bluuebunny/crossref_metadata_2025_split huggingface dataset:
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 multipleMethods: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.