bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐J‐203: Investigation of 1.5T Magnetic Field Dose Effects On Organs of Different Density
Authors: Lee H , Rubinstein A , Ibbott G
Published: June 2015

Purpose:

For the combined 1.5T/6MV MRI‐linac system, the perpendicular magnetic field to the radiation beam results in altered radiation dose distributions. This Monte Carlo study investigates the change in dose at interfaces for common organs neighboring soft tissue.

Methods:

MCNP6 was used to simulate the effects of a 1.5T magnetic field when irradiating tissues with a 6 MV beam. The geometries used in this study were not necessarily anatomically representative in size in order to directly compare quantitative dose effects for each tissue at the same depths. For this purpose, a 512 cm3 cubic material was positioned at the center of a 2744 cm3 cubic soft tissue material phantom. The following tissue materials and their densities were used in this study: lung (0.296 g/cm3), fat (0.95), spinal cord (1.038), soft tissue (1.04), muscle (1.05), eye (1.076), trabecular bone (1.40), and cortical bone (1.85).

Results:

The addition of a 1.5T magnetic field caused dose changes of +46.5%, +2.4%, −0.9%, −0.8%, −1.5%, −6.5%, and −8.8% at the entrance interface between soft tissue and lung, fat, spinal cord, muscle, eye, trabecular bone, and cortical bone tissues respectively. Dose changes of −39.4%, −4.1%, −0.8%, −0.8%, +0.5%, +6.7%, and +10.9% were observed at the second interface between the same tissues respectively and soft tissue. On average, the build‐up distance was reduced by 0.6 cm, and a dose increase of 62.7% was observed at the exit interface between soft tissue and air of the entire phantom.

Conclusion:

The greatest changes in dose were observed at interfaces containing lung and bone tissues. Due to the prevalence and proximity of bony anatomy to soft tissues throughout the human body, these results encourage further examination of these tissues with anatomically representative geometries using multiple beam configurations for safe treatment using the MRI‐linac system.

NSF GRFP Grant Award #LH‐102SPS
SU‐E‐J‐209: Geometric Distortion at 3T in a Commercial 4D MRI‐Compatible Phantom
Authors: Fatemi‐Ardekani A , Wronski M , Kim A , Stanisz G , Sarfehnia A , Keller B
Published: June 2015

Purpose:

There are very few commercial 4D phantoms that are marketed as MRI compatible. We are evaluating one such commercial phantom, made to be used with an MRI‐Linear accelerator. The focus of this work is to characterize the geometric distortions produced in this phantom at 3T using 3 clinical MR pulse sequences.

Methods:

The CIRS MRI‐Linac Dynamic Phantom (CIRSTM) under investigation in this study consists of a softwaredriven moving tumour volume within a thorax phantom body and enables dose accumulation by placing a dosimeter within the tumour volume. Our initial investigation is to evaluate the phantom in static mode prior to examining its 4D capability. The water‐filled thorax phantom was scanned using a wide‐bore Philips 3T Achieva MRI scanner employing a Thoracic xl coil and clinical 2D T1W FFE, 2D T1W TSE and 3D T1W TFE pulse sequences. Each of the MR image sets was rigidly fused with a reference CT image of the phantom employing a rigid registration with 6 degrees of freedom. Geometric distortions between the MR and CT image sets were measured in 3 dimensions at selected points along the periphery of the distortion grid embedded within the phantom body (11.5, 7.5 and 3 cm laterally, ant/post and sup/inf of magnetic isocenter respectively).

Results:

The maximal measured geometric distortions between the MR and reference CT points of interest were 0.9, 1.8 and 1.3 mm in the lateral, anteriorposterior and cranio‐caudal directions, respectively. For all 3 spatial dimensions, the maximal distortions occurred for the FFE pulse sequence. Maximal distortions for the 2D FFE, 2D TSE and 3D TFE sequences were 1, 0.7 and 1.8 mm, respectively.

Conclusion:

Our initial static investigation of this phantom shows minimal geometric distortions at 3T along the periphery of the embedded grid.

CIRS has provided us with a phantom at no charge for evaluation at 3 Tesla.
SU‐E‐J‐212: MR Diffusion Tensor Imaging for Assessment of Tumor and Normal Brain Tissue Responses of Juvenile Pilocytic Astrocytoma Treated by Proton Therapy
Authors: Hou P , Park P , Li H , Zhu X , Mahajan A , Grosshans D
Published: June 2015

Purpose:

Diffusion tensor imaging (DTI) can measure molecular mobility at the cellular level, quantified by the apparent diffusion coefficient (ADC). DTI may also reveal axonal fiber directional information in the white matter, quantified by the fractional anisotropy (FA). Juvenile pilocytic astrocytoma (JPA) is a rare brain tumor that occurs in children and young adults. Proton therapy (PT) is increasingly used in the treatment of pediatric brain tumors including JPA. However, the response of both tumors and normal tissues to PT is currently under investigation. We report tumor and normal brain tissue responses for a pediatric case of JPA treated with PT assessed using DTI.

Methods:

A ten year old male with JPA of the left thalamus received passive scattered PT to a dose of 50.4 Gy (RBE) in 28 fractions. Post PT, the patient has been followed up in seven years. At each follow up, MRI imaging including DTI was performed to assess response. MR images were registered to the treatment planning CT and the GTV mapped onto each MRI. The GTV contour was then mirrored to the right side of brain through the patient's middle line to represent normal brain tissue. ADC and FA were measured within the ROIs.

Results:

Proton therapy can completely spare contra lateral brain while the target volume received full prescribed dose. From a series of MRI ADC images before and after PT at different follow ups, the enhancement corresponding to GTV had nearly disappeared more than 2 years after PT. Both ADC and FA demonstrate that contralateral normal brain tissue were not affect by PT and the tumor volume reverted to normal ADC and FA values.

Conclusion:

DTI allowed quantitative evaluation of tumor and normal brain tissue responses to PT. Further study in a larger cohort is warranted.
SU‐E‐J‐218: Novel Validation Paradigm of MRI to CT Deformation of Prostate
Authors: Padgett K , Pirozzi S , Nelson A , Stoyanova R , Piper J , Horvat M , Dogan N , Pollack A
Published: June 2015

Purpose:

Deformable registration algorithms are inherently difficult to characterize in the multi‐modality setting due to a significant differences in the characteristics of the different modalities (CT and MRI) as well as tissue deformations. We present a unique paradigm where this is overcome by utilizing a planning‐MRI acquired within an hour of the planning‐CT serving as a surrogate for quantifying MRI to CT deformation by eliminating the issues of multi‐modality comparisons.

Methods:

For nine subjects, T2 fast‐spin‐echo images were acquired at two different time points, the first several weeks prior to planning (diagnostic‐MRI) and the second on the same day as the planning‐CT (planning‐MRI). Significant effort in patient positioning and bowel/bladder preparation was undertaken to minimize distortion of the prostate in all datasets. The diagnostic‐MRI was rigidly and deformably aligned to the planning‐CT utilizing a commercially available deformable registration algorithm synthesized from local registrations. Additionally, the quality of rigid alignment was ranked by an imaging physicist. The distances between corresponding anatomical landmarks on rigid and deformed registrations (diagnostic‐MR to planning‐CT) were evaluated.

Results:

It was discovered that in cases where the rigid registration was of acceptable quality the deformable registration didn't improve the alignment, this was true of all metrics employed. If the analysis is separated into cases where the rigid alignment was ranked as unacceptable the deformable registration significantly improved the alignment, 4.62mm residual error in landmarks as compared to 5.72mm residual error in rigid alignments with a p‐value of 0.0008.

Conclusion:

This paradigm provides an ideal testing ground for MR to CT deformable registration algorithms by allowing for inter‐modality comparisons of multi‐modality registrations. Consistent positioning, bowel and bladder preparation may Result in higher quality rigid registrations than typically achieved which limits the impact of deformable registrations. In this study cases where significant differences exist, deformable registrations provide significant value.

This work was funded by an NIH grant# 1R01CA189295‐01. PI: Alan Pollack, MD PhD. “MRI IMAGING AND GENETIC SIGNATURES TO MANAGE PROSTATE CANCER OVERDIAGNOSIS”
SU‐E‐J‐223: A BOLD Contrast Imaging Sequence to Evaluate Oxygenation Changes Due to Breath Holding for Breast Radiotherapy: A Pilot Study
Authors: Adamson J , Chang Z , Cai J , Palta M , Horton J , Yin F , Blitzblau R
Published: June 2015

Purpose:

To develop a robust MRI sequence to measure BOLD breath hold induced contrast in context of breast radiotherapy.

Methods:

Two sequences were selected from prior studies as candidates to measure BOLD contrast attributable to breath holding within the breast: (1) T2* based Gradient Echo EPI (TR/TE = 500/41ms, flip angle = 60°), and (2) T2 based Single Shot Fast Spin Echo (SSFSE) (TR/TE = 3000/60ms). We enrolled ten women post‐lumpectomy for breast cancer who were undergoing treatment planning for whole breast radiotherapy. Each session utilized a 1.5T GE MRI and 4 channel breast coil with the subject immobilized prone on a custom board. For each sequence, 1–3 planes of the lumpectomy breast were imaged continuously during a background measurement (1min) and intermittent breath holds (20–40s per breath hold, 3–5 holds per sequence). BOLD contrast was quantified as correlation of changes in per‐pixel intensity with the breath hold schedule convolved with a hemodynamic response function. Subtle motion was corrected using a deformable registration algorithm. Correlation with breath‐holding was considered significant if p<0.001.

Results:

The percentage of the breast ROI with positive BOLD contrast measured by the two sequences were in agreement with a correlation coefficient of R=0.72 (p=0.02). While both sequences demonstrated areas with strong BOLD response, the response was more systematic throughout the breast for the SSFSE (T2) sequence (% breast with response in the same direction: 51.2%±0.7% for T2* vs. 68.1%±16% for T2). In addition, the T2 sequence was less prone to magnetic susceptibility artifacts, especially in presence of seroma, and provided a more robust image with little distortion or artifacts.

Conclusion:

A T2 SSFSE sequence shows promise for measuring BOLD contrast in the context of breast radiotherapy utilizing a breath hold technique. Further study in a larger patient cohort is warranted to better refine this novel technique.
SU‐E‐T‐88: Acceptance Testing and Commissioning Measurements of a Newly Released InCiseâ„¢ Multileaf Collimator for CyberKnife M6â„¢ System
Authors: Huq M Saiful , Ozhasoglu C , Jang S , Hwang M , Heron D , Lalonde R
Published: June 2015

Purpose:

Accuray recently released a new collimator, the InCise™ Multileaf Collimator (MLC), for clinical use with the CyberKnife M6™ System. This work reports the results of acceptance testing and commissioning measurements for this collimator.

Methods:

The MLC consists of 41 pairs of 2.5 mm wide leaves projecting a clinical maximum field size of 110 mm × 97.5 mm at 800 mm SAD. The leaves are made of tungsten, 90 mm in height and tilted by 0.5 degree. The manufacturer stated leaf positioning accuracy and reproducibility are 0.5 mm and 0.4 mm respectively at 800 mm SAD. The leaf over‐travel is 100% with full interdigitation capability. Acceptance testing included, but are not limited to, the verification of the specifications of various parameters described above, leakage measurements and end‐to‐end tests. Dosimetric measurements included, but not limited to, measurements of output factors, open beam profiles, tissue‐phantom ratios, beam flatness and symmetry, and patient specific QA.

Results:

All measurements were well within the manufacturer specifications. The values of output factors ranged from 0.804 (smallest field size of 7.6 mm × 7.5 mm) to 1.018 (largest field size of 110.0 mm × 97.5 mm). End‐to‐end test results for the various tracking modes are: Skull (0.27mm), fiducial (0.16mm), Xsight Spine (0.4mm), Xsight Lung (0.93 mm) and Synchrony (0.43mm). Measured maximum and average leakage was 0.37% and 0.3%, respectively. Patient‐specific QA measurements with chamber were all within 5% absolute dose agreement, and film measurements all passed 2%/2mm gamma evaluation for more than 95% of measurement points.

Conclusion:

The presented results are the first set of data reported on the InCise™ MLC. The MLC proved to be very reliable and is currently in clinical use.
SU‐E‐T‐110: An Investigation On Monitor Unit Threshold and Effects On IMPT Delivery in Proton Pencil Beam Planning System
Authors: Syh J , Ding X , Syh J , Patel B , Rosen L , Wu H
Published: June 2015

Purpose:

An approved proton pencil beam scanning (PBS) treatment plan might not be able to deliver because of existed extremely low monitor unit per beam spot. A dual hybrid plan with higher efficiency of higher spot monitor unit and the efficacy of less number of energy layers were searched and optimized. The range of monitor unit threshold setting was investigated and the plan quality was evaluated by target dose conformity.

Methods:

Certain limitations and requirements need to be checks and tested before a nominal proton PBS treatment plan can be delivered. The plan needs to be met the machine characterization, specification in record and verification to deliver the beams. Minimal threshold of monitor unit, e.g. 0.02, per spot was set to filter the low counts and plan was re‐computed. Further MU threshold increment was tested in sequence without sacrificing the plan quality. The number of energy layer was also alternated due to elimination of low count layer(s).

Results:

Minimal MU/spot threshold, spot spacing in each energy layer and total number of energy layer and the MU weighting of beam spots of each beam were evaluated. Plan optimization between increases of the spot MU (efficiency) and less energy layers of delivery (efficacy) was adjusted. 5% weighting limit of total monitor unit per beam was feasible. Scarce spreading of beam spots was not discouraging as long as target dose conformity within 3% criteria.

Conclusion:

Each spot size is equivalent to the relative dose in the beam delivery system. The energy layer is associated with the depth of the targeting tumor. Our work is crucial to maintain the best possible quality plan. To keep integrity of all intrinsic elements such as spot size, spot number, layer number and the carried weighting of spots in each layer is important in this study.
SU‐E‐T‐118: Analysis of Variability and Stability Between Two Water Tank Phantoms Utilizing Water Tank Commissioning Procedures
Authors: Roring J , Saenz D , Cruz W , Papanikolaou N , Stathakis S
Published: June 2015

Purpose:

The commissioning criteria of water tank phantoms are essential for proper accuracy and reproducibility in a clinical setting. This study outlines the results of mechanical and dosimetric testing between PTW MP3‐M water tank system and the Standard Imaging Doseview 3D water tank system.

Methods:

Measurements were taken of each axis of movement on the tank using 30 cm calipers at 1, 5, 10, 50, 100, and 200 mm for accuracy and reproducibility of tank movement. Dosimetric quantities such as percent depth dose and dose profiles were compared between tanks using a 6 MV beam from a Varian 23EX LINAC. Properties such as scanning speed effects, central axis depth dose agreement with static measurements, reproducibility of measurements, symmetry and flatness, and scan time between tanks were also investigated.

Results:

Results showed high geometric accuracy within 0.2 mm. Central axis PDD and in‐field profiles agreed within 0.75% between the tanks. These outcomes test many possible discrepancies in dose measurements across the two tanks and form a basis for comparison on a broader range of tanks in the future.

Conclusion:

Both 3D water scanning phantoms possess a high degree of spatial accuracy, allowing for equivalence in measurements regardless of the phantom used. A commissioning procedure when changing water tanks or upon receipt of a new tank is nevertheless critical to ensure consistent operation before and after the arrival of new hardware.
SU‐E‐T‐123: Anomalous Altitude Effect in Permanent Implant Brachytherapy Seeds
Authors: Watt E , Spencer DP , Meyer T
Published: June 2015

Purpose:

Permanent seed implant brachytherapy procedures require the measurement of the air kerma strength of seeds prior to implant. This is typically accomplished using a well‐type ionization chamber. Previous measurements (Griffin et al., 2005; Bohm et al., 2005) of several low‐energy seeds using the air‐communicating HDR 1000 Plus chamber have demonstrated that the standard temperature‐pressure correction factor, PTP, may overcompensate for air density changes induced by altitude variations by up to 18%. The purpose of this work is to present empirical correction factors for two clinically‐used seeds (IsoAid ADVANTAGE™ 103Pd and Nucletron selectSeed 125I) for which empirical altitude correction factors do not yet exist in the literature when measured with the HDR 1000 Plus chamber.

Methods:

An in‐house constructed pressure vessel containing the HDR 1000 Plus well chamber and a digital barometer/thermometer was pumped or evacuated, as appropriate, to a variety of pressures from 725 to 1075 mbar. Current measurements, corrected with PTP, were acquired for each seed at these pressures and normalized to the reading at ‘standard’ pressure (1013.25 mbar).

Results:

Measurements in this study have shown that utilization of PTP can overcompensate in the corrected current reading by up to 20% and 17% for the IsoAid Pd‐103 and the Nucletron I‐125 seed respectively. Compared to literature correction factors for other seed models, the correction factors in this study diverge by up to 2.6% and 3.0% for iodine (with silver) and palladium respectively, indicating the need for seed‐specific factors.

Conclusion:

The use of seed specific altitude correction factors can reduce uncertainty in the determination of air kerma strength. The empirical correction factors determined in this work can be applied in clinical quality assurance measurements of air kerma strength for two previously unpublished seed designs (IsoAid ADVANTAGE™ 103Pd and Nucletron selectSeed 125I) with the HDR 1000 Plus well chamber.
SU‐E‐T‐124: Anthropomorphic Phantoms for Confirmation of Linear Accelerator Based Small Animal Irradiation
Authors: Perks J , Lucero S , Benedict S
Published: June 2015

Purpose:

To document the support of radiobiological small animal research by a modern radiation oncology facility. This study confirms that a standard, human use linear accelerator can cover the range of experiments called for by researchers performing animal irradiation. A number of representative, anthropomorphic murine phantoms were made. The phantoms confirmed the small field photon and electron beams dosimetry validated the use of the linear accelerator for rodents.

Methods:

Laser scanning a model, CAD design and 3D printing produced the phantoms. The phantoms were weighed and CT scanned to judge their compatibility to real animals. Phantoms were produced to specifically mimic lung, gut, brain, and othotopic lesion irradiations. Each phantom was irradiated with the same protocol as prescribed to the live animals. Delivered dose was measured with small field ion chambers, MOS/FETs or TLDs.

Results:

The density of the phantom material compared to density range across the real mice showed that the printed material would yield sufficiently accurate measurements when irradiated. The whole body, lung and gut irradiations were measured within 2% of prescribed doses with A1SL ion chamber. MOSFET measurements of electron irradiations for the orthotopic lesions allowed refinement of the measured small field output factor to better than 2% and validated the immunology experiment of irradiating one lesion and sparing another.

Conclusion:

Linacs are still useful tools in small animal bio‐radiation research. This work demonstrated a strong role for the clinical accelerator in small animal research, facilitating standard whole body dosing as well as conformal treatments down to 1cm field. The accuracy of measured dose, was always within 5%. The electron irradiations of the phantom brain and flank tumors needed adjustment; the anthropomorphic phantoms allowed refinement of the initial output factor measurements for these fields which were made in a large block of solid water.
SU‐E‐T‐125: Application of Jaw‐Tracking Function in VMAT for Upper Thoracic Esophageal Cancer
Authors: Zhang W , Chen J , Zhai T , Yan L , Chen C
Published: June 2015

Purpose:

To explore the effect of the Jaw‐tracking with RapidArc(JT‐RapidArc) plans for upper thoracic esophageal cancer.

Methods:

Treatment planning was designed by using RapidArc and JT‐RapidArc techniques for 11 consecutive patients. The dose‐volume histogram parameters of PTV and the organs at risk(OAR), conformity index(CI), heterogeneity index(HI), low dose volume of normal tissue(B‐P) and monitor units(MUs) were compared between the different techniques.

Results:

JT‐RapidArc plans provided the better coverage of PTV1(64) D98 and HI(P<0.05), lower MLD, D2 of PTV1(64) and PTV2(54), but no statistically difference in CI(P>0.05), which comparison with RapidArc plans. Plans with JT‐ RapidArc had lower Lung of V5, V10, V13, V20, V30, MLD(P<0.05); heart of V20, MLD(P<0.05); and B‐P of V5, V10, V15, V20, V30(P<0.05); but no significantly different in Spinal cord and SC‐PRV as compared with RapidArc plans. JT‐RapidArc plans increaseed the MUs by 1%(P>0.05) as compared with RapidArc plans.

Conclusion:

All of the plans had met the requirements of clinical dosimetry. JT‐RapidArc plans as compared with RapidArc plans, showing better part of target coverage, part of OARS(lung and heart) and heart and B‐P sparing, which MUs was slightly increased.

This work was sponsored by Shantou University Medical College Clinical Research Enhancement Initiative(NO.201424)
SU‐E‐T‐157: CARMEN: A MatLab‐Based Research Platform for Monte Carlo Treatment Planning (MCTP) and Customized System for Planning Evaluation
Authors: Baeza J.A. , Ureba A. , Jimenez‐Ortega E. , Barbeiro A.R. , Lagares J.I. , Plaza A. Leal
Published: June 2015

Purpose:

Although there exist several radiotherapy research platforms, such as: CERR, the most widely used and referenced; SlicerRT, which allows treatment plan comparison from various sources; and MMCTP, a full MCTP system; it is still needed a full MCTP toolset that provides users complete control of calculation grids, interpolation methods and filters in order to “fairly” compare results from different TPSs, supporting verification with experimental measurements.

Methods:

This work presents CARMEN, a MatLab‐based platform including multicore and GPGPU accelerated functions for loading RT data; designing treatment plans; and evaluating dose matrices and experimental data.CARMEN supports anatomic and functional imaging in DICOM format, as well as RTSTRUCT, RTPLAN and RTDOSE. Besides, it contains numerous tools to accomplish the MCTP process, managing egs4phant and phase space files.CARMEN planning mode assist in designing IMRT, VMAT and MERT treatments via both inverse and direct optimization. The evaluation mode contains a comprehensive toolset (e.g. 2D/3D gamma evaluation, difference matrices, profiles, DVH, etc.) to compare datasets from commercial TPS, MC simulations (i.e. 3ddose) and radiochromic film in a user‐controlled manner.

Results:

CARMEN has been validated against commercial RTPs and well‐established evaluation tools, showing coherent behavior of its multiple algorithms. Furthermore, CARMEN platform has been used to generate competitive complex treatment that has been published in comparative studies.

Conclusion:

A new research oriented MCTP platform with a customized validation toolset has been presented. Despite of being coded with a high‐level programming language, CARMEN is agile due to the use of parallel algorithms. The wide‐spread use of MatLab provides straightforward access to CARMEN's algorithms to most researchers. Similarly, our platform can benefit from the MatLab community scientific developments as filters, registration algorithms etc. Finally, CARMEN arises the importance of grid and filtering control in treatment plan comparison.
SU‐E‐T‐315: Dosimetric Effects of Couch Top Shift On VMAT Delivery in Absence of Indexing
Authors: Islam M , Jin H , Ferguson S , Ahmad S
Published: June 2015

Purpose:

To investigate dosimetric effects of couch top shift for volumetric‐modulated arc therapy (VMAT) in absence of indexing of immobilization devices.

Methods:

A total of twelve VMAT treatment plans were selected from three regions (lung, abdomen, and pelvis) to account for the variation of the patient position relative to the couch top. The treatment plans were generated using the Varian Eclipse system. A pinpoint ionization chamber (PTW TN31014) was placed at the center of 16‐cm solid water phantom and the dose was delivered using the Varian TrueBeam STx with BrainLAB ExacTrac couch top. To simulate the day‐to‐day variation of the patient position relative to couch top, the couch top was laterally shifted up to 50 mm, with an increment of 5 mm from 0 to 20 mm; and of 10 mm afterwards, and the phantom was moved back to 0 cm shift for measurement. The dose was also delivered using a Varian tennis racket grid insert at 0 cm shift to simulate the absence of couch top. The treatment plans were delivered with 6, 10, and 15 MV photons using the same leaf sequencing to investigate the energy dependence of couch top shift. The dose difference was normalized to 0 cm shift for the regular couch top for comparison.

Results:

The percent difference of dose was found to increase with lateral shift for all energies; however, the average differences were close to 0% and the maximum difference was within 1% along the lateral shifts. The differences with the absence of couch top were 2.2±0.5% (6MV), 1.7±0.3% (10MV), and 1.6±0.2% (15MV), respectively.

Conclusion:

The inclusion of couch top is recommended in treatment planning to minimize the dosimetric uncertainty between calculated and delivered dose even in absence of indexing of immobilization devices in VMAT delivery.
SU‐E‐T‐321: Dosimetric Evaluation of Non‐Coplanar Arcs in VMAT Planning for SBRT Lung Cases: A Planning Study
Authors: Lu S , Gardner S , Doemer A , Chetty I
Published: June 2015

Purpose:

Investigate use of standardized non‐coplanar arcs to improve plan quality in lung Stereotactic Body Radiation Therapy(SBRT) VMAT planning.

Methods:

VMAT planning was performed for 9 patients previously treated with SBRT for peripheral lung tumors (tumor size:12.7cc to 32.5cc). For each patient, 7 VMAT plans (couch rotation values:0,5,10,15,20,25,and 30 deg) were generated; the coplanar plans were pushed to meet the RTOG0915 constraints and each non‐coplanar plans utilized the same optimization constraints. The following plan dose metrics were used (taken from RTOG 0915): D‐2cm: the maximum dose at 2 cm from the PTV, conformality index (CI), gradient index (GI), lung volume receiving 5 Gy (V5) and lung volume receiving 20 Gy (V20). The couch collision clearance was checked for each plan through a dry run using the couch position from the patient's treatment.

Results:

Of the 9 cases, one coplanar plan failed to meet two protocol guidelines (both gradient index and D‐2cm parameter), and an additional plan failed the D‐2cm parameter. When introducing at least 5 degree couch rotation, all plans met the protocol guidelines. The largest feasible couch angle available was 15 to 20 degrees due to gantry collision issues. Non‐coplanar plans resulted in the average (standard deviation) reduction of the following metrics: GI by 7.3% (3.7%); lung V20 by 11.1% (3.2%); D‐2cm by 12.7% (3.9%). The CI was unchanged (−0.3%±0.6%), and lung V5 increased (3.8%±8.2%).

Conclusion:

The use of couch rotations as little as 5 degrees allows for plan quality that will meet RTOG0915 constraints while reducing D‐2cm, GI, and lung V20. Using default couch rotations while planning SBRT cases will allow for more efficient planning with the stated goal of meeting RTOG0915 criteria for all clinical cases. Gantry clearance checks in the treatment room may be necessary to ensure safe treatments for larger couch rotation values.
SU‐E‐T‐333: Dosimetric Impact of Rotational Error On the Target Coverage in IMPT Lung Cancer Plans
Authors: Rana S , Zheng Y
Published: June 2015

Purpose:

The main purpose of this study was to investigate the impact of rotational (yaw, roll, and pitch) error on the planning target volume (PTV) coverage in lung cancer plans generated by intensity modulated proton therapy (IMPT).

Methods:

In this retrospective study, computed tomography (CT) dataset of previously treated lung case was used. IMPT plan were generated on the original CT dataset using left‐lateral (LL) and posterior‐anterior (PA) beams for a total dose of 74 Gy[RBE] with 2 Gy[RBE] per fraction. In order to investigate the dosimetric impact of rotational error, 12 new CT datasets were generated by re‐sampling the original CT dataset for rotational (roll, yaw, and pitch) angles ranged from −5° to +5°, with an increment of 2.5°. A total of 12 new IMPT plans were generated based on the re‐sampled CT datasets using beam parameters identical to the ones in the original IMPT plan. All treatment plans were generated in XiO treatment planning system. The PTV coverage (i.e., dose received by 95% of the PTV volume, D95) in new IMPT plans were then compared with the PTV coverage in the original IMPT plan.

Results:

Rotational errors caused the reduction in the PTV coverage in all 12 new IMPT plans when compared to the original IMPT lung plan. Specifically, the PTV coverage was reduced by 4.94% to 50.51% for yaw, by 4.04% to 23.74% for roll, and by 5.21% to 46.88% for pitch errors.

Conclusion:

Unacceptable dosimetric results were observed in new IMPT plans as the PTV coverage was reduced by up to 26.87% and 50.51% for rotational error of 2.5° and 5°, respectively. Further investigation is underway in evaluating the PTV coverage loss in the IMPT lung cancer plans for smaller rotational angle change.
SU‐E‐T‐351: Effects of Isocenter Location, MLC Aperture Angle and Gantry Angle On Dose Distribution
Authors: Li R , Xue T , Song C , Xue C
Published: June 2015

Purpose:

Single‐isocenter‐multiple‐target arc plans will have some of the targets displaced from the isocenter. This is a study of the effects of target size, the distance between the isocenter and a target, how many degrees of gantry angle per multileaf collimator (MLC) apertures, and how many degrees of gantry angle per calculation.

Methods:

Dynamic arc plans are generated with the isocenter placed at 0, 5 and 10 cm from the center of the targets with sizes of 1, 2 and 3 cm in Head/Neck phantom CT images respectively. Each arc plan has a 60 degree gantry rotation, calculated with gantry angle resolution of 1, 2 and 5 degrees, and MLC aperture resolution of 1, 2, 5 and 6 degrees. Some Static‐field plans are generated with static fields having gantry angle changes every 1 or 2 degrees and MLC aperture changing every 2 or 6 degrees to simulate actual LINAC delivery situation of arc plans of same gantry and MLC aperture resolutions.

Results:

When the isocenter is placed inside the target, all plans are identical regardless of target size, gantry resolution and MLC aperture resolution. When the isocenter is placed a certain distance outside the target center, the aforementioned parameters will affect the plan results. Decreasing target size and increasing distance from the isocenter will increase the effects. For a 1 cm the target with isocenter 10 cm away, the differences between the arc plan and the static‐field plan are: maximal dose 12.8%, minimal dose 24% and MU 14.9%.

Conclusion:

Arc plan calculation interpolates MLC apertures. This interpolation will cause dose discrepancy between the arc plan and actual delivery when the target size is small and is away from the isocenter. Single‐isocenter‐multiple‐target arc plan should be calculated with finest MLC aperture resolutions and finest gantry angle resolutions to optimize the plan.
SU‐E‐T‐352: Effects of Skull Attenuation and Missing Backscatter On Brain Dose in HDR Treatment of the Head with Surface Applicators
Authors: Cifter F , Dhou S , Altundal Y , Sajo E , Lewis J , Cormack R
Published: June 2015

Purpose:

To calculate the effect of lack of backscatter from air and attenuation of bone on dose distributions in brachytherapy surface treatment of head. Existing treatment planning systems based on TG43 do not account for heterogeneities, and thus may overestimate the dose to the brain. While brachytherapy generally has rapid dose falloff, the dose to the deeper tissues (in this case, the brain) can become significant when treating large curved surfaces.

Methods:

Applicator geometries representing a range of clinical cases were simulated in MCNP5. An Ir‐192 source was modeled using the energy spectrum presented by TG‐43. The head phantom was modeled as a 7.5‐cm radius water sphere, with a 7 ‐mm thick skull embedded 5‐mm beneath the surface. Dose values were calculated at 20 points inside the head, in which 10 of them were on the central axis and the other 10 on the axis connecting the central of the phantom with the second to last source from the applicator edge.

Results:

Central and peripheral dose distributions for a range of applicator and head sizes are presented. The distance along the central axis at which the dose falls to 80% of the prescribed dose (D80) was 7 mm for a representative small applicator and 9 mm for a large applicator. Corresponding D50 and D30 for the same small applicator were 17 mm and 32 mm respectively. D50 and D30 for the larger applicator were 32 mm and 60 mm respectively. These results reflect the slower falloff expected for larger applicators on a curved surface.

Conclusion:

Our results can provide guidance for clinicians to calculate the dose reduction effect due to bone attenuation and the lack of backscatter from air to estimate the brain dose for the HDR treatments of surface lesions.
SU‐E‐T‐353: Effects of Time and Temperature On a Potential Reusable 3D Dosimeter
Authors: Juang T , Miles D , Crockett E , Adamovics J , Oldham M
Published: June 2015

Purpose:

Preliminary studies of a novel, optically‐clearing PRESAGE 3D dosimeter formulation (Presage‐RU) demonstrated potential reusability. This study investigates the effects of time and temperature on the accuracy and reusability of Presage‐RU, and reports on progress toward developing a reusable 3D dosimeter.

Methods:

Presage‐RU was cast as small volume samples (1×1×4.5cm). The effect of dose response sensitivity with reirradiation and time was evaluated by irradiating samples from 0–10Gy, measuring change in optical density (ΔOD), clearing at room temperature (RT) (5–7 days to fully clear), and then repeating for a total of 5 irradiations. Effects of heating on clearing rate were investigated by irradiating samples to 8Gy, then tracking measurements with samples held at RT, 35°C, and 45°C. Two cylindrical dosimeters (11cm diameter, 9.5cm length) were evaluated for dosimetric accuracy when stored at RT and −3°C prior to irradiation. Plans delivered were 2 overlapping AP fields (RT) and VMAT (‐3°C).

Results:

Heating the dosimeters reduced the clearing half‐life from 16.3h at RT to 5.8h (35°C) and 5.1h (45°C), but also increased background ΔOD by 1.7x (35°C) and 2.3x (45°C). Reductions in dose response were more closely linked to age than reirradiation, and storage at RT showed pronounced desensitization from dosimeter edges. These results suggest desensitization from oxygen diffusion. It should be noted that atmospheric diffusion into the dosimeter is not seen in standard, single‐use PRESAGE, and is likely caused by differences in the Presage‐RU polyurethane matrix. The dosimeter kept in cold storage, however, showed no evidence of desensitization and exhibited accuracy on par with standard PRESAGE with a 3%/3mm 3D gamma passing rate of 98.1%.

Conclusions:

Presage‐RU is sensitive to storage temperatures and time, both of which affect oxygen diffusion and subsequent desensitization. Development shows promising progress with further formulation optimization as the next step toward achieving a successful reusable 3D dosimeter.

This work was supported by NIH R01CA100835. John Adamovics is the president of Heuris Inc., which commercializes PRESAGE.
SU‐E‐T‐361: Energy Dependent Radiation/light‐Field Misalignment On Truebeam Linear Accelerator
Authors: Sperling N , Tanny S , Parsai E
Published: June 2015

Purpose:

Verifying the co‐incidence of the radiation and light field is recommended by TG‐142 for monthly and annual checks. On a digital accelerator, it is simple to verify that beam steering settings are consistent with accepted and commissioned values. This fact should allow for physicists to verify radiation‐light‐field co‐incidence for a single energy and accept that Result for all energies. We present a case where the radiation isocenter deviated for a single energy without any apparent modification to the beam steering parameters.

Methods:

The radiation isocenter was determined using multiple

Methods:

Gafchromic film, a BB test, and radiation profiles measured with a diode. Light‐field borders were marked on Gafchromic film and then irradiated for all photon energies. Images of acceptance films were compared with films taken four months later. A phantom with a radio‐opaque BB was aligned to isocenter using the light‐field and imaged using the EPID for all photon energies. An unshielded diode was aligned using the crosshairs and then beam profiles of multiple field sizes were obtained. Field centers were determined using Omni‐Pro v7.4 software, and compared to similar scans taken during commissioning. Beam steering parameter files were checked against backups to determine that the steering parameters were unchanged.

Results:

There were no differences between the configuration files from acceptance. All three tests demonstrated that a single energy had deviated from accepted values by 0.8 mm in the inline direction. The other two energies remained consistent with previous measurements. The deviated energy was re‐steered to be within our clinical tolerance.

Conclusions:

Our study demonstrates that radiation‐light‐field coincidence is an energy dependent effect for modern linacs. We recommend that radiation‐light‐field coincidence be verified for all energies on a monthly basis, particularly for modes used to treat small fields, as these may drift without influencing results from other tests.
SU‐E‐T‐367: Evaluate the Dosimetric Impacts On Patient Specific Treatment Plans Due to Set Up Uncertainties During LINAC Annual QA and Commission
Authors: Nguyen V , Shi C , Wang B
Published: June 2015

Purpose:

To quantify the dosimetric impact on patient's specific treatment plans due to set up uncertainties during LINAC commission and annual QA and to determine the maximum set up uncertainty allowance range.

Methods:

A 60×60×60 cm3 solid water cube was created in Varian Eclipse TPS. Beam data profiles (crossline and diagonal) and PDDs for field sizes ranging from 2×2 cm2 to 40×40 cm2 were simulated. Three main uncertainty scenarios were purposely introduced for gantry position tilts (0–5°), source axis distance changes (100–105 cm), and iso‐center position shifts (0–5 mm) during the simulation. A gamma analysis was used to compare the correct simulated profiles with the profiles for each scenario. Two static IMRT treatment plans (H&N and GYN) with tumors at 5 cm and 15 cm depths were compared using similar set up uncertainties.

Results:

A gamma analysis using ±3%/±3mm with 90% passing rate criteria is included to show the passing rate for each scenario. Crossline and diagonal profiles showed a gamma passing rating of ≥ 90% at depth ≤10 cm for these scenarios: gantry tilted from 0–5°, SAD changed from 100–105 cm, and iso‐center shifted ≤ 4 mm. From 10 to 20 cm depths, all three scenarios failed with gamma passing ≤ 90% excepted for diagonal profiles at Gantry =2°, SAD =1 cm, and iso‐center =1 mm off center. Diagonal profiles showed a higher gamma passing rating compared to crossline profiles for all three scenarios. PDD differences also increased as depth increased. For patient's specific treatment plans, maximum uncertainties allowed to obtain a ≥90% gamma passing rating are: gantry tilts ±1 degree, SAD shifts ±2 cm, and iso‐center moves ±3 mm.

Conclusion:

This study validated AAPM TG 142 recommendations on the mechanical and dosimetry uncertainties and provided proofs on maximum acceptance tolerances for LINAC annual QA and commission.
SU‐E‐T‐371: Evaluating the Convolution Algorithm of a Commercially Available Radiosurgery Irradiator Using a Novel Phantom
Authors: Cates J , Drzymala R
Published: June 2015

Purpose:

The purpose of this study was to develop and use a novel phantom to evaluate the accuracy and usefulness of the Leskell Gamma Plan convolution‐based dose calculation algorithm compared with the current TMR10 algorithm.

Methods:

A novel phantom was designed to fit the Leskell Gamma Knife G Frame which could accommodate various materials in the form of one inch diameter, cylindrical plugs. The plugs were split axially to allow EBT2 film placement. Film measurements were made during two experiments. The first utilized plans generated on a homogeneous acrylic phantom setup using the TMR10 algorithm, with various materials inserted into the phantom during film irradiation to assess the effect on delivered dose due to unplanned heterogeneities upstream in the beam path. The second experiment utilized plans made on CT scans of different heterogeneous setups, with one plan using the TMR10 dose calculation algorithm and the second using the convolution‐based algorithm. Materials used to introduce heterogeneities included air, LDPE, polystyrene, Delrin, Teflon, and aluminum.

Results:

The data shows that, as would be expected, having heterogeneities in the beam path does induce dose delivery error when using the TMR10 algorithm, with the largest errors being due to the heterogeneities with electron densities most different from that of water, i.e. air, Teflon, and aluminum. Additionally, the Convolution algorithm did account for the heterogeneous material and provided a more accurate predicted dose, in extreme cases up to a 7–12% improvement over the TMR10 algorithm. The convolution algorithm expected dose was accurate to within 3% in all cases.

Conclusion:

This study proves that the convolution algorithm is an improvement over the TMR10 algorithm when heterogeneities are present. More work is needed to determine what the heterogeneity size/volume limits are where this improvement exists, and in what clinical and/or research cases this would be relevant.
SU‐E‐T‐487: In VMAT of Spine Stereotactic Radiosurgery a 1 Mm Grid Size Increases Dose Gradient and Lowers Cord Dose Significantly Relative to a 2.5 Mm Grid Size
Authors: Liu M , Snyder K , Zhao B , Wen N , Huang Y , Song K , Li H , Kim J , Chetty I , Siddiqui S
Published: June 2015

Purpose:

Sharp dose gradients between the target and the spinal cord are critical to achieve dose constraints in spine stereotactic radiosurgery (SRS). In this study, the volume averaging effect of grid size (GS) on the dosimetric accuracy of volumetric modulated arc therapy (VMAT) spine SRS plans was investigated.

Methods:

The Eclipse v11.0 Anisotropic Analytical Algorithm (AAA) algorithm was used for dose calculation. Plan qualities of 10 treatment plans were evaluated with GS of 2.5mm (AAA's default value) and 1mm. All plans were prescribed 18Gy to the 90% isodose line. Parameters used for comparison included the distance between 18Gy and 10Gy isodose levels in the axial plane, maximum cord dose (Dmax, defined as dose to 0.035cc), dose to 10% of the cord (D10%) and 0.35cc of the cord (D0.35cc), film gamma pass rate (3%, 1mm), film line profile through the cord, and calculation time. Paired t‐test was used to investigate the statistical significance.

Results:

The 18Gy−10Gy distance was shorter for all plans with 1mm compared to 2.5mm GS (0.32±0.07mm vs. 0.38±0.07mm, p<0.001). In addition, 1mm GS plans showed lower cord Dmax (11.11±1.74Gy vs. 11.96±1.6Gy, p<0.001), D10% (8.26±1.08Gy vs. 9.11±1.18Gy, p<0.001) and D0.35cc (8.55±1.28Gy vs. 9.43±1.33Gy, p<0.001). Film analysis demonstrated better agreement in the high dose gradient near the cord. Calculation times for 1mm GS plans increased significantly (14:00 vs. 2:30, p<0.001).

Conclusion:

Due to more accurate dose calculations at the sharp dose gradient near the cord, we recommend the use of 1mm grid size for final dose calculation for VMAT SRS spine plans. This not only leads to more accurate dose gradient calculation near the cord, and lower spinal cord dose, but also, affords more room to balance between the needs of PTV coverage and cord sparing.
SU‐E‐T‐495: Influence of Reduced Target‐To‐Nozzle Distance On Secondary Neutron Dose Equivalent in Proton and Carbon Ion Radiotherapy
Authors: Sheng Y , Shahnazi K , Wang W , Moyers M , Deng Y , Huang Z , Liu X
Published: June 2015

Purpose:

Ion beams have an unavoidable lateral spread due to nuclear interactions interacting with the air and monitoring systems. To minimize this spread, the distance between the nozzle and the patient should be kept as small as possible.The purpose of this work was to determine the impact of the target‐to‐nozzle distance reduction on the secondary neutron dose equivalent in proton and carbon ion radiotherapy.

Methods:

In this study, abdominal and head phantoms were scanned with our CT scanner. Cubical targets with side lengths of 3 cm to 10 cm and 1 cm to 5 cm were drawn in the abdominal and head phantoms respectively. Two intensity‐modulated plans were made for each phantom and ion. The first of these plans placed the target at the isocenter while the other shifted the phantom 30 cm towards the nozzle. The plans at both phantom locations were optimized to provide identical dose coverage to the PTVs.Secondary neutron dose equivalent at 50 cm lateral to the center of target.

Results:

The neutron dose equivalent was higher for the larger field size from 0.25µSv per Gy (RBE) to 72µSv per Gy (RBE). The neutron dose equivalent was smaller when the phantom was placed at the upstream target location versus at the isocenter location by 8.9% to 10.4% and 11.0% to 22.1% for proton plans of the abdominal and head phantoms respectively. Differences for carbon plans with different target‐to‐nozzle locations were less than 3% for both phantoms.

Conclusion:

A reduction of target‐to‐nozzle distance can lead to benefits for proton radiotherapy. In this study, a reduction of secondary neutron dose equivalent was found for proton plans with a smaller target‐to‐nozzle distance. A greater impact was found for a head phantom with a smaller field size; however, a reduction of the target‐to‐nozzle distance had little effect for carbon therapy.