SU-E-J-110: Dosimetric Analysis of Respiratory Motion Based On Four-Dimensional Dose Accumulation in Liver Stereotactic Body Radiotherapy

Purpose:

Respiratory motion in thoracic and abdominal region could lead to significant underdosing of target and increased dose to healthy tissues. The aim of this study is to evaluate the dosimetric effect of respiratory motion in conventional 3D dose by comparing 4D deformable dose in liver stereotactic body radiotherapy (SBRT).

Methods:

Five patients who had previously treated liver SBRT were included in this study. Four-dimensional computed tomography (4DCT) images with 10 phases for all patients were acquired on multi-slice CT scanner (Siemens, Somatom definition). Conventional 3D planning was performed using the average intensity projection (AIP) images. 4D dose accumulation was calculated by summation of dose distribution for all phase images of 4DCT using deformable image registration (DIR) . The target volume and normal organs dose were evaluated with the 4D dose and compared with those from 3D dose. And also, Index of achievement (IOA) which assesses the consistency between planned dose and prescription dose was used to compare target dose distribution between 3D and 4D dose.

Results:

Although the 3D dose calculation considered the moving target coverage, significant differences of various dosimetric parameters between 4D and 3D dose were observed in normal organs and PTV. The conventional 3D dose overestimated dose to PTV, however, there was no significant difference for GTV. The average difference of IOA which become ‘1’ in an ideal case was 3.2% in PTV. The average difference of liver and duodenum was 5% and 16% respectively.

Conclusion:

4D dose accumulation which can provide dosimetric effect of respiratory motion has a possibility to predict the more accurate delivered dose to target and normal organs and improve treatment accuracy.

This work was supported by the Radiation Technology R&D program (No. 2013M2A2A7043498) and the Mid-career Researcher Program (2014R1A2A1A10050270) through the National Research Foundation of Korea funded by the Ministry of Science, ICT&Future Planning (MSIP) of Korea.

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This article by S Kang, S Kim, D Kim, T Kim, S Park, K Kim, M Cho, D Shin and T Suh originally appeared on scitation.aip.org on June 26, 2015 at 08:13PM

SU-E-J-113: Effects of Deformable Registration On First-Order Texture Maps Calculated From Thoracic Lung CT Scans

Purpose:

To determine the stability of eight first-order texture features following the deformable registration of serial computed tomography (CT) scans.

Methods:

CT scans at two different time points from 10 patients deemed to have no lung abnormalities by a radiologist were collected. Following lung segmentation using an in-house program, texture maps were calculated from 32×32-pixel regions of interest centered at every pixel in the lungs. The texture feature value of the ROI was assigned to the center pixel of the ROI in the corresponding location of the texture map. Pixels in the square ROI not contained within the segmented lung were not included in the calculation. To quantify the agreement between ROI texture features in corresponding pixels of the baseline and follow-up texture maps, the Fraunhofer MEVIS EMPIRE10 deformable registration algorithm was used to register the baseline and follow-up scans. Bland-Altman analysis was used to compare registered scan pairs by computing normalized bias (nBias), defined as the feature value change normalized to the mean feature value, and normalized range of agreement (nRoA), defined as the range spanned by the 95% limits of agreement normalized to the mean feature value.

Results:

Each patient’s scans contained between 6.8–15.4 million ROIs. All of the first-order features investigated were found to have an nBias value less than 0.04% and an nRoA less than 19%, indicating that the variability introduced by deformable registration was low.

Conclusion:

The eight first-order features investigated were found to be registration stable. Changes in CT texture maps could allow for temporal-spatial evaluation of the evolution of lung abnormalities relating to a variety of diseases on a patient-by-patient basis.

SGA and HA receives royalties and licensing fees through the University of Chicago for computer-aided diagnosis technology. Research reported in this publication was supported by the National Institute Of General Medical Sciences of the National Institutes of Health under Award Number R25GM109439.

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This article by C Smith, A Cunliffe, H Al-Hallaq and S Armato originally appeared on scitation.aip.org on June 26, 2015 at 08:13PM

SU-E-J-160: Motion Analysis and 3D Plane Fit Through the Tumor Center of Mass Positions in the 4DCT Data Set

Purpose:

To model GTV’s center of mass (COM) motion planes based on 4DCT lung data sets and establish possible correlation with the GTV’s size and/or location as an aid in defining adequate GTV margins based on the most prominent motion amplitude in the 3D space.

Methods:

For the modeling of the COM motion throughout the respiratory phases, eleven lung cancer patients were used and the least square fitting was applied. The least square fitting generally involves either data smoothening or apparent trend identification. Quadratic minimization, singular value decomposition, orthogonal projections (orthogonal regression), are ways to find a least square solution. The linear least squares method was initially used for the planar fitting of the 3D point data. Evaluation of the goodness of fit was performed through the root mean square deviations (root mean square error, RMSE) and normalized RMSE (NRMSE) values.

Results:

The summed square of residuals of the fit method varied from 0.02 to 0.26 cm and the coefficient of determination from 0.04 to 0.89, while RMSE ranged from 0.05 to 0.13 and NRMSE from 0.11 to 0.29. The most prominent motion was in the transverse and sagittal planes. A correlation was found between the best fit motion plane and the GTV size and/or location. In the lower GTVs, when maximum SI was 1.5, AP and RL motions were 0.87 and 1.0 cm, respectively. This Result confirms the need for margins larger than 0.5 cm for these GTVs.

Conclusion:

The lower RMSE and NRMSE values showed that for each patient a good planar fit can be achieved for the COM motion path. Knowing the GTV size and location, a good estimate of the motion plane in the 3D space can be made. The required dosimetric margins can be easily calculated knowing only one motion extent and motion plane angle.

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This article by I Jurkovic, S Stathakis, Y Li, A Patel, J Vincent, N Papanikolaou and P Mavroidis originally appeared on scitation.aip.org on June 26, 2015 at 08:11PM

SU-E-J-105: Trusting Dose Deformation and Accumulation for GYN Brachytherapy

Purpose:

In brachytherapy, structures surrounding the target have the potential to move between treatments and receive unknown dose. Deformable image registration could overcome challenges through dose accumulation. This study uses two possible deformable dose summation techniques and compares the results to point dose summation currently performed in clinic.

Methods:

Data for ten patients treated with a Syed template was imported into the MIM software (Cleveland, OH). The deformable registration was applied to structures by masking other image data to a single intensity. The registration flow consisted of the following steps: 1) mask CTs so that each of the structures-of-interest had one unique intensity; 2) perform applicator — based rigid registration; 3) Perform deformable registration; 4) Refine registration by changing local alignments manually; 5) Repeat steps 1 to 3 until desired structure adequately deformed; 5) Transfer each deformed contours to the first CT. The deformed structure accuracy was determined by a dice similarity coefficient (DSC) comparison with the first fraction. Two dose summation techniques were investigated: a deformation and recalculation on the structure; and a dose deformation and accumulation method. Point doses were used as a comparison value.

Results:

The Syed deformations have DSC ranging from 0.53 to 0.97 and 0.75 and 0.95 for the bladder and rectum, respectively. For the bladder, contour deformation addition ranged from −34.8% to 0.98% and dose deformation accumulation ranged from −35% to 29.3% difference from clinical calculations. For the rectum, contour deformation addition ranged from −5.2% to 16.9% and the dose deformation accumulation ranged from −29.1% to 15.3% change.

Conclusion:

Deforming dose for summation leads to different volumetric doses than when dose is recalculated on deformed structures, raising concerns about the accuracy of the deformed dose. DSC alone cannot be used to establish the accuracy of a deformation for brachy dose summation purpose.

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This article by L Poplawski, T Li, J Chino and O Craciunescu originally appeared on scitation.aip.org on June 26, 2015 at 08:11PM

SU-E-J-79: Evaluation of Prostate Volume Changes During Radiotherapy Using Implanted Markers and On-Board Imaging

Purpose:

To evaluate prostate volume changes during radiation therapy using implanted gold markers and on-board imaging.

Methods:

Twenty-five patients were included who underwent an implantation of three gold markers. Cartesian coordinates of markers were assessed in kV-images. The coordinates of centers of two markers were measured on kV-images from the center of the marker at the apex which was reference. The distances between the markers were extrapolated from the coordinates using the Euclid formula. The radius of the sphere through markers was calculated using sinus theorem. The prostate volume for the first and last fraction was substituted with a sphere model and was calculated for each patient. The t-test was used for analysis.

Results:

The mean prostate volume for first and last fraction was 24.65 and 20.87 cc, respectively (p≤0.05). The prostate volume was smaller for 23 patients, whereas there was an expansion for 2 patients. Fifteen patients had androgen deprivation during radiotherapy (H group) and ten did not (NH group). The mean prostate volume for the first and last fraction for the NH group was 30.73 cc and 24.89 cc and for the H group 20.84 cc and 18.19 cc, respectively. There was a 15.8% volume change during treatment for the NH group and 12.2% for the H group, but the difference was not statistically significant. The radius difference of the theoretical sphere for the first and last fraction was 0.98 mm (range, 0.09–2.95 mm) and remained below 2 mm in 88% of measurements.

Conclusion:

There was a significant volume change during prostate radiotherapy. The difference between H group and NH group was not significant. The radius changes did not exceed 3 mm and it was below adaptive treatment requirements. Our results indicate that prostate volume changes during treatment should be taken into account during contouring and treatment planning.

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This article by B Ispir, G Ugurluer, Y Akdeniz, A Eken, T Arpaci and M Serin originally appeared on scitation.aip.org on June 26, 2015 at 08:09PM

SU-E-J-168: Investigation of the Effect of Modulation Factor and Treatment Delivery Time for Liver VMAT SBRT

Purpose:

To investigate the effect on PTV target coverage of liver VMAT SBRT treatment plans by varying the IMRT modulation factor (MF) and length of delivery time in the presence of induced respiratory motion.

Methods:

Four different VMAT SBRT plans (MFs 2.1 to 3.5) were created for a liver phantom within the treatment planning system (Eclipse, Varian Oncology Systems). For each plan, the dose prescription was set to 5 different values (1000cGy to 5000cGy in 5 fractions); these variations force a change in the treatment delivery time due to reduce gantry speed required to deliver higher doses. To simulate respiratory motion, an in-house program was developed that shifts the positions of the MLC leaves at each control point according to population-based respiratory curves. This was done for 4 respiratory traces of comparable peak-to-peak amplitudes and breathing cycle lengths, with one trace exhibiting a baseline drift. A PTV margin of 5mm was used for all treatment plans.

Results:

A two-way repeatedmeasures ANOVA looked at changes in PTV V95 and GTV V95 coverage, based on IMRT factor and treatment time. The GTV V95 coverage and mean liver dose showed no significant difference (p values >0.05). The PTV V95 coverage showed a significant decrease with increasing IMRT factor and decreasing treatment time (p values < 0.01 for both). The patient curve with baseline drift exhibited a decrease in PTV coverage with increasing delivery time.

Conclusion:

Decreased PTV coverage was seen with increasing modulation factor and decreasing treatment times for patients with regular breathing curves. With the advent of faster treatment times it is recommended that lower modulation factors for SBRT treatment plans be utilized to mitigate the possibility of interplay. Additionally, longer treatment times could be advantageous to blur out the interplay effect for SBRT treatments. Further investigation is required in patient plans.

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This article by E Hubley, G Pierce and G Ecclestone originally appeared on scitation.aip.org on June 26, 2015 at 08:09PM

SU-E-J-103: Propagation of Rectum and Bladder Contours for Tandem and Ring (T&R) HDR Treatment Using Deformable Image Registration

Purpose:

To investigate the feasibility of using DIR to propagate the manually contoured rectum and bladder from the 1st insertion to the new CT images on subsequent insertions and evaluate the segmentation performance.

Methods:

Ten cervical cancer patients, who were treated by T&R brachytherapy in 3–4 insertions, were retrospectively collected. In each insertion, rectum and bladder were manually delineated on the planning CT by a physicist and verified by a radiation oncologist. Using VelocityAI (Velocity Medical Solutions, Atlanta, GA), a rigid registration was firstly employed to match the bony structures between the first insertion and each of the following insertions, then a multi-pass B-spine DIR was carried out to further map the sub volume that encompasses rectum and bladder. The resultant deformation fields propagated contours, and dice similarity coefficient (DSC) was used to quantitatively evaluate the agreement between the propagated contours and the manually-delineated organs. For the 3rd insertion, we also evaluated if the segmentation performance could be improved by propagating the contours from the most recent insertion, i.e., the 2nd insertion.

Results:

On average, the contour propagation took about 1 minute. The average and standard deviation of DSC over all insertions and patients was 0.67±0.10 (range: 0.44–0.81) for rectum, and 0.78±0.07 (range: 0.63–0.87) for bladder. For the 3rd insertion, propagating contours from the 2nd insertion could improve the segmentation performance in terms of DSC from 0.63±0.10 to 0.72±0.08 for rectum, and from 0.77±0.07 to 0.79±0.06 for bladder. A Wilcoxon signed rank test indicated that the improvement was statistically significant for rectum (p = 0.004).

Conclusion:

The preliminary results demonstrate that deformable image registration could efficiently and accurately propagate rectum and bladder contours between CT images in different T&R brachytherapy fractions. We are incorporating the propagated contours into our learning-based method to further segment these organs.

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This article by Y Yuan, M Chao, R Sheu, V Dumane, V Gupta and Y Lo originally appeared on scitation.aip.org on June 26, 2015 at 08:07PM

SU-E-J-198: Out-Of-Field Dose and Surface Dose Measurements of MRI-Guided Cobalt-60 Radiotherapy

Purpose:

To measure quantities of dosimetric interest in an MRI-guided cobalt radiotherapy machine that was recently introduced to clinical use.

Methods:

Out-of-field dose due to photon scatter and leakage was measured using an ion chamber and solid water slabs mimicking a human body. Surface dose was measured by irradiating stacks of radiochromic film and extrapolating to zero thickness. Electron out-of-field dose was characterized using solid water slabs and radiochromic film.

Results:

For some phantom geometries, up to 50% of Dmax was observed up to 10 cm laterally from the edge of the beam. The maximum penetration was between 1 and 2 mm in solid water, indicating an electron energy not greater than approximately 0.4 MeV. Out-of-field dose from photon scatter measured at 1 cm depth in solid water was found to fall to less than 10% of Dmax at a distance of 1.2 cm from the edge of a 10.5 × 10.5 cm field, and less that 1% of Dmax at a distance of 10 cm from field edge. Surface dose was measured to be 8% of Dmax.

Conclusion:

Surface dose and out-of-field dose from the MRIguided cobalt radiotherapy machine was measured and found to be within acceptable limits. Electron out-of-field dose, an effect unique to MRI-guided radiotherapy and presumed to arise from low-energy electrons trapped by the Lorentz force, was quantified.

Dr. Low is a member of the scientific advisory board of ViewRay, Inc.

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This article by J Lamb, N Agazaryan, M Cao, D Low, D Thomas and Y Yang originally appeared on scitation.aip.org on June 26, 2015 at 08:07PM

SU-E-J-145: Validation of An Analytical Model for in Vivo Range Verification Using GATE Monte Carlo Simulation in Proton Therapy

Purpose:

Predicted PET images on the basis of analytical filtering approach for proton range verification has been successful developed and validated using FLUKA Monte Carlo (MC) codes and phantom measurements. The purpose of the study is to validate the effectiveness of analytical filtering model for proton range verification on GATE/GEANT4 Monte Carlo simulation codes.

Methods:

In this study, we performed two experiments for validation of predicted β+-isotope by the analytical model with GATE/GEANT4 simulations. The first experiments to evaluate the accuracy of predicting β+-yields as a function of irradiated proton energies. In second experiment, we simulate homogeneous phantoms of different materials irradiated by a mono-energetic pencil-like proton beam. The results of filtered β+-yields distributions by the analytical model is compared with those of MC simulated β+-yields in proximal and distal fall-off ranges.

Results:

The results investigate the distribution between filtered β+-yields and MC simulated β+-yields distribution in different conditions. First, we found that the analytical filtering can be applied over the whole range of the therapeutic energies. Second, the range difference between filtered β+-yields and MC simulated β+-yields at the distal fall-off region are within 1.5mm for all materials used. The findings validated the usefulness of analytical filtering model on range verification of proton therapy on GATE Monte Carlo simulations. In addition, there is a larger discrepancy between filtered prediction and MC simulated β+-yields using GATE code, especially in proximal region. This discrepancy might Result from the absence of wellestablished theoretical models for predicting the nuclear interactions.

Conclusion:

Despite the fact that large discrepancies of the distributions between MC-simulated and predicted β+-yields were observed, the study prove the effectiveness of analytical filtering model for proton range verification using GATE Monte Carlo simulation.

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This article by C Lee, H Lin, T Chao, I Hsiao and K Chuang originally appeared on scitation.aip.org on June 26, 2015 at 08:05PM

SU-E-J-135: Feasibility of Using Quantitative Cone Beam CT for Proton Adaptive Planning

Purpose:

To investigate the feasibility of using scatter corrected cone beam CT (CBCT) for proton adaptive planning.

Methods:

Phantom study was used to evaluate the CT number difference between the planning CT (pCT), quantitative CBCT (qCBCT) with scatter correction and calibrated Hounsfield units using adaptive scatter kernel superposition (ASKS) technique, and raw CBCT (rCBCT). After confirming the CT number accuracy, prostate patients, each with a pCT and several sets of weekly CBCT, were investigated for this study. Spot scanning proton treatment plans were independently generated on pCT, qCBCT and rCBCT. The treatment plans were then recalculated on all images. Dose-volume-histogram (DVH) parameters and gamma analysis were used to compare between dose distributions.

Results:

Phantom study suggested that Hounsfield unit accuracy for different materials are within 20 HU for qCBCT and over 250 HU for rCBCT. For prostate patients, proton dose could be calculated accurately on qCBCT but not on rCBCT. When the original plan was recalculated on qCBCT, tumor coverage was maintained when anatomy was consistent with pCT. However, large dose variance was observed when patient anatomy change. Adaptive plan using qCBCT was able to recover tumor coverage and reduce dose to normal tissue.

Conclusion:

It is feasible to use qu antitative CBCT (qCBCT) with scatter correction and calibrated Hounsfield units for proton dose calculation and adaptive planning in proton therapy.

Partly supported by Varian Medical Systems.

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This article by W Jingqian, T Tsui, J Wei, Q Wang, X Zhang, Z Wen, X Zhu, S Frank, H Li and L Zhu originally appeared on scitation.aip.org on June 26, 2015 at 08:05PM

SU-E-J-107: The Impact of the Tumor Location to Deformable Image Registration

Purpose:

For four-dimensional planning and adaptive radiotherapy, the accuracy of deformable image registration (DIR) is essential. We evaluated the accuracy of an in-house program with the free-downloadable DIR software library package (NiftyReg) and two commercially available DIR software programs (MIM Maestro and Velocity AI) in lung SBRT cancer patients. In addition to it, the relationship between the tumor location and the accuracy of the DIRs was investigated.

Methods:

The free-form deformation was implemented in the in-house program and the MIM. The Velocity was based on the B-spline algorithm. The accuracy of the three programs was evaluated in comparison for the structures on 4DCT image datasets between at the peak-inhale and at the peak-exhale. The dice similarity coefficient (DSC) and normalized DSC (NDSC) were measured for the gross tumor volumes from 19 lung SBRT patients.

Results:

The DSC measurement showed the median values of the DSC were 0.885, 0.872 and 0.798 for the In-house program, the MIM and the Velocity, respectively. The Velocity showed significant difference compared to the others. The median NDSC values were 1.027, 1.005 and 0.946 for the In-house, the MIM and the Velocity, respectively. This indicated that the spatial overlap agreement between the reference and the deformed structure for the in-house and MIM was comparable with the accuracy within 1mm uncertainty. There was larger discrepancy within 1–2mm uncertainty for the Velocity. The In-house and the MIM showed the higher NDSC values than the median values when the GTV was not attached to the chest wall and diaphragm(p < 0.05). However, there is no relationship between the accuracy and the tumor location in the Velocity.

Conclusion:

The difference of the DIR program would affect different accuracy and the accuracy may be reduced when the tumor is located or attached to chest wall or diaphragm.

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This article by Y Sugawara, H Tachibana, N Kadoya, N Kitamura and K Jingu originally appeared on scitation.aip.org on June 26, 2015 at 08:02PM

SU-E-J-162: Systematic Feasibility Analysis of a Quantitative Elasticity Estimation for Breast Anatomy Using Supine/prone Model Postures

Purpose:

The purpose of this project is to estimate an elasticity distribution of breast tissue using supine/prone patient postures. Using biomechanical high-resolution breast models, a systematic assessment was performed in order to determine the feasibility of using patient standard clinical CT data.

Methods:

A GPU-based framework was used to generate a mass-spring model with a hemispherical geometry to represent a breast in the prone position. The model was anchored to the base of the hemisphere mimicking the chest-wall/breast tissue interface. The model then was deformed with several ground-truth elasticity distributions. Both isotropic and anisotropic elasticity distributions were used to demonstrate a tumor’s presence in the breast tissue. Solving the inverse elasticity problem was accomplished with a forward iterative approach by resetting the model to a non-deformed position and iteratively deforming it. A gradient-based iterative optimization scheme was used to update elasticity distribution in order to minimize the discrepancy between ground-truth and computed mechanical responses (displacement) for each voxel. For the purpose of our feasibility study, convergence was obtained when each voxel of tissue was within 1 mm of the ground-truth deformation. The model resolution was systematically varied to verify convergence at different volume representations.

Results:

For a simulated hemisphere running at 200 deformations per second (dps) with about 100,000 mass elements and 26 mass-spring connections, convergence occurred within minutes for each elastic distribution. Using apriori information about the underlying anatomy, these elasticity distribution were estimated in half the computation time with the accuracy improving from 89% to 93%. Though the effective elasticity estimation varied slightly from the ground-truth, displacement magnitude difference was well within 0.1 mm.

Conclusion:

By utilizing a forward iterative approach to solve the inverse elasticity problem, this work indicates the feasibility and potential of the fast reconstruction of breast tissue elasticity distribution using supine/prone postures.

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This article by K Hasse, J Neylon and A Santhanam originally appeared on scitation.aip.org on June 26, 2015 at 08:02PM

SU-E-J-156: Velocity-Based Reference Scan Selection for Motion-Model Error Reduction of 5DCT

Purpose:

A recently proposed 5DCT protocol uses fast helical CT scans to model lung tissue motion that requires one of the scans to be used as a reference for deformable image registration. We have found that, while the CT scans are acquired using fast helical acquisition, residual motionartifacts remain for scans acquired during mid inspiration or exhalation. Selecting the scan with minimal motion artifacts as the reference scan will improve image registration and model accuracy.

Methods:

A free-breathing patient was scanned 25 successive times in alternating directions with a Siemens 64-slice CT scanner using a low-mA fast-helical protocol. Each of the 25 CT scans was successively used as the reference scan with the other 24 images registered to it. Motion-model parameters were determined using the breathing-motion model. Voxel specific tissue velocity maps were generated for each of the 25 scan based on the breathing amplitude surrogate and its time derivative. The resulting 24 model error maps were registered to the first CT scan for comparison. The relationships between regional motion-model error, reference scan motion-artifacts and tissue-velocity were analyzed using linear-regression.

Results:

Motion-model errors were linearly correlated with reference scan blurring artifacts and doubling artifacts. The effects of blurring-artifacts on motion model errors were stronger than doubling artifacts due to their impact on registration accuracy. Selecting the lowest tissue motion velocity scan as the reference scan decreased the regional motion model error by 33.67%±43.12%.

Conclusion:

Motion-artifacts, especially blurring artifacts, that exist in the reference image induce motion model errors in 5DCT. A carefully selected reference scan based on the lowest tissue velocities can minimize the regional motion model error.

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This article by Liu (Lisa) Yang, Dylan O’Connell, Tai Dou, David Thomas, James Lamb and Daniel A. Low originally appeared on scitation.aip.org on June 26, 2015 at 08:00PM

SU-E-J-140: Simulation Study Using Thermoacoustics to Image Proton Dose and Range in Water and Skull Phantom

Purpose:

In this study, thermoacoustic pressure signals generated from a proton beam were simulated in water and currently within a skull phantom to investigate the sensitivity of radioacoustic CT imaging in the brain.

Methods:

Thermoacoustically generated pressure signals from a pulse pencil proton beam (12, 15, 20, and 27cm range) were simulated in water. These simulated pressure signal are detected using a (71) transducer array placed along the surface of a cylinder (30cm × 40cm) and rotated over 2π (in 2 degree increments), where the normal vector to the surface of each transducer intersects the isocenter of the scanner. Currently, a software skull phantom is positioned at isocenter, where the scattering, absorption and speed of dispersion of the thermoacoustic signal through a three layer cortical-trabecular-cortical structure is being simulated. Based on data obtained from the literature, the effects of acoustic attenuation and speed-of-sound (dispersion) will be applied within the 3D FBP algorithm to obtain dosimetric images.

Results:

Based on hydrophone detector specifications, a 0.5MHz bandwidth and 50dB re 1μPa per Hz^1/2, a 1.6cGy sensitivity at the Bragg peak was demonstrated while maintaining a 1.0 mm (FWHM) range resolution along the central axis of the beam. Utilizing this same information, the integral dose within the Bragg peak and distal edge compared to MC had a 2% (statistical) and 5% voxel-based RMS at this same dose sensitivity. We plan to present preliminary data determining the range sensitivity for a head phantom for this scanner design and the feasibility of imaging the proton dose in patients with a brain tumor undergoing therapy.

Conclusion:

RACT scanner provides 3D dosimetric images with 1.6cGy (Bragg peak) sensitivity with 1mm range sensitivity. Simulations will be performed to determine feasibility to treat brain cancer patients.

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This article by K Stantz and V Moskvin originally appeared on scitation.aip.org on June 26, 2015 at 08:00PM

SU-E-J-152: Synthetic Gated CBCT Imaging for Lung SBRT

Purpose:

Due to slow scanning speed, when free breathing CBCT (FBCBCT) is used to localize target in lung SBRT treatment, respiratory motion induced image artifacts may degrade the image quality and compromise the target positioning accuracy. The purpose of this study is to investigate the feasibility of synthetic gated CBCT (SGCBCT) imaging for lung SBRT treatment.

Methods:

Two Lung SBRT patients were selected for this study. The amplitudes of tumor respiratory motion were 1.2cm and 3.0cm (Superior-Inferior).Each Lung SBRT patient underwent FBCBCT scan, and then the acquired projection images were exported to an in-house program for off-line analysis. Firstly a semi-automatic algorithm was used to trace the diaphragm movement from those projection images in order to correlate respiratory phases with each projection image. Secondly, a Feldkamp-Davis-Kress (FDK) algorithm was utilized to reconstruct the gated CBCT (GCBCT) images based on the projection images within the gating windows (35∼65% for exhale; 85∼15% for inhale). Thirdly, the bony structures were automatically segmented from FBCBCT images using Fuzzy C-mean segmentation method. Finally, the SGCBCT was generated by replacing the HU value of the GCBCT image with the value of FBCBCT image for the voxels that belong to the bony structure based on the above segmentation results.

Results:

Due to the respiratory motion effect, the targets determined by FBCBCT have reduced image contrast and blurred boundary between tumor and surrounding normal lung tissues. Compared to FBCBCT, the SGCBCT images at exhale and inhale not only delineate the target with enhanced image contrast and much clearer boundary with less motion artifacts, but also determine the amplitudes of tumor respiratory motion.

Conclusion:

SGCBCT provides better representation of the moving lung tumor with less motion artifacts, and has a potential to improve the positioning accuracy in lung SBRT treatment.

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This article by X Li, P Zhang, R Gewanter, M Chan, X Tang and M Hunt originally appeared on scitation.aip.org on June 26, 2015 at 07:58PM

SU-E-J-192: Comparative Effect of Different Respiratory Motion Management Systems

Purpose:

Irregular breathing can influence the outcome of four-dimensional computed tomography imaging for causing artifacts. Audio-visual biofeedback systems associated with patient-specific guiding waveform are known to reduce respiratory irregularities. In Japan, abdomen and chest motion self-control devices (Abches), representing simpler visual coaching techniques without guiding waveform are used instead; however, no studies have compared these two systems to date. Here, we evaluate the effectiveness of respiratory coaching to reduce respiratory irregularities by comparing two respiratory management systems.

Methods:

We collected data from eleven healthy volunteers. Bar and wave models were used as audio-visual biofeedback systems. Abches consisted of a respiratory indicator indicating the end of each expiration and inspiration motion. Respiratory variations were quantified as root mean squared error (RMSE) of displacement and period of breathing cycles.

Results:

All coaching techniques improved respiratory variation, compared to free breathing. Displacement RMSEs were 1.43 ± 0.84, 1.22 ± 1.13, 1.21 ± 0.86, and 0.98 ± 0.47 mm for free breathing, Abches, bar model, and wave model, respectively. Free breathing and wave model differed significantly (p < 0.05). Period RMSEs were 0.48 ± 0.42, 0.33 ± 0.31, 0.23 ± 0.18, and 0.17 ± 0.05 s for free breathing, Abches, bar model, and wave model, respectively. Free breathing and all coaching techniques differed significantly (p < 0.05). For variation in both displacement and period, wave model was superior to free breathing, bar model, and Abches. The average reduction in displacement and period RMSE compared with wave model were 27% and 47%, respectively.

Conclusion:

The efficacy of audio-visual biofeedback to reduce respiratory irregularity compared with Abches. Our results showed that audio-visual biofeedback combined with a wave model can potentially provide clinical benefits in respiratory management, although all techniques could reduce respiratory irregularities.

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This article by Y Nakajima, N Kadoya, S Kida, K Ito, T Kanai, K Kishi, K Sato, S Dobashi, K Takeda and K Jingu originally appeared on scitation.aip.org on June 26, 2015 at 07:58PM

SU-E-J-164: Estimation of DVH Variation for PTV Due to Interfraction Organ Motion in Prostate VMAT Using Gaussian Error Function

Purpose:

We developed a method to predict the change of DVH for PTV due to interfraction organ motion in prostate VMAT without repeating the CT scan and treatment planning. The method is based on a pre-calculated patient database with DVH curves of PTV modelled by the Gaussian error function (GEF).

Methods:

For a group of 30 patients with different prostate sizes, their VMAT plans were recalculated by shifting their PTVs 1 cm with 10 increments in the anterior-posterior, left-right and superior-inferior directions. The DVH curve of PTV in each replan was then fitted by the GEF to determine parameters describing the shape of curve. Information of parameters, varying with the DVH change due to prostate motion for different prostate sizes, was analyzed and stored in a database of a program written by MATLAB.

Results:

To predict a new DVH for PTV due to prostate interfraction motion, prostate size and shift distance with direction were input to the program. Parameters modelling the DVH for PTV were determined based on the pre-calculated patient dataset. From the new parameters, DVH curves of PTVs with and without considering the prostate motion were plotted for comparison. The program was verified with different prostate cases involving interfraction prostate shifts and replans.

Conclusion:

Variation of DVH for PTV in prostate VMAT can be predicted using a pre-calculated patient database with DVH curve fitting. The computing time is fast because CT rescan and replan are not required. This quick DVH estimation can help radiation staff to determine if the changed PTV coverage due to prostate shift is tolerable in the treatment. However, it should be noted that the program can only consider prostate interfraction motions along three axes, and is restricted to prostate VMAT plan using the same plan script in the treatment planning system.

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This article by C Lewis, R Jiang and J Chow originally appeared on scitation.aip.org on June 26, 2015 at 07:56PM

SU-E-J-172: Bio-Physical Effects of Patients Set-Up Errors According to Whole Breast Irradiation Techniques

Purpose:

The dose-related effects of patient setup errors on biophysical indices were evaluated for conventional wedge (CW) and field-in-field (FIF) whole breast irradiation techniques.

Methods:

The treatment plans for 10 patients receiving whole left breast irradiation were retrospectively selected. Radiobiological and physical effects caused by dose variations were evaluated by shifting the isocenters and gantry angles of the treatment plans. Dose-volume histograms of the planning target volume (PTV), heart, and lungs were generated, and conformity index (CI), homogeneity index (HI), tumor control probability (TCP), and normal tissue complication probability (NTCP) were determined.

Results:

For “isocenter shift plan” with posterior direction, the D95 of the PTV decreased by approximately 15% and the TCP of the PTV decreased by approximately 50% for the FIF technique and by 40% for the CW; however, the NTCPs of the lungs and heart increased by about 13% and 1%, respectively, for both techniques. Increasing the gantry angle decreased the TCPs of the PTV by 24.4% (CW) and by 34% (FIF). The NTCPs for the two techniques differed by only 3%. In case of CW, the CIs and HIs were much higher than that of the FIF in all cases. It had a significant difference between two techniques (p<0.01). According to our results, however, the FIF had more sensitive response by set up errors rather than CW in bio-physical aspects.

Conclusions:

The radiobiological-based analysis can detect significant dosimetric errors then, can provide a practical patient quality assurance method to guide the radiobiological and physical effects.

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This article by S Lee, T Suh, S Park, M Kim, J Park and M Lee originally appeared on scitation.aip.org on June 26, 2015 at 07:56PM

SU-E-J-175: Comparison of the Treatment Reproducibility of Tumors Affected by Breathing Motion

Purpose:

The aim of the dose distribution simulations was to form a global idea of intensity-modulated radiation therapy (IMRT) realization, by its comparison to three-dimensional conformal radiation therapy (3DCRT) delivery for tumors affected by respiratory motion.

Methods:

In the group of 10patients both 3DCRT and IMRT plans were prepared.For each field the motion kernel was generated with the largest movement amplitude of 4;6 and 8mm.Additionally,the sets of reference measurements were made in no motion conditions(0 mm).The evaluation of plan delivery,using a diode array placed on moving platform,was based on the Gamma Index analysis with distance to agreement of 3mm and dose difference of 3%.

Results:

IMRT plans tended to spare doses delivered to lungs compared to 3DCRT.Nonetheless,analyzed volumes showed no significant difference between the static and dynamic techniques,except for the volumes of both lungs receiving 10 and 15Gy.After adding the components associated with the respiratory movement,all IMRT lung parameters evaluated for the ipsilateral,contralateral and both lungs together,revealed considerable differences between the 0vs.6, 0vs.8 and 4vs.8-mm amplitudes.Similar results were obtained for the 3DCRT lung measurements,but without significance between the 0vs.6-mm amplitude.Taking into account the CTV score parameter in 3DCRT and IMRT plans,there was no statistically significant difference between the motion patterns with the smallest amplitudes.The differences were found for the 8-mm amplitude when it was compared both with static conditions and 4-mm amplitude (for 3DCRT) and between 0vs.6, 0vs.8 and 4vs.8-mm amplitudes (for IMRT).All accepted and measured 3DCRT and IMRT doses to spinal cord,esophagus and heart were always below the QUANTEC limits.

Conclusion:

The application of IMRT technique in lung radiotherapy affords possibilities for reducing the lung doses.For maximal amplitudes of breathing trajectory below 4mm,the disagreement between CTV planned and delivered dose distribution can be neglected.Whereas respiratory motion with an amplitude above 5mm leads to significant changes in IMRT delivery.

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This article by M Adamczyk, T Piotrowski and S Adamczyk originally appeared on scitation.aip.org on June 26, 2015 at 07:56PM

SU-E-J-108: Solving the Chinese Postman Problem for Effective Contour Deformation

Purpose:

To develop a practical approach for accurate contour deformation when deformable image registration (DIR) is used for atlas-based segmentation or contour propagation in image-guided radiotherapy.

Methods:

A contour deformation approach was developed on the basis of 3D mesh operations. The 2D contours represented by a series of points in each slice were first converted to a 3D triangular mesh, which was deformed by the deformation vectors resulting from DIR. A set of parallel 2D planes then cut through the deformed 3D mesh, generating unordered points and line segments, which should be reorganized into a set of 2D contour points. It was realized that the reorganization problem was equivalent to solving the Chinese Postman Problem (CPP) by traversing a graph built from the unordered points with the least cost. Alternatively, deformation could be applied to a binary mask converted from the original contours. The deformed binary mask was then converted back into contours at the CT slice locations. We performed a qualitative comparison to validate the mesh-based approach against the image-based approach.

Results:

The DIR could considerably change the 3D mesh, making complicated 2D contour representations after deformation. CPP was able to effectively reorganize the points in 2D planes no matter how complicated the 2D contours were. The mesh-based approach did not require a post-processing of the contour, thus accurately showing the actual deformation in DIR. The mesh-based approach could keep some fine details and resulted in smoother contours than the image-based approach did, especially for the lung structure. Image-based approach appeared to over-process contours and suffered from image resolution limits. The mesh-based approach was integrated into in-house DIR software for use in routine clinic and research.

Conclusion:

We developed a practical approach for accurate contour deformation. The efficiency of this approach was demonstrated in both clinic and research applications.

This work was partially supported by Cancer Prevention & Research Institute of Texas (CPRIT) RP110562.

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This article by J Yang, L Zhang, Y Zhang, L Dong, P Balter and L Court originally appeared on scitation.aip.org on June 26, 2015 at 07:56PM