bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐F‐J‐109: Generate Synthetic CT From Cone Beam CT for CBCT‐Based Dose Calculation
Authors: Wang H , Barbee D , Wang W , Pennell R , Hu K , Osterman K
Published: July 2016

Purpose:

The use of CBCT for dose calculation is limited by its HU inaccuracy from increased scatter. This study presents a method to generate synthetic CT images from CBCT data by a probabilistic classification that may be robust to CBCT noise. The feasibility of using the synthetic CT for dose calculation is evaluated in IMRT for unilateral H&N cancer.

Methods:

In the training phase, a fuzzy c‐means classification was performed on HU vectors (CBCT, CT) of planning CT and registered day‐1 CBCT image pair. Using the resulting centroid CBCT and CT values for five classified “tissue” types, a synthetic CT for a daily CBCT was created by classifying each CBCT voxel to obtain its probability belonging to each tissue class, then assigning a CT HU with a probability‐weighted summation of the classes’ CT centroids. Two synthetic CTs from a CBCT were generated: s‐CT using the centroids from classification of individual patient CBCT/CT data; s2‐CT using the same centroids for all patients to investigate the applicability of group‐based centroids. IMRT dose calculations for five patients were performed on the synthetic CTs and compared with CT‐planning doses by dose‐volume statistics.

Results:

DVH curves of PTVs and critical organs calculated on s‐CT and s2‐CT agree with those from planning‐CT within 3%, while doses calculated with heterogeneity off or on raw CBCT show DVH differences up to 15%. The differences in PTV D95% and spinal cord max are 0.6±0.6% and 0.6±0.3% for s‐CT, and 1.6±1.7% and 1.9±1.7% for s2‐CT. Gamma analysis (2%/2mm) shows 97.5±1.6% and 97.6±1.6% pass rates for using s‐CTs and s2‐CTs compared with CT‐based doses, respectively.

Conclusion:

CBCT‐synthesized CTs using individual or group‐based centroids resulted in dose calculations that are comparable to CT‐planning dose for unilateral H&N cancer. The method may provide a tool for accurate dose calculation based on daily CBCT.
SU‐F‐J‐113: Multi‐Atlas Based Automatic Organ Segmentation for Lung Radiotherapy Planning
Authors: Kim J , Han J , Ailawadi S , Baker J , Hsia A , Xu Z , Ryu S
Published: July 2016

Purpose:

Normal organ segmentation is one time‐consuming and labor‐intensive step for lung radiotherapy treatment planning. The aim of this study is to evaluate the performance of a multi‐atlas based segmentation approach for automatic organs at risk (OAR) delineation.

Methods:

Fifteen Lung stereotactic body radiation therapy patients were randomly selected. Planning CT images and OAR contours of the heart ‐ HT, aorta ‐ AO, vena cava ‐ VC, pulmonary trunk ‐ PT, and esophagus – ES were exported and used as reference and atlas sets. For automatic organ delineation for a given target CT, 1) all atlas sets were deformably warped to the target CT, 2) the deformed sets were accumulated and normalized to produce organ probability density (OPD) maps, and 3) the OPD maps were converted to contours via image thresholding. Optimal threshold for each organ was empirically determined by comparing the auto‐segmented contours against their respective reference contours. The delineated results were evaluated by measuring contour similarity metrics: DICE, mean distance (MD), and true detection rate (TD), where DICE=(intersection volume/sum of two volumes) and TD = {1.0 ‐ (false positive + false negative)/2.0}. Diffeomorphic Demons algorithm was employed for CT‐CT deformable image registrations.

Results:

Optimal thresholds were determined to be 0.53 for HT, 0.38 for AO, 0.28 for PT, 0.43 for VC, and 0.31 for ES. The mean similarity metrics (DICE[%], MD[mm], TD[%]) were (88, 3.2, 89) for HT, (79, 3.2, 82) for AO, (75, 2.7, 77) for PT, (68, 3.4, 73) for VC, and (51,2.7, 60) for ES.

Conclusion:

The investigated multi‐atlas based approach produced reliable segmentations for the organs with large and relatively clear boundaries (HT and AO). However, the detection of small and narrow organs with diffused boundaries (ES) were challenging. Sophisticated atlas selection and multi‐atlas fusion algorithms may further improve the quality of segmentations.
SU‐F‐J‐118: On‐Treatment 4D CT Reconstruction From Planning 4D CT Using Linear Amplitude Scaling
Authors: Park S , Jung J , Kim J , Yeo I , Yi B
Published: July 2016

Purpose:

To reconstruct on‐treatment 4D CT images from planning 4D CT images by adapting deformation vector field (DVF) of the planning CT to the on‐treatment condition, while the adaptation is based on the scaling of two amplitudes that are motion characteristics at the times of treatment and planning CT acquisition, respectively.

Methods:

An anthropomorphic digital phantom (XCAT) was used to generate 4D image sets with 1‐cm and 2‐cm tumor motions simulating conditions of planning CT and treatment, respectively. DVFs were acquired from the planning CT image set. The DRR images were acquired simulating setup kV images from the two CT image sets. On the DRR images, tumor positions and their motion amplitudes were quantified. The DVFs were scaled linearly by the amplitude ratio between the treatment and the planning CT times, assuming the elasticity of lung. The scaled DVFs were used to resample the planning 4D CT images generating on‐treatment 4D CT images. The on‐treatment 4D CT images thus acquired were compared with the reference on‐treatment images (2‐cm motion).

Results:

The resampled images showed good agreement within 1 mm residual errors with the reference images. The normalized cross correlation was 0.995.

Conclusion:

A linear model of amplitude scaling was developed to reconstruct on‐treatment 4D CT images from planning 4D CT images using the setup KV images acquired during treatment. The model was validated on a digital phantom. For the model to fully work, a further research needs to be followed, that aims at utilizing a phase‐specific CT image set that is geometrically identical between pretreatment and treatment conditions.
SU‐F‐J‐119: Pilot Study On the Location‐Based Lung Motion Assessment
Authors: Lee TK , Ewald A
Published: July 2016

Purpose:

In most of lung treatment cases with various radiotherapy beam modalities, 4DCT images are obtained in order to define ITV. ITV is defined with the signal from motion monitoring system, e.g. RPM. However, the signal is not consistent with tumor motion because it varies with location, its size, age, gender, etc. In the present study, the location‐based motion assessment is presented.

Methods:

4DCT images of 70 patients were reviewed: 28‐left‐lung and 42‐right‐lung patients; 36‐female and 34‐male patients; the age range of 51.2–89.9; tumor‐size range of 0.75–9.50cm with 25% of these adherent to bony‐anatomy. Philips Big‐Bore Simulation CT and RPM systems were used. The study was performed as follows. First, RPM signal and tumor motion in superior‐inferior direction was compared. Second, the tumor size and its motion amplitude in all directions were measured at multiple locations. Third, the average tumor motion was calculated to assess general motion amplitudes at various locations.

Results:

RPM amplitude is not consistent with lung tumor motion amplitude. The tumors of similar sizes at similar location present various motion amplitude up to 1.1cm difference, but in average, the standard deviation was <0.5cm. Almost regardless of tumor sizes, the tumor motion was greatest at lower lobe location (>=1.0cm), and the smallest at upper lobe location and when adherent to bony‐anatomy (<=0.5cm).

Conclusion:

The tumor size affects the motion amplitude less than does the tumor location. However, as the study results indicate that tumor motion has noticeable variation and so further study with more patient cases is needed. Also, for the same patient, the RPM signal presents instability of breathing, and clinically the patient with the instability of RPM breathing of <=10% is selected for respiratory‐gated radiotherapy and ∼25% of patients under current study was treated. Patient‐specific motion‐uncertainty margins are considered to be added following further study.
SU‐F‐J‐124: Reduction in Dosimetric Impact of Motion Using VMAT Compared to IMRT in Hypofractionated Prostate Cancer Patients
Authors: Ravindranath B , Xiong J , Happersett L , Mageras G , Zhang P , Hunt M
Published: July 2016

Purpose:

To quantify and compare the dosimetric impact of motion management correction strategies during VMAT and IMRT for hypofractionated prostate treatment.

Methods:

Two arc VMAT and 9 field IMRT plans were generated for two prostate cancer patients undergoing hypofractionated radiotherapy (7.5Gy × 5 and 8Gy × 5). 212 motion traces were retrospectively extracted from treatment records of prostate cancer patients with implanted Calypso beacons. Dose to the CTV and normal tissues was reconstructed for each trace and plan taking into account the actual treatment delivery time. Following motion correction scenarios were simulated: (1) VMAT plan – (a) No correction, (b) correction between arcs, (c) correction every 20 degrees of gantry rotation and (2) IMRT plan ‐ (a) No correction,(b) correction between fields. Two mm action threshold for position correction was assumed. The 5–95% confidence interval (CI) range was extracted from the family of DVHs for each correction scenario.

Results:

Treatment duration for 8Gy plan (VMAT vs IMRT) was 3 vs 12 mins and for 7.5Gy plan was 3 vs 9 mins. In the absence of correction, the VMAT 5–−95% CI dose spread was, on average, less than the IMRT dose spread by 2% for CTVD95, 9% for rectalwall (RW) D1cc and 9% for bladderwall (BW) D53. Further, VMAT b/w arcs correction strategy reduced the spread about the planned value compared to IMRT b/w fields correction by: 1% for CTVD95, 2.6% for RW1cc and 2% for BWD53. VMAT 20 degree strategy led to greater reduction in dose spread compared to IMRT by: 2% for CTVD95, 4.5% for RW1cc and 6.7% for BWD53.

Conclusion:

In the absence of a correction strategy, the limited motion during VMAT's shorter delivery times translates into less motion‐induced dosimetric degradation than IMRT. Performing limited periodic motion correction during VMAT can yield excellent conformity to planned values that is superior to IMRT.

This work was partially supported by Varian Medical Systems
SU‐F‐J‐134: Evaluation of Deformation Field Accuracy Using Regional Rigid Registrations with Variable Size
Authors: Ahunbay E , Li X
Published: July 2016

Purpose:

How to quickly assess the uncertainty in the Deformation Field (DF) generated from Deformable Image Registration (DIR) over the imaged anatomy is a major issue. Here we introduce a method to determine the DF uncertainty using local rigid registrations of varying box size.

Methods:

A series of 6‐degree rigid registrations are performed with a cubic ROI centered on the voxel of the interest. The size of the registration box is varied from a large value, e.g. 30mm to 3mm. The translation vector obtained from the registration of the largest box is assumed to be most reliable and the reliability of the registrations with smaller regions is assumed to be good as long as the variation in the translation vector, as the box size is reduced, is less than the reduction amount in the box size at each step. In this way a smallest reliable box (SRB) size is determined. A distance uncertainty (DU) is calculated for each voxel based on (1) half size of SRB and (2) the magnitude difference between the SRB translation vector to the voxel's DF. This DU is compared to other DIR reliability measures such as Jacobian determinant and root mean square.

Results:

The DU for selected voxels was found to agree with those expected from various organ types. For example, mean DU from the voxels in rectum and bladder was 8.4 and 8.9mm respectively, while the DU of the mean femoral head was 2.2mm. 88% of voxels with small DU (<3mm) resulted in reliable Jacobian determinant range (0.5–2), while DU>5mm voxels had 69% of voxels in this range.

Conclusion:

A method to assess DIR reliability is introduced. The method's ability to provide uncertainty information in terms of distance could be an advantage over other measures such as Jacobian or intensity difference.
SU‐F‐J‐137: Intrafractional Change of the Relationship Between Internal Fiducials and External Breathing Signal in Pancreatic Cancer Stereotactic Body Radiation Therapy
Authors: Pettersson N , Murphy J , Simpson D , Cervino L
Published: July 2016

Purpose:

The use of respiratory gating for management of breathing motion during stereotactic body radiation therapy (SBRT) relies on a consistent relationship between the breathing signal and the actual position of the internal target. This relationship was investigated in patients treated for pancreatic cancer.

Methods:

Four patients with pancreatic cancer undergoing SBRT that had implanted fiducials in the tumor were included in this study. Treatment plans were generated based on the exhale phases (30–70%) from the pre‐treatment 4DCT. The margin between the internal target volume (ITV) and the planning target volume was three mm. After patient setup using cone‐beam CT, simultaneous fluoroscopic imaging and breathing motion monitoring were used during at least three breathing cycles to verify the fiducial position and to optimize the gating window. After treatment, fluoroscopic images were acquired for verification purposes and exported for retrospective analyses. Fiducial positions were determined using a template‐matching algorithm. For each dataset, we established a linear relationship between the fiducial position and the anterior‐posterior (AP) breathing signal. The relationships before and after treatment were compared and the dose distribution impact evaluated.

Results:

Seven pre‐ and post‐treatment fluoroscopic pairs were available for fiducial position analyses in the superior‐inferior (SI) and left‐right (LR) directions, and five in the AP direction. Time between image acquisitions was typically six to eight minutes. An average absolute change of 1.2±0.7 mm (range: 0.1–1.7) of the SI fiducial position relative to the external signal was found. Corresponding numbers for the LR and AP fiducial positions were 0.9±1.0 mm (range: 0.2–3.0) and 0.5±0.4 mm (range: 0.2–1.2), respectively. The dose distribution impact was small in both the ITV and organs‐at‐risk.

Conclusion:

The relationship change between fiducial position and external breathing signal has been observed to be about 1 mm in four pancreas SBRT patients, leading to small dose distribution impact.

Pettersson and Cervino are funded by a Varian Medical Systems grant.
SU‐F‐J‐140: Using Handheld Stereo Depth Cameras to Extend Medical Imaging for Radiation Therapy Planning
Authors: Jenkins C , Xing L , Yu S
Published: July 2016

Purpose:

A correct body contour is essential for the accuracy of dose calculation in radiation therapy. While modern medical imaging technologies provide highly accurate representations of body contours, there are times when a patient's anatomy cannot be fully captured or there is a lack of easy access to CT/MRI scanning. Recently, handheld cameras have emerged that are capable of performing three dimensional (3D) scans of patient surface anatomy. By combining 3D camera and medical imaging data, the patient's surface contour can be fully captured.

Methods:

A proof‐of‐concept system matches a patient surface model, created using a handheld stereo depth camera (DC), to the available areas of a body contour segmented from a CT scan. The matched surface contour is then converted to a DICOM structure and added to the CT dataset to provide additional contour information. In order to evaluate the system, a 3D model of a patient was created by segmenting the body contour with a treatment planning system (TPS) and fabricated with a 3D printer. A DC and associated software were used to create a 3D scan of the printed phantom. The surface created by the camera was then registered to a CT model that had been cropped to simulate missing scan data. The aligned surface was then imported into the TPS and compared with the originally segmented contour.

Results:

The RMS error for the alignment between the camera and cropped CT models was 2.26 mm. Mean distance between the aligned camera surface and ground truth model was −1.23 +/−2.47 mm. Maximum deviations were < 1 cm and occurred in areas of high concavity or where anatomy was close to the couch.

Conclusion:

The proof‐of‐concept study shows an accurate, easy and affordable method to extend medical imaging for radiation therapy planning using 3D cameras without additional radiation.

Intel provided the camera hardware used in this study.
SU‐F‐J‐143: Initial Assessment of Image Quality of An Integrated MR‐Linac System with ACR Phantom
Authors: Wang J , Yung J , Kadbi M , Ding Y , Fuller C , Ibbott G
Published: July 2016

Purpose/Objective(s):

To assess the image quality of an integrated MR‐Linac system and compare with other MRI systems that are primarily used for diagnostic purposes.

Materials/Methods:

An ACR MRI quality control (QC) phantom was used to evaluate the image quality of a fully integrated 1.5T MRI‐Linac system recently installed at our institution. This system has a new split magnet design which gives the magnetic field strength of 1.5T. All images were acquired with a set of phased‐array surface coils which are designed to have minimal attention of radiation beam. The anterior coil rests on a coil holder which keeps the anterior coil's position consistent for QA purposes. The posterior coil is imbedded in the patient couch. Multiple sets of T1, T2/PD images were acquired using the protocols as prescribed by the ACR on three different dates, ranging 3 months apart.

Results:

The geometric distortion are within 0.5 mm in the axial scans and within 1mm in the saggital (z‐direction) scans. Slice thickness accuracy, image uniformity, ghosting ratio, high contrast detectability are comparable to other 1.5T diagnostic MRI scanners. The low‐contrast object detectability are lower comparatively, which is a result of using the body array coil. Additionally, the beam's‐eye‐view images (oblique coronal and saggital images) have minimal geometric distortion at all linac gantry angles tested. No observable changes or drift in image quality is found from images acquired 3 month apart.

Conclusion:

Despite the use of a body array surface coil, the image quality is comparable to that of an 1.5T MRI scanner and is of sufficient quality to pass the ACR MRI accreditation program. The geometric distortion of the MRI system of the integrated MR‐Linac is within 1mm for an object size similar to the ACR phantom, sufficient for radiation therapy treatment purpose.

The authors received corporate sponsored research grants from Elekta which is the vendor for the MR‐Linac evaluated in this study.
SU‐F‐J‐144: Scatter and Leakage Survey of An Integrated MR‐Linac System
Authors: Wang J , Bosco G , Darenbourg B , Ibbott G
Published: July 2016

Purpose:

To assess the scatter and leakage radiation of an integrated 1.5T MRI‐Linac system.

Methods:

A 150cc chamber (model 96020C, Inovision) was used in all the scatter and leakage measurements, after being recalibrated for MV energy by the Accredited Dosimetry Calibration Laboratory at MD Anderson. The scatter radiation was measured by placing a 25 cm stack of solid‐water materials at iso‐center on the patient couch to simulate patient scatter. Gantry angles were positioned at 0 degree (beam pointing downward) and 270 (beam pointing laterally). Scatter radiation was measured at selective locations inside the RF room. Beam stopper leakage was measured at the exterior panel of the gantry. The head leakage was measured at 1 meter away from the Linac head in the direction which was determined to be the area of maximum leakage by wrapped films test. All measurements were repeated with the 1.5T magnetic field turned off to study the effect of magnetic field.

Results:

When the magnet was on (B=1.5T), the maximum head leakage at 1 meter was 191.6mR/1000MU. The scatter radiation at 1 meter from the iso‐center was 1.091R/1000MU when the radiation beam was pointing downward, 1.296R/1000MU when the beam pointed laterally. The beam stopper leakage was measured as 299.4 mR/1000MU at the exterior panel of the gantry. When magnet was off (B=0), the head leakage was measured as 198.6mR/1000MU. The scatter radiation at 1 meter was 1.153R/1000MU when beam pointed downward, 1.287R/1000MU when beam pointed laterally. The beam stopper leakage was measured as 309.4 mR/1000MU at the exterior panel of the gantry.

Conclusion:

The measurements indicate that the scatter and leakage radiation from the integrated MR‐Linac system are in‐line with the expected values. The beam stopper leakage is approximately 300 mR/1000MU. The leakage and scatter difference with the magnetic field ON and OFF was within 5%.

The authors received a corporate sponsored grant from Elekta which is the vendor of the MR‐Linac system studied in this work.
SU‐F‐J‐146: Experimental Validation of 6 MV Photon PDD in Parallel Magnetic Field Calculated by EGSnrc
Authors: Ghila A , Steciw S , Fallone B , Rathee S
Published: July 2016

Purpose:

Integrated linac‐MR systems are uniquely suited for real time tumor tracking during radiation treatment. Understanding the magnetic field dose effects and incorporating them in treatment planning is paramount for linac‐MR clinical implementation. We experimentally validated the EGSnrc dose calculations in the presence of a magnetic field parallel to the radiation beam travel.

Methods:

Two cylindrical bore electromagnets produced a 0.21 T magnetic field parallel to the central axis of a 6 MV photon beam. A parallel plate ion chamber was used to measure the PDD in a polystyrene phantom, placed inside the bore in two setups: phantom top surface coinciding with the magnet bore center (183 cm SSD), and with the magnet bore's top surface (170 cm SSD). We measured the field of the magnet at several points and included the exact dimensions of the coils to generate a 3D magnetic field map in a finite element model. BEAMnrc and DOSXYZnrc simulated the PDD experiments in parallel magnetic field (i.e. 3D magnetic field included) and with no magnetic field.

Results:

With the phantom surface at the top of the electromagnet, the surface dose increased by 10% (compared to no‐magnetic field), due to electrons being focused by the smaller fringe fields of the electromagnet. With the phantom surface at the bore center, the surface dose increased by 30% since extra 13 cm of air column was in relatively higher magnetic field (>0.13T) in the magnet bore. EGSnrc Monte Carlo code correctly calculated the radiation dose with and without the magnetic field, and all points passed the 2%, 2 mm Gamma criterion when the ion chamber's entrance window and air cavity were included in the simulated phantom.

Conclusion:

A parallel magnetic field increases the surface and buildup dose during irradiation. The EGSnrc package can model these magnetic field dose effects accurately.

Dr. Fallone is a co‐founder and CEO of MagnetTx Oncology Solutions (under discussions to license Alberta bi‐planar linac MR for commercialization).
SU‐F‐J‐148: A Collapsed Cone Algorithm Can Be Used for Quality Assurance for Monaco Treatment Plans for the MR‐Linac
Authors: Hackett S , van Asselen B , Feist G , Pencea S , Akhiat H , Wolthaus J , Kotte A , Bol G , Lagendijk J , Raaymakers B
Published: July 2016

Purpose:

Treatment plans for the MR‐linac, calculated in Monaco v5.19, include direct simulation of the effects of the 1.5T B0‐field. We tested the feasibility of using a collapsed‐cone (CC) algorithm in Oncentra, which does not account for effects of the B0‐field, as a fast online, independent 3D check of dose calculations.

Methods:

Treatment plans for six patients were generated in Monaco with a 6 MV FFF beam and the B0‐field. All plans were recalculated with a CC model of the same beam. Plans for the same patients were also generated in Monaco without the B0‐field. The mean dose (Dmean) and doses to 10% (D10%) and 90% (D90%) of the volume were determined, as percentages of the prescribed dose, for target volumes and OARs in each calculated dose distribution. Student's t‐tests between paired parameters from Monaco plans and corresponding CC calculations were performed.

Results:

Figure 1 shows an example of the difference between dose distributions calculated in Monaco, with the B0‐field, and the CC algorithm. Figure 2 shows distributions of (absolute) difference between parameters for Monaco plans, with the B0‐field, and CC calculations. The Dmean and D90% values for the CTVs and PTVs were significantly different, but differences in dose distributions arose predominantly at the edges of the target volumes. Inclusion of the B0‐field had little effect on agreement of the Dmean values, as illustrated by Figure 3, nor on agreement of the D10% and D90% values.

Conclusion:

Dose distributions recalculated with a CC algorithm show good agreement with those calculated with Monaco, for plans both with and without the B0‐field, indicating that the CC algorithm could be used to check online treatment planning for the MRlinac. Agreement for a wider range of treatment sites, and the feasibility of using the γ‐test as a simple pass/fail criterion, will be investigated.
SU‐F‐J‐150: Development of An End‐To‐End Chain Test for the First‐In‐Man MR‐Guided Treatments with the MRI Linear Accelerator by Using the Alderson Phantom
Authors: Hoogcarspel S , Kerkmeijer L , Lagendijk J , Van Vulpen M , Raaymakers B
Published: July 2016

The Alderson phantom is a human shaped quality assurance tool that has been used for over 30 years in radiotherapy. The phantom can provide integrated tests of the entire chain of treatment planning and delivery. The purpose of this research was to investigate if this phantom can be used to chain test a treatment on the MRI linear accelerator (MRL) which is currently being developed at the UMC Utrecht, in collaboration with Elekta and Philips. The latter was demonstrated by chain testing the future First‐in‐Man treatments with this system.An Alderson phantom was used to chain test an entire treatment with the MRL. First, a CT was acquired of the phantom with additional markers that are both visible on MR and CT. A treatment plan for treating bone metastases in the sacrum was made. The phantom was consecutively placed in the MRL. For MRI imaging, an 3D volume was acquired. The initially developed treatment plan was then simulated on the new MRI dataset. For simulation, both the MR and CT data was used by registering them together. Before treatment delivery a MV image was acquired and compared with a DRR that was calculated form the MR/CT registration data. Finally, the treatment was delivered. Figure 1 shows both the T1 weighted MR‐image of the phantom and the CT that was registered to the MR image. Figure 2 shows both the calculated and measured MV image that was acquired by the MV panel. Figure 3 shows the dose distribution that was simulated. The total elapsed time for the entire procedure excluding irradiation was 13:35 minutes.The Alderson Phantom yields sufficient MR contrast and can be used for full MR guided radiotherapy treatment chain testing. As a result, we are able to perform an end‐to‐end chain test of the future First‐in‐Man treatments.
SU‐F‐J‐152: Accuracy of Charge Particle Transport in Magnetic Fields Using EGSnrc
Authors: Mainegra‐Hing E , Bouchard H , Tessier F , Walters B
Published: July 2016

Purpose:

Determine accuracy of the current implementation of electron transport under magnetic fields in EGSnrc by means of single scattering (SS) and Fano convergence tests, and establish quantitatively the electron step size restriction required to achieve a desired level of accuracy for ionization chamber dosimetry.

Methods:

Condensed history (CH) dose calculations are compared to SS results for a PTW30013 ionization chamber irradiated in air by a 60Co photon beam. CH dose results for this chamber irradiated in a water phantom by a source of mono‐energetic electrons are compared to the prediction of Fano's theorem for step size restrictions EM ESTEPE from 0.01 to 0.1 and strengths of 0.5 T, 1.0 T, and 1.5 T.

Results:

CH calculations in air for 60Co photons using an EM ESTEPE of 0.25 overestimate SS values by 6% for a 1.5 T field and by 1.5% for a 0.5 T field. Agreement improves with decreasing EM ESTEPE reducing this difference at 0.02 to 0.13% and 0.04% for 1.5 T and 0.5 T respectively. CH results converge with decreasing EM ESTEPE reaching an agreement of 0.2% at a value of EM ESTEPE of 0.01 for 100 keV electrons. SS results at 100 keV for 1.5 T show the same EM ESTEPE dependency as the CH results.

Conclusion:

Accurate transport of charged particles in magnetic fields is only possible if the step size is significantly restricted. An EM ESTEPE value of 0.02 is required to reproduce SS results at the 0.1% level for a calculation in air. The EM ESTEPE dependency of the SS results suggests SS is bypassed when simulating the transport of charged particles in magnetic fields. Fano test results for in water calculation suggest that only a 0.2% accuracy can be achieved with the current implementation.
SU‐F‐J‐158: Respiratory Motion Resolved, Self‐Gated 4D‐MRI Using Rotating Cartesian K‐Space Sampling
Authors: Han F , Zhou Z , Yang Y , Sheng K , Hu P
Published: July 2016

Purpose:

Dynamic MRI has been used to quantify respiratory motion of abdominal organs in radiation treatment planning. Many existing 4D‐MRI methods based on 2D acquisitions suffer from limited slice resolution and additional stitching artifacts when evaluated in 3D1. To address these issues, we developed a 4D‐MRI (3D dynamic) technique with true 3D k‐space encoding and respiratory motion self‐gating.

Methods:

The 3D k‐space was acquired using a Rotating Cartesian K‐space (ROCK) pattern, where the Cartesian grid was reordered in a quasi‐spiral fashion with each spiral arm rotated using golden angle2. Each quasi‐spiral arm started with the k‐space center‐line, which were used as self‐gating3 signal for respiratory motion estimation. The acquired k‐space data was then binned into 8 respiratory phases and the golden angle ensures a near‐uniform k‐space sampling in each phase. Finally, dynamic 3D images were reconstructed using the ESPIRiT technique4. 4D‐MRI was performed on 6 healthy volunteers, using the following parameters (bSSFP, Fat‐Sat, TE/TR=2ms/4ms, matrix size=500×350×120, resolution=1×1×1.2mm, TA=5min, 8 respiratory phases). Supplemental 2D real‐time images were acquired in 9 different planes. Dynamic locations of the diaphragm dome and left kidney were measured from both 4D and 2D images. The same protocol was also performed on a MRI‐compatible motion phantom where the motion was programmed with different amplitude (10–30mm) and frequency (3–10/min).

Results:

High resolution 4D‐MRI were obtained successfully in 5 minutes. Quantitative motion measurements from 4D‐MRI agree with the ones from 2D CINE (<5% error). The 4D images are free of the stitching artifacts and their near‐isotropic resolution facilitates 3D visualization and segmentation of abdominal organs such as the liver, kidney and pancreas.

Conclusion:

Our preliminary studies demonstrated a novel ROCK 4D‐MRI technique with true 3D k‐space encoding and respiratory motion self‐gating. The technique leads to high‐resolution and artifacts‐free 4D images for improved abdominal organ motion studies.

K.S acknowledges funding support from NIH R01CA188300
SU‐F‐J‐163: In Vivo Quantification of Sequence Parameter Effect On Geometric Distortion Caused by Implanted Titanium Brachytherapy Applicator
Authors: Sullivan T , Diak A , Surucu M , Yacoub J , Harkenrider M , Shea S
Published: July 2016

Purpose:

The use of MR to plan and evaluate brachytherapy treatment for cervical cancer is increasing given the availability of MR conditional or safe applicators and MRI's proven superiority to CT for characterizing soft tissue lesions. The titanium applicators, however, cause geometric distortions or imaging artifacts, which reduce the utility of MRI for dosimetry. We sought to quantify the observed volume of the same applicator on a previously optimized T2 sequence in comparison to the conventional T2 sequence and CT obtained for brachytherapy planning.

Methods:

Prior work with testing in phantoms showed that increases in readout bandwidth yielded reductions in artifact area and distortion measurements even with voxel increases. Following IRB approval, nine patients with titanium tandem & ovoid applicator (Varian Medical Systems) in place were scanned with a standard periprocedural protocol which included sagittal T2 fast spin echo (FSE) acquisition (res 0.98×0.78×4.0 mm3; BW 200Hz). An additional T2‐weighted FSE sequence (res 0.98×0.98×3–4 mm3; BW500Hz) with increased readout bandwidth, readout voxel size, and echo train length was added to the protocol. Volume measurements of the applicator (from tip to cervical stop) were hand‐segmented in Velocity AI 3.1 (Velocity Medical Solutions) for the two T2 FSE sequences and a planning CT obtained shortly after MRI. Differences were analyzed using a paired t‐test.

Results:

Average apparent volumes of the applicator on standard T2 sequence, decreased bandwidth T2 sequence and CT were 5.922±1.283 cm3, 4.544±1.524 cm3, and 2.304±0.509 cm3 respectively.

Conclusion:

Apparent volumes of a brachytherapy applicator can be compared in vivo. The modified sequence results in decreased apparent size of the cervical applicator. Both MR sequence volumes were larger than the planning CT, which was expected. Future work will focus on the diagnostic quality of the new sequence and quantifying any geometric shifts after CT to MRI registration based on anatomical landmarks.
SU‐F‐J‐165: Spectral Characterization of Tissues in High Spectral and Spatial Resolution MR Images in Vitro: Implications for Classification‐Based Synthetic CT Algorithm
Authors: Diak A , Shea S , Surucu M , Gros S , Medved M , Karczmar G , Small W , Roeske J
Published: July 2016

Purpose:

To investigate features of spectral parameters of tissue for use in a synthetic CT algorithm.

Methods:

A phantom of fresh beef leg embedded in 1% agarose gel was imaged at 1.5T on a Siemens Aero using HiSS MRI with 1.0mm resolution and 11.1Hz spectral resolution. Post‐processing was performed off‐line and included labeling of fat and water peaks, and fitting the spectra. All spectra were normalized by the noise within each voxel; thus all parameters are reported in units of signal‐to‐noise (SNR). Images of the peak height (PH) and peak integral (PI) of both the water and fat resonances were analyzed. Several regions‐of‐interest (ROIs) were identified: bone marrow, cortical bone, adipose tissue, muscle, agar gel, and air. Distributions and scatterplots were generated. Water PH in cortical bone was compared to air using a t‐test. Composition of the various ROIs was also reported.

Results:

Bone marrow was 97% fat (mean PH=4.5SNR). Adipose tissue was mostly mixed fat/water voxels (66%); mean fat and water PH were 4.1 and 7.8 SNR, respectively. Cortical bone was 70% water (PH=2.3SNR). Muscle was 85% water (PH=4.2SNR, with fat PH=1.6SNR). For the air ROI, mean fat and water PH were 1.1SNR and 1.1SNR, respectively. The scatterplot of PH (water vs. fat) showed fair separation of bone marrow and adipose tissue. Water vs. fat PI scatterplot showed good separation of muscle and cortical bone. Cortical bone water PH was significantly different than air (p<0.05). A scatterplot of the water PH vs. pixel value of a summation of the first three echoes showed a different distribution with slightly better separation of air and bone.

Conclusion:

This study shows promising results for utilizing HiSS imaging in a classification‐based syn‐CT algorithm. Reducing early echo times would improve SNR of bone and will be investigated along with further optimization.
SU‐F‐J‐171: Robust Atlas Based Segmentation of the Prostate and Peripheral Zone Regions On MRI Utilizing Multiple MRI System Vendors
Authors: Padgett K , Swallen A , Nelson A , Pollack A , Stoyanova R
Published: July 2016

Purpose:

Automatically generated prostate MRI contours can be used to aid in image registration with CT or ultrasound and to reduce the burden of contouring for radiation treatment planning. In addition, prostate and zonal contours can assist to automate quantitative imaging features extraction and the analyses of longitudinal MRI studies. These potential gains are limited if the solutions are not compatible across different MRI vendors. The goal of this study is to characterize an atlas based automatic segmentation procedure of the prostate collected on MRI systems from multiple vendors.

Methods:

The prostate and peripheral zone (PZ) were manually contoured by an expert radiation oncologist on T2‐weighted scans acquired on both GE (n=31) and Siemens (n=33) 3T MRI systems. A leave‐one‐out approach was utilized where the target subject is removed from the atlas before the segmentation algorithm is initiated. The atlas‐segmentation method finds the best nine matched atlas subjects and then performs a normalized intensity‐based free‐form deformable registration of these subjects to the target subject. These nine contours are then merged into a single contour using Simultaneous Truth and Performance Level Estimation (STAPLE). Contour comparisons were made using Dice similarity coefficients (DSC) and Hausdorff distances.

Results:

Using the T2 FatSat (FS) GE datasets the atlas generated contours resulted in an average DSC of 0.83±0.06 for prostate, 0.57±0.12 for PZ and 0.75±0.09 for CG. Similar results were found when using the Siemens data with a DSC of 0.79±0.14 for prostate, 0.54±0.16 and 0.70±0.9. Contrast between prostate and surrounding anatomy and between the PZ and CG contours for both vendors demonstrated superior contrast separation; significance was found for all comparisons p‐value < 0.0001.

Conclusion:

Atlas‐based segmentation yielded promising results for all contours compared to expertly defined contours in both Siemens and GE 3T systems providing fast and automatic segmentation of the prostate.

Funding Support, Disclosures, and Conflict of Interest: AS Nelson is a partial owner of MIM Software, Inc. AS Nelson, and A Swallen are current employees at MIM Software, Inc.
SU‐F‐J‐174: A Series of Computational Human Phantoms in DICOM‐RT Format for Normal Tissue Dose Reconstruction in Epidemiological Studies
Authors: Pyakuryal A , Moroz B , Lee C , Pelletier C , Jung J , Lee C
Published: July 2016

Purpose:

Epidemiological studies of second cancer risk in radiotherapy patients often require individualized dose estimates of normal tissues. Prior to 3D conformal radiation therapy planning, patient anatomy information was mostly limited to 2D radiological images or not even available. Generic patient CT images are often used in commercial radiotherapy treatment planning system (TPS) to reconstruct normal tissue doses. The objective of the current work was to develop a series of reference size computational human phantoms in DICOM‐RT format for direct use in dose reconstruction in TPS.

Methods:

Contours of 93 organs and tissues were extracted from a series of pediatric and adult hybrid computational human phantoms (newborn, 1‐, 5‐, 10‐, 15‐year‐old, and adult males and females) using Rhinoceros software. A MATLAB script was created to convert the contours into the DICOM‐RT structure format. The simulated CT images with the resolution of 1×1×3 mm3 were also generated from the binary phantom format and coupled with the DICOM‐structure files. Accurate volumes of the organs were drawn in the format using precise delineation of the contours in converted format. Due to complex geometry of organs, higher resolution (1×1×1 mm3) was found to be more efficient in the conversion of newborn and 1‐year‐old phantoms.

Results:

Contour sets were efficiently converted into DICOM‐RT structures in relatively short time (about 30 minutes for each phantom). A good agreement was observed in the volumes between the original phantoms and the converted contours for large organs (NRMSD<1.0%) and small organs (NRMSD<7.7%).

Conclusion:

A comprehensive series of computational human phantoms in DICOM‐RT format was created to support epidemiological studies of second cancer risks in radiotherapy patients. We confirmed the DICOM‐RT phantoms were successfully imported into the TPS programs of major vendors.
SU‐F‐J‐176: Development of a Patient‐Specific 3D Couplant Pad for Ultrasound IGRT
Authors: Kim H , Chang A , Ye S
Published: July 2016

Purpose:

to overcome the several issues of ultrasound image‐guided radiation therapy (US IGRT) such as probe pressure and optical tracking disability by using a patient‐specific three‐dimensional couplant pad (CP) fabricated by a patient's skin mold using a 3D printing technique.

Methods:

A CP was then fabricated by pouring gelatin solution into a fixed‐shape container accommodating the patient skin mold fabricated by a 3D printer. A breast phantom was fabricated with the compound of gelatin and agarose and a phantom study was carried out. From four patients who underwent US IGRT, total 486 ultrasound images with and without a CP were acquired before treatment. Effectiveness of the use of the CP was evaluated.

Results:

The positioning accuracies in the phantom study were 0.9 ± 0.3 mm and 1.3 ± 0.4 mm with and without the CP in 3D vector amplitude, respectively. In the patient study, the use of CP reduced the mean target shift from 4.7 mm to 3.7 mm in 3D vector amplitude and the one standard deviation from 2.2 mm to 1.7 mm. It also improved the image contrast around the treatment target by 10 %. The centroid offset of the target volume affected from the US scanning coverage and the target deformation due to the excessive probe pressure was decreased from 4.4 mm to 2.9 mm due to the use of CP. Its difference among three different users was statistically significant (p=0.020) without the use of CP but not significantly different (p=0.133) with the use of CP.

Conclusion:

Our patient‐specific 3D CP using a mold by 3D printing technique is a promising strategy for improving tracking accuracy, image quality, and inter‐observer variation for ultrasound‐based image guided radiotherapy. In addition to its conventional advantage of non‐invasiveness, US can be more facilitated in radiotherapy by the developed CP.
SU‐F‐J‐177: A Novel Image Analysis Technique (center Pixel Method) to Quantify End‐To‐End Tests
Authors: Wen N , Snyder K , Scheib S , Qin Y , Li H , Chetty I
Published: July 2016

Purpose:

To implement a novel image analysis technique, “center pixel method”, to quantify end‐to‐end tests accuracy of a frameless, image guided stereotactic radiosurgery system.

Methods:

The localization accuracy was determined by delivering radiation to an end‐to‐end prototype phantom. The phantom was scanned with 0.8 mm slice thickness. The treatment isocenter was placed at the center of the phantom. In the treatment room, CBCT images of the phantom (kVp=77, mAs=1022, slice thickness 1 mm) were acquired to register to the reference CT images. 6D couch correction were applied based on the registration results. Electronic Portal Imaging Device (EPID)‐based Winston Lutz (WL) tests were performed to quantify the errors of the targeting accuracy of the system at 15 combinations of gantry, collimator and couch positions. The images were analyzed using two different methods. a) The classic method. The deviation was calculated by measuring the radial distance between the center of the central BB and the full width at half maximum of the radiation field. b) The center pixel method. Since the imager projection offset from the treatment isocenter was known from the IsoCal calibration, the deviation was determined between the center of the BB and the central pixel of the imager panel.

Results:

Using the automatic registration method to localize the phantom and the classic method of measuring the deviation of the BB center, the mean and standard deviation of the radial distance was 0.44 ± 0.25, 0.47 ± 0.26, and 0.43 ± 0.13 mm for the jaw, MLC and cone defined field sizes respectively. When the center pixel method was used, the mean and standard deviation was 0.32 ± 0.18, 0.32 ± 0.17, and 0.32 ± 0.19 mm respectively.

Conclusion:

Our results demonstrated that the center pixel method accurately analyzes the WL images to evaluate the targeting accuracy of the radiosurgery system.

The work was supported by a Research Scholar Grant, RSG‐15‐137‐01‐CCE from the American Cancer Society.
SU‐F‐J‐178: A Computer Simulation Model Observer for Task‐Based Image Quality Assessment in Radiation Therapy
Authors: Dolly S , Yu L , Mutic S , Anastasio M , Li H
Published: July 2016

Purpose:

Traditionally, image quality in radiation therapy is assessed subjectively or by utilizing physically‐based metrics. Some model observers exist for task‐based medical image quality assessment, but almost exclusively for diagnostic imaging tasks. As opposed to disease diagnosis, the task for image observers in radiation therapy is to utilize the available images to design and deliver a radiation dose which maximizes patient disease control while minimizing normal tissue damage. The purpose of this study was to design and implement a new computer simulation model observer to enable task‐based image quality assessment in radiation therapy.

Methods:

A modular computer simulation framework was developed to resemble the radiotherapy observer by simulating an end‐to‐end radiation therapy treatment. Given images and the ground‐truth organ boundaries from a numerical phantom as inputs, the framework simulates an external beam radiation therapy treatment and quantifies patient treatment outcomes using the previously defined therapeutic operating characteristic (TOC) curve. As a preliminary demonstration, TOC curves were calculated for various CT acquisition and reconstruction parameters, with the goal of assessing and optimizing simulation CT image quality for radiation therapy. Sources of randomness and bias within the system were analyzed.

Results:

The relationship between CT imaging dose and patient treatment outcome was objectively quantified in terms of a singular value, the area under the TOC (AUTOC) curve. The AUTOC decreases more rapidly for low‐dose imaging protocols. AUTOC variation introduced by the dose optimization algorithm was approximately 0.02%, at the 95% confidence interval.

Conclusion:

A model observer has been developed and implemented to assess image quality based on radiation therapy treatment efficacy. It enables objective determination of appropriate imaging parameter values (e.g. imaging dose). Framework flexibility allows for incorporation of additional modules to include any aspect of the treatment process, and therefore has great potential for both assessment and optimization within radiation therapy.