bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐402: Evaluation of the Accuracy of a Novel Open Mask System for Immobilization of Cranial Stereotactic Radiosurgery Patients
Authors: Song K , Wen N , Qin Y , Kim J , Li H , Snyder K , Gordon J , Chetty I , Siddiqui S
Published: June 2015

Purpose:

To evaluate the accuracy of a novel open mask for intracranial SRS and to investigate the capability of monitoring intrafraction motion of the patient within this mask using the optical surface monitoring system (OSMS) on a linac‐based platform.

Methods:

Efficiency was evaluated by measuring mask fabrication time during CT simulation and setup time in the treatment room. Mask shrinkage was assessed by comparing shim settings during treatment to simulation and also by measuring the distance between anterior and posterior sections of the mask on CBCT and Sim CT. Mask comfort was evaluated qualitatively post‐treatment by surveying patients. Intrafraction motion was examined using 2D‐3D autofusion of orthogonal kV images at mid‐treatment. The intrafraction motion accuracy of the OSMS was analyzed by measuring the surface area of mask opening in Eclipse and pictures from the OSMS cameras.

Results:

The average preparation time for mask setup during simulation and treatment was 12.7 ± 2.3 min and 1.8 ± 0.9 min, respectively. Shims needed to be increased on average 0.8 mm, 0.8 mm, and 0.6 mm for right, left, and superior positions, respectively. CBCT measurements showed mask shrinkage, 0.8 mm both anteriorly and posteriorly. Greatest discomfort was reported on forehead followed by neck and chin. Intrafraction motion was less than 1 mm/1°. Accuracy of OSMS monitoring depended on the selected ROI. The OSMS monitored motion within 1 mm and 1° when the open surface area measured in Eclipse and captured by the camera > 147 cm2 and 33 cm2, respectively. When such conditions were not met, the accuracy somewhat degraded, an issue which can be mitigated by increasing the superior area or by angling the mask to expose greater surface to the camera.

Conclusion:

The new mask system rigidly immobilizes with sub‐mm accuracy during treatment according to optical camera (OSMS) measurements.

Research supported in part by a grant from QFix (Avondale, PA) and Varian Medical System (Palo Alto, CA)
SU‐E‐T‐403: Evaluation of the Beam Performance of a Varian TrueBeam Linear Accelerator Under External Device‐Based Gated Delivery Conditions
Authors: Kobulnicky K , Pawlak D , Purwar A
Published: June 2015

Purpose:

To examine the beam performance of a Varian TrueBeam linear accelerator under external device‐based gated delivery conditions.

Methods:

Six gating cycles were used to evaluate the gating performance of a standard production TrueBeam system that was not specially tuned in any way. The system was equipped with a factory installed external gating interface (EXGI). An in‐house EXGI tester box was used to simulate the input gating signals. The gating cycles were selected based on long beam‐on and short beam‐off times, short beam‐on and long beam‐off times, or equal beam on and off times to check linac performance. The beam latencies were measured as the time difference between the logic high gating signal and the first or last target pulses with an oscilloscope. Tissue‐Phantom Ratio, beam flatness, and dose distributions from 5 different plans were measured using the 6 different gating durations and the un‐gated irradiation. A PTW 729 2‐D array was used to compare 5 plans versus the un‐gated delivery with a 1%/1mm gamma index passing criteria.

Results:

The beam latencies of the linac were based off of 20 samples for beam‐on and beam‐off, for each gating cycle. The average beam‐on delays were measured to be between 57 and 66msec, with a maximum of 88 msec. The beam off latencies averaged between 19 and 26msec, with a maximum of 48 msec. TPR20,10 measurements showed beam energy stability within 0.5% of the un‐gated delivery. Beam flatness was better than 2.5% for all gated cycles. All but two deliveries, the open field with 4 seconds on, 1 second off, and a five field IMRT plan with 0.5 seconds on, 2.5 seconds off, had >90% passing rate.

Conclusion:

TrueBeam demonstrates excellent beam stability with minimal beam latencies under external device‐based gated operations. Dosimetric measurements show minimal variation in beam energy, flatness, and plan delivery.

Authors are employees of Varian Medical Systems, Inc.
SU‐E‐T‐404: Evaluation of the Effect of Spine Hardware for CyberKnife Spinal Stereotactic Radiosurgery
Authors: Yuan J , Zhang Y , Zheng Y , Wessels B , Machtay M , Yao M , Lo S
Published: June 2015

Purpose:

Spine hardware made of high‐Z materials such as titanium has the potential to affect the dose distribution around the metal rods in CyberKnife spinal stereotactic radiosurgery (SRS) treatments. The purpose of this work was to evaluate the magnitude of such effect retrospectively for clinical CyberKnife plans.

Methods:

The dose calculation was performed within the MultiPlan treatment planning system using the ray tracing (RT) and Monte Carlo (MC) method. A custom density model was created by extending the CT‐to‐Density table to titanium density of 4.5 g/cm3 with the CT number of 4095. To understand the dose perturbation caused by the titanium rod, a simple beam setup (7.5 mm IRIS collimator) was used to irradiate a mimic rod (5 mm) with overridden high density. Five patient spinal SRS cases were found chronologically from 2010 to 2015 in our institution. For each case, the hardware was contoured manually. The original plan was re‐calculated using both RT and MC methods with and without rod density override without changing clinical beam parameters.

Results:

The simple beam irradiation shows that there is 10% dose increase at the interface because of electron backscattering and 7% decrease behind the rod because of photon attenuation. For actual clinical plans, the iso‐dose lines and DVHs are almost identical (<2%) for calculations with and without density override for both RT and MC methods. However, there is a difference of more than 10% for D90 between RT and MC method.

Conclusion:

Although the dose perturbation around the metal rods can be as large as 10% for a single beam irradiation, for clinical treatments with complex beam composition the effect of spinal hardware to the PTV and spinal dose is minimal. As such, the MC dose algorithm without rod density override for CyberKnife spinal SRS is acceptable.
SU‐E‐T‐408: Evaluation of the Type and Frequency of Variations Discovered During Routine Secondary Patient Chart Review
Authors: Hardin M , Harrison A , Lockamy V , Li J , Peng C , Potrebko P , Yu Y , Doyle L , Cao J
Published: June 2015

Purpose:

Desire to improve efficiency and throughput inspired a review of our physics chart check procedures. Departmental policy mandates plan checks pre‐treatment, after first treatment and weekly every 3–5 days. This study examined the effectiveness of the “after first” check with respect to improving patient safety and clinical efficiency. Type and frequency of variations discovered during this redundant secondary review was examined over seven months.

Methods:

A community spreadsheet was created to record variations in care discovered during chart review following the first fraction of treatment and before the second fraction (each plan reviewed prior to treatment). Entries were recorded from August 2014 through February 2015, amounting to 43 recorded variations out of 906 reviewed charts. The variations were divided into categories and frequencies were assessed month‐to‐month.

Results:

Analysis of recorded variations indicates an overall variation rate of 4.7%. The initial rate was 13.5%; months 2–7 average 3.7%. The majority of variations related to discrepancies in documentation at 46.5%, followed by prescription, plan deficiency, and dose tracking related variations at 25.5%, 12.8%, and 12.8%, respectively. Minor variations (negligible consequence on patient treatment) outweighed major variations 3 to 1.

Conclusion:

This work indicates that this redundant secondary check is effective. The first month spike in rates could be due to the Hawthorne/observer effect, but the consistent 4% variation rate suggests the need for periodical re‐training on variations noted as frequent to improve awareness and quality of the initial chart review process, which may lead to improved treatment quality, patient safety and increased clinical efficiency. Utilizing these results, a continuous quality improvement process following Deming's Plan‐Do‐Study‐Act (PDSA) methodology was generated. The first iteration of this PDSA was adding a specific dose tracking checklist item in the pre‐treatment plan check assessment; the ramification of which will be assessed in future data.
SU‐E‐T‐409: Evaluation of Tissue Composition Effect On Dose Distribution in Radiotherapy with 6 MV Photon Beam of a Medical Linac
Authors: Ghorbani M , Tabatabaei Z , Noghreiyan A Vejdani , Meigooni A Soleimani
Published: June 2015

Purpose:

The aim of this study is to evaluate soft tissue composition effect on dose distribution for various soft tissues and various depths in radiotherapy with 6 MV photon beam of a medical linac.

Methods:

A phantom and Siemens Primus linear accelerator were simulated using MCNPX Monte Carlo code. In a homogeneous cubic phantom, six types of soft tissue and three types of tissue‐equivalent materials were defined separately. The soft tissues were muscle (skeletal), adipose tissue, blood (whole), breast tissue, soft tissue (9‐component) and soft tissue (4‐component). The tissue‐equivalent materials included: water, A‐150 tissue‐equivalent plastic and perspex. Photon dose relative to dose in 9‐component soft tissue at various depths on the beam's central axis was determined for the 6 MV photon beam. The relative dose was also calculated and compared for various MCNPX tallies including,F8, F6 and,F4.

Results:

The results of the relative photon dose in various materials relative to dose in 9‐component soft tissue and using different tallies are reported in the form of tabulated data. Minor differences between dose distributions in various soft tissues and tissue‐equivalent materials were observed. The results from F6 and F4 were practically the same but different with,F8 tally.

Conclusion:

Based on the calculations performed, the differences in dose distributions in various soft tissues and tissue‐equivalent materials are minor but they could be corrected in radiotherapy calculations to upgrade the accuracy of the dosimetric calculations.
SU‐E‐T‐414: Experimental Correction of High‐Z Electrode Effect in Mini‐Ionization Chambers for Small Beam Dosimetry
Authors: Larraga‐Gutierrez J
Published: June 2015

Purpose:

To correct for the over‐response of mini‐ionization chambers with high‐Z central electrodes. The hypothesis is that by applying a negative/reverse voltage, it is possible to suppress the signal generated in the high‐Z central electrode by low‐energy photons.

Methods:

The mini‐ionization chambers used in the experiments were a PTW‐31014, PTW‐31006 and IBA‐CC01. The PTW‐31014 has an aluminum central electrode while the PTW‐31006 and IBA‐CC01 have a steel one. Total scatter factors (Scp) were measured for a 6 MV photon beam down to a square field size of 0.5 cm. The measurements were performed in water at 10 cm depth with SAD of 100 cm. The Scp were measured with the dosimeters with +400V bias voltage. In the case of the PTW‐31006 and IBA‐CC01, the measurements were repeated with −400V bias voltage. Also, the field factors in water were calculated with Monte Carlo simulations for comparison.

Results:

The measured Scp at +400V with the PTW‐31006 and IBA‐CC01 detectors were in agreement within 0.2% down to a field size of 1.5 cm. Both dosimeters shown a systematic difference about 2.5% with the Scp measured with the PTW‐31014 and the Monte Carlo calculated field factors. The measured Scp at −400V with the PTW‐31006 and IBA‐CC01 detectors were in close agreement with the PTW‐31014 measured Scp and the field factors within 0.3 and 1.0%, respectively. In the case of the IBA‐CC01 it was found a good agreement (1%) down to field size of 1.0 cm. All the dosimeters shown differences up to 17% between the measured Scp and the field factor for the 0.5 cm field size.

Conclusion:

By applying a negative/reverse voltage to the mini‐ionization chambers with high‐Z central electrode it was possible to correct for their over‐response to low energy photons.
SU‐E‐T‐415: Experimental Determination of the Relative Stopping Power of TRUFILL N‐BCA Liquid Embolic System in a Clinical Proton Beam
Authors: Mandapaka A , Ghebremedhin A , Patyal B
Published: June 2015

Purpose:

Patients who undergo n‐BCA glue embolization as part of treatment for AVMs are later referred for proton therapy. Knowing the relative stopping power of the glue accurately allows us to perform accurate dose calculations. In this study we experimentally determine the relative stopping power of an n‐BCA mixture in a 126 MeV and 149.6 MeV proton beams.

Methods:

One unit of the TRUFILL™ n‐BCA liquid embolic system consists of 1g unit of n‐BCA, 1g unit of Tantalum powder and one 10mL vial of Ethiodized oil. The physician mixed 3:1 Ethiodized oil to n‐BCA. Five units (20cc) of the n‐BCA liquid embolic glue were prepared and placed in a 6cm × 3cm x3cm Lucite container. The container was placed in front of a water tank in the proton beam path. A diamond detector (active volume 0.004mm3) was used to measure distal edge of depth dose of a modulated 126 MeV proton beam collimated using a 3cm brass aperture. The procedure was repeated with a container carrying the same amount of water placed in front of the water tank. The difference in the depth dose measured with glue and with water was used to determine the relative stopping power of the glue. The same determination was done earlier at 149.6 MeV using a different smaller sample (4cc) of n‐BCA.

Results:

The relative stopping power of this particular n‐BCA mixture was determined to be 1.06 at both 126 MeV and 149.6 MeV. We are working on obtaining the composition data of the n‐BCA glue so we can perform Monte Carlo calculations.

Conclusion:

Accurate value of the stopping power of the n‐BCA glue in the proton beam was determined to be 1.06. It will improve the accuracy of dose calculations in proton radiosurgery procedures on AVM patients with n‐BCA embolization.
SU‐E‐T‐417: Experimental Verification of Robustness Optimized Intensity Modulated Proton Therapy Plans for Craniospinal Irradiation
Authors: Yu J , Liao L , Sahoo N , Zhu X , Gillin M , Zhang X
Published: June 2015

Purpose:

The robustness‐optimized intensity modulated proton therapy (RO‐IMPT) plans takes into account the setup errors to eliminate the hot/cold spots caused by setup errors at the junction for craniospinal (CSI) irradiation fields. The RO‐IMPT requires a large area for the junction of two fields. This work is to verify the treatment planning system calculated dose with introduction of setup errors by two‐dimensional (2D) dose measurement. Two fields of total lengths of 45 cm with junctions of 21 cm were measured.

Methods:

2D dose distributions in the coronal plane were measured by a commercial 2D ion‐chamber‐array detector in a cubic solid water phantom. A total spine field consisting of an upper and a lower spine field was measured at the depths of 5.4 and 6.9 cm. The active area for the measurement was 24×24 cm22. The junction and the area outside the junction of the spine fields were measured separately to complete the verification for the total field. Six different amounts of set‐up errors along the superior‐inferior direction up to 1.4 cm were intentionally applied. The measured dose with and without set‐up errors were compared with the calculated dose with and without set‐up errors using Gamma‐index analysis. 3%/3mm and 2%/2mm dose/distance agreement criteria were used. The point dose at the center of the junction was also compared.

Results:

For all fields with various introduced setup errors, 100 % of pixels passed the 3%/3mm criteria and on average 96% of pixels passed 2%/2mm criteria. Differences in the measured and calculated doses were within 1.1%.

Conclusion:

The efficacy of the robustness for the CSI RO‐IMPT plans were verified by measurement. The measurement method can be used for patient treatment field specific QA for CSI RO‐IMPT plans in our institution in the near future.
SU‐E‐T‐418: Explore the Sensitive of the Planar Quality Assurance to the MLC Error with Different Beam Complexity in Intensity‐Modulate Radiation Therapy
Authors: Wang J , Peng J , Xie J , Hu W
Published: June 2015

Purpose:

The purpose of this study is to investigate the sensitivity of the planar quality assurance to MLC errors with different beam complexities in intensity‐modulate radiation therapy.

Methods:

sixteen patients’ planar quality assurance (QA) plans in our institution were enrolled in this study, including 10 dynamic MLC (DMLC) IMRT plans measured by Portal Dosimetry and 6 static MLC (SMLC) IMRT plans measured by Mapcheck. The gamma pass rate was calculated using vender's software. The field numbers were 74 and 40 for DMLC and SMLC, respectively. A random error was generated and introduced to these fields. The modified gamma pass rate was calculated by comparing the original measured fluence and modified fields’ fluence. A decreasing gamma pass rate was acquired using the original gamma pass rate minus the modified gamma pass rate. Eight complexity scores were calculated in MATLAB based on the fluence and MLC sequence of these fields. The complexity scores include fractal dimension, monitor unit of field, modulation index, fluence map complexity, weighted average of field area, weighted average of field perimeter, and small aperture ratio ( <5cm2 and <50cm2). The Spearman's rank correlation coefficient was implemented to analyze the correlation between these scores and decreasing gamma rate.

Results:

The relation between the decreasing gamma pass rate and field complexity was insignificant for most complexity scores. The most significant complexity score was fluence map complexity for SMLC, which have ρ =0.4274 (p‐value=0.0063). For DMLC, the most significant complex score was fractal dimension, which have ρ=−0.3068 (p‐value=0.0081).

Conclusions:

According to the primarily Result of this study, the sensitivity gamma pass rate was not strongly relate to the field complexity.
SU‐E‐T‐419: Fabricating Cerrobend Grids with 3D Printing for Spatially Modulated Radiation Therapy: A Feasibility Study
Authors: Zhu X , Driewer J , Lei Y , Zheng D , Li S , Cullip T , Chang S , Zhang Q , Zhang M , Zhou S
Published: June 2015

Purpose:

Grid therapy has promising applications in the radiation treatment of bulky and large tumors. However, research and applications of grid therapy is limited by the accessibility of the specialized blocks that produce the grid of pencil‐like radiation beams. In this study, a Cerrobend grid block was fabricated using a 3D printing technique.

Methods:

A grid block mold was designed with divergent tubes following beam central rays. The mold was printed using a resin with the working temperature below 230 °C. The melted Cerrobend liquid at 120°oC was cast into the resin mold to yield a block with a thickness of 7.4 cm. The grid had a hexagonal pattern, with each pencil beam diameter of 1.4 cm at the iso‐center plane; the distance between the beam centers was 2 cm. The dosimetric properties of the grid block were studied using radiographic film and small field dosimeters.

Results:

the grid block was fabricated to be mounted at the third accessory mount of a Siemens Oncor linear accelerator. Fabricating a grid block using 3D printing is similar to making cutouts for traditional radiotherapy photon blocks, with the difference being that the mold was created by a 3D printer rather than foam. In this study, the valley‐to‐peak ratio for a 6MV photon grid beam was 20% at dmax, and 30% at 10 cm depth, respectively.

Conclusion:

We have demonstrated a novel process for implementing grid radiotherapy using 3D printing techniques. Compared to existing approaches, our technique combines reduced cost, accessibility, and flexibility in customization with efficient delivery. This lays the groundwork for future studies to improve our understanding of the efficacy of grid therapy and apply it to improve cancer treatment.
SU‐E‐T‐434: Fixed Margin Or Online Adaptation for Intermediate‐Risk Prostate Stereotactic Body Radiation Therapy? A Dosimetric Study
Authors: Sheng Y , Li T , Yin F , Wu Q
Published: June 2015

Purpose:

To investigate the choice of fixed margin or online adaptation when treating intermediate‐risk prostate cancer including seminal vesicles (SV) using stereotactic body radiation therapy (SBRT).

Methods:

9 prostate SBRT patients were retrospectively studied. All patients were implanted with fiducial markers in the prostate for daily localization and verification. Each patient had 5 pairs of pre‐treatment and post‐treatment cone‐beam CT (CBCT) per protocol. SVs were contoured on planning CT and all CBCTs by one attending physician. Simultaneous integral boost (SIB) IMRT plans were developed to deliver 25Gy/5fx to the SV while delivering 37Gy/5fx to the prostate. A 3mm isotropic margin was added to the prostate while a 5 mm isotropic margin was used for the SV. The planning CT was registered to daily pre‐treatment and post‐treatment CBCT based on fiducial markers in the prostate to mimic online prostate localization; and the SV on daily CBCT was transferred to the CT structure set after the prostates were aligned. Daily pre‐treatment and post‐treatment SV dose coverage and the organ‐at‐risk (OAR) sparing were evaluated for the SIB regimen. At least 95% of the SV need to receive the prescription dose (5Gy per fraction).

Results:

For the total of 90 daily SVs analyzed (ten CBCTs for each of nine patients), only 45 daily SVs (50%) were able to meet the coverage that 95% of the SV received 25Gy. The OAR sparing performance was acceptable for most of the dosimetric constraints in low‐risk prostate SBRT protocol with only two exceptions in bladder V100 (cc).

Conclusion:

A fixed 5mm margin for SV is not sufficient to provide consistent daily dose coverage due to SV's substantial inter‐ and intra‐fractional motion relative to the prostate.

This finding calls for innovative strategies in margin design as well as online treatment adaptation.

This work is partially supported a master research grant from Varian Medical Systems.
SU‐E‐T‐436: Fluence‐Based Trajectory Optimization for Non‐Coplanar VMAT
Authors: Smyth G , Evans PM , Bamber JC , Saran FH , Mandeville HC , Bedford JL
Published: June 2015

Purpose:

To investigate a fluence‐based trajectory optimization technique for non‐coplanar VMAT for brain cancer.

Methods:

Single‐arc non‐coplanar VMAT trajectories were determined using a heuristic technique for five patients. Organ at risk (OAR) volume intersected during raytracing was minimized for two cases: absolute volume and the sum of relative volumes weighted by OAR importance. These trajectories and coplanar VMAT formed starting points for the fluence‐based optimization method. Iterative least squares optimization was performed on control points 24° apart in gantry rotation. Optimization minimized the root‐mean‐square (RMS) deviation of PTV dose from the prescription (relative importance 100), maximum dose to the brainstem (10), optic chiasm (5), globes (5) and optic nerves (5), plus mean dose to the lenses (5), hippocampi (3), temporal lobes (2), cochleae (1) and brain excluding other regions of interest (1). Control point couch rotations were varied in steps of up to 10° and accepted if the cost function improved. Final treatment plans were optimized with the same objectives in an in‐house planning system and evaluated using a composite metric ‐ the sum of optimization metrics weighted by importance.

Results:

The composite metric decreased with fluence‐based optimization in 14 of the 15 plans. In the remaining case its overall value, and the PTV and OAR components, were unchanged but the balance of OAR sparing differed. PTV RMS deviation was improved in 13 cases and unchanged in two. The OAR component was reduced in 13 plans. In one case the OAR component increased but the composite metric decreased ‐ a 4 Gy increase in OAR metrics was balanced by a reduction in PTV RMS deviation from 2.8% to 2.6%.

Conclusion:

Fluence‐based trajectory optimization improved plan quality as defined by the composite metric. While dose differences were case specific, fluence‐based optimization improved both PTV and OAR dosimetry in 80% of cases.
SU‐E‐T‐438: Frameless Cranial Stereotactic Radiosurgery Immobilization Effectiveness Evaluation
Authors: Tseng T , Green S , Sheu R , Lo Y
Published: June 2015

Purpose:

To evaluate immobilization effectiveness of Brainlab frameless mask in cranial stereotactic radiosurgery (SRS).

Methods:

Two sets of setup images were collected pre‐and post‐treatment for 24 frameless SRS cases. The pre‐treatment images were obtained after applying 2D‐2D kV image‐guided shifts with patients in treatment position and approved by physicians; the post‐treatment images were taken immediately after treatment completion. All cases were treated on a Novalis linac with ExacTrac positioning system and Exact Couch. The two image sets were compared with the correctional shifts measured by ExacTrac 6D auto‐fusion. The shift differences were considered patient motion within the frameless mask and were used to evaluate its effectiveness for immobilization. Two‐tailed paired t‐test was applied for significance comparison.

Results:

The correctional shifts (mean±STD, median) of pre‐and post‐treatment images were 0.33±0.27mm, 0.26mm and 0.34±0.27mm, 0.23mm (p=0.740) in lateral direction; 0.32±0.29mm, 0.22mm and 0.48±0.30mm, 0.50mm (p=0.012) in longitudinal direction; 0.31±0.22mm, 0.24mm and 0.33±0.21mm, 0.36mm (p=0.623) in vertical direction. The radial correctional shifts (mean±STD, median) of pre ‐and post‐treatment images were 0.60±0.38mm, 0.45mm and 0.75±0.31mm, 0.66mm (p=0.033). The shift differences (mean±STD, median, maximum) were 0.35±0.28mm, 0.3mm, 1.05mm, 0.34±0.28mm, 0.3mm, 1.00mm, 0.24±0.15mm, 0.21mm, 0.60mm and 0.61±0.32mm, 0.57mm, 1.40mm in lateral, longitudinal, vertical and radial direction, respectively. Two shifts greater than 1 mm (1.06mm and 1.02mm) were acquired from post‐treatment images. However, the shift differences were only 0.09 and 0.19mm for these two shifts. Two patients with shift differences greater than 1mm (1.05 and 1.04mm) were observed and didn't coincide with those two who had post‐correctional shifts greater than 1mm.

Conclusion:

Image‐guided SRS allowed us to set up patients with sub‐millimeter accuracy relative to simulation position. However, patient motion during treatment could affect treatment accuracy. Our Result shows that Brainlab frameless mask provides reasonable patient immobilization and maintains the mean post‐treatment position within sub‐millimeter accuracy with some borderline results observed.
SU‐E‐T‐445: Gross Error Detection Based On EPID‐Image‐Extracted MLC‐Leaf‐Position Monitoring
Authors: Kathuria K , Watkins W , Siebers J
Published: June 2015

Purpose:

To develop a robust real‐time, log‐file independent MLC position validation metric capable of preventing gross treatment delivery errors.

Methods:

MLC leaf positions are identified on individual during‐treatment EPID images acquired in real time at 10 Hz during VMAT/IMRT patient treatments. MLC positions are identified as follows: (1) the image is rotated counterclockwise by 90°‐collimatorAngle so that MLC motion direction is vertical; (2) pixel‐columns corresponding to individual leaves are assigned based upon the imager SDD and known leaf positions on the isocenter plane; (3) inflection points are identified and averaged across pixel‐column intensity‐profiles for all central (50 % of total) pixel‐columns of each leaf; (4) an empirical position‐based offset is added to each leaf's average inflection point to arrive at its final estimated position. These leaf positions are appropriately interpolated between EPID images and compared to their planned positions at every control point and statistically analyzed for in‐field active (moving) leaves on image. Machine log and planned positions were compared for the same deliveries.

Results:

The mean EPID‐detected leaf offset from 51 fractions was 0.03±0.25 cm. 90.3 % of EPID‐detected leaves were within 3 mm. Machine‐log‐based analysis found an average of 0.0002±0.02 cm and 100% within.7 mm. Large (>3mm) EPID‐detected deviations resulted from algorithm failure in cases where multiple inflection points persisted in the profiles.

Conclusions:

Real‐time EPID‐based quality control can provide an independent validation of MLC leaf positions with sufficient resolution for gross error detection. Although further algorithmic improvements are required to match log‐file‐based accuracy, the current EPID‐based leaf position algorithm is suitable for clinical gross error detection.
SU‐E‐T‐449: Hippocampal Sparing Radiotherapy Using Intensity Modulated Radiotherapy and Volumetric Modulated Arc Therapy
Authors: Moon S , Kim D , Chung W , Yoon M
Published: June 2015

Purpose:

The hippocampus sparing during the cranial irradiation has become interesting because it may mitigate radiation‐induced neurocognitive toxicity. Herein we report our preliminary study for sparing the hippocampus with and without tilling condition for patient with brain metastases.

Methods:

Ten patients previously treated with whole brain were reviewed. Five patients tilted the head to around 30 degrees and others were treated without tilting. Treatment plans of linear accelerator (Linac)‐based volumetric modulated arc therapy (VMAT) and intensity modulated radiotherapy (IMRT) were generated for prescription dose of 30 Gy in 15 fractions. Hippocampal avoidance regions were created with 5‐mm volumetric expansion around the hippocampus. Whole brain, hippocampus and hippocampal avoidance volume were 1372cm3, 6cm3 and 30cm3 and hippocampal avoidance volume was 2.2% of the whole brain planned target volume in average. Organs at risk (OARs) are hippocampus, eyes, lens, and cochleae. Coverage index (CVI), conformity index (CI), homogeneity index (HI) and mean dose to OARs were used to compare dose characteristic of tilted and non‐tilted cases.

Results:

In IMRT, when CI, CVI and HI of whole brain were 0.88, 0.09 and 0.98 in both tilted and non‐tilted cases, absorbed dose of hippocampal avoidance volume in tilted cases were 10% lower than non‐tilted cases. Doses in other OARs such as eyes, lens, and cochleae were also decreased about 20% when tilting the head. When CI, HI and CVI in VMAT were 0.9, 0.08 and 0.99, the dose‐decreased ratio of OARs in both with and without tilting cases were almost the same with IMRT. But absolute dose of hippocampal avoidance volume in VMAT was 30% lower than IMRT.

Conclusion:

This study confirms that dose to hippocampus decreases if patients tilt the head. When treating the whole brain with head tilted, patients can acquire the same successful treatment Result and also preserve their valuable memory.
SU‐E‐T‐450: How Important Is a Reproducible Breath Hold for DIBH Breast Radiotherapy?
Authors: Liu H , Wentworth S , Sintay B , Wiant D
Published: June 2015

Purpose:

Deep inspiration breath hold (DIBH) for left‐sided breast cancer has been shown to reduce heart dose. Surface imaging helps to ensure accurate breast positioning, but does not guarantee a reproducible breath hold (BH) at DIBH treatments. We examine the effects of variable BH positions for DIBH treatments.

Methods:

Twenty‐Five patients with free breathing (FB) and DIBH scans were reviewed. Four plans were created for each patient: 1) FB, 2) DIBH, 3) FB_DIBH – the DIBH plans were copied to the FB images and recalculated (image registration was based on breast tissue), and 4) P_DIBH – a partial BH with the heart shifted midway between the FB and DIBH positions. The FB_DIBH plans give “worst case” scenarios for surface imaging DIBH, where the breast is aligned by surface imaging but the patient is not holding their breath. Students t‐tests were used to compare dose metrics.

Results:

The DIBH plans gave lower heart dose and comparable breast coverage versus FB in all cases. The FB_DIBH plans showed no significant difference versus FB plans for breast coverage, mean heart dose, or maximum heart dose (p >= 0.10). The mean heart dose differed between FB_DIBH and FB by < 2 Gy for all cases, the maximum heart dose differed by < 2 Gy for 21 cases. The P_DIBH plans showed significantly lower mean heart dose than FB (p = 0.01). The mean heart doses for the P_DIBH plans were < FB for 22 cases, the maximum dose < FB for 18 cases.

Conclusions:

A DIBH plan delivered to a FB patient set‐up with surface imaging will yield similar dosimetry to a plan created and delivered FB. A DIBH plan delivered with even a partial BH can give reduced heart dose compared to FB techniques when the breast tissue is well aligned.
SU‐E‐T‐461: Impact of the Radiation Sensitivity of Tumor and Geometric Design of the GRID Block On the Clinical Response of Grid Therapy
Authors: Gholami S , Meigooni A S. , Nedaie H.A , Longo F , Ay M.R
Published: June 2015

Purpose:

The spatially fractionated radiation therapy (Grid therapy) is a novel technique that has been recently introduced for treatments of the patients with advanced bulky tumors. The purpose of this study is to investigate the effect of the radiation sensitivity of tumor and Grid block design on the clinical response of the Grid therapy.

Methods:

The Geant4 toolkit is used to simulate a Varian2100C linear accelerator with the block‐based Grid. The dose profile has been utilized to calculate the Therapeutic Ratio (TR) and Equivalent Uniform Dose (EUD) for different types of tumors. These calculations were performed with respect to their radiation sensitivities using Linear Quadratic model (LQ). Clinical responses of few hundred patients from different publications that were treated with Grid therapy, have been used to validate correlation between tumor radiation sensitivity and clinical response of Grid therapy. Moreover, the influence of Grid design on clinical response of this radiation technique was investigated with using Monte Carlo simulation of a Grid block with different hole‐diameters of 0.5 cm, 0.75 cm, 1.25 cm, and 1.5 cm. The Geometrical Sparing Factor (GSF) was considered to compare the results from different Grid designs.

Results:

The higher clinical response of Grid therapy for more radio‐resistant tumors was confirmed in both theoretical assumptions and clinical outcomes. In addition, the results indicate that the Grid hole diameter have more effect on increasing the TR in comparison with the hole center‐to‐center distance effect. From this Monte Carlo study, a Grid design with the hole diameter between 1.25 cm and 1.5 cm with GSF value of 0.90 is recommended.

Conclusion:

Grid Therapy is more effective for more radio‐resistant tumors.The optimum results of the Grid therapy depend on selection of the appropriate Grid design.This study is a first step toward clarifying some important aspects in a clinical response of spatially fractionated radiation therapy.
SU‐E‐T‐466: Implementation of An Extension Module for Dose Response Models in the TOPAS Monte Carlo Toolkit
Authors: Ramos‐Mendez J , Perl J , Schuemann J , Shin J , Paganetti H , Faddegon B
Published: June 2015

Purpose:

To develop and verify an extension to TOPAS for calculation of dose response models (TCP/NTCP). TOPAS wraps and extends Geant4.

Methods:

The TOPAS DICOM interface was extended to include structure contours, for subsequent calculation of DVH's and TCP/NTCP. The following dose response models were implemented: Lyman‐Kutcher‐Burman (LKB), critical element (CE), population based critical volume (CV), parallel‐serials, a sigmoid‐based model of Niemierko for NTCP and TCP, and a Poisson‐based model for TCP. For verification, results for the parallel‐serial and Poisson models, with 6 MV x‐ray dose distributions calculated with TOPAS and Pinnacle v9.2, were compared to data from the benchmark configuration of the AAPM Task Group 166 (TG166). We provide a benchmark configuration suitable for proton therapy along with results for the implementation of the Niemierko, CV and CE models.

Results:

The maximum difference in DVH calculated with Pinnacle and TOPAS was 2%. Differences between TG166 data and Monte Carlo calculations of up to 4.2%±6.1% were found for the parallel‐serial model and up to 1.0%±0.7% for the Poisson model (including the uncertainty due to lack of knowledge of the point spacing in TG166). For CE, CV and Niemierko models, the discrepancies between the Pinnacle and TOPAS results are 74.5%, 34.8% and 52.1% when using 29.7 cGy point spacing, the differences being highly sensitive to dose spacing. On the other hand, with our proposed benchmark configuration, the largest differences were 12.05%±0.38%, 3.74%±1.6%, 1.57%±4.9% and 1.97%±4.6% for the CE, CV, Niemierko and LKB models, respectively.

Conclusion:

Several dose response models were successfully implemented with the extension module. Reference data was calculated for future benchmarking. Dose response calculated for the different models varied much more widely for the TG166 benchmark than for the proposed benchmark, which had much lower sensitivity to the choice of DVH dose points.

This work was supported by National Cancer Institute Grant R01CA140735
SU‐E‐T‐467: Implementation of Monte Carlo Dose Calculation for a Multileaf Collimator Equipped Robotic Radiotherapy System
Authors: Li JS , Fan J , Ma C‐M
Published: June 2015

Purpose:

To improve the treatment efficiency and capabilities for full‐body treatment, a robotic radiosurgery system has equipped with a multileaf collimator (MLC) to extend its accuracy and precision to radiation therapy. To model the MLC and include it in the Monte Carlo patient dose calculation is the goal of this work.

Methods:

The radiation source and the MLC were carefully modeled to consider the effects of the source size, collimator scattering, leaf transmission and leaf end shape. A source model was built based on the output factors, percentage depth dose curves and lateral dose profiles measured in a water phantom. MLC leaf shape, leaf end design and leaf tilt for minimizing the interleaf leakage and their effects on beam fluence and energy spectrum were all considered in the calculation. Transmission/leakage was added to the fluence based on the transmission factors of the leaf and the leaf end. The transmitted photon energy was tuned to consider the beam hardening effects. The calculated results with the Monte Carlo implementation was compared with measurements in homogeneous water phantom and inhomogeneous phantoms with slab lung or bone material for 4 square fields and 9 irregularly shaped fields.

Results:

The calculated output factors are compared with the measured ones and the difference is within 1% for different field sizes. The calculated dose distributions in the phantoms show good agreement with measurements using diode detector and films. The dose difference is within 2% inside the field and the distance to agreement is within 2mm in the penumbra region. The gamma passing rate is more than 95% with 2%/2mm criteria for all the test cases.

Conclusion:

Implementation of Monte Carlo dose calculation for a MLC equipped robotic radiosurgery system is completed successfully. The accuracy of Monte Carlo dose calculation with MLC is clinically acceptable.

This work was supported by Accuray Inc.
SU‐E‐T‐468: Implementation of the TG‐142 QA Process for Seven Linacs with Enhanced Beam Conformance
Authors: Woollard J , Ayan A , DiCostanzo D , Grzetic S , Hessler J , Gupta N
Published: June 2015

Purpose:

To develop a TG‐142 compliant QA process for 7 Varian TrueBeam linear accelerators (linacs) with enhanced beam conformance and dosimetrically matched beam models. To ensure consistent performance of all 7 linacs, the QA process should include a common set of baseline values for use in routine QA on all linacs.

Methods:

The TG 142 report provides recommended tests, tolerances and frequencies for quality assurance of medical accelerators. Based on the guidance provided in the report, measurement tests were developed to evaluate each of the applicable parameters listed for daily, monthly and annual QA. These tests were then performed on each of our 7 new linacs as they came on line at our institution.

Results:

The tolerance values specified in TG‐142 for each QA test are either absolute tolerances (i.e. ±2mm) or require a comparison to a baseline value. The results of our QA tests were first used to ensure that all 7 linacs were operating within the suggested tolerance values provided in TG −142 for those tests with absolute tolerances and that the performance of the linacs was adequately matched. The QA test results were then used to develop a set of common baseline values for those QA tests that require comparison to a baseline value at routine monthly and annual QA. The procedures and baseline values were incorporated into a spreadsheets for use in monthly and annual QA.

Conclusion:

We have developed a set of procedures for daily, monthly and annual QA of our linacs that are consistent with the TG‐142 report. A common set of baseline values was developed for routine QA tests. The use of this common set of baseline values for comparison at monthly and annual QA will ensure consistent performance of all 7 linacs.
SU‐E‐T‐469: Implementation of VAs Web‐Based Radiotherapy Incident Reporting and Analysis System (RIRAS)
Authors: Kapoor R , Malik G , Palta J , Hagan M
Published: June 2015

Purpose:

This Web‐based Radiotherapy Incident Reporting and Analysis System (RIRAS) is a tool to improve quality of care for radiation therapy patients. This system is an important facet of continuing effort by our community to maintain and improve safety of radiotherapy.Material and

Methods:

VA's National Radiation Oncology Program office has embarked on a program to electronically collect adverse events and good‐catch data of radiation treatment of over 25,000 veterans treated with radiotherapy annually. This VA‐Intranet based software design has made use of dataset taxonomies and data dictionaries defined in AAPM/ASTRO reports on error reporting. We used proven industrial and medical event reporting techniques to avoid several common problems faced in effective data collection such as incomplete data due to data entry fatigue by the reporters, missing data due to data difficult to obtain or not familiar to most reporters, missing reports due to fear of reprisal etc. This system encompasses the entire feedback loop of reporting an incident, analyzing it for salient details, and developing interventions to prevent it from happening again. The analysis reports with corrective, learning actions are shared with the reporter/facility and made public to the community (after deidentification) as part of the learning process.

Results:

Till date 50 incident/good catches have been reported in RIRAS and we have completed analysis on 100% of these reports. This is done due to the fact that each reported incidents is investigated and a complete analysis/patient‐safety‐work‐product report is generated by radiation oncology domain‐experts. Conclusions Because of the completeness of the data, the system has enabled us to analyze process steps and track trends of major errors which in the future will lead to implementing system wide process improvement steps and safe standard operating procedures for each radiotherapy treatment modality/technique and fulfills our goal of “Effecting Quality While Treating Safely”.

RIRAS developed and copyrighted by TSG Innovations Inc.
SU‐E‐T‐470: Importance of HU‐Mass Density Calibration Technique in Proton Pencil Beam Dose Calculation
Authors: Penfold S , Miller A
Published: June 2015

Purpose:

Stoichiometric calibration of Hounsfield Units (HUs) for conversion to proton relative stopping powers (RStPs) is vital for accurate dose calculation in proton therapy. However proton dose distributions are not only dependent on RStP, but also on relative scattering power (RScP) of patient tissues. RScP is approximated from material density but a stoichiometric calibration of HU‐density tables is commonly neglected. The purpose of this work was to quantify the difference in calculated dose of a commercial TPS when using HU‐density tables based on tissue substitute materials and stoichiometric calibrated ICRU tissues.

Methods:

Two HU‐density calibration tables were generated based on scans of the CIRS electron density phantom. The first table was based directly on measured HU and manufacturer quoted density of tissue substitute materials. The second was based on the same CT scan of the CIRS phantom followed by a stoichiometric calibration of ICRU44 tissue materials. The research version of Pinnacle3 proton therapy was used to compute dose in a patient CT data set utilizing both HU‐density tables.

Results:

The two HU‐density tables showed significant differences for bone tissues; the difference increasing with increasing HU. Differences in density calibration table translated to a difference in calculated RScP of −2.5% for ICRU skeletal muscle and 9.2% for ICRU femur. Dose‐volume histogram analysis of a parallel opposed proton therapy prostate plan showed that the difference in calculated dose was negligible when using the two different HU‐density calibration tables.

Conclusion:

The impact of HU‐density calibration technique on proton therapy dose calculation was assessed. While differences were found in the calculated RScP of bony tissues, the difference in dose distribution for realistic treatment scenarios was found to be insignificant.
SU‐E‐T‐471: Improvement of Gamma Knife Treatment Planning Through Tumor Control Probability for Metastatic Brain Tumors
Authors: Huang Z , Feng Y , Lo S , Grecula J , Mayr N , Yuh W
Published: June 2015

Purpose:

The dose–volume histogram (DVH) has been normally accepted as a tool for treatment plan evaluation. However, spatial information is lacking in DVH. As a supplement to the DVH in three‐dimensional treatment planning, the differential DVH (DDVH) provides the spatial variation, the size and magnitude of the different dose regions within a region of interest, which can be incorporated into tumor control probability model. This study was to provide a method in evaluating and improving Gamma Knife treatment planning.

Methods:

10 patients with brain metastases from different primary tumors including melanoma (#1,#4,#5, #10), breast cancer (#2), prostate cancer (#3) and lung cancer (#6–9) were analyzed. By using Leksell GammaPlan software, two plans were prepared for each patient. Special attention was given to the DDVHs that were different for different plans and were used for a comparison between two plans. Dose distribution inside target and tumor control probability (TCP) based on DDVH were calculated, where cell density and radiobiological parameters were adopted from literature. The plans were compared based on DVH, DDVH and TCP.

Results:

Using DVH, the coverage and selectivity were the same between plans for 10 patients. DDVH were different between two plans for each patient. The paired t‐test showed no significant difference in TCP between the two plans. For brain metastases from melanoma (#1, #4–5), breast cancer (#2) and lung cancer (#6–8), the difference in TCP was less than 5%. But the difference in TCP was about 6.5% for patient #3 with the metastasis from prostate cancer, 10.1% and 178.7% for two patients (#9–10) with metastasis from lung cancer.

Conclusion:

Although DVH provides average dose–volume information, DDVH provides differential dose– volume information with respect to different regions inside the tumor. TCP provides radiobiological information and adds additional information on improving treatment planning as well as adaptive radiotherapy. Further clinical validation is necessary.