bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐J‐165: Ghost Marker Prediction, Detection and Elimination in Marker‐Based Optical Tracking Systems
Authors: Yan G , Li J , Mittauer K , Huang Y , Lu B , Liu C
Published: July 2014

Purpose:

In optical tracking system (OTS), ghost marker (GM) limits users' flexibility in marker placement and can lead to incorrect patient setup or system execution failure. This work aims to model the phenomenon of GM and propose robust algorithms for GM prediction, detection and elimination.

Methods:

Two GMs occur when two markers and two sensors of the OTS are coplanar. However, when one marker gradually moves out of the plane and breaks the coplanar condition, both GMs briefly remain before they completely disappear. To explain this effect, we expanded line‐line intersection definition and the coplanar condition: two lines intersect when their distance is less than threshold Ω and involved points are said to be coplanar within Ω. We fixed one marker and moved the other through the plane with a 0.1 mm step length. Sensor positions were calculated using a back projection method when coplanar condition was strictly satisfied. Ω was determined as line‐line distance when ghost marker first appeared. The prediction algorithm checks whether any two given markers are coplanar with the two sensors within Ω; the detection algorithm checks whether any marker is near the intersection of lines connecting sensors and two other markers. A backtracking pattern matching algorithm matches markers to reference markers and eliminates unmatched ones as GMs. The algorithms were validated with two phantoms and 40 SBRT patients using 5~8 markers.

Results:

The prediction success rates were 97.7% and 89.6% and the detection success rates were 100% on the two phantoms (1544 ' 2045 samples). The matching success rates were 100% on both phantoms and patients.

Conclusion:

The proposed model precisely explains the phenomenon of GMs in marker‐based OTS. The proposed detection and matching algorithms guarantee complete elimination of GMs, thus avoiding errors in SBRT tracking and providing user maximum flexibility in marker placement.
SU‐E‐J‐168: Automated Pancreas Segmentation Based On Dynamic MRI
Authors: Gou S , Rapacchi S , Hu P , Sheng K
Published: July 2014

Purpose:

MRI guided radiotherapy is particularly attractive for abdominal targets with low CT contrast. To fully utilize this modality for pancreas tracking, automated segmentation tools are needed. A hybrid gradient, region growth and shape constraint (hGReS) method to segment 2D upper abdominal dynamic MRI is developed for this purpose.

Methods:

2D coronal dynamic MR images of 2 healthy volunteers were acquired with a frame rate of 5 f/second. The regions of interest (ROIs) included the liver, pancreas and stomach. The first frame was used as the source where the centers of the ROIs were annotated. These center locations were propagated to the next dynamic MRI frame. 4‐neighborhood region transfer growth was performed from these initial seeds for rough segmentation. To improve the results, gradient, edge and shape constraints were applied to the ROIs before final refinement using morphological operations.

Results from hGReS and 3 other automated segmentation methods using edge detection, region growth and level set were compared to manual contouring. Results:

For the first patient, hGReS resulted in the organ segmentation accuracy as measure by the Dices index (0.77) for the pancreas. The accuracy was slightly superior to the level set method (0.72), and both are significantly more accurate than the edge detection (0.53) and region growth methods (0.42). For the second healthy volunteer, hGReS reliably segmented the pancreatic region, achieving a Dices index of 0.82, 0.92 and 0.93 for the pancreas, stomach and liver, respectively, comparing to manual segmentation. Motion trajectories derived from the hGReS, level set and manual segmentation methods showed high correlation to respiratory motion calculated using a lung blood vessel as the reference while the other two methods showed substantial motion tracking errors. hGReS was 10 times faster than level set.

Conclusion:

We have shown the feasibility of automated segmentation of the pancreas anatomy based on dynamic MRI.
SU‐E‐J‐175: Proton Dose Calculation On Scatter‐Corrected CBCT Image: Feasibility Study for Adaptive Proton Therapy
Authors: Park Y , Winey B , Sharp G
Published: July 2014

Purpose:

To demonstrate feasibility of proton dose calculation on scattercorrected CBCT images for the purpose of adaptive proton therapy.

Methods:

Two CBCT image sets were acquired from a prostate cancer patient and a thorax phantom using an on‐board imaging system of an Elekta infinity linear accelerator. 2‐D scatter maps were estimated using a previously introduced CT‐based technique, and were subtracted from each raw projection image. A CBCT image set was then reconstructed with an open source reconstruction toolkit (RTK). Conversion from the CBCT number to HU was performed by soft tissue‐based shifting with reference to the plan CT. Passively scattered proton plans were simulated on the plan CT and corrected/uncorrected CBCT images using the XiO treatment planning system. For quantitative evaluation, water equivalent path length (WEPL) was compared in those treatment plans.

Results:

The scatter correction method significantly improved image quality and HU accuracy in the prostate case where large scatter artifacts were obvious. However, the correction technique showed limited effects on the thorax case that was associated with fewer scatter artifacts. Mean absolute WEPL errors from the plans with the uncorrected and corrected images were 1.3 mm and 5.1 mm in the thorax case and 13.5 mm and 3.1 mm in the prostate case. The prostate plan dose distribution of the corrected image demonstrated better agreement with the reference one than that of the uncorrected image.

Conclusion:

A priori CT‐based CBCT scatter correction can reduce the proton dose calculation error when large scatter artifacts are involved. If scatter artifacts are low, an uncorrected CBCT image is also promising for proton dose calculation when it is calibrated with the soft‐tissue based shifting.
SU‐E‐J‐176: Results of Images Acquired with Backscattered MV Radiation Using a Pinhole Collimator
Authors: Bernard D , Turian J , Wu Z , Darwish N , Chu J , Bernard D
Published: July 2014

Purpose:

To ascertain the feasibility of acquiring real time images of small lung tumors from scattered photons while undergoing radiation treatment. There are several methodologies currently used to track tumor location such as MV‐cine acquisition and kV fluoroscopy. However, MVcine offers no information parallel to the beam axis while kV fluoroscopy offers little potential for soft tissue discernability while also increasing the patient dose. This study investigates the feasibility of observing an actual simulated tumor while exploring techniques that may improve image quality.

Methods:

A prototype imager consisting of a gamma camera pinhole collimator and a computed radiography (CR) plate were used in conjunction with a Varian TrueBeam linac. One study consisted of a 2.5 cm diameter solid water cylinder representing a solid tumor imbedded within a lung equivalent material slab. The cylinder with the lung slab was sandwiched between 1 cm lung equivalent slabs and these were sandwiched between 2 slabs of solid water. The top water slab was 1 cm thick. The other imaging study consisted of three different density plugs, 0.46, 1.09, and 1.82 g/cm3 placed on the accelerator couch. The gantry was orientated 70° relative to the CR plate. The slabs and plugs were irradiated with 2000 MU and 500 MU respectively using the 6FFF mode.

Results:

The solid water plug was visually discernible in the slab phantom. The ratio of the signal coming from the higher density plugs (placed on the treatment couch) to that between the plugs increased from 1.02 to about 3.0 after subtracting the background image acquired with no plugs present.

Conclusion:

Preliminary results indicate that a lung tumor could be visualized with scattered radiation during treatment. Improvements in discerning an object can be enhanced by filtering out the head leakage and background scattered radiation not emanating from the imaged object.
SU‐E‐J‐177: Understanding the Seminal Vesicle's Motion Relative to the Prostate: Range and Pattern
Authors: Sheng Y , Li T , Yin F , Ge Y , Wu Q
Published: July 2014

Purpose:

To investigate inter‐fractional (inter‐fx) and intra‐fractional (intra‐fx) seminal vesicle (SV) motion to determine motion management strategies for prostate SBRT.

Methods:

9 prostate SBRT patients (37Gy/5fx) were studied. All patients were implanted with fiducial markers in the prostate for soft tissue tracking. SVs were contoured on 5 pairs of preand post‐treatment CBCTs and the planning CT. All CT/CBCT images were registered according to fiducial markers. Inter‐fx motion was analyzed by comparing pre‐treatment CBCT with planning CT; intra‐fx motion was analyzed by comparing pre‐ and post‐treatment CBCTs. Both the center of mass (COM) and boarders of SV were assessed to determine motion ranges. SV were segmented into left SV (LSV) and right SV (RSV) to analyze independent motion patterns.

Results:

Inter‐fx SV motion is significant in all directions. The 95% range of COM is 9.4, 14.3 and 10.3 mm for LR, AP and SI direction. Larger motion variation is observed with SV boarder. A 5 mm margin can provide 60% and 58% CTV coverage for COM and border. Intra‐fx SV motion is smaller than inter‐fx. A 5 mm margin can provide 93% and 84% CTV coverage for COM and boarder. LSV and RSV show different motion patterns in LR direction with correlation of −0.194 for COM while AP motion is strongly correlated (0.837). SV COM shows weak correlation with RSV COM, LSV COM, right boarder of RSV and left boarder of LSV (coefficient of 0.326, 0.450, 0.087, −0.192). SV COM's AP motion is strongly correlated with both LSV and RSV.

Conclusion:

SV moves substantially both inter‐ and intra‐fractionally, indicating the difficulty of covering CTV with fixed 5mm margin. Additional considerations/strategies in margin design are required. SV boarder motion shows independence from COM, indicating potential underdose to CTV if tracking with COM. Soft tissue imaging is highly desired for SV localization.
SU‐E‐J‐178: A Normalization Method Can Remove Discrepancy in Ventilation Function Due to Different Breathing Patterns
Authors: Qu H , Yu N , Stephans K , Xia P
Published: July 2014

Purpose:

To develop a normalization method to remove discrepancy in ventilation function due to different breathing patterns.

Methods:

Twenty five early stage non‐small cell lung cancer patients were included in this study. For each patient, a ten phase 4D‐CT and the voluntarily maximum inhale and exhale CTs were acquired clinically and retrospectively used for this study. For each patient, two ventilation maps were calculated from voxel‐to‐voxel CT density variations from two phases of the quiet breathing and two phases of the extreme breathing. For the quiet breathing, 0% (inhale) and 50% (exhale) phases from 4D‐CT were used. An in‐house tool was developed to calculate and display the ventilation maps. To enable normalization, the whole lung of each patient was evenly divided into three parts in the longitude direction at a coronal image with a maximum lung cross section. The ratio of cumulated ventilation from the top one‐third region to the middle one‐third region of the lung was calculated for each breathing pattern. Pearson's correlation coefficient was calculated on the ratios of the two breathing patterns for the group.

Results:

For each patient, the ventilation map from the quiet breathing was different from that of the extreme breathing. When the cumulative ventilation was normalized to the middle one‐third of the lung region for each patient, the normalized ventilation functions from the two breathing patterns were consistent. For this group of patients, the correlation coefficient of the normalized ventilations for the two breathing patterns was 0.76 (p < 0.01), indicating a strong correlation in the ventilation function measured from the two breathing patterns.

Conclusion:

For each patient, the ventilation map is dependent of the breathing pattern. Using a regional normalization method, the discrepancy in ventilation function induced by the different breathing patterns thus different tidal volumes can be removed.
SU‐E‐J‐179: Prediction of Pelvic Nodal Coverage Using Mutual Information Between Cone‐Beam and Planning CTs
Authors: Jani S , Kishan A , O' Connell D , King C , Steinberg M , Low D , Lamb J
Published: July 2014

Purpose:

To investigate if pelvic nodal coverage for prostate patients undergoing intensity modulated radiotherapy (IMRT) can be predicted using mutual image information computed between planning and cone‐beam CTs (CBCTs).

Methods:

Four patients with high‐risk prostate adenocarcinoma were treated with IMRT on a Varian TrueBeam. Plans were designed such that 95% of the nodal planning target volume (PTV) received the prescription dose of 45 Gy (N=1) or 50.4 Gy (N=3). Weekly CBCTs (N=25) were acquired and the nodal clinical target volumes and organs at risk were contoured by a physician. The percent nodal volume receiving prescription dose was recorded as a ground truth. Using the recorded shifts performed by the radiation therapists at the time of image acquisition, CBCTs were aligned with the planning kVCT. Mutual image information (MI) was calculated between the CBCT and the aligned planning CT within the contour of the nodal PTV. Due to variable CBCT fields‐of‐view, CBCT images covering less than 90% of the nodal volume were excluded from the analysis, resulting in the removal of eight CBCTs.

Results:

A correlation coefficient of 0.40 was observed between the MI metric and the percent of the nodal target volume receiving the prescription dose. One patient's CBCTs had clear outliers from the rest of the patients. Upon further investigation, we discovered image artifacts that were present only in that patient's images. When those four images were excluded, the correlation improved to 0.81.

Conclusion:

This pilot study shows the potential of predicting pelvic nodal dosimetry by computing the mutual image information between planning CTs and patient setup CBCTs. Importantly, this technique does not involve manual or automatic contouring of the CBCT images. Additional patients and more robust exclusion criteria will help validate our findings.
SU‐E‐J‐180: Monitoring Thermochemical Ablation Using Fast Chemical Shift Magnetic Resonance Imaging
Authors: MacLellan C , Maier F , Cressman E , Stafford R
Published: July 2014

Purpose:

To investigate the feasibility of using fast chemical shift imaging (fCSI) to directly image the chemical species associated with exothermic acid/base reactions during thermochemical ablation (TCA) injections.

Methods:

Five different mixtures of 10M acetic acid and 10M sodium hydroxide representing pure reagents, partial neutralization, and complete neutralization (1:0, 3:1, 1:1, 1:3, 0:1) were prepared in 15mL centrifuge tubes to assess impact on fCSI. An MR spectroscopy (MRS) acquisition provided corroborative data. An fCSI acquisition used a multi gradient echo acquisition (n=16) with autoregressive moving average model based spectral estimation. The acquisition was repeated 8 times with shifted echo trains to effectively increase the number of echoes per k‐space line to 128 and the spectral bandwidth from 2.82 ppm to 22.7 ppm. The T1 of each sample was measured using inversion recovery FSE.

Results:

The 1H chemical shift of acetate measured via MRS was 2.65 ppm and 2.40 ppm to 2.87 ppm (mean=2.63 ppm) on fCSI, with 2.8 ppm expected. The difference between the chemical shift of water and acetate within the same sample and the T1 in each sample increase with the concentration of acetate ion and range from 2.82ppm to 3.90 ppm and 323ms to 1743ms, respectively.

Conclusion:

In this work we evaluated a fCSI acquisition for measuring the MR parameters of acid and base solutions that have been investigated for use in TCA. The importance of shimming the main magnetic field was highlighted by the variation in the position of the acetate peak in the fCSI acquisition. The changes in the chemical shift difference between water and acetate and T1 relaxation suggest a dependence on acetate ion concentration that may be useful and must be accounted for in the monitoring of these procedures.
SU‐E‐J‐181: Magnetic Resonance Image‐Guided Radiation Therapy Workflow: Initial Clinical Experience
Authors: Green O , Kashani R , Santanam L , Hand T , Steele C , Victoria J , Wooten H , Li H , Rodriguez V , Hu Y , Mutic S
Published: July 2014

Purpose:

The aims of this work are to describe the workflow and initial clinical experience treating patients with an MRI‐guided radiotherapy (MRIGRT) system.

Methods:

Patient treatments with a novel MR‐IGRT system started at our institution in mid‐January. The system consists of an on‐board 0.35‐T MRI, with IMRT‐capable delivery via doubly‐focused MLCs on three 6 0 Co heads. In addition to volumetric MR‐imaging, real‐time planar imaging is performed during treatment. So far, eleven patients started treatment (six finished), ranging from bladder to lung SBRT. While the system is capable of online adaptive radiotherapy and gating, a conventional workflow was used to start, consisting of volumetric imaging for patient setup using visible tumor, evaluation of tumor motion outside of PTV on cine images, and real‐time imaging. Workflow times were collected and evaluated to increase efficiency and evaluate feasibility of adding the adaptive and gating features while maintaining a reasonable patient throughput.

Results:

For the first month, physicians attended every fraction to provide guidance on identifying the tumor and an acceptable level of positioning and anatomical deviation. Average total treatment times (including setup) were reduced from 55 to 45 min after physician presence was no longer required and the therapists had learned to align patients based on soft‐tissue imaging. Presently, the source strengths were at half maximum (7.7K Ci each), therefore beam‐on times will be reduced after source replacement. Current patient load is 10 per day, with increase to 25 anticipated in the near future.

Conclusion:

On‐board, real‐time MRI‐guided RT has been incorporated into clinical use. Treatment times were kept to reasonable lengths while including volumetric imaging, previews of tumor movement, and physician evaluation. Workflow and timing is being continuously evaluated to increase efficiency. In near future, adaptive and gating capabilities of the system will be implemented.
SU‐E‐J‐192: Verification of 4D‐MRI Internal Target Volume Using Cine MRI
Authors: Lafata K , Czito B , Palta M , Bashir M , Yin F , Cai J
Published: July 2014

Purpose:

To investigate the accuracy of 4D‐MRI in determining the Internal Target Volume (ITV) used in radiation oncology treatment planning of liver cancers. Cine MRI is used as the standard baseline in establishing the feasibility and accuracy of 4D‐MRI tumor motion within the liver.

Methods:

IRB approval was obtained for this retrospective study. Analysis was performed on MR images from four patients receiving external beam radiation therapy for liver cancer at our institution. Eligible patients received both Cine and 4D‐MRI scans before treatment. Cine images were acquired sagittally in real time at a slice bisecting the tumor, while 4D images were acquired volumetrically. Cine MR DICOM headers were manipulated such that each respiratory frame was assigned a unique slice location. This approach permitted the treatment planning system (Eclipse, Varian Medical Systems) to recognize a complete respiratory cycle as a “volume”, where the gross tumor was contoured temporally. Software was developed to calculate the union of all frame contours in the structure set, resulting in the corresponding plane of the ITV projecting through the middle of the tumor, defined as the Internal Target Area (ITA). This was repeated for 4D‐MRI, at the corresponding slice location, allowing a direct comparison of ITAs obtained from each modality.

Results:

Four patients have been analyzed. ITAs contoured from 4D‐MRI correlate with contours from Cine MRI. The mean error of 4D values relative to Cine values is 7.67 +/− 2.55 %. No single ITA contoured from 4D‐MRI demonstrated more than 10.5 % error compared to its Cine MRI counterpart.

Conclusion:

Motion management is a significant aspect of treatment planning within dynamic environments such as the liver, where diaphragmatic and cardiac activity influence plan accuracy. This small pilot study suggests that 4D‐MRI based ITA measurements agree with Cine MRI based measurements, an important step towards clinical implementation.

NIH 1R21CA165384‐01A1
SU‐E‐J‐193: Feasibility of MRI‐Only Based IMRT Planning for Pancreatic Cancer
Authors: Prior P , Botros M , Chen X , Paulson E , Erickson B , Li X
Published: July 2014

Purpose:

With the increasing use of MRI simulation and the advent of MRI‐guided delivery, it is desirable to use MRI only for treatment planning. In this study, we assess the dosimetric difference between MRI‐ and CTbased IMRT planning for pancreatic cancer.

Methods:

Planning CTs and MRIs acquired for a representative pancreatic cancer patient were used. MRI‐based planning utilized forced relative electron density (rED) assignment of organ specific values from IRCU report 46, where rED = 1.029 for PTV and a rED = 1.036 for non‐specified tissue (NST). Six IMRT plans were generated with clinical dose‐volume (DV) constraints using a research Monaco planning system employing Monte Carlo dose calculation with optional perpendicular magnetic field (MF) of 1.5T. The following five plans were generated and compared with the planning CT: 1.) CT plan with MF and dose recalculation without optimization; 2.) MRI (T2) plan with target and OARs redrawn based on MRI, forced rED, no MF, and recalculation without optimization; 3.) Similar as in 2 but with MF; 4.) MRI plan with MF but without optimization; and 5.) Similar as in 4 but with optimization.

Results:

Generally, noticeable differences in PTV point doses and DV parameters (DVPs) between the CT‐and MRI‐based plans with and without the MF were observed. These differences between the optimized plans were generally small, mostly within 2%. Larger differences were observed in point doses and mean doses for certain OARs between the CT and MRI plan, mostly due to differences between image acquisition times.

Conclusion:

MRI only based IMRT planning for pancreatic cancer is feasible. The differences observed between the optimized CT and MRI plans with or without the MF were practically negligible if excluding the differences between MRI and CT defined structures.
SU‐E‐J‐197: A Moving Least Squares Approach for Computing Spatially Accurate Transformations That Satisfy Strict Physiologic Constraints
Authors: Castillo E , Castillo R , Fuentes D , Guerrero T
Published: July 2014

Purpose:

To efficiently compute a physiologically realistic spatial transformation from a sparse point cloud of displacement estimates using Moving Least Squares (MLS) and any combination of upper bound, lower bound, or equality constraints placed on the Jacobian. Whereas diffeomorphic deformable image registration (DIR) requires the transformation's Jacobian determinant to be positive, within the context of thoracic CT, a more appropriate constraint is to require positive Jacobian values that reflect a strictly contracting volume (inhale to exhale DIR) or expanding volume (exhale to inhale DIR).

Methods:

Jacobian constraints manifest as nonlinear inequality constraints that considerably complicate DIR computation. However, MLS defines a spatial transformation from a sparse point cloud of estimated displacements and provides simple analytic derivative estimates for all voxel locations. Given displacement estimates from automated block matching algorithm, an optimal MLS spatial transformation is determined by computing the minimal perturbation to the displacement data required to yield an MLS transformation that satisfies the Jacobian constraints.

Results:

Five publicly available (cases 6‐10 from www.dir‐lab.com) inhale/exhale thoracic CT image pairs each with 300 landmarks (for DIR validation) were registered by first obtaining a sparse point cloud of displacement estimates via block matching. Two MLS transformations were then computed, one with no Jacobian constraints and the other with strict contraction Jacobian constraints. Both MLS fields achieved similarly low average millimeter error on all five cases (between 1.16 – 1.26). However, the constrained MLS yielded a strict contraction (all Jacobian values between 0 and 1) while the unconstrained MLS resulted in regions of expansion (Jacobian values larger than 1) despite registering from inhale to exhale.

Conclusion:

The proposed MLS approach is capable of producing Jacobian constrained transformations without degrading spatial accuracy. Though applied to block match estimates, the approach can be employed in conjunction with displacement estimates from any DIR algorithm.
SU‐E‐J‐199: Evaluation of Motion Tracking Effects On Stereotactic Body Radiotherapy of Abdominal Targets
Authors: Monterroso M , Dogan N , Yang Y
Published: July 2014

Purpose:

To evaluate the effects of respiratory motion on the delivered dose distribution of CyberKnife motion tracking‐based stereotactic body radiotherapy (SBRT) of abdominal targets.

Methods:

Four patients (two pancreas and two liver, and all with 4DCT scans) were retrospectively evaluated. A plan (3D plan) using CyberKnife Synchrony was optimized on the end‐exhale phase in the CyberKnife's MultiPlan treatment planning system (TPS), with 40Gy prescribed in 5 fractions. A 4D plan was then created following the 4D planning utility in the MultiPlan TPS, by recalculating dose from the 3D plan beams on all 4DCT phases, with the same prescribed isodose line. The other seven phases of the 4DCT were then deformably registered to the end‐exhale phase for 4D dose summation. Doses to the target and organs at risk (OAR) were compared between 3D and 4D plans for each patient. The mean and maximum doses to duodenum, liver, spinal cord and kidneys, and doses to 5cc of duodenum, 700cc of liver, 0.25cc of spinal cord and 200cc of kidneys were used.

Results:

Target coverage in the 4D plans was about 1% higher for two patients and about 9% lower in the other two. OAR dose differences between 3D and 4D varied among structures, with doses as much as 8.26Gy lower or as much as 5.41Gy higher observed in the 4D plans.

Conclusion:

The delivered dose can be significantly different from the planned dose for both the target and OAR close to the target, which is caused by the relative geometry change while the beams chase the moving target. Studies will be performed on more patients in the future. The differences of motion tracking versus passive motion management with the use of internal target volumes will also be investigated.
SU‐E‐J‐208: Fast and Accurate Auto‐Segmentation of Abdominal Organs at Risk for Online Adaptive Radiotherapy
Authors: Gupta V , Wang Y , Romero A , Myronenko A , Jordan P , Heijmen B , Hoogeman M
Published: July 2014

Purpose:

Various studies have demonstrated that online adaptive radiotherapy by real‐time re‐optimization of the treatment plan can improve organs‐at‐risk (OARs) sparing in the abdominal region. Its clinical implementation, however, requires fast and accurate auto‐segmentation of OARs in CT scans acquired just before each treatment fraction. Autosegmentation is particularly challenging in the abdominal region due to the frequently observed large deformations. We present a clinical validation of a new auto‐segmentation method that uses fully automated non‐rigid registration for propagating abdominal OAR contours from planning to daily treatment CT scans.

Methods:

OARs were manually contoured by an expert panel to obtain ground truth contours for repeat CT scans (3 per patient) of 10 patients. For the non‐rigid alignment, we used a new non‐rigid registration method that estimates the deformation field by optimizing local normalized correlation coefficient with smoothness regularization. This field was used to propagate planning contours to repeat CTs. To quantify the performance of the auto‐segmentation, we compared the propagated and ground truth contours using two widely used metrics‐ Dice coefficient (Dc) and Hausdorff distance (Hd). The proposed method was benchmarked against translation and rigid alignment based auto‐segmentation.

Results:

For all organs, the auto‐segmentation performed better than the baseline (translation) with an average processing time of 15 s per fraction CT. The overall improvements ranged from 2% (heart) to 32% (pancreas) in Dc, and 27% (heart) to 62% (spinal cord) in Hd. For liver, kidneys, gall bladder, stomach, spinal cord and heart, Dc above 0.85 was achieved. Duodenum and pancreas were the most challenging organs with both showing relatively larger spreads and medians of 0.79 and 2.1 mm for Dc and Hd, respectively.

Conclusion:

Based on the achieved accuracy and computational time we conclude that the investigated auto‐segmentation method overcomes an important hurdle to the clinical implementation of online adaptive radiotherapy.

Partial funding for this work was provided by Accuray Incorporated as part of a research collaboration with Erasmus MC Cancer Institute.
SU‐E‐J‐212: Identifying Bones From MRI: A Dictionary Learnign and Sparse Regression Approach
Authors: Ruan D , Yang Y , Cao M , Hu P , Low D
Published: July 2014

Purpose:

To develop an efficient and robust scheme to identify bony anatomy based on MRI‐only simulation images.

Methods:

MRI offers important soft tissue contrast and functional information, yet its lack of correlation to electron‐density has placed it as an auxiliary modality to CT in radiotherapy simulation and adaptation. An effective scheme to identify bony anatomy is an important first step towards MR‐only simulation/treatment paradigm and would satisfy most practical purposes. We utilize a UTE acquisition sequence to achieve visibility of the bone. By contrast to manual + bulk or registration‐to identify bones, we propose a novel learning‐based approach for improved robustness to MR artefacts and environmental changes. Specifically, local information is encoded with MR image patch, and the corresponding label is extracted (during training) from simulation CT aligned to the UTE. Within each class (bone vs. nonbone), an overcomplete dictionary is learned so that typical patches within the proper class can be represented as a sparse combination of the dictionary entries. For testing, an acquired UTE‐MRI is divided to patches using a sliding scheme, where each patch is sparsely regressed against both bone and nonbone dictionaries, and subsequently claimed to be associated with the class with the smaller residual.

Results:

The proposed method has been applied to the pilot site of brain imaging and it has showed general good performance, with dice similarity coefficient of greater than 0.9 in a crossvalidation study using 4 datasets. Importantly, it is robust towards consistent foreign objects (e.g., headset) and the artefacts relates to Gibbs and field heterogeneity.

Conclusion:

A learning perspective has been developed for inferring bone structures based on UTE MRI. The imaging setting is subject to minimal motion effects and the post‐processing is efficient. The improved efficiency and robustness enables a first translation to MR‐only routine. The scheme generalizes to multiple tissue classes.
SU‐E‐J‐273: Simulation of the Radiation Response of Hypoxic Tumors
Authors: Espinoza I , Peschke P , Karger C
Published: July 2014

Purpose:

In radiotherapy, it is important to predict the response of tumour to irradiation prior to the treatment. Mathematical modelling of tumour control probability (TCP) based on the dose distribution, medical imaging and other biological information may help to improve this prediction and to optimize the treatment plan. The aim of this work is to develop an image based 3D multiscale radiobiological model, which describes the growth and the response to radiotherapy of hypoxic tumors.

Methods:

The computer model is based on voxels, containing tumour, normal (including capillary) and dead cells. Killing of tumour cells due to irradiation is calculated by the Linear Quadratic Model (extended for hypoxia), and the proliferation and resorption of cells are modelled by exponential laws. The initial shape of the tumours is taken from CT images and the initial vascular and cell density information from PET and/or MR images. Including the fractionation regime and the physical dose distribution of the radiation treatment, the model simulates the spatial‐temporal evolution of the tumor. Additionally, the dose distribution may be biologically optimized.

Results:

The model describes the appearance of hypoxia during tumour growth and the reoxygenation processes during radiotherapy. Among other parameters, the TCP is calculated for different dose distributions. The results are in accordance with published results.

Conclusion:

The simulation model may contribute to the understanding of the influence of biological parameters on tumor response during treatment, and specifically on TCP. It may be used to implement dose‐painting approaches. Experimental and clinical validation is needed.

This study is supported by a grant from the Ministry of Education of Chile, Programa Mece Educacion Superior (2).
SU‐E‐T‐06: A Comparison of IMRT Treatment of Esophageal Carcinoma in Elekta‐Precise and Varian23EX Linac
Authors: Bai W , Fan X , qiu r , Qiao X , Zhang R
Published: July 2014

Purpose:

To compare and analyze the characteristics of static intensity‐modulated radiotherapy (IMRT) plans designed on Elekta and Varian Linac in different esophageal cancer(EC), exploring advantages and disadvantages of different vendor Linac, thus can be better serve for clinical.

Methods:

Twenty‐four patients with EC were selected, including 6 cases located in the cervical, upper, middle and the lower thorax, respectively. Two IMRT plans were generated with the Oncentra planning system: in Elekta and Varian Linac, prescription dose of 60Gy in 30 fractions to the PTV. We examined the dose‐volume histogram parameters of PTV and the organs at risk (OAR) such as lungs, spinal cord and heart, and additional Monitor units(MU), treatment time, Homogeneity index(HI), Conformity index(CI) and Gamma index comparisons were performed.

Results:

All plans resulted in abundant dose coverage of PTV for EC of different locations. The doses to PTV, HI and OAR in Elekta plans were not statistically different in comparison with Varian plans, with the following exceptions: in cervical, upper and lower thoracic EC the PTV's CI, and in middle thorax EC PTV's D2, D50, V105 and PTV‐average were better in Elekta plans than in Varian plans. In the cervical, upper and the middle thorax EC, treatment time were significantly decreased in Varian plans as against Elekta plans, while in the lower thoracic EC treatment time were no striking difference. MUs and gamma index were similar between the two Linac plans.

Conclusion:

For the the middle thorax EC Varian plans is better than Elekta plans, not only in treatment time but in the PTV dose; while for the lower thorax EC Elekta plans is the first choice for better CI; for the other part of the EC usually Elekta plans can increase the CI, while Varian plans can reduce treatment time, can be selected according to the actual situation of the patient treatment.
SU‐E‐T‐08: A Convolution Model for Head Scatter Fluence in the Intensity Modulated Field
Authors: Chen M , Mo X , Chen Y , Parnell D , Key S , Olivera G , Galmarini W , Lu W
Published: July 2014

Purpose:

To efficiently calculate the head scatter fluence for an arbitrary intensity‐modulated field with any source distribution using the source occlusion model.

Method:

The source occlusion model with focal and extra focal radiation (Jaffray et al, 1993) can be used to account for LINAC head scatter. In the model, the fluence map of any field shape at any point can be calculated via integration of the source distribution within the visible range, as confined by each segment, using the detector eye's view. A 2D integration would be required for each segment and each fluence plane point, which is time‐consuming, as an intensity‐modulated field contains typically tens to hundreds of segments. In this work, we prove that the superposition of the segmental integrations is equivalent to a simple convolution regardless of what the source distribution is. In fact, for each point, the detector eye's view of the field shape can be represented as a function with the origin defined at the point's pinhole reflection through the center of the collimator plane. We were thus able to reduce hundreds of source plane integration to one convolution. We calculated the fluence map for various 3D and IMRT beams and various extra‐focal source distributions using both the segmental integration approach and the convolution approach and compared the computation time and fluence map results of both approaches.

Results:

The fluence maps calculated using the convolution approach were the same as those calculated using the segmental approach, except for rounding errors (<0.1%). While it took considerably longer time to calculate all segmental integrations, the fluence map calculation using the convolution approach took only ∼1/3 of the time for typical IMRT fields with ∼100 segments.

Conclusions:

The convolution approach for head scatter fluence calculation is fast and accurate and can be used to enhance the online process.
SU‐E‐T‐14: A Feasibility Study of Using Modified AP Proton Beam for Post‐Operative Pancreatic Cancer Therapy
Authors: Ding X , Witztum A , Kenton O , Younan F , Dormer J , Kremmel E , Lin H , Liu H , Tang S , Both S , Kassaee A , Avery S
Published: July 2014

Purpose:

Due to the unpredictability of bowel gas movement, the PA beam direction is always favored for robust proton therapy in post‐operative pancreatic cancer treatment. We investigate the feasibility of replacing PA beam with a modified AP beam to take the bowel gas uncertainty into account.

Methods:

Nine post‐operative pancreatic cancer patients treated with proton therapy (5040cGy, 28 fractions) in our institution were randomly selected. The original plan uses PA and lateral direction passive‐scattering proton beams. Beam weighting is about 1:1. All patients received weekly verification CTs to assess the daily variations(total 17 verification CTs). The PA direction beam was replaced by two other groups of AP direction beam. Group AP: takes 3.5% range uncertainty into account. Group APmod: compensates the bowel gas uncertainty by expanding the proximal margin to 2cm more. The 2cm margin was acquired from the average bowel diameter in from 100 adult abdominal CT scans near pancreatic region (+/‐ 5cm superiorly and inferiorly). Dose Volume Histograms(DVHs) of the verification CTs were acquired for robustness study.

Results:

Without the lateral beam, Group APmod is as robust as Group PA. In Group AP, more than 10% of iCTV D98/D95 were reduced by 4–8%. LT kidney and Liver dose robustness are not affected by the AP/PA beam direction. There is 10% of chance that RT kidney and cord will be hit by AP proton beam due to the bowel gas. Compared to Group PA, APmod plan reduced the dose to kidneys and cord max significantly, while there is no statistical significant increase in bowel mean dose.

Conclusion:

APmod proton beam for the target coverage could be as robust as the PA direction without sacrificing too much of bowel dose. When the AP direction beam has to be selected, a 2cm proximal margin should be considered.
SU‐E‐T‐36: An Investigation of the Margin From CTV to PTV Using Retraction Method for Cervical Carcinoma
Authors: Wu D , Chen J , Hao Y , Liao C , Huang Y , Mo Y , Wei Y
Published: July 2014

Purpose:

This work employs the retraction method to compute and evaluate the margin from CTV to PTV based on the influence of target dosimetry of setup errors during cervical carcinoma patients treatment.

Methods:

Sixteen patients with cervical cancer were treated by Elekta synergy and received a total of 305 KV‐CBCT images. The iso‐center of the initial plans were changed according to the setup errors to simulate radiotherapy and then recalculated the dose distribution using leaf sequences and MUs for individual plans. The margin from CTV to PTV will be concluded both by the method of retracting (Fixed the PTV of the original plan, and retract PTV a certain distance defined as simulative organization CTVnx. The minimum distance value from PTV to CTVnx which get specified doses, namely guarantee at least 99% CTV volume can receive the dose of 95%, is the margin CTV to PTV we found) and the former formula method.

Results:

(1)The setup errors of 16 patients in X, Y and Z directions were(1.13±2.94) mm,(−1.63±7.13) mm,(−0.65±2.25) mm. (2) The distance between CTVx and PTV was 5, 9 and 3mm in X, Y and Z directions According to 2.5+0.7σ. (3) Transplantation plans displayed 99% of CTVx10‐ CTVx7 and received 95% of prescription dose, but CTVx6‐ CTVx3 departed from standard of clinic.In order to protect normal tissues, we selected 7mm as the minimum value of the margin from CTV to PTV.

Conclusion:

We have test an retraction method for the margin from CTV to PTV evaluation. The retraction method is more reliable than the formula method for calculating the margin from the CTV to the PTV, because it represented practice of treatment, and increasing a new method in this field.
SU‐E‐T‐41: Analysis of GI Dose Variability Due to Intrafraction Setup Variance
Authors: Phillips J , Wolfgang J
Published: July 2014

Purpose:

Proton SBRT (stereotactic body radiation therapy) can be an effective modality for treatment of gastrointestinal tumors, but limited in practice due to sensitivity with respect to variation in the RPL (radiological path length). Small, intrafractional shifts in patient anatomy can lead to significant changes in the dose distribution. This study describes a tool designed to visualize uncertainties in radiological depth in patient CT's and aid in treatment plan design.

Methods:

This project utilizes the Shadie toolkit, a GPU‐based framework that allows for real‐time interactive calculations for volume visualization. Current SBRT simulation practice consists of a serial CT acquisition for the assessment of inter‐ and intra‐fractional motion utilizing patient specific immobilization systems. Shadie was used to visualize potential uncertainties, including RPL variance and changes in gastric content. Input for this procedure consisted of two patient CT sets, contours of the desired organ, and a pre‐calculated dose. In this study, we performed rigid registrations between sets of 4DCT's obtained from a patient with varying setup conditions. Custom visualizations are written by the user in Shadie, permitting one to create color‐coded displays derived from a calculation along each ray.

Results:

Serial CT data acquired on subsequent days was analyzed for variation in RPB and gastric content. Specific shaders were created to visualize clinically relevant features, including RPL (radiological path length) integrated up to organs of interest. Using pre‐calculated dose distributions and utilizing segmentation masks as additional input allowed us to further refine the display output from Shadie and create tools suitable for clinical usage.

Conclusion:

We have demonstrated a method to visualize potential uncertainty for intrafractional proton radiotherapy. We believe this software could prove a useful tool to guide those looking to design treatment plans least insensitive to motion for patients undergoing proton SBRT in the GI tract.
SU‐E‐T‐42: Analysis of Spatial Trade‐Offs for a Spinal SRS Case
Authors: Viulet T , Blanck O , Schlaefer A
Published: July 2014

Purpose:

To study whether the spatially limited relaxation of dose constraints could improve the target coverage for spinal lesions. Particularly, to illustrate that the localized relaxation of areas identified as restricting can be used to consider critical trade‐offs.

Methods:

We used an in‐house planning system based on constrained optimization to explore potential spatial dose distributions. A three‐dimensional visualization of the iso‐dose surfaces and the dose bounds allowed studying the location of limiting constraints. An interactive tool was used to set optimization objectives and to selectively relax constraints. Considering a spinal tumor treatment which resulted in a relapse close to the spinal cord we studied trade‐offs that could increase the coverage of the original GTV. Starting from a plan similar to the original treatment our goal was to cover the GTV facing the spinal cord and including the area of recurrence. Subsequently, different trade‐offs are analyzed. For comparison, a global relaxation of the bounds was studied.

Results:

We identified that the upper bounds on spinal cord, esophagus and PTV are partially limiting for the optimization. Relaxing limiting constraints in lateral directions, the upper bound in part of the GTV, and on the surface of the spine increased the coverage of the selected region from 5.6 to 87.4 % with 97.3% coverage of the recurrence. Only the explicitly relaxed areas received higher doses. In comparison, relaxing the upper bounds globally yielded 94.5% and 98.5% coverage for the selected and the recurrence regions, respectively. However, hot spots extended even outside the PTV and the 8.5 Gy iso‐dose cut substantially deeper into the spinal cord.

Conclusion:

Highlighting the constraining regions helped to quickly identify possible trade‐offs. Spatially localized relaxation allows more fine grained compromises when planning cases where conflicting goals prevent solutions close to the original goals.
SU‐E‐T‐47: Application of the Repair‐Misrepair‐Fixation RBE Model to Describe the Results of High Resolution Proton Irradiation Cell Survival Experiments
Authors: Peeler C , Taleei R , Guan F , Bronk L , Grosshans D , Mirkovic D , Titt U , Mohan R
Published: July 2014

Purpose:

To develop a system to rapidly and accurately calculate RBE with the repair‐misrepair‐fixation (RMF) model for proton therapy data sets and to evaluate its effectiveness in modeling RBE for cell survival experiments performed with the H460 cell line for a range of proton LET.

Methods:

A system for using the Monte Carlo Damage Simulation (MCDS) software with high performance computing was developed. Input for the MCDS software for a range of proton energies in increments of 0.1 MeV was generated and simulated. The output data were then used to determine doseaveraged quantities for the RMF model based on equivalently binned proton energy spectra. The method was applied to calculate RBE at 50% survival for experimental cell survival data. Experimental data were obtained using a system which allowed for the accumulation of cell survival data at known values of dose‐averaged proton LETs at a range of doses. RBE was calculated based directly on a Cs‐137 reference experiment and, additionally, according to fitted values of the θ and κ terms of the RMF model.

Results:

Dose‐averaged RMF model quantities were calculated using the HPC system. Compared to experimental RBE determined using a Cs‐137 irradiation as a reference, the RBE from the model differed by at most 49%. RBE based on the fitted values of θ and κ differed by at most 18% for the highest LET.

Conclusion:

A system for rapidly generating data necessary to calculate RBE with the RMF model has been developed. For the H460 cell line, the RMF model could not reproduce the experimentally determined RBE based solely on the photon reference data. Fitting of the θ and κ terms of the RMF model indicates that their values increase for proton LET exceeding approximately 10 keV/µm.

NIH Program Project Grant P01CA021239