bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐F‐I‐75: Half‐Value Layer Thicknesses and Homogeneity Coefficients for Fluoroscopic X‐Ray Beam Spectra Incorporating Spectral Filtration
Authors: Wunderle K , Godley A , Shen Z , Rakowski J , Dong F
Published: July 2016

Purpose:

The purpose of this investigation is to quantify various first half‐value‐layers (HVLs), second HVLs and homogeneity coefficients (HCs) for a state‐of‐the‐art fluoroscope utilizing spectral (copper) filtration.

Methods:

A Radcal (Monrovia, Ca) AccuPro dosimeter with a 10×6‐6 calibrated ionization chamber was used to measure air kerma for radiographic x‐ray exposures made on a Siemens (Erlangen, Germany) Artis ZeeGo fluoroscope operated in the service mode. The ionization chamber was centered in the x‐ray beam at 72 cm from the focal spot with a source‐to‐image‐distance of 120 cm. The collimators were introduced to limit the x‐ray field to approximately 5 cm × 5 cm at the ionization chamber plane. Type‐1100 aluminum filters, in 0.5 mm increments, were used to determine the HVL. Two HVL calculation methods were used, log‐linear interpolation and Lambert‐W interpolation as described by Mathieu [Med Phys, 38(8), 4546 (2011)]. Multiple measurements were made at 60, 80, 100, 120 kVp at spectral filtration thicknesses of 0, 0.1, 0.3, 0.6 and 0.9 mm.

Results:

First HVL, second HVL, and HCs are presented for the fluoroscopic x‐ray beam spectra indicated above, with nearly identical results from the two interpolation methods. Accuracy of the set kVp was also determined and deviated less than 2%. First HVLs for fluoroscopic x‐ray beam spectra without spectral filtration determined in our study were 7%–16% greater than previously published data by Fetterly et al. [Med Phys, 28, 205 (2001)]. However, the FDA minimum HVL requirements changed since that publication, requiring larger HVLs as of 2006. Additionally, x‐ray tube and generator architecture have substantially changed over the last 15 years providing different beam spectra.

Conclusion:

X‐ray beam quality characteristics for state‐of‐the‐art fluoroscopes with spectral filtration have not been published. This study provides reference data which will be useful for defining beam qualities encountered on fluoroscopes using spectral filtration.
SU‐F‐I‐79: Validation of a Monte Carlo Radiographic Simulation
Authors: Gearhart A , Carver D , Stabin M
Published: July 2016

Purpose:

To validate a radiographic simulation in order to estimate patient dose due to clinically‐used radiography protocols.

Methods:

A Monte Carlo simulation was created to simulate a radiographic x‐ray beam using GEANT4. Initial validation was performed according to a portion of TG 195. Computational NURBS‐based phantoms were used simulate patients of varying ages and sizes. The deposited energy in the phantom is output by the simulation. The exposure in air from a clinically used radiography unit was measured at 100 cm for various tube potentials. 10 million photons were simulated with 1 cubic centimeter of air located 100 cm from the source, and the total absorbed dose was noted. The normalization factor was determined by taking a ratio of the measured dose in air to the simulated dose in air. Dose to individual voxels is calculated using the energy deposition map along with the voxelized and segmented phantom and the normalization factor. Finally, the effective dose is calculated using the ICRP methodology and tissue weighting factors.

Results:

This radiography simulation allows for the calculation and visualization of the energy deposition map within a voxelized phantom. The ratio of exposure, measured using an ionization chamber, to air in the simulation was determined. Since the simulation output is calibrated to match the exposure of a given clinical radiographic x‐ray tube, the dose map may be visualized. This will also allow for absorbed dose estimation in specific organs or tissues as well as a whole body effective dose estimation.

Conclusion:

This work indicates that our Monte Carlo simulation may be used to estimate the radiation dose from clinical radiographic protocols. This will allow for an estimate of radiographic dose from various examinations without the use of traditional methods such as thermoluminescent dosimeters and body phantoms.
SU‐F‐J‐02: Flexible Training of MR‐Guided Laser Ablation Models Via Global Optimization
Authors: Fahrenholtz SJ , Stafford RJ , Madankan R , Hazle JD , Fuentes D
Published: July 2016

Purpose:

MR‐guided laser ablation is currying interest in neurosurgery as a minimally invasive procedure. The procedure is planned via an assumed lesion overlaid on various anatomic and functional images. Computational models of heat transfer may further inform planning the intervention, reducing surgical morbidity and ensuring therapeutic effect. These models require physical parameter data, via less relevant literature values. Using commercialized laser ablation equipment combined with MR temperature imaging (MRTI) feedback, recent work has demonstrated the use of inverse problems as a means to gain the necessary information via retrospective analysis. Here, performance of a global inverse problem solution is explored that allows for simultaneous optimization of multiple objective functions.

Methods:

A steady state bioheat model is trained on MRTI patient datasets (n=20); the MRTI regions exceeding a 57°C threshold are considered lethally damaged. Optical and blood perfusion parameters are densely sampled, allowing for global optimization of the model. There are two objective functions to optimize with respect to damage prediction: Dice similarity coefficient (DSC) and Hausdorff distance (HD). DSC is an overlap metric; HD is the distance‐to‐agreement in physical distance. Predictive performance is simulated via leave‐one‐out cross‐validation (LOOCV) and is compared to the same model using naive literature values.

Results:

Post‐optimization and during LOOCV, mean and median DSC are 0.824 and 0.850 respectively; mean and median HD are 4.64mm and 4.03mm respectively. These compare favorably to the model using literature values. Mean and median DSC are 0.6685 and 0.6564 respectively; mean and median HD are 5.85mm and 5.58mm respectively. A two‐tailed paired Student's t‐test for the mean DSC reported p=0.0002. For mean HD, p=0.0064.

Conclusion:

Clinical MRTI feedback provides useful information for designing optimized laser ablation models. Global optimization facilitates simultaneous optimization of multiple objective functions. Trained models are considerably more accurate than naive literature based models as measured by DSC and HD.
SU‐F‐J‐14: Kilovoltage Cone‐Beam CT Dose Estimation of Varian On‐Board Imager Using GMctdospp Monte Carlo Framework
Authors: Kim S , Rangaraj D
Published: July 2016

Purpose:

Although cone‐beam CT (CBCT) imaging became popular in radiation oncology, its imaging dose estimation is still challenging. The goal of this study is to assess the kilovoltage CBCT doses using GMctdospp ‐ an EGSnrc based Monte Carlo (MC) framework.

Methods:

Two Varian OBI x‐ray tube models were implemented in the GMctpdospp framework of EGSnrc MC System. The x‐ray spectrum of 125 kVp CBCT beam was acquired from an EGSnrc/BEAMnrc simulation and validated with IPEM report 78. Then, the spectrum was utilized as an input spectrum in GMctdospp dose calculations. Both full and half bowtie pre‐filters of the OBI system were created by using egs_prism module. The x‐ray tube MC models were verified by comparing calculated dosimetric profiles (lateral and depth) to ion chamber measurements for a static x‐ray beam irradiation to a cuboid water phantom. An abdominal CBCT imaging doses was simulated in GMctdospp framework using a 5‐year‐old anthropomorphic phantom. The organ doses and effective dose (ED) from the framework were assessed and compared to the MOSFET measurements and convolution/superposition dose calculations.

Results:

The lateral and depth dose profiles in the water cuboid phantom were well matched within 6% except a few areas ‐ left shoulder of the half bowtie lateral profile and surface of water phantom. The organ doses and ED from the MC framework were found to be closer to MOSFET measurements and CS calculations within 2 cGy and 5 mSv respectively.

Conclusion:

This study implemented and validated the Varian OBI x‐ray tube models in the GMctdospp MC framework using a cuboid water phantom and CBCT imaging doses were also evaluated in a 5‐year‐old anthropomorphic phantom. In future study, various CBCT imaging protocols will be implemented and validated and consequently patient CT images will be used to estimate the CBCT imaging doses in patients.
SU‐F‐J‐15: The Use of CBCT for Dose Calculations in H/N Patients: Impact of Limited FOV
Authors: Rosen B , Lee C , Brock K
Published: July 2016

Purpose:

To investigate the impact of limited superior and inferior (SI) field‐of‐view (FOV) associated with on‐board kV CBCT images for dose calculation.

Methods:

First, a (10×10) cm2 continuous arc 6 MV treatment plan was calculated for fourteen virtual water phantoms of varying SI thicknesses (5 to 15 cm) to replicate the FOV effect. In addition, 70 Gy VMAT plans from two H/N patients were calculated on their corresponding planning CT (pCT) and first fraction CBCT image volumes. For the phantom, the mean dose difference (full vs. limited FOV phantom) was calculated for each axial plane for each size phantom to determine the error with distance from the SI boundary, as a function of dose at the FOV edge. For the clinical cases, the absolute difference between CBCT and pCT dose was calculated for each voxel within the CBCT FOV and evaluated relative to CBCT SI boundary.

Results:

Phantom calculations showed that mean error was less than 5% for all planes whenever the CBCT border received less than 35 Gy or, otherwise, in planes at least 1.5 cm inside the CBCT SI FOV. Maximum pCT‐calculated doses differences in the axial plane of the superior CBCT FOV border was 4 Gy for Patient 1 and 55 Gy for Patient 2, and the corresponding maximum error in the same planes were 2% and 24%, respectively. Maximum pCT‐calculated dose 1 cm inferior to the superior FOV border was 7 Gy and 72 Gy, with a corresponding error of 0.5% and 2.5%.

Conclusion:

CBCT images may be used as a surrogate for full CT images with the following limitations: low dose (<50% of prescription dose) at the FOV edge, or greater than 1.5 cm material thickness between the CBCT border and the axial plane of interest.

This work was partially funded by NIH P01CA059827.
SU‐F‐J‐23: Field‐Of‐View Expansion in Cone‐Beam CT Reconstruction by Use of Prior Information
Authors: Haga A , Magome T , Nakano M , Kotoku J , Nakagawa K
Published: July 2016

Purpose:

Cone‐beam CT (CBCT) has become an integral part of online patient setup in an image‐guided radiation therapy (IGRT). In addition, the utility of CBCT for dose calculation has actively been investigated. However, the limited size of field‐of‐view (FOV) and resulted CBCT image with a lack of peripheral area of patient body prevents the reliability of dose calculation. In this study, we aim to develop an FOV expanded CBCT in IGRT system to allow the dose calculation.

Methods:

Three lung cancer patients were selected in this study. We collected the cone‐beam projection images in the CBCT‐based IGRT system (X‐ray volume imaging unit, ELEKTA), where FOV size of the provided CBCT with these projections was 410 × 410 mm2 (normal FOV). Using these projections, CBCT with a size of 728 × 728 mm2 was reconstructed by a posteriori estimation algorithm including a prior image constrained compressed sensing (PICCS). The treatment planning CT was used as a prior image. To assess the effectiveness of FOV expansion, a dose calculation was performed on the expanded CBCT image with region‐of‐interest (ROI) density mapping method, and it was compared with that of treatment planning CT as well as that of CBCT reconstructed by filtered back projection (FBP) algorithm.

Results:

A posteriori estimation algorithm with PICCS clearly visualized an area outside normal FOV, whereas the FBP algorithm yielded severe streak artifacts outside normal FOV due to under‐sampling. The dose calculation result using the expanded CBCT agreed with that using treatment planning CT very well; a maximum dose difference was 1.3% for gross tumor volumes.

Conclusion:

With a posteriori estimation algorithm, FOV in CBCT can be expanded. Dose comparison results suggested that the use of expanded CBCTs is acceptable for dose calculation in adaptive radiation therapy.

This study has been supported by KAKENHI (15K08691).
SU‐F‐J‐34: Automatic Target‐Based Patient Positioning Framework for Image‐Guided Radiotherapy in Prostate Cancer Treatment
Authors: Sasahara M , Arimura H , Shibayama Y , Hirose T , Ohga S , Umezu Y , Honda H , Sasaki T
Published: July 2016

Purpose:

Current image‐guided radiotherapy (IGRT) procedure is bonebased patient positioning, followed by subjective manual correction using cone beam computed tomography (CBCT). This procedure might cause the misalignment of the patient positioning. Automatic target‐based patient positioning systems achieve the better reproducibility of patient setup. Our aim of this study was to develop an automatic target‐based patient positioning framework for IGRT with CBCT images in prostate cancer treatment.

Methods:

Seventy‐three CBCT images of 10 patients and 24 planning CT images with digital imaging and communications in medicine for radiotherapy (DICOM‐RT) structures were used for this study. Our proposed framework started from the generation of probabilistic atlases of bone and prostate from 24 planning CT images and prostate contours, which were made in the treatment planning. Next, the gray‐scale histograms of CBCT values within CTV regions in the planning CT images were obtained as the occurrence probability of the CBCT values. Then, CBCT images were registered to the atlases using a rigid registration with mutual information. Finally, prostate regions were estimated by applying the Bayesian inference to CBCT images with the probabilistic atlases and CBCT value occurrence probability. The proposed framework was evaluated by calculating the Euclidean distance of errors between two centroids of prostate regions determined by our method and ground truths of manual delineations by a radiation oncologist and a medical physicist on CBCT images for 10 patients.

Results:

The average Euclidean distance between the centroids of extracted prostate regions determined by our proposed method and ground truths was 4.4 mm. The average errors for each direction were 1.8 mm in anteroposterior direction, 0.6 mm in lateral direction and 2.1 mm in craniocaudal direction.

Conclusion:

Our proposed framework based on probabilistic atlases and Bayesian inference might be feasible to automatically determine prostate regions on CBCT images.
SU‐F‐J‐35: Moving Towards Isocentric Prone Breast Setup with Contralateral Leveling Tattoo and Couch Move Assistant (CMA)
Authors: Yang K , Downey C , Suter N , Gott L , Naughton L , Aufiero K , Day S , McGovern N , Brock J , Andreou K , Strasser J , Koprowski C , Raben A , Chen H , Mourtada F
Published: July 2016

Purpose:

Large setup variability has been observed for prone breast patients due to rotation error. To reduce this uncertainty, we propose a novel implementation of isocentric prone breast method on Elekta linac with couch move assistant (CMA) and on‐line KV CBCT.

Methods:

Daily CBCT is used to evaluate the prone breast patient positioning uncertainty of proposed isocentric technique against our routine manual clinical setup. Clinical setup involves a manual AP shift from the ipsilateral torso tattoos (2PT) to the treatment iso directly underneath and SSD and flash check. While the proposed isocentric prone breast method features an additional contralateral leveling tattoo positioned at mid‐level of torso during CT simulation to determine correct patient obliqueness, and an automatic couch shift using Elekta CMA to correct both known distance from 2PT to the iso and daily setup uncertainty. Summary statistics were calculated for a cohort of prone breast patients from our clinic (n=5), and will be updated as more patients get administrated.

Results:

Small field of view partial CBCT acquisition is optimized to achieve clearance and minimize nominal imaging dose to 0.6cGy per scan to a 16cm phantom. Routine clinical setup uncertainty is 0.3±0.2cm, 0.7±0.4cm and 0.4±0.3cm in S/I, L/R and A/P respectively. Using the isocentric prone breast method, the setup uncertainty could be reduced to 0±0.3cm, 0.3±0.4cm and 0.1±0.3cm if the average couch shift in the 1st week of treatment is applied by CMA to the rest of treatment days. The uncertainty could be further reduced to 0.3±0.1cm, 0±0.1cm and 0±0.2cm with additional contralateral leveling tattoo. The improvement in precision is found to be statistically significant (p<0.05) using unpaired student t‐tests.

Conclusion:

Contralateral leveling tattoo is essential to determine correct patient obliqueness. More consistent and accurate isocentric prone breast patient positioning is achievable on Elekta linac with CMA and CBCT.
SU‐F‐J‐42: Comparison of Varian TrueBeam Cone‐Beam CT and BrainLab ExacTrac X‐Ray for Cranial Radiotherapy
Authors: Li J , Shi W , Andrews D , Werner‐Wasik M , Lu B , Yu Y , Dicker A , Liu H
Published: July 2016

Purpose:

To compare online image registrations of TrueBeam cone‐beam CT (CBCT) and BrainLab ExacTrac x‐ray imaging systems for cranial radiotherapy.

Method:

Phantom and patient studies were performed on a Varian TrueBeam STx linear accelerator (Version 2.5), which is integrated with a BrainLab ExacTrac imaging system (Version 6.1.1). The phantom study was based on a Rando head phantom, which was designed to evaluate isocenter‐location dependence of the image registrations. Ten isocenters were selected at various locations in the phantom, which represented clinical treatment sites. CBCT and ExacTrac x‐ray images were taken when the phantom was located at each isocenter. The patient study included thirteen patients. CBCT and ExacTrac x‐ray images were taken at each patient's treatment position. Six‐dimensional image registrations were performed on CBCT and ExacTrac, and residual errors calculated from CBCT and ExacTrac were compared.

Results:

In the phantom study, the average residual‐error differences between CBCT and ExacTrac image registrations were: 0.16±0.10 mm, 0.35±0.20 mm, and 0.21±0.15 mm, in the vertical, longitudinal, and lateral directions, respectively. The average residual‐error differences in the rotation, roll, and pitch were: 0.36±0.11 degree, 0.14±0.10 degree, and 0.12±0.10 degree, respectively. In the patient study, the average residual‐error differences in the vertical, longitudinal, and lateral directions were: 0.13±0.13 mm, 0.37±0.21 mm, 0.22±0.17 mm, respectively. The average residual‐error differences in the rotation, roll, and pitch were: 0.30±0.10 degree, 0.18±0.11 degree, and 0.22±0.13 degree, respectively. Larger residual‐error differences (up to 0.79 mm) were observed in the longitudinal direction in the phantom and patient studies where isocenters were located in or close to frontal lobes, i.e., located superficially.

Conclusion:

Overall, the average residual‐error differences were within 0.4 mm in the translational directions and were within 0.4 degree in the rotational directions.
SU‐F‐J‐51: A Cone‐Based Scintillator Detector for IGRT QA for Scattered and Scanning Proton Therapy
Authors: Oesten H , Clasie B , Nelson B , Jee K
Published: July 2016

Purpose:

IGRT commissioning and QA are critical components for precise delivery of proton treatment beams to patients. In order to ensure high quality IGRT, a new cone‐based scintillator detector was evaluated for our QA activities for double‐scattered and scanning proton modalities. This allows a routine evaluation of the gantry‐angle dependent position offset between the radiation and imaging.

Methods:

The cone‐based scintillator detector (XRV‐124, Logos Systems, Int'l CA, USA) features a unique configuration of measuring stereotactic paths of proton and x‐ray beams in a single setup with arbitrary gantry angles. For the beams‐eye‐view (BEV) analysis of x‐ray crosshair images, a cylindrical representation of the cone image was newly developed. The calibration accuracy was evaluated using different CT resolutions for a range of 55 – 95mm in patient's cranial direction and ±9mm in the lateral direction. Energy‐dependent spot sizes (σ) of pencil beams were characterized and compared to measurements by the MatriXX detector (IBA, Germany). Iso‐centric deviations between radiation and x‐ray imaging were characterized as a function of gantry angle.

Results:

The position calibration of the detector was successfully verified with a reproducible positioning by x‐ray imaging. The measurements were reproducible within clinical tolerances (±1mm). The spot size vs. energy at zero gantry angle measured with the scintillating cone detector agreed with the MatriXX detector measurements within 17%.

Conclusion:

The new approach to investigate the accuracy of IGRT and pencil beam properties could successfully be implemented into the QA program. The system will improve efficiency in our QA activities for proton treatments.
SU‐F‐J‐59: Assessment of Dose Response Distribution in Individual Human Tumor
Authors: Yan D , Chen S , Krauss D , Chen P , Wilson G
Published: July 2016

Purpose:

To fulfill precision radiotherapy via adaptive dose painting by number, voxel‐by‐voxel dose response or radio‐sensitivity in individual human tumor needs to be determined in early treatment to guide treatment adaptation. In this study, multiple FDG PET images obtained pre‐ and weekly during the treatment course were utilized to determine the distribution/spectrum of dose response parameters in individual human tumors.

Methods:

FDG PET/CT images of 18 HN cancer patients were used in the study. Spatial parametric image of tumor metabolic ratio (dSUV) was created following voxel by voxel deformable image registration. Each voxel value in dSUV was a function of pre‐treatment baseline SUV and treatment delivered dose, and used as a surrogate of tumor survival fraction (SF). Regression fitting with break points was performed using the LQ‐model with tumor proliferation for the control and failure group of tumors separately. The distribution and spectrum of radiation sensitivity and growth in individual tumors were determined and evaluated.

Results:

Spectrum of tumor dose‐sensitivity and proliferation in the controlled group was broad with α in tumor survival LQ‐model from 0.17 to 0.8. It was proportional to the baseline SUV. Tlag was about 21∼25 days, and Tpot about 0.56∼1.67 days respectively. Commonly tumor voxels with high radio‐sensitivity or larger α had small Tlag and Tpot. For the failure group, the radio‐sensitivity α was low within 0.05 to 0.3, but did not show clear Tlag. In addition, tumor voxel radio‐sensitivity could be estimated during the early treatment weeks.

Conclusion:

Dose response distribution with respect to radio‐sensitivity and growth in individual human tumor can be determined using FDG PET imaging based tumor metabolic ratio measured in early treatment course. The discover is critical and provides a potential quantitative objective to implement tumor specific precision radiotherapy via adaptive dose painting by number.
SU‐F‐J‐61: The Effect of CBCT Volume Cutoff On Dose Calculations for Adaptive Planning
Authors: Byers K , Godley A
Published: July 2016

Purpose:

In adaptive re‐planning, the CBCT is used to recalculate the dose and adjust the treatment plan accordingly. The limited field of view of a CBCT can cause external volume cutoff of the patient, which may cause the dose in re‐planning to be miscalculated. This research determined how often this volume cutoff occurs, and how much the dose calculations are affected by this cutoff.

Methods:

30 consecutive prostate patients treated on the Synergy S (Elekta AB, Stockholm, Sweden) and 28 treated on the Edge (Varian, Palo Alto, CA) linacs were included. For three patients, external contours were added to the original CTs. The external contours added included the true external volume and five others created to mimic the cutoff seen in the CBCTs. The original treatment plans were recalculated using these cutoff contours.

Results:

30% of Synergy patients were affected by volume cutoff in their CBCTs, in which 75% of their CBCTs had volume cutoff that was less than 700cc, or 6% of the total patient volume imaged. At this 6% change in volume the target dose increases by <2% although there is up to a 13% increase in organ at risk dose. 11% of Edge patients were affected by volume cutoff, all of these CBCTs were affected by a volume cutoff less than 300cc, or 3% of the total imaged volume. At this 3% change in volume the target dose increases by <1% and there is up to a 7% increase in organ at risk dose. The Edge CBCT field of view diameter is 46 cm, whereas the Synergy is only 41 cm, which explains this discrepancy.

Conclusion:

There is no cutoff observed for 79% of patients studied. For 76% of the patients with cutoff, the target dose increases by <2%. Godley receives research grant from Elekta
SU‐F‐J‐62: Assessment of Dose Changes Due to Anterior‐Posterior Patient Separation During Daily MVCT Scans
Authors: Leney M , Nalichowski A , Patel S
Published: July 2016

Purpose:

To determine the effects of patient separation on absolute dose and dose distribution in patients undergoing pelvic radiotherapy on TomoTherapy.

Methods:

An Alderson RANDO phantom with 4cm of bolus was imaged on a CT simulator and the resulting scans were contoured as a whole pelvic case. Using TomoTherapy Planning Station, the plan was designed to give 45 Gy to 95% of the treatment volume in 25 fractions. TomoTherapy MVCT scans were performed on the RANDO phantom with 2cm and 4cm of bolus removed to simulate visible changes in a patient's anatomy. The MVCT images were rigidly registered with planning CT images on TomoTherapy Planned Adaptive. The original fluence was recalculated on the MVCT images and changes in dose distribution due to patient separation were quantified by the changes in DVHs for the target volume and the organs at risk.

Results:

Patient separation difference equivalent to 2cm and 4cm in anterior‐posterior direction resulted in an increase of the PTV D50 and maximum PTV dose of 5.6%, 6.2% for 2cm and 7.7%, 10.4% for 4cm, respectively. For the 2cm change, D50 and maximum doses to organs at risk increased by 6.5%, 7.1% in the bladder, 4.9%, 4.8% in the rectum, and 5.3%, 6.6% in the bowel. For the 4cm change, D50 and maximum doses increased by 10.7%, 12.2% in the bladder, 5.9%, 6.1% in the rectum, and 7.7%, 10.1% in the bowel.

Conclusion:

This research indicates that, without any changes to the structures, patient separation in the anterior‐posterior direction can affect the dose distribution for the PTV and organs at risk. These results can assist physicians in determining if obtaining a new CT simulation set and replanning is necessary for pelvic patients on TomoTherapy.
SU‐F‐J‐63: Abdominal Diameter Changes in Children During Volumetric Modulated Arc Therapy (VMAT): Is Re‐Planning Needed?
Authors: Guerreiro F , Janssens G , Seravalli E , Raaymakers B
Published: July 2016

Purpose:

To investigate the dosimetric impact of daily changes in patient's diameter, due to weight gain/loss and air in the bowel, based on CBCT information during radiotherapy treatment of pediatric abdominal tumors.

Methods:

10 pediatric patients with neuroblastoma (n=6) and Wilms’ (n=4) tumors were included. Available CBCTs were affinely registered to the planning CT for daily set‐up variations corrections. A density override approach assigning air‐density to the random air pockets and water‐density to the remaining anatomy was used to determine the CBCT and CT dose. Clinical VMAT plans, with a PTV prescribed dose ranging between (14.4‐ 36) Gy, were re‐optimized on the density override CT and re‐calculated on each CBCT. Dose‐volume statistics of the PTV and kidneys, delineated on each CBCT, were used to compare the daily and cumulative CBCT dose with the reference CT dose.

Results:

The average patient diameter variation was (0.5 ± 0.7) cm (maximum daily difference of 2.3 cm). The average PTV mean dose difference (MDD) between the CT and the cumulative CBCT plans was (0.1 ± 1.1) % (maximum daily MDD of 2%). A reduction in target coverage up to 3% and 7% was observed for the cumulative and daily CBCT plans, respectively. The average kidneys’ cumulative MDD was (−2.7 ± 3.6) % (maximum daily MDD of −12%), corresponding to an overdosage.

Conclusion:

Due to patient's diameter changes, a target underdosage was assessed. Given the high local tumor control of neuroblastoma and Wilms’ diseases, the need of re‐planning might be discarded. However, the assessed kidneys overdosage could represent a problem when the normal tissue tolerance is reached. The necessity of re‐planning should then be considered to reduce the risk of long‐term renal complications. Due to the poor softtissue contrast on CBCT, MRI‐guidance is required to obtain a better assessment of the accumulated dose on the remaining OARs.
SU‐F‐J‐66: Anatomy Deformation Based Comparison Between One‐Step and Two‐Step Optimization for Online ART
Authors: Feng Z , Yu G , Qin S , Ma C , Zhu J , Yin Y , Li D
Published: July 2016

Purpose:

This study investigated that how the quality of adapted plan was affected by inter‐fractional anatomy deformation by using one‐step and two‐step optimization for on line adaptive radiotherapy (ART) procedure.

Methods:

10 lung carcinoma patients were chosen randomly to produce IMRT plan by one‐step and two‐step algorithms respectively, and the prescribed dose was set as 60 Gy on the planning target volume (PTV) for all patients. To simulate inter‐fractional target deformation, four specific cases were created by systematic anatomy variation; including target superior shift 0.5 cm, 0.3cm contraction, 0.3 cm expansion and 45‐degree rotation. Based on these four anatomy deformation, adapted plan, regenerated plan and non‐adapted plan were created to evaluate quality of adaptation. Adapted plans were generated automatically by using one‐step and two‐step algorithms respectively to optimize original plans, and regenerated plans were manually created by experience physicists. Non‐adapted plans were produced by recalculating the dose distribution based on corresponding original plans. The deviations among these three plans were statistically analyzed by paired T‐test.

Results:

In PTV superior shift case, adapted plans had significantly better PTV coverage by using two‐step algorithm compared with one‐step one, and meanwhile there was a significant difference of V95 by comparison with adapted and non‐adapted plans (p=0.0025). In target contraction deformation, with almost same PTV coverage, the total lung received lower dose using one‐step algorithm than two‐step algorithm (p=0.0143,0.0126 for V20, Dmean respectively). In other two deformation cases, there were no significant differences observed by both two optimized algorithms.

Conclusion:

In geometry deformation such as target contraction, with comparable PTV coverage, one‐step algorithm gave better OAR sparing than two‐step algorithm. Reversely, the adaptation by using two‐step algorithm had higher efficiency and accuracy as target occurred position displacement.

We want to thank Dr. Lei Xing and Dr. Yong Yang in the Stanford University School of Medicine for this work. This work was jointly supported by NSFC (61471226), Natural Science Foundation for Distinguished Young Scholars of Shandong Province (JQ201516), and China Postdoctoral Science Foundation (2015T80739, 2014M551949).
SU‐F‐J‐70: Monte Carlo Study On a Cone‐Beam Computed Tomography Using a Cross‐Type Carbon Fiber Antiscatter Grid
Authors: Usui K , Inoue T , Kurokawa C , Sugimoto S , Sasai K , Ogawa K
Published: July 2016

Purpose:

If a cone‐beam computed tomography (CBCT) image can be used for adaptive radiotherapy (ART), an irradiated actual dose distribution can be verified on every treatment day. However, a CBCT image is commonly degraded by scattered photons and thus cannot be directly used for dose calculation. The purpose of the present study is to evaluate whether a cross‐type carbon fiber antiscatter grid improves CBCT image quality using a Monte Carlo (MC) method for achieving CBCT‐based ART.

Methods:

We designed a new carbon fiber (C6H10O5) interspaced grid, which allows the grid ratio to be increased while maintaining the transmission of primary photons. The grid ratio was 20:1, and the grid frequency was 20 lp/cm, with lead strips oriented along the cone beam projections. The MC method was used to simulate a projection data acquisition system of a material phantom (water, lung, bone, cartilage, and adipose) with a bow‐tie filter. The parameters used to evaluate the image quality were uniformity, contrast, and contrast‐to‐noise ratio (CNR). Moreover, to evaluate the accuracy of the CT number, we compared CT numbers between images reconstructed by only primary photons and those obtained using grids.

Results:

The proposed grid had a beneficial impact on the image quality and accuracy of the CT number. The uniformity was improved by approximately 70% compared to the case without the grid. The average contrast and CNR improved by 14.2% and 22.6%, respectively, when using the proposed grid. CT numbers using the proposed grid were most consistent with nominal CT numbers obtained with only primary photons.

Conclusion:

The proposed cross‐type carbon fiber grid improves CBCT image quality and achieves an accurate CT number, without increasing the imaging doses. Therefore, the cross‐type carbon fiber antiscatter grid should be considered to achieve CBCT‐based ART.

This work was supported by Japan Society for the Promotion of Science KAKENHI.

Grant Number 15K19211.
SU‐F‐J‐78: Characterization of Deformable Image Registration for the Pelvic Region Based On Prostate Shifting, Image Noise and the Existence of Implanted Fiducial Markers
Authors: Ikeda T , Takemura A , Shinichi U , Kojima H , Noto K
Published: July 2016

Purpose:

To assess the impact of prostate shifting, image noise and the existence of fiducial markers on deformable image registration (DIR) performance in the pelvic region using original digital phantoms.

Methods:

A digital phantom of the pelvic region of a prostate cancer patient was created using contoured anatomy as a base image set. Two simulated fiducial markers were implanted at the apex and base of the prostate. With and without markers, the prostate was shifted by 6.00 and 10.0 mm superior, 5.63 and 9.38 mm posterior, and 5.63 and 9.38 mm left, respectively. We also added image noise with a standard deviation (SD) of 9, 21, or 28. A phantom with a noise SD of 9 and zero prostate shift was defined as the target, and the other phantoms were defined as the sources. DIR was performed using MIM Maestro (MIM Software Inc., Cleveland, OH, USA) on the digital phantoms. A dice similarity coefficient (DSC) was used as the index of correspondence level of the prostates on the target images and the results of the DIR. Also, we calculated the motion distance of the fiducial markers.

Results:

The DSC decreased with increasing prostate distance. The degree of correspondence of the fiducial markers and the DIR performance were worse when the bladder volume changed after the prostate shifted in the superior direction than when it shifted in the other directions. Changes in the image noise and the existence of fiducial markers produced minimal impact on the DIR results.

Conclusion:

We found that DIR was not affected by image noise, and the reproduction of prostate position by maintaining the bladder volume was important to preserve DIR performance. Further, the existence of two fiducial markers had no impact on DIR performance.

This work was supported by Japan Society for the Promotion of Science (JSPS) KAKEN Grant‐in‐Aid for Scientific Research (C) Grant number 26460724.
SU‐F‐J‐101: Stereotactic Body Radiation Therapy Planning for Primary Prostate Cancer with Selective Intraprostatic Boost Determined by 18F‐Choline PET/CT
Authors: Wu L , Wang H , Hirata E , Kwee S , Kuang Y
Published: July 2016

Purpose:

To investigate the utility of 18F‐choline positron emission tomography (PET) scans guidance for SBRT dose painting in patients with prostate cancer and its impact on tumor control probability (TCP) and normal tissue complication probability (NTCP).

Methods:

Twenty seven patients with localized prostate cancer who had 18F‐choline PET/CT scan prior to treatment were included. A pair of nested intraprostatic dominant lesion (IDL) contours (IDLsuv60% and IDLsuv70%) were generated for each patient based on 60% and 70% of maximum prostate uptake on the 18F‐choline PET images. GTVreg was delineated on prostate according to the gland boundary seen on CT images. The PTVs (PTVsuv60% and PTVsuv70%) were defined as respective IDLs with a 3‐mm margin posteriorly and 5 mm in all other dimensions. Two 5‐fraction SBRT plans using VMAT technique along with 10 MV FFF beams, plan36Gy and plan50–55Gy, were generated for each patient. All plans included a dose of 36.25 Gy prescribed to PTVreg. The Plan50–55Gy also included a simultaneous boost dose of 50 Gy and 55 Gy prescribed to the PTVsuv60% and PTVsuv70%, respectively. The utility of 18F‐Choline PET‐guided SBRT dose escalation was evaluated by its ability to achieve the prescription dose objectives while adhering to organ‐at‐risk (OAR) dose constraints. The TCP and NTCP calculated by radiological models were also compared between two plans for each patient.

Results:

In all 54 SBRT plans generated, the planning objectives and dose constraints were met without exception. Plan50–55Gy had a significantly higher dose in PTVsuv60% and PTVsuv70% than those in Plan36Gy (p < 0.05), respectively, while still maintaining a safe OAR sparing profile. In addition, plan50–55Gy had significantly higher TCP than plan36Gy.

Conclusion:

Using VMAT with FFF beams to incorporate a simultaneous 18F‐choline PET‐guided radiation boost dose up to 55 Gy into a SBRT plan is technically feasible.

This work was supported in part by Congressionally Directed Medical Research Programs Prostate Cancer Research Program grant PC04130, National Institutes of Health/National Cancer Institute grant R41CA110121, and the UNLV Lincy Endowed Assistant Professorship.
SU‐F‐J‐102: Lower Esophagus Margin Implications Based On Rapid Computational Algorithm for SBRT
Authors: Cardenas M , Mazur T , Li H , Mutic S , Bradley J , Tsien C , Green O
Published: July 2016

Purpose:

To quantify inter‐fraction esophagus‐variation.

Methods:

Computed tomography and daily on‐treatment 0.3‐T MRI data sets for 7 patients were analyzed using a novel Matlab‐based (Mathworks, Natick, MA) rapid computational method. Rigid registration was performed from the cricoid to the gastro‐esophageal junction. CT and MR‐based contours were compared at slice intervals of 3mm. Variation was quantified by “expansion,” defined as additional length in any radial direction from CT contour to MR contour. Expansion computations were performed with 360° of freedom in each axial slice. We partitioned expansions into left anterior, right anterior, right posterior, and left posterior quadrants (LA, RA, RP, and LP, respectively). Sample means were compared by analysis of variance (ANOVA) and Fisher's Protected Least Significant Difference test.

Results:

Fifteen fractions and 1121 axial slices from 7 patients undergoing SBRT for primary lung cancer (3) and metastatic lung disease (4) were analyzed, generating 41,970 measurements. Mean LA, RA, RP, and LP expansions were 4.30±0.05 mm, 3.71±0.05mm, 3.17±0.07, and 3.98±0.06mm, respectively. 50.13% of all axial slices showed variation > 5 mm in one or more directions. Variation was greatest in lower esophagus with mean LA, RA, RP, and LP expansion (5.98±0.09 mm, 4.59±0.09 mm, 4.04±0.16 mm, and 5.41±0.16 mm, respectively). The difference was significant compared to mid and upper esophagus (p<.0001). The 95th percentiles of expansion for LA, RA, RP, LP were 13.36 mm, 9.97 mm, 11.29 mm, and 12.19 mm, respectively.

Conclusion:

Analysis of on‐treatment MR imaging of the lower esophagus during thoracic SBRT suggests margin expansions of 13.36 mm LA, 9.97 mm RA, 11.29 mm RP, 12.19 mm LP would account for 95% of measurements. Our novel algorithm for rapid assessment of margin expansion for critical structures with 360° of freedom in each axial slice enables continuously adaptive patient‐specific margins which may reduce overlap with high‐dose regions.
SU‐F‐J‐108: TMR Correction Factor Based Online Adaptive Radiotherapy for Stereotactic Radiosurgery (SRS) of L‐Spine Tumors Using Cone Beam CT
Authors: Ghaffar I , Balik S , Zhuang T , Chao S , Xia P
Published: July 2016

Purpose:

To investigate the feasibility of using TMR ratio correction factors for a fast online adaptive plan to compensate for anatomical changes in stereotactic radiosurgery (SRS) of L‐spine tumors.

Methods:

Three coplanar treatment plans were made for 11 patients: Uniform (9 IMRT beams equally distributed around the patient); Posterior (IMRT with 9 posterior beams every 20 degree) and VMAT (2 360° arcs). For each patient, the external body and bowel gas were contoured on the planning CT and pre‐treatment CBCT. After registering CBCT and the planning CT by aligning to the tumor, the CBCT contours were transferred to the planning CT. To estimate the actual delivered dose while considering patient's anatomy of the treatment day, a hybrid CT was created by overriding densities in planning CT using the differences between CT and CBCT external and bowel gas contours. Correction factors (CF) were calculated using the effective depth information obtained from the planning system using the hybrid CT: CF = TMR (delivery)/TMR (planning). The adaptive plan was generated by multiplying the planned Monitor Units with the CFs.

Results:

The mean absolute difference (MAD) in V16Gy of the target between planned and estimated delivery with and without TMR correction was 0.8 ± 0.7% vs. 2.4 ± 1.3% for Uniform and 1.0 ± 0.9% vs. 2.6 ± 1.3% for VMAT plans(p<0.05), respectively. For V12Gy of cauda‐equina with and without TMR correction, MAD was 0.24 ± 0.19% vs. 1.2 ± 1.02% for Uniform and 0.23 ± 0.20% vs. 0.78 ± 0.79% for VMAT plans(p<0.05), respectively. The differences between adaptive and original plans were not significant for posterior plans.

Conclusion:

The online adaptive strategy using TMR ratios and pre‐treatment CBCT information was feasible strategy to compensate for anatomical changes for the patients treated for L‐spine tumors, particularly for equally spaced IMRT and VMAT plans.
SU‐F‐J‐111: A Novel Distance‐Dose Weighting Method for Label Fusion in Multi‐ Atlas Segmentation for Prostate Radiation Therapy
Authors: Chang J , Gu X , Lu W , Song T , Jiang S
Published: July 2016

Purpose:

A novel distance‐dose weighting method for label fusion was developed to increase segmentation accuracy in dosimetrically important regions for prostate radiation therapy.

Methods:

Label fusion as implemented in the original SIMPLE (OS) for multi‐atlas segmentation relies iteratively on the majority vote to generate an estimated ground truth and DICE similarity measure to screen candidates. The proposed distance‐dose weighting puts more values on dosimetrically important regions when calculating similarity measure. Specifically, we introduced distance‐to‐dose error (DDE), which converts distance to dosimetric importance, in performance evaluation. The DDE calculates an estimated DE error derived from surface distance differences between the candidate and estimated ground truth label by multiplying a regression coefficient. To determine the coefficient at each simulation point on the rectum, we fitted DE error with respect to simulated voxel shift. The DEs were calculated by the multi‐OAR geometry‐dosimetry training model previously developed in our research group.

Results:

For both the OS and the distance‐dose weighted SIMPLE (WS) results, the evaluation metrics for twenty patients were calculated using the ground truth segmentation. The mean difference of DICE, Hausdorff distance, and mean absolute distance (MAD) between OS and WS have shown 0, 0.10, and 0.11, respectively. In partial MAD of WS which calculates MAD within a certain PTV expansion voxel distance, the lower MADs were observed at the closer distances from 1 to 8 than those of OS. The DE results showed that the segmentation from WS produced more accurate results than OS. The mean DE error of V75, V70, V65, and V60 were decreased by 1.16%, 1.17%, 1.14%, and 1.12%, respectively.

Conclusion:

We have demonstrated that the method can increase the segmentation accuracy in rectum regions adjacent to PTV. As a result, segmentation using WS have shown improved dosimetric accuracy than OS. The WS will provide dosimetrically important label selection strategy in multi‐atlas segmentation.

CPRIT grant RP150485
SU‐F‐J‐112: Clinical Feasibility Test of An RF Pulse‐Based MRI Method for the Quantitative Fat‐Water Segmentation
Authors: Yee S , Wloch J , Pirkola M
Published: July 2016

Purpose:

Quantitative fat‐water segmentation is important not only because of the clinical utility of fat‐suppressed MRI images in better detecting lesions of clinical significance (in the midst of bright fat signal) but also because of the possible physical need, in which CT‐like images based on the materials’ photon attenuation properties may have to be generated from MR images; particularly, as in the case of MR‐only radiation oncology environment to obtain radiation dose calculation or as in the case of hybrid PET/MR modality to obtain attenuation correction map for the quantitative PET reconstruction. The majority of such fat‐water quantitative segmentations have been performed by utilizing the Dixon's method and its variations, which have to enforce the proper settings (often predefined) of echo time (TE) in the pulse sequences. Therefore, such methods have been unable to be directly combined with those ultrashort TE (UTE) sequences that, taking the advantage of very low TE values (∼ 10's microsecond), might be beneficial to directly detect bones. Recently, an RF pulse‐based method (http://dx.doi.org/10.1016/j.mri.2015.11.006), termed as PROD pulse method, was introduced as a method of quantitative fat‐water segmentation that does not have to depend on predefined TE settings. Here, the clinical feasibility of this method is verified in brain tumor patients by combining the PROD pulse with several sequences.

Methods:

In a clinical 3T MRI, the PROD pulse was combined with turbo spin echo (e.g. TR=1500, TE=16 or 60, ETL=15) or turbo field echo (e.g. TR=5.6, TE=2.8, ETL=12) sequences without specifying TE values.

Results:

The fat‐water segmentation was possible without having to set specific TE values.

Conclusion:

The PROD pulse method is clinically feasible. Although not yet combined with UTE sequences in our laboratory, the method is potentially compatible with UTE sequences, and thus, might be useful to directly segment fat, water, bone and air.