict them and reduce linac downtime.

Methods:

A Varian HD120 MLC was used in our study. The hyperterminal MLC errors logged from 06/2012 to 12/2014 were collected. Along with the hyperterminal errors, the MLC motor changes and all other MLC interventions by the linear accelerator engineer were recorded. The MLC dynalog files were also recorded on a daily basis for each treatment and during linac QA. The dynalog files were analyzed to calculate root mean square errors (RMS) and cumulative MLC travel distance per motor. An in‐house MatLab code was used to analyze all dynalog files, record RMS errors and calculate the distance each MLC traveled per day.

Results:

A total of 269 interventions were recorded over a period of 18 months. Of these, 146 included MLC motor leaf change, 39 T‐nut replacements, and 84 MLC cleaning sessions. Leaves close to the middle of each side required the most maintenance. In the A bank, leaves A27 to A40 recorded 73% of all interventions, while the same leaves in the B bank counted for 52% of the interventions. On average, leaves in the middle of the bank had their motors changed approximately every 1500m of travel. Finally, it was found that the number of RMS errors increased prior to an MLC motor change.

Conclusion:

An MLC dynalog file analysis software was developed that can be used to log daily MLC usage. Our eighteen‐month data analysis showed that there is a correlation between the distance an MLC travels, the RMS and the life of the MLC motor. We plan to use this tool to predict MLC motor failures and with proper and timely intervention, reduce the downtime of the linac during clinical hours.
SU‐E‐T‐550: Modulation Index for VMAT
Authors: Park J , Park S , Kim J , Kim H , Kim J , Carlson J , Ye S
Published: June 2015

Purpose:

To present modulation indices (MIs) for volumetric modulated arc therapy (VMAT).

Methods:

A total of 40 VMAT plans were retrospectively selected. To investigate the delivery accuracy of each VMAT plan, gamma passing rates, differences in modulating parameters between plans and log files, and differences between the original plans and the plans reconstructed with the log files were acquired. A modulation index (MIt) was designed by multiplications of the weighted quantifications of MLC speeds, MLC accelerations, gantry accelerations and dose‐rate variations. Textural features including angular second moment, inverse difference moment, contrast, variance, correlation and entropy were calculated from the fluences of each VMAT plan. To test the performance of suggested MIs, Spearman's rank correlation coefficients (r) with the plan delivery accuracy were calculated. Conventional modulation indices for VMAT including the modulation complexity score for VMAT (MCSv), leaf travel modulation complexity score (LTMCS) and MI by Li & Xing were calculated, and their correlations were also analyzed in the same way.

Results:

The r values of contrast (particular displacement distance, d = 1), variance (d = 1), MIt, MCSv, LTMCS and MI by Li&Xing to the local gamma passing rates with 2%/2 mm were 0.547 (p < 0.001), 0.519 (p < 0.001), −0.658 (p < 0.001), 0.186 (p = 0.251), 0.312 (p = 0.05) and −0.455 (p = 0.003), respectively. The r values of those to the MLC errors were −0.863, −0.828, 0.917, −0.635, − 0.857 and 0.795, respectively (p < 0.001). For dose‐volumetric parameters, MIt showed higher statistically significant correlations than did the conventional modulation indices.

Conclusion:

The MIt, contrast (d = 1) and variance (d = 1) showed good performance to predict the VMAT delivery accuracy showing higher correlations to the results of various types of verification methods for VMAT.

This work was in part supported by the National Research Foundation of Korea (NRF) grant (no. 490‐20140029 and no. 490‐20130047) funded by the Korea government.
SU‐E‐T‐552: Monte Carlo Calculation of Correction Factors for a Free‐Air Ionization Chamber in Support of a National Air‐Kerma Standard for Electronic Brachytherapy
Authors: Mille M , Bergstrom P
Published: June 2015

Purpose:

To use Monte Carlo radiation transport methods to calculate correction factors for a free‐air ionization chamber in support of a national air‐kerma standard for low‐energy, miniature x‐ray sources used for electronic brachytherapy (eBx).

Methods:

The NIST is establishing a calibration service for well‐type ionization chambers used to characterize the strength of eBx sources prior to clinical use. The calibration approach involves establishing the well‐chamber's response to an eBx source whose air‐kerma rate at a 50 cm distance is determined through a primary measurement performed using the Lamperti free‐air ionization chamber. However, the free‐air chamber measurements of charge or current can only be related to the reference air‐kerma standard after applying several corrections, some of which are best determined via Monte Carlo simulation. To this end, a detailed geometric model of the Lamperti chamber was developed in the EGSnrc code based on the engineering drawings of the instrument. The egs_fac user code in EGSnrc was then used to calculate energy‐dependent correction factors which account for missing or undesired ionization arising from effects such as: (1) attenuation and scatter of the x‐rays in air; (2) primary electrons escaping the charge collection region; (3) lack of charged particle equilibrium; (4) atomic fluorescence and bremsstrahlung radiation.

Results:

Energy‐dependent correction factors were calculated assuming a monoenergetic point source with the photon energy ranging from 2 keV to 60 keV in 2 keV increments. Sufficient photon histories were simulated so that the Monte Carlo statistical uncertainty of the correction factors was less than 0.01%. The correction factors for a specific eBx source will be determined by integrating these tabulated results over its measured x‐ray spectrum.

Conclusion:

The correction factors calculated in this work are important for establishing a national standard for eBx which will help ensure that dose is accurately and consistently delivered to patients.
SU‐E‐T‐554: Monte Carlo Calculation of Source Terms and Attenuation Lengths for Neutrons Produced by 50–200 MeV Protons On Brass
Authors: Ramos‐Mendez J , Paganetti H , Faddegon B
Published: June 2015

Purpose:

We used TOPAS (TOPAS wraps and extends Geant4 for medical physicists) to compare Geant4 physics models with published data for neutron shielding calculations. Subsequently, we calculated the source terms and attenuation lengths (shielding data) of the total ambient dose equivalent (TADE) in concrete for neutrons produced by protons in brass.

Methods:

Stage1: The Bertini and Binary nuclear models available in Geant4 were compared with published attenuation at depth of the TADE in concrete and iron. Stage2: Shielding data of the TADE in concrete was calculated for 50– 200 MeV proton beams on brass. Stage3: Shielding data from Stage2 was extrapolated for 235 MeV proton beams. This data was used in a point‐line‐source analytical model to calculate the ambient dose per unit therapeutic dose at two locations inside one treatment room at the Francis H Burr Proton Therapy Center. Finally, we compared these results with experimental data and full TOPAS simulations.

Results:

At larger angles (∼130o) the TADE in concrete calculated with the Bertini model was about 9 times larger than that calculated with the Binary model. The attenuation length in concrete calculated with the Binary model agreed with published data within 7%±0.4% (statistical uncertainty) for the deepest regions and 5%±0.1% for shallower regions. For iron the agreement was within 3%±0.1%. The ambient dose per therapeutic dose calculated with the Binary model, relative to the experimental data, was a ratio of 0.93±0.16 and 1.23±0.24 for two locations. The analytical model overestimated the dose by four orders of magnitude. These differences are attributed to the complexity of the geometry.

Conclusion:

The Binary and Bertini models gave comparable results, with the Binary model giving the best agreement with published data at large angle. Shielding data we calculated using the Binary model is useful for fast shielding calculations with other analytical models.

This work was supported by National Cancer Institute Grant R01CA140735
SU‐E‐T‐560: Monte Carlo Simulation of the Neutron Radiation Field Around a Medical 18 MV Linac
Authors: Horst F , Czarnecki D , Zink K
Published: June 2015

Purpose:

Today the majority of radiation therapy treatments are performed at medical electron linear accelerators (linacs). The accelerated electrons are used for the generation of bremsstrahlung photons. The use of higher electron respectively photon energies has some advantages over lower energies such as the longer dose build‐up. However photons with energies higher than ∼7 MeV can additionally to the interaction with bound electrons undergo inelastic reactions with nuclei. These photonuclear reactions lead to the emission of fast neutrons which contaminate the primary photon field. The neutrons might penetrate through the collimators and deliver out‐of‐field dose to the patient. Furthermore the materials inside the linac head as well as the air inside the treatment room get activated which might deliver dose to the medical employees even when the linac is not in operation. A detailed knowledge of these effects is essential for adequate radiation protection of the employees and an optimal patient treatment.

Methods:

It is a common method to study the radiation fields of such linacs by means of Monte Carlo simulations. For the investigation of the effects caused by photonuclear reactions a typical linac in high energy mode (Varian Clinac 18 MV‐X) as well as the surrounding bunker were modelled and simulated using the Monte Carlo code FLUKA which includes extensive nuclear reaction and neutron transport models additional to electron‐photon transport as well as capabilities for a detailed study of effective dose distributions and activation yields.

Results:

Neutron spectra as well as neutron effective dose distributions within the bunker were obtained, reaching up to some mSv/Gy in the patient's plane. The results are normalized per Gy in the depth dose maximum at 10×10 cm2 field size. Therefore an absolute interpretation is possible.

Conclusion:

The obtained data gives a better understanding of the photonuclear reaction caused effects.
SU‐E‐T‐563: Multi‐Fraction Stereotactic Radiosurgery with Extend System of Gamma Knife: Treatment Verification Using Indigenously Designed Patient Simulating Multipurpose Phantom
Authors: Bisht R , Kale S , Gopishankar N , Rath G , Julka P , Agarwal D , Singh M , Garg A , Kumar P , Thulkar S , Sharma B
Published: June 2015

Purpose:

Aim of the study is to evaluate mechanical and radiological accuracy of multi‐fraction regimen and validate Gamma knife based fractionation using newly developed patient simulating multipurpose phantom.

Methods:

A patient simulating phantom was designed to verify fractionated treatments with extend system (ES) of Gamma Knife however it could be used to validate other radiotherapy procedures as well. The phantom has options to insert various density material plugs and mini CT/MR distortion phantoms to analyze the quality of stereotactic imaging. An additional thorax part designed to predict surface doses at various organ sites. The phantom was positioned using vacuum head cushion and patient control unit for imaging and treatment. The repositioning check tool (RCT) was used to predict phantom positioning under ES assembly. The phantom with special inserts for film in axial, coronal and sagittal plane were scanned with X‐Ray CT and the acquired images were transferred to treatment planning system (LGP 10.1). The focal precession test was performed with 4mm collimator and an experimental plan of four 16mm collimator shots was prepared for treatment verification of multi‐fraction regimen. The prescription dose of 5Gy per fraction was delivered in four fractions. Each fraction was analyzed using EBT3 films scanned with EPSON 10000XL Scanner.

Results:

The measurement of 38 RCT points showed an overall positional accuracy of 0.28mm. The mean deviation of 0.28% and 0.31 % were calculated as CT and MR image distortion respectively. The radiological focus accuracy test showed its deviation from mechanical center point of 0.22mm. The profile measurement showed close agreement between TPS planned and film measured dose. At tolerance criteria of 1%/1mm gamma index analysis showed a pass rate of > 95%.

Conclusion:

Our results show that the newly developed multipurpose patient simulating phantom is highly suitable for the verification of fractionated stereotactic radiosurgery using ES of Gamma knife.

The study is a part of intramural research project of Research Section, All India Institute of Medical Sciences New Delhi India (A 247).
SU‐E‐T‐565: Multiple Channel Radiochromic Film Calibration with Spatial Dose Gradient Regularization
Authors: Li Darrell T H , Lee Louis K Y , Chan Anthony T C
Published: June 2015

Purpose:

To develop a new algorithm for multiple channel radiochromic film calibration and to evaluate the accuracy of the algorithm with virtual simulated film images.

Methods:

The proposed calibration method was based on least‐square optimization of discrepancies between measured and estimated optical density of the three colour channels, with the inclusion of the weighted dose gradient as penalty to this minimization problem. The regularization parameter λ was selected using the L‐curve method to reduce the L2‐norm of the dose gradient while preserving the data consistency. The situation of λ = 0 represented no regularization and the least‐square solution would be obtained. The algorithm was tested with 4 virtual film images which were produced from dose planes exported from the planning system. Zero‐mean Gaussian noise were added to simulate the real situation. The resultant dose maps calculated by both least‐square method and regularization method were then compared with the original dose planes.

Results:

Comprehensive study of the 4 simulated cases with the L‐curve method yielded an optimal regularization parameters of 2×10−8. The dose planes generated by least‐square method showed significant noise retained when compared with that from the regularization method. In performing Gamma Index analysis with tolerance of 1% and 1 mm, it was observed that in the least‐square solution, failed points exist everywhere on the dose plane, while only a small amount in the sharp dose gradient region of the regularized solution. It was also noted that the number of failed point in the least‐square solution can be 20 to 150 times more than that in the regularized solution.

Conclusion:

The calibration algorithm proposed in this study successfully recovered the original dose plane from the noise contaminated film image with the regularization method. Regularization should be a better option to the least‐square one in solving the radiochromic film calibration problem.
SU‐E‐T‐568: Neutron Dose Survey of a Compact Single Room Proton Machine
Authors: Chen Y , Prusator M , Islam M , Johnson D , Ahmad S
Published: June 2015

Purpose:

To ensure acceptable radiation limits are maintained for those working at and near the machine during its operation, a comprehensive radiation survey was performed prior to the clinical release of Mevion S250 compact proton machine at Stephenson Oklahoma Cancer Center.

Methods:

The Mevion S250 proton therapy system consists of the following: a superconducting cyclotron to accelerate the proton particles, a passive double scattering system for beam shaping, and paired orthogonal x‐ray imaging systems for patient setup and verification via a 6D robotic couch. All equipment is housed within a single vault of compact design. Two beam delivery applicators are available for patient treatment, offering field sizes of as great as 14 cm and 25 cm in diameter, respectively. Typical clinical dose rates are between 1 and 2 Gy/min with a fixed beam energy of 250 MeV. The large applicator (25 cm in diameter) was used in conjunction with a custom cut brass aperture to create a 20 cm × 20 cm field size at beam isocenter. A 30 cm − 30 cm − 35 cm high density plastic phantom was placed in the beam path to mimic the conditions creating patient scatter. Measurements integrated‐ambient‐neutron‐dose‐equivalence were made with a SWENDII detector. Gantry angles of 0, 90 and 180 degrees, with a maximum dose rate of 150 MU/min (for large applicator) and beam configuration of option 1 (range 25 cm and 20 cm modulation), were selected as testing conditions. At each point of interest, the highest reading was recorded at 30 cm from the barrier surface.

Results:

The highest neutron dose was estimated to be 0.085 mSv/year at the console area.

Conclusion:

All controlled areas are under 5 mSv/year and the uncontrolled areas are under 1 mSv/year. The radiation protection provided by the proton vault is of sufficient quality.
SU‐E‐T‐570: New Quality Assurance Method Using Motion Tracking for 6D Robotic Couches
Authors: Cheon W , Cho J , Ahn S , Han Y , Choi D
Published: June 2015

Purpose:

To accommodate geometrically accurate patient positioning, a robotic couch that is capable of 6‐degrees of freedom has been introduced. However, conventional couch QA methods are not sufficient to enable the necessary accuracy of tests. Therefore, we have developed a camera based motion detection and geometry calibration system for couch QA.

Methods:

Employing a Visual‐Tracking System (VTS, BonitaB10, Vicon, UK) which tracks infrared reflective(IR) markers, camera calibration was conducted using a 5.7 × 5.7 × 5.7 cm3 cube attached with IR markers at each corner. After positioning a robotic‐couch at the origin with the cube on the table top, 3D coordinates of the cube's eight corners were acquired by VTS in the VTS coordinate system. Next, positions in reference coordinates (roomcoordinates) were assigned using the known relation between each point. Finally, camera calibration was completed by finding a transformation matrix between VTS and reference coordinate systems and by applying a pseudo inverse matrix method. After the calibration, the accuracy of linear and rotational motions as well as couch sagging could be measured by analyzing the continuously acquired data of the cube while the couch moves to a designated position. Accuracy of the developed software was verified through comparison with measurement data when using a Laser tracker (FARO, Lake Mary, USA) for a robotic‐couch installed for proton therapy.

Results:

VTS system could track couch motion accurately and measured position in room‐coordinates. The VTS measurements and Laser tracker data agreed within 1% of difference for linear and rotational motions. Also because the program analyzes motion in 3‐Dimension, it can compute couch sagging.

Conclusion:

Developed QA system provides submillimeter/ degree accuracy which fulfills the high‐end couch QA.

This work was supported by the National Research Foundation of Korea funded by Ministry of Science, ICT & Future Planning. (2013M2A2A7043507 and 2012M3A9B6055201)
SU‐E‐T‐571: Newly Emerging Integrated Transmission Detector Systems Provide Online Quality Assurance of External Beam Radiation Therapy
Authors: Hoffman D , Chung E , Hess C , Stern R , Benedict S
Published: June 2015

Purpose:

Two newly emerging transmission detectors positioned upstream from the patient have been evaluated for online quality assurance of external beam radiotherapy. The prototype for the Integral Quality Monitor (IQM), developed by iRT Systems GmbH (Koblenz, Germany) is a large‐area ion chamber mounted on the linac accessory tray to monitor photon fluence, energy, beam shape, and gantry position during treatment. The ion chamber utilizes a thickness gradient which records variable response dependent on beam position. The prototype of Delta4 Discover™, developed by ScandiDos (Uppsala, Sweden) is a linac accessory tray mounted 4040 diode array that measures photon fluence during patient treatment. Both systems are employable for patient specific QA prior to treatment delivery.

Methods:

Our institution evaluated the reproducibility of measurements using various beam types, including VMAT treatment plans with both the IQM ion chamber and the Delta4 Discover diode array. Additionally, the IQM's effect on photon fluence, dose response, simulated beam error detection, and the accuracy of the integrated barometer, thermometer, and inclinometer were characterized. The evaluated photon beam errors are based on the annual tolerances specified in AAPM TG‐142.

Results:

Repeated VMAT treatments were measured with 0.16% reproducibility by the IQM and 0.55% reproducibility by the Delta4 Discover. The IQM attenuated 6, 10, and 15 MV photon beams by 5.43±0.02%, 4.60±0.02%, and 4.21±0.03% respectively. Photon beam profiles were affected <1.5% in the non‐penumbra regions. The IQM's ion chamber's dose response was linear and the thermometer, barometer, and inclinometer agreed with other calibrated devices. The device detected variations in monitor units delivered (1%), field position (3mm), single MLC leaf positions (13mm), and photon energy.

Conclusion:

We have characterized two new transmissions detector systems designed to provide in‐vivo like measurements upstream from the patient. Both systems demonstrate substantial utility for online treatment verification and QA of photon external beam radiotherapy.
SU‐E‐T‐573: Normal Tissue Dose Effect of Prescription Isodose Level Selection in Lung Stereotactic Body Radiation Therapy
Authors: Zhang Q , Lei Y , Zheng D , Zhu X , Wahl A , Lin C , Zhou S , Zhen W
Published: June 2015

Purpose:

To evaluate dose fall‐off in normal tissue for lung stereotactic body radiation therapy (SBRT) cases planned with different prescription isodose levels (IDLs), by calculating the dose dropping speed (DDS) in normal tissue on plans computed with both Pencil Beam (PB) and Monte‐Carlo (MC) algorithms.

Methods:

The DDS was calculated on 32 plans for 8 lung SBRT patients. For each patient, 4 dynamic conformal arc plans were individually optimized for prescription isodose levels (IDL) ranging from 60% to 90% of the maximum dose with 10% increments to conformally cover the PTV. Eighty non‐overlapping rind structures each of 1mm thickness were created layer by layer from each PTV surface. The average dose in each rind was calculated and fitted with a double exponential function (DEF) of the distance from the PTV surface, which models the steep‐ and moderate‐slope portions of the average dose curve in normal tissue. The parameter characterizing the steep portion of the average dose curve in the DEF quantifies the DDS in the immediate normal tissue receiving high dose. Provided that the prescription dose covers the whole PTV, a greater DDS indicates better normal tissue sparing. The DDS were compared among plans with different prescription IDLs, for plans computed with both PB and MC algorithms.

Results:

For all patients, the DDS was found to be the lowest for 90% prescription IDL and reached a highest plateau region for 60% or 70% prescription. The trend was the same for both PB and MC plans.

Conclusion:

Among the range of prescription IDLs accepted by lung SBRT RTOG protocols, prescriptions to 60% and 70% IDLs were found to provide best normal tissue sparing.
SU‐E‐T‐578: On Definition of Minimum and Maximum Dose for Target Volume
Authors: Gong Y , Yu J , Xiao Y
Published: June 2015

Purpose:

This study aims to investigate the impact of different minimum and maximum dose definitions in radiotherapy treatment plan quality evaluation criteria by using tumor control probability (TCP) models.

Methods:

Dosimetric criteria used in RTOG 1308 protocol are used in the investigation. RTOG 1308 is a phase III randomized trial comparing overall survival after photon versus proton chemoradiotherapy for inoperable stage II‐IIIB NSCLC. The prescription dose for planning target volume (PTV) is 70Gy. Maximum dose (Dmax) should not exceed 84Gy and minimum dose (Dmin) should not go below 59.5Gy in order for the plan to be “per protocol” (satisfactory).A mathematical model that simulates the characteristics of PTV dose volume histogram (DVH) curve with normalized volume is built. The Dmax and Dmin are noted as percentage volumes Dη% and D(100‐δ)%, with η and d ranging from 0 to 3.5. The model includes three straight line sections and goes through four points: D95%= 70Gy, Dη%= 84Gy, D(100‐δ)%= 59.5 Gy, and D100%= 0Gy. For each set of η and δ, the TCP value is calculated using the inhomogeneously irradiated tumor logistic model with D50= 74.5Gy and γ50=3.52.

Results:

TCP varies within 0.9% with η; and δ values between 0 and 1. With η and η varies between 0 and 2, TCP change was up to 2.4%. With η and δ variations from 0 to 3.5, maximum of 8.3% TCP difference is seen.

Conclusion:

When defined maximum and minimum volume varied more than 2%, significant TCP variations were seen. It is recommended less than 2% volume used in definition of Dmax or Dmin for target dosimetric evaluation criteria.

This project was supported by NIH grants U10CA180868, U10CA180822, U24CA180803, U24CA12014 and PA CURE Grant.
SU‐E‐T‐579: On the Relative Sensitivity of Monte Carlo and Pencil Beam Dose Calculation Algorithms to CT Metal Artifacts in Volumetric‐Modulated Arc Spine Radiosurgery (RS)
Authors: Wong M , Lee V , Blanck O , Leung R , Lee K , Law G , Tung S , Chan M
Published: June 2015

Purpose:

Investigating the relative sensitivity of Monte Carlo (MC) and Pencil Beam (PB) dose calculation algorithms to low‐Z (titanium) metallic artifacts is important for accurate and consistent dose reporting in post¬operative spinal RS.

Methods:

Sensitivity analysis of MC and PB dose calculation algorithms on the Monaco v.3.3 treatment planning system (Elekta CMS, Maryland Heights, MO, USA) was performed using CT images reconstructed without (plain) and with Orthopedic Metal Artifact Reduction (OMAR; Philips Healthcare system, Cleveland, OH, USA). 6MV and 10MV volumetric‐modulated arc (VMAT) RS plans were obtained for MC and PB on the plain and OMAR images (MC‐plain/OMAR and PB‐plain/OMAR).

Results:

Maximum differences in dose to 0.2cc (D0.2cc) of spinal cord and cord +2mm for 6MV and 10MV VMAT plans were 0.1Gy between MC‐OMAR and MC‐plain, and between PB‐OMAR and PB‐plain. Planning target volume (PTV) dose coverage changed by 0.1±0.7% and 0.2±0.3% for 6MV and 10MV from MC‐OMAR to MC‐plain, and by 0.1±0.1% for both 6MV and 10 MV from PB‐OMAR to PB‐plain, respectively. In no case for both MC and PB the D0.2cc to spinal cord was found to exceed the planned tolerance changing from OMAR to plain CT in dose calculations.

Conclusion:

Dosimetric impacts of metallic artifacts caused by low‐Z metallic spinal hardware (mainly titanium alloy) are not clinically important in VMAT‐based spine RS, without significant dependence on dose calculation methods (MC and PB) and photon energy ≥ 6MV. There is no need to use one algorithm instead of the other to reduce uncertainty for dose reporting. The dose calculation method that should be used in spine RS shall be consistent with the usual clinical practice.
SU‐E‐T‐581: On the Value of LET as a Radiation Quality Descriptor for RBE
Authors: Pater P , Backstrom G , Enger S , Villegas F , Ahnesjo A , Seuntjens J , Naqa I El
Published: June 2015

Purpose:

To investigate the relationship between linear energy transfer (LET) and relative biological effectiveness (RBE) for protons and light ions, and the corresponding role of LET as a descriptor of radiation quality of hadron therapy.

Methods:

Monte Carlo (MC) proton and light ion (He, Li, C) tracks with LET < 30 eV nm‐1 were generated in an event‐by‐event mode. They were overlaid on a cell nucleus model containing 6×109 nucleotide base pairs using an isotropic irradiation procedure that provides electronic equilibrium. Strand breaks (sbs) were scored in the DNA sugar‐phosphate groups and further sub‐classified into single or double sbs (ssbs or dsbs). Distributions of ssbs and dsbs for 2 Gy fractions were calculated to estimate RBE for the induction of initial dsbs with reference to 60Co. Additionally, sbs were classified based on their complexity (i.e. the number of sbs in each cluster).

Results:

An increase in LET for light ions of the same atomic number or a decrease in atomic number for ions of the same LET resulted in a lower kinetic energy of emitted secondary electrons. The clustering of DNA damage was more pronounced as reflected by the increase in proton RBE from ∼ 1.75 to 4 for LET values of 7 to 28 eV nm‐1. A significant RBE decrease between protons, He, Li and C ions of the same LET was also noticed as function of the atomic number. Significant differences in ssbs and dsbs complexities were also seen for particles with the same LET, potentially supporting a clustering‐based radiation quality descriptor.

Conclusion:

The LET‐RBE relationships were simulated for proton and light ions and exhibited expected trends, including different RBEs for particles with the same LET but different atomic numbers. A complexity based radiation quality descriptor may allow better differentiation of RBE between radiation fields of similar LET.

We would like to acknowledge support from the Fonds de recherche du Quebec Sante (FRQS), from the CREATE Medical Physics Research Training Network grant (number 432290) of NSERC, support from NSERC under grants RGPIN 397711‐11 and RGPIN‐2014‐06475 and support from CIHR under grants MOP‐114910, MOP‐136774 and MOP‐102550.
SU‐E‐T‐584: Optical Tracking Guided Patient‐Specific VMAT QA with ArcCHECK
Authors: Fan Q , Park C , Lu B , Barraclough B , Lebron S , Li J , Liu C , Yan G
Published: June 2015

Purpose:

To investigate the novel use of an in‐house optical tracking system (OTS) to improve the efficacy of VMAT QA with a cylindrical dosimeter (ArcCHECK™).

Methods:

The translational and rotational setup errors of ArcCHECK are convoluted which makes it challenging to position the device efficiently and accurately. We first aligned the ArcCHECK to the machine cross‐hair at three cardinal angles (0°, 90°, and 270°) to establish a reference position. Four infrared reflective markers were attached to the back of the device. An OTS with 0.2mm/0.2° accuracy was used to control its setup uncertainty. Translational uncertainties of 1 mm and 2 mm in three directions (in, right, and up) were applied on the device. Four open beams of various field sizes and six clinical VMAT arcs were delivered and measured for all simulated setup errors. The measurements were compared with Pinnacle™ calculations using Gamma analysis to evaluate the impact of setup uncertainty. This study also evaluated the improvement in setup reproducibility and efficiency with the aid of the OTS.

Results:

For open beams, with 3%/3mm, the mean passing rates dropped by less than 5% for all shifts; with 2%/2mm, two significant drops(>5%) were observed: 15.38±6.75% for 2 mm lateral shift and 9.35±4.88% for 2 mm longitudinal shift. For VMAT arcs, the mean passing rates using 2%/2mm dropped by 10.47±7.46% and 22.02±11.39% for 1 and 2 mm shift, respectively. With 3%/3mm, significant drop only occurred with 2 mm longitudinal shift (13.73±8.30%). Setup time could be reduced by >15 min with the aid of the OTS.

Conclusion:

OTS is an effective tool for separating translational and rotational uncertainties. The current VMAT QA solution was not strongly sensitive to translation errors of 2mm with widely accepted criterion (3%/3mm). This finding raises concerns regarding the efficacy of such QA system in detecting errors in the dynamic VMAT delivery.
SU‐E‐T‐591: Optimizing the Flattening Filter Free Beam Selection in RapidArc‐Based Stereotactic Body Radiotherapy for Stage I Lung Cancer
Authors: Huang B‐T , Lu J‐Y
Published: June 2015

Purpose:

To optimize the flattening filter free (FFF) beam energy selection in stereotactic body radiotherapy (SBRT) treatment for stage I lung cancer with different fraction schemes.

Methods:

Twelve patients suffering from stage I lung cancer were enrolled in this study. Plans were designed using 6XFFF and 10XFFF beams with the most widely used fraction schemes of 4*12 Gy, 3*18 Gy and 1*34 Gy, respectively. The plan quality was appraised in terms of planning target volume (PTV) coverage, conformity of the prescribed dose (CI100%), intermediate dose spillage (R50% and D2cm), organs at risk (OARs) sparing and beam‐on time.

Results:

The 10XFFF beam predicted 1% higher maximum, mean dose to the PTV and 4–5% higher R50% compared with the 6XFFF beam in the three fraction schemes, whereas the CI100% and D2cm was similar. Most importantly, the 6XFFF beam exhibited 3–10% lower dose to all the OARs. However, the 10XFFF beam reduced the beam‐on time by 31.9±7.2%, 38.7±2.8% and 43.6±4.0% compared with the 6XFFF beam in the 4*12 Gy, 3*18 Gy and 1*34 Gy schemes, respectively. Beam‐on time was 2.2±0.2 vs 1.5±0.1, 3.3±0.9 vs 2.0±0.5 and 6.3±0.9 vs 3.5±0.4 minutes for the 6XFFF and 10XFFF one in the three fraction schemes.

Conclusion:

The 6XFFF beam obtains better OARs sparing in SBRT treatment for stage I lung cancer, but the 10XFFF one provides improved treatment efficiency. To balance the OARs sparing and intrafractional variation as a function of prolonged treatment time, the authors recommend to use the 6XFFF beam in the 4*12 Gy and 3*18 Gy schemes for better OARs sparing. However, for the 1*34 Gy scheme, the 10XFFF beam is recommended to achieve improved treatment efficiency.
SU‐E‐T‐593: Outcomes and Toxicities From a Clinical Trial of APBI Using MERT+IMRT with the Same XMLC
Authors: Jimenez‐Ortega E. , Miguez‐Sanchez C. , Palma B. , Ureba A. , Miras H. , Arrans R. , Barbeiro A.R. , Baeza J.A. , Carrasco F. , Plaza A. Leal
Published: June 2015

Purpose:

We present the results from a clinical trial of accelerated partial breast irradiation (APBI), using mixed modulated photon and electron beams (MERT+IMRT) with the same photon multileaf collimator (xMLC).

Methods:

Seven patients were enrolled in the first year of the APBI clinical trial. Patients were selected following the conditions included in the NSABP B‐39/RTOG 0413 protocol. The targets and clinically relevant normal structures were contoured on the CT images following this protocol for APBI‐EBRT. All treatments were delivered using combined modulated electron and photon beams by means of the same xMLC installed in a SIEMENS Primus linac, with a reduced SSD equal to 60 cm for electron beams. The plans were performed with a treatment planning system based on full Monte Carlo simulations, called CARMEN, developed by our group. Simultaneously, an alternative IMRT plan was calculated with the commercial TPS PINNACLE v8.0m (Philips), and both plans were compared. An ad‐hoc breast phantom with semi‐spherical geometry called NAOMI was designed for a specific QA protocol. Patients received a total dose of 38.5 Gy, delivered in 10 fractions over 5 consecutive days, with a twice‐a‐day hypofractionated schema.Follow‐up visits during 2.5 years on average were repeated at 1 month post‐treatment, every 3 months for the first year, and every 6 months for the second year. Toxicity was scored according to National Cancer Institute Common Terminology Criteria for Adverse Events (CTCAE version 3.0).

Results:

This APBI technique achieved high loco‐regional control rates and showed low acute toxicity (grade 1 of CTCAE) and no toxicities from first month onwards. Photographic assessment of cosmesis showed skin excellent results.

Conclusion:

The clinical results achieved with MERT+IMRT by using the same xMLC are comparable or even better than those obtained with other APBI techniques, thanks to a software solution without any additional equipment or specific device.
SU‐E‐T‐607: Performance Quantification of the Nine Detectors Used for Dosimetry Measurements in Advanced Radiation Therapy Treatments
Authors: Markovic M , Stathakis S , Jurkovic I , Mavroidis P , Papanikolaou N
Published: June 2015

Purpose:

The purpose of this study was to quantify performance of the nine detectors used for dosimetry measurements in advanced radiation therapy treatments.

Methods:

The 6 MV beam was utilized for measurements of the field sizes with the lack of lateral charge particle equilibrium. For dose fidelity aspect, energy dependence was studied by measuring PDD and profiles at different depths. The volume effect and its influence on the measured dose profiles have been observed by measuring detector's response function. Output factor measurements with respect to change in energy spectrum have been performed and collected data has been analyzed. The linearity of the measurements with the dose delivered has been evaluated and relevant comparisons were done.

Results:

The measured values of the output factors with respect to change in energy spectrum indicated presence of the energy dependence. The detectors with active volume size ≤ 0.3 mm3 maximum deviation from the mean is 5.6% for the field size 0.5 × 0.5 cm2 while detectors with active volume size > 0.3 mm3 have maximum deviation from the mean 7.1%. Linearity with dose at highest dose rate examined for diode detectors showed maximum deviation of 4% while ion chambers showed maximum deviation of 2.2%. Dose profiles showed energy dependence at shallow depths (surface to dmax) influenced by low energy particles with 12 % maximum deviation from the mean for 5 mm2 field size. In relation to Monte Carlo calculation, the detector's response function σ values were between (0.42±0.25) mm and (1.2±0.25) mm.

Conclusion:

All the detectors are appropriate for the dosimetry measurements in advanced radiation therapy treatments. The choice of the detectors has to be determined by the application and the scope of the measurements in respect to energy dependence and ability to accurately resolve dose profiles as well as to it's intrinsic characteristics.
SU‐E‐T‐608: Perturbation Corrections for Alanine Dosimeters in Different Phantom Materials in High‐Energy Photon Beams
Authors: von Voigts‐Rhetz P , Anton M , Czarnecki D , Zink K
Published: June 2015

Purpose:

Alanine dosimeters are often used for in‐vivo dosimetry purposes in radiation therapy. In a Monte Carlo study the influence of 20 different surrounding/phantom materials for alanine dosimeters was investigated. The investigations were performed in high‐energy photon beams, covering the whole range from 60Co up to 25 MV‐X. The aim of the study is the introduction of a perturbation correction kenv for alanine dosimeters accounting for the environmental material.

Methods:

The influence of different surrounding materials on the response of alanine dosimeters was investigated with Monte Carlo simulations using the EGSnrc code. The photon source was adapted with BEAMnrc to a 60Co unit and an Elekta (Enom=6, 10, 25 MV‐X) linear accelerator. Different tissue‐equivalent materials ranging from cortical bone to lung were investigated. In addition to available phantom materials, some material compositions were taken and scaled to different electron densities. The depth of the alanine detectors within the different phantom materials corresponds to 5 cm depth in water, i.e. the depth is scaled according to the electron density (ne/ne,w) of the corresponding phantom material. The dose was scored within the detector volume once for an alanine/paraffin mixture and once for a liquid water voxel. The relative response, the ratio of the absorbed dose to alanine to the absorbed dose to water, was calculated and compared to the corresponding ratio under reference conditions. Results: For each beam quality the relative response r and the correction factor for the environment kenv was calculated. kenv=0.9991+0.0049 *((ne/ne,w)−0.7659)3

Conclusion:

A perturbation correction factor kenv accounting for the phantom environment has been introduced. The response of the alanine dosimeter can be considered independent of the surrounding material for relative electron densities (ne/ne,w) between 1 and 1.4. For denser materials such as bone or much less dense surroundings such as lung, a small correction would be appropriate.
SU‐E‐T‐610: Phosphor‐Based Fiber Optic Probes for Proton Beam Characterization
Authors: Darafsheh A , Soldner A , Liu H , Kassaee A , Zhu T , Finlay J
Published: June 2015

Purpose:

To investigate feasibility of using fiber optics probes with rare‐earth‐based phosphor tips for proton beam radiation dosimetry. We designed and fabricated a fiber probe with submillimeter resolution (<0.5 mm3) based on TbF3 phosphors and evaluated its performance for measurement of proton beam including profiles and range.

Methods:

The fiber optic probe with TbF3 phosphor tip, embedded in tissue‐mimicking phantoms was irradiated with double scattering proton beam with energy of 180 MeV. Luminescence spectroscopy was performed by a CCD‐coupled spectrograph to analyze the emission spectra of the fiber tip. In order to measure the spatial beam profile and percentage depth dose, we used singular value decomposition method to spectrally separate the phosphors ionoluminescence signal from the background Cerenkov radiation signal.

Results:

The spectra of the TbF3 fiber probe showed characteristic ionoluminescence emission peaks at 489, 542, 586, and 620 nm. By using singular value decomposition we found the contribution of the ionoluminescence signal to measure the percentage depth dose in phantoms and compared that with measurements performed with ion chamber. We observed quenching effect at the spread out Bragg peak region, manifested as under‐responding of the signal, due to the high LET of the beam. However, the beam profiles were not dramatically affected by the quenching effect.

Conclusion:

We have evaluated the performance of a fiber optic probe with submillimeter resolution for proton beam dosimetry. We demonstrated feasibility of spectral separation of the Cerenkov radiation from the collected signal. Such fiber probes can be used for measurements of proton beams profile and range. The experimental apparatus and spectroscopy method developed in this work provide a robust platform for characterization of proton‐irradiated nanophosphor particles for ultralow fluence photodynamic therapy or molecular imaging applications.
SU‐E‐T‐613: Physics Considerations for Single‐Isocenter Volumetric Modulated Arc Radiosurgery for Treatment of Multiple Intracranial Targets
Authors: Stanhope C , Chang Z , Wang Z , Yin F , Kim G , Salama J , Kirkpatrick J , Adamson J
Published: June 2015

Purpose:

To address physics challenges associated with single isocenter radiosurgery for multiple intracranial metastases (SIRMIT). Because the Varian High Definition MLC has thicker leaves >4cm from the isocenter, we predict inferior dose falloff and plan quality for small targets located >4cm from isocenter. To address this concern, we evaluate various isocenter placement strategies including one that places the isocenter closer to smaller targets. We also evaluate the significance of arc geometry avoidance angles to limit lens dose.

Methods:

11 SIRMIT plans with isocenter at centroid were retrospectively analyzed to determine the relationship between relevant dosimetric indices and distance from isocenter (e.g. relative tumor volume). We investigated three isocenter placements for four SIRMIT patients whose lesions had larger variations in target volume; placements included: centroid, Eclipse's built‐in method, and an “inverse center of mass” (ICM) method that weights the isocenter towards smaller lesions. Three VMAT SIRMIT plans were prepared according to institutional and Clark et al.'s guidelines to investigate the effect of arc geometry on lens dose.

Results:

Dose conformity and gradient fall‐off tended to be better for larger tumors (>1cc) close to isocenter. For the four largest tumors (per plan), centroid CI, GI, and HI values were 9±9%, 12±4%, and 3.8±4.9% better than the next best isocenter placement strategy. None of the methods yielded significantly higher conformity for small tumors, however ICM GI and HI values were better for three of the small tumors by 6±12% and 1.8±2.2% respectively. Lens dose range decreased from [3.3%, 11.4%] to [0.1%, 0.5%] by avoiding arc geometries that enter/exit through the eyes.

Conclusion:

CI and GI values were poorer for small distal targets, especially >6cm. Large tumors benefited from centroid placement more than small tumors did from ICM. It is necessary to consider arc geometry avoidance angles to adequately reduce lens dose.
SU‐E‐T‐614: Plan Averaging for Multi‐Criteria Navigation of Step‐And‐Shoot IMRT
Authors: Guo M , Craft D , Gao H
Published: June 2015

Purpose:

Step‐and‐shoot IMRT is fundamentally discrete in nature, while multi‐criteria optimization (MCO) is fundamentally continuous: the MCO planning consists of continuous sliding across the Pareto surface (the set of plans which represent the tradeoffs between organ‐at‐risk doses and target doses). In order to achieve close to real‐time dose display during this sliding, it is desired that averaged plans share many of the same apertures as the pre‐computed plans, since dose computation for apertures generated on‐the‐fly would be expensive. We propose a method to ensure that neighboring plans on a Pareto surface share many apertures.

Methods:

Our baseline step‐and‐shoot sequencing method is that of K. Engel (a method which minimizes the number of segments while guaranteeing the minimum number of monitor units), which we customize to sequence a set of Pareto optimal plans simultaneously. We also add an error tolerance to study the relationship between the number of shared apertures, the total number of apertures needed, and the quality of the fluence map re‐creation.

Results:

We run tests for a 2D Pareto surface trading off rectum and bladder dose versus target coverage for a clinical prostate case. We find that if we enforce exact fluence map recreation, we are not able to achieve much sharing of apertures across plans. The total number of apertures for all seven beams and 4 plans without sharing is 217. With sharing and a 2% error tolerance, this number is reduced to 158 (73%).

Conclusion:

With the proposed method, total number of apertures can be decreased by 42% (averaging) with no increment of total MU, when an error tolerance of 5% is allowed. With this large amount of sharing, dose computations for averaged plans which occur during Pareto navigation will be much faster, leading to a real‐time what‐you‐see‐is‐what‐you‐get Pareto navigation experience.

Minghao Guo and Hao Gao were partially supported by the NSFC (#11405105), the 973 Program (#2015CB856000) and the Shanghai Pujiang Talent Program (#14PJ1404500).