bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐F‐T‐475: An Evaluation of the Overlap Between the Acceptance Testing and Commissioning Processes for Conventional Medical Linear Accelerators
Authors: Morrow A , Rangaraj D , Perez‐Andujar A , Krishnamurthy N
Published: July 2016

Purpose:

This work's objective is to determine the overlap of processes, in terms of sub‐processes and time, between acceptance testing and commissioning of a conventional medical linear accelerator and to evaluate the time saved by consolidating the two processes.

Method:

A process map for acceptance testing for medical linear accelerators was created from vendor documentation (Varian and Elekta). Using AAPM TG‐106 and inhouse commissioning procedures, a process map was created for commissioning of said accelerators. The time to complete each sub‐process in each process map was evaluated. Redundancies in the processes were found and the time spent on each were calculated.

Results:

Mechanical testing significantly overlaps between the two processes ‐ redundant work here amounts to 9.5 hours. Many beam non‐scanning dosimetry tests overlap resulting in another 6 hours of overlap. Beam scanning overlaps somewhat ‐ acceptance tests include evaluating PDDs and multiple profiles but for only one field size while commissioning beam scanning includes multiple field sizes and depths of profiles. This overlap results in another 6 hours of rework. Absolute dosimetry, field outputs, and end to end tests are not done at all in acceptance testing. Finally, all imaging tests done in acceptance are repeated in commissioning, resulting in about 8 hours of rework. The total time overlap between the two processes is about 30 hours.

Conclusion:

The process mapping done in this study shows that there are no tests done in acceptance testing that are not also recommended to do for commissioning. This results in about 30 hours of redundant work when preparing a conventional linear accelerator for clinical use. Considering these findings in the context of the 5000 linacs in the United states, consolidating acceptance testing and commissioning would have allowed for the treatment of an additional 25000 patients using no additional resources.
SU‐F‐T‐476: Performance of the AS1200 EPID for Periodic Photon Quality Assurance
Authors: DeMarco J , Moran J , Barnes M , Greer P , Kim G , Fraass B , Yang W , McKenzie Boehnke E
Published: July 2016

Purpose:

To assess the dosimetric performance of a new amorphous silicon flat‐panel electronic portal imaging device (EPID) suitable for high‐intensity, flattening‐filter‐free delivery mode.

Methods:

An EPID‐based QA suite was created with automation to periodically monitor photon central‐axis output and two‐dimensional beam profile constancy as a function of gantry angle and dose‐rate. A Varian TrueBeamTM linear accelerator installed with Developer Mode was used to customize and deliver XML script routines for the QA suite using the dosimetry mode image acquisition for an aS1200 EPID. Automatic post‐processing software was developed to analyze the resulting DICOM images.

Results:

The EPID was used to monitor photon beam output constancy (central‐axis), flatness, and symmetry over a period of 10 months for four photon beam energies (6x, 15x, 6xFFF, and 10xFFF). EPID results were consistent to those measured with a standard daily QA check device. At the four cardinal gantry angles, the standard deviation of the EPID central‐axis output was <0.5%. Likewise, EPID measurements were independent for the wide range of dose rates (including up to 2400 mu/min for 10xFFF) studied with a standard deviation of <0.8% relative to the nominal dose rate for each energy. Also, profile constancy and field size measurements showed good agreement with the reference acquisition of 0° gantry angle and nominal dose rate. XML script files were also tested for MU linearity and picket‐fence delivery. Using Developer Mode, the test suite was delivered in <60 minutes for all 4 photon energies with 4 dose rates per energy and 5 picket‐fence acquisitions.

Conclusion:

Dosimetry image acquisition using a new EPID was found to be accurate for standard and high‐intensity photon beams over a broad range of dose rates over 10 months. Developer Mode provided an efficient platform to customize the EPID acquisitions by using custom script files which significantly reduced the time.

This work was funded in part by Varian Medical Systems.
SU‐F‐T‐479: Estimation of the Accuracy in Respiratory‐Gated Radiotherapy
Authors: Kurosawa T , Tachibana H , Moriya S , Miyakawa S , Sato M
Published: July 2016

Purpose:

Irregular respiratory patterns affects dose outputs in respiratorygated radiotherapy and there is no commercially available quality assurance (QA) system for it. We designed and developed a patient specific QA system for respiratory‐gated radiotherapy to estimate irradiated output.

Methods:

Our in‐house QA system for gating was composed of a personal computer with the USB‐FSIO electronic circuit connecting to the linear accelerator (ONCOR‐K, Toshiba Medical Systems). The linac implements a respiratory gating system (AZ‐733V, Anzai Medical). During the beam was on, 4.2 V square‐wave pulses were continually sent to the system. Our system can receive and count the pulses. At first, our system and an oscilloscope were compared to check the performance of our system. Next, basic estimation models were generated when ionization‐chamber measurements were performed in gating using regular sinusoidal wave patterns (2.0, 2.5, 4.0, 8.0, 15 sec/cycle). During gated irradiation with the regular patterns, the number of the pulses per one gating window was measured using our system. Correlation between the number of the pulses per one gating and dose per the gating window were assessed to generate the estimation model. Finally, two irregular respiratory patterns were created and the accuracy of the estimation was evaluated.

Results:

Compared to the oscilloscope, our system worked similarly. The basic models were generated with the accuracy within 0.1%. The results of the gated irradiations with two irregular respiratory patterns show good agreement within 0.4% estimation accuracy.

Conclusion:

Our developed system shows good estimation for even irregular respiration patterns. The system would be a useful tool to verify the output for respiratory‐gated radiotherapy.
SU‐F‐T‐482: Using Cerenkov Light Emission for the Measurement of Output Factors at Clinical Linear Accelerators
Authors: Zink K , Burg J
Published: July 2016

Purpose:

A potential method of quality assurance for clinical accelerators is using the signal from Cerenkov light generated during irradiation. The example of measuring output factors (OF) of a clinical accelerator was used to investigate the potential of Cerenkov light as a dosimetry tool.

Methods:

OF as a function of field size were measured under reference conditions using ionization chambers at 18 MV‐X. Furthermore, for selected field sizes, the produced Cerenkov light within the water Phantom was imaged by a CCD‐Camera. In order to improve the optical signal, any light source in the treatment room was blocked. The measurement was repeated three times for better statistics. Images were median filtered to reduced errors due to direct ionization of the CCD‐chip. Afterwards, raw images were averaged and evaluated at ROI in 10 cm depth. Considering cone shaped geometrical corrections, the measured Cerenkov signal in relation to the signal from 10×10 cm field was compared with OF measured under reference conditions.

Results:

Comparing the OF as a function of field size with the measured signal change in Cerenkov images, a comparable trend is observable. A limiting factor was the small signal to noise ratio caused by the insufficient sensitivity of the CCD‐Camera which results in large error bars.

Conclusion:

The correlation between Cerenkov light production and dose deposition has already been published. The geometrical correction of the optical system for the non‐isotropic Cerenkov signal represents a big challenge of defining correction factors for the system. A highly developed image system is required to image and use Cerenkov signals as a dosimetry tool with an acceptable signal to noise ratio. Moreover, correction factors for the measurement system have to be established. In further work, Monte Carlo simulation should be used in order to define correction factors for the given measuring setup.
SU‐F‐T‐487: On‐Site Beam Matching of An Elekta Infinity with Agility MLC with An Elekta Versa HD
Authors: Nelson C , Garcia M , Mason B , Kirsner S
Published: July 2016

Purpose:

Historically, beam matching of similar Linear Accelerators has been accomplished by sending beam data to the manufacturer to match at their factory. The purpose of this work is to demonstrate that fine beam matching can be carried out on‐site as part of the acceptance test, with similar or better results.

Methods:

Initial scans of a 10 × 10 Percent depth dose (PDD) and a 40 × 40 beam profile at the depth of Dmax, for 6MV and 10 MV were taken to compare with the standard beam data from the Versa. The energy was then adjusted and the beam steered to achieve agreement between the depth dose and the horns of the beam profile. This process was repeated until the best agreement between PDD and profiles was achieved. Upon completion, all other clinical data were measured to verify match. This included PDD, beam profiles, output factors and Wedge factors. For electron beams PDD's were matched and the beam profiles verified for the final beam energy. Confirmatory PDD and beam profiles for clinical field sizes, as well as Output Factors were measured.

Results:

The average difference in PDD's for 6MV and 10MV were within 0.4% for both wedged and open fields. Beam profile comparisons over the central 80% of the field, at multiple depths, show agreement of 0.8% or less for both wedged and open fields. Average output factor agreement over all field sizes was 0.4% for 6MV and 0.2 % for 10MV. Wedge factors agreement was less than 0.6% for both photon energies over all field sizes. Electron PDD agreed to 0.5mm. Cone ratios agreed to 1% or less.

Conclusion:

This work indicates that beam matching can be carried out on‐site simply and quickly. The results of this beam matching can achieve similar or better results than factory matching.
SU‐F‐T‐492: The Impact of Water Temperature On Absolute Dose Calibration
Authors: Islam N , Podgorsak M
Published: July 2016

Purpose:

The Task Group 51 (TG 51) protocol prescribes that dose calibration of photon beams be done by irradiating an ionization chamber in a water tank at pre‐defined depths. Methodologies are provided to account for variations in measurement conditions by applying correction factors. However, the protocol does not completely account for the impact of water temperature. It is well established that water temperature will influence the density of air in the ion chamber collecting volume. Water temperature, however, will also influence the size of the collecting volume via thermal expansion of the cavity wall and the density of the water in the tank. In this work the overall effect of water temperature on absolute dosimetry has been investigated.

Methods:

Dose measurements were made using a Farmer‐type ion chamber for 6 and 23 MV photon beams with water temperatures ranging from 10 to 40°C. A reference ion chamber was used to account for fluctuations in beam output between successive measurements.

Results:

For the same beam output, the dose determined using TG 51 was dependent on the temperature of the water in the tank. A linear regression of the data suggests that the dependence is statistically significant with p‐values of the slope equal to 0.003 and 0.01 for 6 and 23 MV beams, respectively. For a 10 degree increase in water phantom temperature, the absolute dose determined with TG 51 increased by 0.27% and 0.31% for 6 and 23 MV beams, respectively.

Conclusion:

There is a measurable effect of water temperature on absolute dose calibration. To account for this effect, a reference temperature can be defined and a correction factor applied to account for deviations from this reference temperature during beam calibration. Such a factor is expected to be of similar magnitude to most of the existing TG 51 correction factors.
SU‐F‐T‐495: Method for Simultaneous Commissioning of Multiple Truebeam (TB) Linacs: Small Field Data and the Choice of Dosimetric Lead Gap (DLG)
Authors: Mihailidis D , Mallah J
Published: July 2016

Purpose:

Many times a set of multiple Varian‐Truebeam (TB) linacs are acquired by an institution. Since “beam matching” is an important requirement for many facilities, we developed a strategy to perform a “simultaneous” commissioning between multiple linacs.Methods and Materials: We first commissioned the required photon beam data for eclipse on the 1st TB for all the energy modalities with a beam scanning system, while integrated measurements for output factors, of all field sizes (from 1×1 to 40×40cm2) were conducted on the 2nd TB. Care was exercised during small field dosimetry so the appropriate detectors were used with data taken between two detectors be “linked” to a larger field size (4×4cm2) with the “daisy‐chaining” technique via: OF=[M‐PTW(fs)×(M‐PTW(4×4))‐1]×[MA12s(4×4)×(M‐A12S(10×10))−1]. For all energy modalities, data that span the entire range of field size, was repeated on the next TB linac, for verification. The primarily energy‐dependent dosimetric leaf gap (DLG) which was measured separately on each TB. The modeled data was validated with special measurements conducted on both linacs during commissioning.

Results:

Our data agreed with the “TB representative beam data” to within 0.5% for all energy modalities and field sizes ≥3×3cm2. Sample depth‐doses and cross‐profiles of a 3×3cm2 between the linacs agreeing to within 1% between linacs. The measured DLGs were quite different with a uniform difference of 1.3% between the two linacs. The measured DLG values are independent of the average dose rate and medium used for the measurements.

Conclusion:

A comprehensive method of commissioning identical Varian‐TB linacs, outlining the critical issues, especially small field dosimetry and DLG. The dosimetric effect of different DLG values, when it comes to, dynamic delivery and data comparisons will be presented. The dependence of DLG value on the measurement medium (in‐air vs. water) or dose rate used will also be discussed.

This work was supported by CAMC Cancer Center and Alliance Oncology
SU‐F‐T‐502: FFF Beams, Jaw‐Tracking and Treatment Techniques: Out of Field Dose Considerations for Pediatric Radiation Therapy Delivery
Authors: Ben Bouchta Y , Bergman A
Published: July 2016

Purpose:

To compare the extended dose profile delivered by 3DCRT and VMAT techniques for flattened and flattening‐filter‐free(FFF) photon beams (6X, 6XFFF,10XFFF), with and without jaw‐tracking (JT) on Varian TrueBeam linac. The goal is to determine which treatment technique/modality will minimize the peripheral photon dose exposure (and ultimately minimize the risk of second malignant neoplasms (SMN)) in pediatric patients.

Methods:

3DCRT, VMAT, and jaw tracking VMAT (JTVMAT) plans with 6X, 6XFFF and 10XFFF x‐ray beams were created on a 30×60×22.5cm solid water phantom with a 551 cc PTV. The 3DCRT plans consisted of a 4FLD arrangement. The optimization objectives for the single‐arc VMAT plans was V95%Rx=98% to PTV and minimize dose to a 5cm diameter organ at risk (OAR). The OAR to PTV distance varied from 0–30cm along the long axis at 7.5cm depth. The dose to the center of the OAR was measured using a 0.6cc ion chamber.

Results:

Relative to the 6X flattened beam, the 10XFFF photon beam had the lowest dose in the penumbra and peripheral region (>15 cm) region by up to 20% and 40%, respectively for all modalities (3DCRT, VMAT, JTVMAT). The 6XFFF beams only showed a dose reduction in the peripheral region (by up to 20%). JT did not significantly affect the peripheral dose for all modalities and energies.

Conclusion:

Treating pediatric patients with a 10XFFF beam is the most effective way to reduce photon scatter dose in both the penumbra and peripheral regions. However, the neutron dose contribution resulting from the 10MV beam still needs to be considered. For all modalities, 6XFFF was the next effective method to reduce peripheral photon doses. 3DCRT beams had the lowest peripheral doses for all energies compared to VMAT and JTVMAT, however previous publications have shown that this comes at the expense of PTV conformity and OAR sparing.
SU‐F‐T‐520: Dosimetric Comparison of Radiation Treatment Plans for Whole Breast Irradiation Between 3D Conformal in Prone and Supine Positions Vs. VMAT and IMRT in Supine Positions
Authors: Bejarano Buele A , Parsai E
Published: July 2016

Purpose:

The target volume for Whole Breast Irradiation (WBI) is dictated by location of tumor mass, breast tissue distribution, and involvement of lymph nodes. Dose coverage and Organs at Risk (OARs) sparing can be difficult to achieve in patients with unfavorable thoracic geometries. For these cases, inverse‐planned and 3D‐conformal prone treatments can be alternatives to traditional supine 3D‐conformal plans. A dosimetric comparison can determine which of these techniques achieve optimal target coverage while sparing OARs.

Methods:

This study included simulation datasets for 8 patients, 5 of whom were simulated in both supine and prone positions. Positioning devices included breast boards and Vaclok bags for the supine position, and prone breast boards for the prone position. WBI 3‐D conformal plans were created for patients simulated in both positions. Additional VMAT and IMRT WBI plans were made for all patients in the supine position.

Results:

Prone and supine 3D conformal plans had comparable PTV coverage. Prone 3D conformal plans received a significant 50% decrease to V20, V10, V5 and V30% for the ipsilateral lung in contrast to the supine plans. The heart also experienced a 10% decrease in maximum dose in the prone position, and V20, V10, V5 and V2 had significantly lower values than the supine plan. Supine IMRT and VMAT breast plans obtained comparable PTV coverage. The heart experienced a 10% decrease in maximum dose with inverse modulated plans when compared to the supine 3D conformal plan, while V20, V10, V5 and V2 showed higher values with inverse modulated plans than with supine 3D conformal plans.

Conclusion:

Prone 3D‐conformal, and supine inverse planned treatments were generally superior in sparing OARs to supine plans with comparable PTV coverage. IMRT and VMAT plans offer sparing of OARs from high dose regions with an increase of irradiated volume in the low dose regions.
SU‐F‐T‐554: Dark Current Effect On CyberKnife Beam Dosimetry
Authors: Kim H , Chang A
Published: July 2016

Purpose:

All RF linear accelerators produce dark current to varying degrees when an accelerating voltage and RF input is applied in the absence of electron gun injection. This study is to evaluate how dark current from the linear accelerator of CyberKnife affect the dose in the reference dosimetry.

Methods:

The G4 CyberKnife system with 6MV photon beam was used in this study. Using the ion chamber and the diode detector, the dose was measured in water with varying time delay between acquiring charges and staring beam‐on after applying high‐voltage into the linear accelerator. The dose was measured after the time delay with over the range of 0 to 120 seconds in the accelerating high‐voltage mode without beam‐on, applying 0, 10, 50, 100, and 200 MUs. For the measurements, the collimator of 60 mm was used and the detectors were placed at the depths of 10 cm with the source‐to‐surface distance of 80 cm.

Results:

The dark current was constant over time regardless of MU. The dose due to the dark current increased over time linearly with the R‐squared value of 0.9983 up to 4.4 cGy for the time 120 seconds. In the dose rate setting of 720 MU/min, the relative dose when applying the accelerating voltage without beam‐on was increased over time up to 0.6% but it was less than the leakage radiation resulted from the accelerated head. As the reference dosimetry condition, when 100 MU was delivered after 10 seconds time delay, the relative dose increased by 0.7% but 6.7% for the low MU (10 MU).

Conclusion:

In the dosimetry using CyberKnife system, the constant dark current affected to the dose. Although the time delay in the accelerating high‐voltage mode without beam‐on is within 10 seconds, the dose less than 100 cGy can be overestimated more than 1%.
SU‐F‐T‐555: Accurate Stereotactic Cone TMRs Converted from PDDs Scanned with Ray Trace
Authors: Li H , Zhong H , Qin Y , Snyder K , Chetty I , Wen N
Published: July 2016

Purpose:

To investigate whether the accuracy of TMRs for stereotactic cones converted from PDDs scanned with Ray Trace can be improved, when compared against the TMRs converted from the traditional PDDs.

Methods:

Ray Trace measurement in Sun Nuclear 3D Scanner is for accurate scan of small field PDDs. The system detects the center of field at two depths, for example, at 3 and 20 cm in our study, and then performs scan along the line passing the two centers. With both Ray Trace and the traditional method, PDDs for conical cones of 4, 5, 7.5, 10, 12.5, 15, and 17.5 mm diameter (jaws set to 5×5 cm) were obtained for 6X FFF and 10X FFF energies on a Varian Edge linac, using Edge detectors. The formalism of converting PDD to TMR given in Khan's book (4th Edition, p.161) was applied. Sp values at dmax were obtained by measuring cone Scp and Sc. Continuous direct measurement of TMR by filling/draining water to/from the tank and spot measurement by moving the tank and detector were also performed with the same equipment, using 100 cm SDD.

Results:

For 6XFFF energy and all the cones, TMRs converted from Ray Trace were very close to the continuous and spot measurement, while TMRs converted from traditional PDDs had larger deviation. Along the central axis beyond dmax, 1.7% of TMR data points calculated from Ray Trace had more 3% deviation from measurement, with maximal deviation of 5.2%. Whereas, 34% of TMR points calculated from traditional PDDs had more than 3% deviation, with maximum of 5.7%. In this initial study, Ray Trace scans for 10XFFF beam were noisy, further measurement is warranted.

Conclusion:

The Ray Trace could improve the accuracy of PDDs measurement and the calculated TMRs for stereotactic cones, which was within 3% of the measured TMRs.
SU‐F‐T‐562: Validation of EPID‐Based Dosimetry for FSRS Commissioning
Authors: Song Y , Saleh Z , Obcemea C , Chan M , Tang X , Lim S , Lovelock D , Ballangrud A , Mueller B , Zinovoy M , Gelblum D , Mychalczak B , Both S
Published: July 2016

Purpose:

The prevailing approach to frameless SRS (fSRS) small field dosimetry is Gafchromic film. Though providing continuous information, its intrinsic uncertainties in fabrication, response, scan, and calibration often make film dosimetry subject to different interpretations. In this study, we explored the feasibility of using EPID portal dosimetry as a viable alternative to film for small field dosimetry.

Methods:

Plans prescribed a dose of 21 Gy were created on a flat solid water phantom with Eclipse V11 and iPlan for small static square fields (1.0 to 3.0 cm). In addition, two clinical test plans were computed by employing iPlan on a CIRS Kesler head phantom for target dimensions of 1.2cm and 2.0cm. Corresponding portal dosimetry plans were computed using the Eclipse TPS and delivered on a Varian TrueBeam machine. EBT‐XD film dosimetry was performed as a reference. The isocenter doses were measured using EPID, OSLD, stereotactic diode, and CC01 ion chamber.

Results:

EPID doses at the center of the square field were higher than Eclipse TPS predicted portal doses, with the mean difference being 2.42±0.65%. Doses measured by EBT‐XD film, OSLD, stereotactic diode, and CC01 ion chamber revealed smaller differences (except OSLDs), with mean differences being 0.36±3.11%, 4.12±4.13%, 1.7±2.76%, 1.45±2.37% for Eclipse and −1.36±0.85%, 2.38±4.2%, −0.03±0.50%, −0.27±0.78% for iPlan. The profiles measured by EPID and EBT‐XD film resembled TPS (Eclipse and iPlan) predicted ones within 3.0%. For the two clinical test plans, the EPID mean doses at the center of field were 2.66±0.68% and 2.33±0.32% higher than TPS predicted doses.

Conclusion:

We found that results obtained with EPID portal dosimetry were slightly higher (∼2%) than those obtained with EBT‐XD film, diode, and CC01 ion chamber with the exception of OSLDs, but well within IROC tolerance (5.0%). Therefore, EPID has the potential to become a viable real‐time alternative method to film dosimetry.
SU‐F‐T‐622: Comparative Analysis of Pencil Beam and Anisotropic Analytical Algorithm (AAA) for Stereotactic Body Radiation Therapy (SBRT) of Thoracic Spine
Authors: Badkul R , Nicolai W , Pokhrel D , Ramanjappa T , Jiang H , Wang F , Lominskac C
Published: July 2016

Purpose:

To compare the impact of Pencil Beam(PB) and Anisotropic Analytic Algorithm(AAA) dose calculation algorithms on OARs and planning target volume (PTV) in thoracic spine stereotactic body radiation therapy (SBRT).

Methods:

Ten Spine SBRT patients were planned on Brainlab iPlan system using hybrid plan consisting of 1–2 non‐coplanar conformal‐dynamic arcs and few IMRT beams treated on NovalisTx with 6MV photon. Dose prescription varied from 20Gy to 30Gy in 5 fractions depending on the situation of the patient. PB plans were retrospectively recalculated using the Varian Eclipse with AAA algorithm using same MUs, MLC pattern and grid size(3mm).Differences in dose volume parameters for PTV, spinal cord, lung, and esophagus were analyzed and compared for PB and AAA algorithms. OAR constrains were followed per RTOG‐0631.

Results:

Since patients were treated using PB calculation, we compared all the AAA DVH values with respect to PB plan values as standard, although AAA predicts the dose more accurately than PB. PTV(min), PTV(Max), PTV(mean), PTV(D99%), PTV(D90%) were overestimated with AAA calculation on average by 3.5%, 1.84%, 0.95%, 3.98% and 1.55% respectively as compared to PB. All lung DVH parameters were underestimated with AAA algorithm mean deviation of lung V20, V10, V5, and 1000cc were 42.81%,19.83%, 18.79%, and 18.35% respectively. AAA overestimated Cord(0.35cc) by mean of 17.3%; cord (0.03cc) by 12.19% and cord(max) by 10.5% as compared to PB. Esophagus max dose were overestimated by 4.4% and 5cc by 3.26% for AAA algorithm as compared to PB.

Conclusion:

AAA overestimated the PTV dose values by up to 4%.The lung DVH had the greatest underestimation of dose by AAA versus PB. Spinal cord dose was overestimated by AAA versus PB. Given the critical importance of accuracy of OAR and PTV dose calculation for SBRT spine, more accurate algorithms and validation of calculated doses in phantom models are indicated.
SU‐F‐T‐628: An Evaluation of Grid Size in Eclipse AcurosXB Dose Calculation Algorithm for SBRT Lung
Authors: Pokharel S , Rana S
Published: July 2016

Purpose:

purpose of this study is to evaluate the effect of grid size in Eclipse AcurosXB dose calculation algorithm for SBRT lung.

Methods:

Five cases of SBRT lung previously treated have been chosen for present study. Four of the plans were 5 fields conventional IMRT and one was Rapid Arc plan. All five cases have been calculated with five grid sizes (1, 1.5, 2, 2.5 and 3mm) available for AXB algorithm with same plan normalization. Dosimetric indices relevant to SBRT along with MUs and time have been recorded for different grid sizes. The maximum difference was calculated as a percentage of mean of all five values. All the plans were IMRT QAed with portal dosimetry.

Results:

The maximum difference of MUs was within 2%. The time increased was as high as 7 times from highest 3mm to lowest 1mm grid size. The largest difference of PTV minimum, maximum and mean dose were 7.7%, 1.5% and 1.6% respectively. The highest D2‐Max difference was 6.1%. The highest difference in ipsilateral lung mean, V5Gy, V10Gy and V20Gy were 2.6%, 2.4%, 1.9% and 3.8% respectively. The maximum difference of heart, cord and esophagus dose were 6.5%, 7.8% and 4.02% respectively. The IMRT Gamma passing rate at 2%/2mm remains within 1.5% with at least 98% points passing with all grid sizes.

Conclusion:

This work indicates the lowest grid size of 1mm available in AXB is not necessarily required for accurate dose calculation. The IMRT passing rate was insignificant or not observed with the reduction of grid size less than 2mm. Although the maximum percentage difference of some of the dosimetric indices appear large, most of them are clinically insignificant in absolute dose values. So we conclude that 2mm grid size calculation is best compromise in light of dose calculation accuracy and time it takes to calculate dose.
SU‐F‐T‐639: Use of a 6D Couch for Real‐Time Motion Correction During Frameless Stereotactic Radiosurgery
Authors: Liu X , Belcher AH , Wiersma R
Published: July 2016

Purpose:

High precision patient positioning is critical in stereotactic radiosurgery (SRS). A 6 Degree‐of‐freedom (6D) robotic serial couch has the potential to correct both translational and rotational target motion deviations during image‐guided radiosurgery procedures. An inverse kinematics algorithm based on the structure of the couch is presented, and a feedback control algorithm is discussed. The feasibility of motion compensation with a 6D couch is demonstrated by simulation with prerecorded volunteer head motion data.

Methods:

The proposed 6D couch motion compensation system included a robotic 6D couch, an image tracking system, and a control computer. For the control, the desired couch top plane was computed based on coordinate frames transformation, and the actuator outputs were calculated by 6D couch inverse kinematics algorithm. During the control, the rotation and translation were coupled, leading to undesired coupling motion of the lesion. Although such coupling was eventually cancelled out, it nonetheless represented unnecessary displacements of the target. An optimal decoupling control algorithm was designed to resolve this problem. The lever effect of the distance of between the target and the pivot of 6D couch roll/pitch were also discussed.

Results:

The pre‐recorded volunteer head motion data was used in a computer‐based motion compensation simulation, by 6D couch motion compensation, the corrected target was within a 0.3 mm of translational errors and 0.1 degree in the rotational errors about 99.5% of time.

Conclusion:

We presented an optimal decoupling control algorithm for motion compensation by using a robotic 6D couch. The simulation also showed that reducing the distance between the target and the pivot of roll/pitch of the couch can also improve the accuracy of the compensation. The simulation showed that the 6D robotic system can achieve submillimeter/subdegree position accuracies.
SU‐F‐T‐640: Feasibility of Using a Commercially Available Surface Guided Radiotherapy System with An Open‐Face SRS Immobilization System
Authors: Chinsky B , Patel R , Roeske J , Surucu M
Published: July 2016

Purpose:

To evaluate the inherent accuracy of using a surface guided radiotherapy system (SGRT) in the setup and monitoring of patients receiving stereotactic radiosurgery with an open‐face SRS immobilization system.

Methods:

An anthropomorphic head phantom was set up using the Qfix Encompass SRS Immobilization System on a Varian Edge with OSMS and Varian TrueBeam with AlignRT. The phantom was positioned at 0° gantry and couch. A reference image was acquired using the SGRT system and an ROI was created over the mask opening. The couch and gantry were rotated to different combinations focusing on clinically used SRS gantry/couch combinations and those blocking the SGRT cameras. Perceived surface deviation by the SGRT system from the reference image was recorded. A Winston‐Lutz test was performed on couch angles tested and used to exclude couch walkout. The deviation magnitude was calculated using translational values and rotational raw values were recorded.

Results:

The maximum couch walkouts were: 0.4mm (Edge) and 0.5mm (TB). Solely rotating the gantry resulted in a median couch deviation of 0.2mm and range of 0.1–0.3mm for both linacs. Only rotating the couch (0° gantry) resulted in median deviations of 0.6mm and 0.5mm with ranges of 0.3–1.0mm and 0.3–0.7mm for the Edge and TB, respectively. Combining gantry and couch rotations, the median deviations were 0.7mm and 0.9mm with ranges of 0.3–1.1mm and 0.2–1.9mm for the Edge and TB, respectively. Including all combinations, rotation, roll, and pitch median deviations ranged from 0.1–0.3° with pitch demonstrating consistently higher values and a maximum deviation of 1.0° (both linacs).

Conclusion:

SGRT is a reliable monitoring tool, though taking into account system fluctuations, 1mm is too restrictive a site tolerance to use with the Qfix Encompass mask. Gantry rotation has little effect on system fluctuation even with camera blockage, whereas couch rotation has a larger effect.
SU‐F‐T‐641: Comparative Study On Planning and Delivery Efficiency of Volumetric Modulated Arc Therapy Between Varian and Elekta Platforms for SBRT Lung Treatment
Authors: Varadhan  , Arentsen L , Way S , Overbeck C , Yerich S , Garrity B , Fox M
Published: July 2016

Purpose:

To compare the plan quality between Eclipse (Varian) and Monaco (Elekta) TPS. To ascertain, if SBRT lung treatment could be delivered in a single coplanar arc (360 degrees) with both Elekta and Varian platforms. To assess if the smaller leaf width in Varian Millennium and Elekta Agility MLC heads have a dosimetric advantage over Elekta MLCi2 head

Methods:

Ten SBRT lung patients (PTV volumes ranging from 11 cc to 103cc) who were previously treated on Varian Linac with non‐coplanar arcs and received 50Gy in 5 fractions were chosen for this study. The patients were replanned in Eclipse TPS (AAA algorithm) using a 360 degree coplanar single arc (SA) delivery technique and 2 partial complimentary 180 degree arcs (PA). Treatment planning using single coplanar arc (360 degree arc) was also done on Monaco TPS (Montecarlo) for both Agility (160 leaf) and MLCi2 (80 leaf) Elekta MLC heads

Results:

The average monitor units to deliver 10 Gy across all delivery methods were 3000 ± 474 MU and did not vary with PTV size. Coplanar single arc and partial arc techniques did not compromise either the RTOG 0813 or 0915 low dose spillage criteria for R50% or the maximum dose to any point 2cm away from the PTV. OAR doses to spinal cord, heart, great vessels, esophagus, rib and lung were comparable on both Eclipse (Varian) and Monaco (Elekta) platforms regardless of the delivery method.

Conclusion:

SBRT lung tumors can be treated with a single coplanar 360 degree arc in both Varian and Elekta platforms. Non coplanar arcs and increasing arc degrees more than 360 degrees had no benefit in this study regardless of the volume of PTV. 0.5 cm leaf width used in Millennium and Agility MLC heads had no significant dosimetric improvement over 1 cm leaves in the MLCi2 head.
SU‐F‐T‐656: Monte Carlo Study On Air Activation Around a Medical Electron Linac
Authors: Horst F , Fehrenbacher G , Zink K
Published: July 2016

Purpose:

In high energy photon therapy, several radiation protection issues result from photonuclear reactions. The activation of air ‐ directly by photonuclear reactions as well as indirectly by capture of photoneutrons generated inside the linac head ‐ is a major point of concern for the medical staff. The purpose of this study was to estimate the annual effective dose to medical workers due to activated air around a medical high energy electron linac by means of Monte Carlo simulations.

Methods:

The treatment head of a Varian Clinac in 18 MV‐X mode as well as the surrounding concrete bunker were modeled and the radiation transport was simulated using the Monte Carlo code FLUKA, starting from the primary electron striking the bremsstrahlung target. The activation yields in air from photo‐disintegration of O‐16 and N‐14 nuclei as well as from neutron capture on Ar‐40 nuclei were obtained from the simulations. The activation build‐up, radioactive decay and air ventilation were studied using a mathematical model. The annual effective dose to workers was estimated by using published isotope specific conversion factors.

Results:

The oxygen and nitrogen activation yields were in contrast to the argon activation yield found to be field size dependent. The impact of the treatment room ventilation on the different air activation products was investigated and quantified. An estimate with very conservative assumptions gave an annual effective dose to workers of < 1 mSv/a.

Conclusion:

From the results of this study it can be concluded that the contribution of air activation to the radiation exposure to medical workers should be negligible in modern photon therapy, especially when it is compared to the dose due to prompt neutrons and the activation of heavy solid materials such as the jaws and the collimators inside the linac head.
SU‐F‐T‐659: Nanoparticle‐Aided Eye Plaque Radiotherapy
Authors: Chin J , Ngwa W
Published: July 2016

Purpose:

Eye plaque brachytherapy is one of the approaches for radiotherapy treatment for ocular cancers: retinoblastoma and choroidal melanoma. This study, investigates the potential benefits of using gold nanoparticles to enhance therapeutic efficacy during eye plaque brachytherapy.

Methods:

The EYE PHYSICS Inc. Plaque Simulator program distributed by IsoAid, LLC, Port Richey, Florida was used. It is based on the superposition of dose contributions from individual seeds following the TG–43 formalism. Dose enhancement factor (DEF) values for feasible nanoparticle concentrations from previous studies was used to investigate the benefit of using nanoparticles to enhance dose to tumour or reduce dose to healthy tissue. The dose enhancement factor (DEF) represents the ratio of the dose deposited in tumour with nanoparticles divided by dose deposited in the tumour without nanoparticles. The investigation was done for I–125 and Pd–103 typical sources employed for eye plaque brachytherapy. The prescription dose used is 85 Gy.

Results:

Lower dose enhancement values were obtained for Pd–103. With DEF of 2 due to gold nanoparticles, critical structure doses reduce by a factor of 2. Optic disc dose is 6.69 Gy and 4.571 Gy, opposite retina dose is 4.064 and 2.484 Gy, lens dose is 12.66 Gy and 9.870 Gy, and fovea dose is 9.85 Gy and 7.275 Gy. With DEF of 3 due to gold nanoparticles, critical structure doses reduce by a factor of 3. Optic disc dose is 4.352 Gy and 2.975 Gy, opposite retina dose is 2.644 Gy and 1.618 Gy, lens dose is 8.322 Gy and 6.427 Gy, and fovea dose is 4.815 Gy and 4.737 Gy.

Conclusion:

The results of this research predict that using gold nanoparticles will lead to major sparing of dose to critical structures. The finding provides more impetus for the development of nanoparticle–aided brachytherapy.
SU‐F‐T‐660: Evaluating the Benefit of Using Dual‐Function Fiducial Markers for In‐Situ Delivery of Radiosenistizing Gold Nanoparticles During Image‐Guided Radiotherapy
Authors: AlMansour S , Chin J , Sajo E , Ngwa W
Published: July 2016

Purpose:

Dual‐function fiducials loaded with radiosensitizers, like gold nanoparticles (GNP), offer an innovative approach for ensuring geometric accuracy during image‐guided radiotherapy (IGRT) and significantly increasing therapeutic efficacy due to controlled in‐situ release of the radiosensitizers. This study retrospectively investigates the dosimetric benefit of using up to two such dual‐function fiducial markers instead of traditional single function fiducials during IGRT.

Methods:

A computational code was developed to investigate the dosimetric benefit for 10 real patient tumor volumes of up to 6.5 cm diameter. The intra‐tumoral space‐time biodistribution of the GNP was modeled as in previous studies based on Fick's second law. The corresponding dose‐enhancement for each tumor voxel due to the GNP was also calculated for clinical 6MV beam configurations. Various loading concentrations (25–50 mg/g) were studied, as a function of GNP size, to determine potential for clinically significant dose enhancement. The time between initial implantation of dual‐function fiducials to the beginning of radiotherapy was assumed to be 14 days as typical for many clinics.

Results:

A single dual‐function fiducial could achieve at least a DEF of 1.2 for patients with tumors less than 1.4 cm diameter after 14 days. Replacing two single function fiducials with dual‐function ones at the same locations achieved at least the required minimal DEF for tumors that are 2 cm diameter in 3 patients. The results also revealed dosimetrically better fiducial locations which could enable significant DEF when using one or two dual function fiducials. 2 nm sizes showed the most feasibility.

Conclusion:

The results highlight the potential of tumor sub‐volume radiation boosting using GNP released from fiducials, and the ability to customize the DEF throughout the tumor by using two dual‐function fiducials, varying the initial concentration and nanoparticle size. The results demonstrate potential for employing dual‐function fiducials in the development of GNP‐aided radiotherapy.
SU‐F‐T‐668: Irradiating Mouse Brain with a Clinical Linear Accelerator
Authors: Perez‐Torres C
Published: July 2016

Purpose:

To design and construct a “mouse jig” device that would allow for irradiation of the mouse brain with a clinical Varian 6 MeV Linear Accelerator. This device must serve as a head immobilizer, gaseous anesthesia delivery, and radiation bolus concurrently.

Methods:

The mouse jig was machined out of nylon given that it is inexpensive, easy to machine, and has similar electron density to water. A cylindrical opening with diameter of 16 mm and 40 mm depth was drilled into a nylon block sized 56×56×50 mm (width, length, depth). Additional slots were included in the block for ear bars and a tooth bar to serve as a three‐point immobilization device as well as for anesthesia delivery and scavenging. For ease of access when loading the mouse into the holder, there is a removable piece at the top of the block that is 15 mm in depth. This serves a dual purpose, as with the proper extra shielding, the mouse jig could be used with lower linear energy transfer photons with this piece removed. A baseplate was then constructed with five square slots where the mouse jig can securely be inserted plus additional slots that would allow the baseplate to be mounted on a standard lock bar in the treatment couch. This maximizes the reproducibility of placement between imaging and treatment and between treatment sessions.

Results:

CT imaging and radiation treatment planning was performed that showed acceptable coverage and uniformity of radiation dose in the mouse brain while sparing the throat and eyes.

Conclusion:

We have designed and manufactured a device that fulfills our criteria allowing us to selectively irradiate the mouse brain with a clinical linear accelerator. This setup will be used for generating mouse models of radiation‐induced brain injury.
SU‐F‐T‐670: From the OR to the Radiobiology Lab: The Journey of a Small X‐Ray Source
Authors: Lehmann J , Barry M , Jones R , Fay M
Published: July 2016

Purpose:

Irradiation of small animal tumor models within laboratories is vital to radiobiological experiments. Often the animals are not able to be brought back into the lab after being taken out for irradiation. Cell biology laboratories benefit from irradiation capability available around the clock without regard to patient load in an associated radiotherapy clinic. Commercial systems are available, but bulky and expensive.

Methods:

An intraoperative kV irradiation system (IntraBeam™) designed to deliver spherical dose distributions to surgical cavities has been repurposed for the irradiation of cell plates and small laboratory animals. An applicator has been altered to allow for simple, open fields. Special collimators are being developed. BEAMnrc Monte Carlo simulations with the “NRC swept BEAM” source model have been performed to characterize the dose distributions, to develop optimal collimators and as basis for dose prescription. Measurements with radiochromic film and with an ionization chamber were performed to characterize the beam and to validate the simulations.

Results:

Using its highest setting (50 kV and 40 µA) the x‐ray unit is capable of delivering dose rates over 1 Gy/min homogeneously to standard cell plates even without an optimized collimator. Smaller areas (tumors in animals) can be irradiated with significantly higher dose rates (> 20 Gy/min) depending on distance of the source to the tumor. The HVL was found to be 0.21 mm Al which means the shielding requirements for the device are easily achievable in the lab.

Conclusion:

A mobile irradiation facility is feasible. It will allow easier access to radiation for radiobiology experiments. The modified system is versatile in that for cell plates homogenous irradiations can be achieved through distance from the source, while for high dose rate small field irradiations the source can be brought in close proximity to the target.
SU‐F‐T‐673: Effects of Cardiac Induced Brain Pulsations On Proton Minibeams
Authors: Eagle J , Marsh S , Lee E , Meyer J
Published: July 2016

Purpose:

To quantify the dosimetric impact of internal motion within the brain on spatially modulated proton minibeam radiation therapy (pMRT) for small animal research.

Methods:

The peak‐to‐valley dose ratio (PVDR) is an essential dosimetric factor for pMRT. Motion of an animal brain caused by cardiac‐induced pulsations (CIP) can impact dose deposition. For synchrotron generated high dose rate X‐ray microbeams this effect is evaded due to the quasi‐instantaneous delivery. By comparison, pMRT potentially suffers increased spread due to lower dose rates. However, for a given dose rate it is less susceptible to beam spread than microbeams, due to the spatial modulation being an order of magnitude larger. Monte Carlo simulations in TOPAS were used to model the beam spread for a 50.5MeV pMRT beam. Motion effects were simulated for a 50mm thick brass collimator with 0.3mm slit width and 1.0mm center‐to‐center spacing in a water phantom. The maximum motion in a rat brain due to CIP has been reported to be 0.06mm. Motion was simulated with a peak amplitude in the range 0–0.2mm.

Results:

The impact of 0.06mm peak motion was minimal and reduced the PVDR by about 1% at a depth of 10mm. For 0.2mm peak motion the PVDR was reduced by 16% at a depth of 10mm.

Conclusion:

For the pMRT beam the magnitude of cardiac‐induced brain motion has minimal impact on the PVDR for the investigated collimator geometry. For more narrow beams the effect is likely to be larger. This indicates that delivery of pMRT to small animal brains should not be affected considerably by beamlines with linac compatible dose rates.