Engineering topics

Wong, John

Publications and source records attributed to Wong, John.

14 records

DOE OSTI2016

Long-Term Update of NRG Oncology RTOG 0319: A Phase 1 and 2 Trial to Evaluate 3-Dimensional Conformal Radiation Therapy Confined to the Region of the Lumpectomy Cavity for Stage I and II Breast Carcinoma

Purpose: NRG Oncology RTOG 0319 was the first cooperative group trial in the United States to evaluate 3-dimensional conformal radiation therapy (3D-CRT) accelerated partial breast irradiation (APBI). This report updates secondary endpoints of toxicity and efficacy. Methods and Materials: Patients with stage I or II invasive breast cancer (tumor size ≤3 cm, ≤3 positive lymph nodes, negative margins) were eligible for 3D-CRT APBI: 38.5 Gy in 10 twice-daily fractions. Patient characteristics and treatment details have previously been reported. Adverse events were graded with CTCAE v3.0 (National Cancer Institute Common Terminology Criteria for Adverse Events version 3.0). This analysis updates the rates of ipsilateral breast recurrence (IBR), contralateral breast recurrence, ipsilateral node recurrence (INR), metastatic sites (distant metastases [DM]), mastectomy, disease-free survival, mastectomy-free survival, and overall survival. Results: Of 58 enrolled patients, 52 were eligible, with a median age of 61 years; 94% had stage I cancer and 83% had estrogen receptor positive disease. The median follow-up period was 8 years (minimum-maximum, 1.7-9.0 years). The 7-year estimate of isolated IBR (no DM) was 5.9%. The 7-year estimates of all IBRs, INR, mastectomy rate, and DM were 7.7%, 5.8%, 7.7%, and 7.7%, respectively. All 4 IBRs were invasive, of which 3 had a component within the planning target volume. The patterns of failure were as follows: 3 IBRs, 1 INR, 2 DM, 1 INR plus DM, and 1 IBR plus INR plus DM. The 7-year estimates of mastectomy-free survival, disease-free survival, and overall survival were 71.2%, 71.2%, and 78.8%, respectively. Thirteen patients died: 3 of breast cancer and 10 of other causes. Grade 3 (G3) treatment-related adverse events were reported by 4 patients (7.7%). No G3 pain or pulmonary or cardiac toxicities were reported. Conclusions: This phase 1 and 2 trial of 3D-CRT APBI continues to show durable tumor control and minimal G3 toxicity, comparable to other APBI techniques. Mature phase 3 results will determine the appropriateness and limitations of this noninvasive APBI technique.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2016

Technical Note: SCUDA: A software platform for cumulative dose assessment

Purpose: Accurate tracking of anatomical changes and computation of actually delivered dose to the patient are critical for successful adaptive radiation therapy (ART). Additionally, efficient data management and fast processing are practically important for the adoption in clinic as ART involves a large amount of image and treatment data. The purpose of this study was to develop an accurate and efficient Software platform for CUmulative Dose Assessment (SCUDA) that can be seamlessly integrated into the clinical workflow. Methods: SCUDA consists of deformable image registration (DIR), segmentation, dose computation modules, and a graphical user interface. It is connected to our image PACS and radiotherapy informatics databases from which it automatically queries/retrieves patient images, radiotherapy plan, beam data, and daily treatment information, thus providing an efficient and unified workflow. For accurate registration of the planning CT and daily CBCTs, the authors iteratively correct CBCT intensities by matching local intensity histograms during the DIR process. Contours of the target tumor and critical structures are then propagated from the planning CT to daily CBCTs using the computed deformations. The actual delivered daily dose is computed using the registered CT and patient setup information by a superposition/convolution algorithm, and accumulated using the computed deformation fields. Both DIR and dose computation modules are accelerated by a graphics processing unit. Results: The cumulative dose computation process has been validated on 30 head and neck (HN) cancer cases, showing 3.5 ± 5.0 Gy (mean±STD) absolute mean dose differences between the planned and the actually delivered doses in the parotid glands. On average, DIR, dose computation, and segmentation take 20 s/fraction and 17 min for a 35-fraction treatment including additional computation for dose accumulation. Conclusions: The authors developed a unified software platform that provides accurate and efficient monitoring of anatomical changes and computation of actually delivered dose to the patient, thus realizing an efficient cumulative dose computation workflow. Evaluation on HN cases demonstrated the utility of our platform for monitoring the treatment quality and detecting significant dosimetric variations that are keys to successful ART.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2016

MO-FG-CAMPUS-JeP3-04: Feasibility Study of Real-Time Ultrasound Monitoring for Abdominal Stereotactic Body Radiation Therapy

Purpose: Ultrasound is ideal for real-time monitoring in radiotherapy with high soft tissue contrast, non-ionization, portability, and cost effectiveness. Few studies investigated clinical application of real-time ultrasound monitoring for abdominal stereotactic body radiation therapy (SBRT). This study aims to demonstrate the feasibility of real-time monitoring of 3D target motion using 4D ultrasound. Methods: An ultrasound probe holding system was designed to allow clinician to freely move and lock ultrasound probe. For phantom study, an abdominal ultrasound phantom was secured on a 2D programmable respiratory motion stage. One side of the stage was elevated than another side to generate 3D motion. The motion stage made periodic breath-hold movement. Phantom movement tracked by infrared camera was considered as ground truth. For volunteer study three healthy subjects underwent the same setup for abdominal SBRT with active breath control (ABC). 4D ultrasound B-mode images were acquired for both phantom and volunteers for real-time monitoring. 10 breath-hold cycles were monitored for each experiment. For phantom, the target motion tracked by ultrasound was compared with motion tracked by infrared camera. For healthy volunteers, the reproducibility of ABC breath-hold was evaluated. Results: Volunteer study showed the ultrasound system fitted well to the clinical SBRT setup. The reproducibility for 10 breath-holds is less than 2 mm in three directions for all three volunteers. For phantom study the motion between inspiration and expiration captured by camera (ground truth) is 2.35±0.02 mm, 1.28±0.04 mm, 8.85±0.03 mm in LR, AP, SI directly, respectively. The motion monitored by ultrasound is 2.21±0.07 mm, 1.32±0.12mm, 9.10±0.08mm, respectively. The motion monitoring error in any direction is less than 0.5 mm. Conclusion: The volunteer study proved the clinical feasibility of real-time ultrasound monitoring for abdominal SBRT. The phantom and volunteer ABC studies demonstrated sub-millimeter accuracy of 3D motion movement monitoring.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2015

Quantitative correlational study of microbubble-enhanced ultrasound imaging and magnetic resonance imaging of glioma and early response to radiotherapy in a rat model

Purpose: Radiotherapy remains a major treatment method for malignant tumors. Magnetic resonance imaging (MRI) is the standard modality for assessing glioma treatment response in the clinic. Compared to MRI, ultrasound imaging is low-cost and portable and can be used during intraoperative procedures. The purpose of this study was to quantitatively compare contrast-enhanced ultrasound (CEUS) imaging and MRI of irradiated gliomas in rats and to determine which quantitative ultrasound imaging parameters can be used for the assessment of early response to radiation in glioma. Methods: Thirteen nude rats with U87 glioma were used. A small thinned skull window preparation was performed to facilitate ultrasound imaging and mimic intraoperative procedures. Both CEUS and MRI with structural, functional, and molecular imaging parameters were performed at preradiation and at 1 day and 4 days postradiation. Statistical analysis was performed to determine the correlations between MRI and CEUS parameters and the changes between pre- and postradiation imaging. Results: Area under the curve (AUC) in CEUS showed significant difference between preradiation and 4 days postradiation, along with four MRI parameters, T{sub 2}, apparent diffusion coefficient, cerebral blood flow, and amide proton transfer-weighted (APTw) (all p < 0.05). The APTw signal was correlated with three CEUS parameters, rise time (r = − 0.527, p < 0.05), time to peak (r = − 0.501, p < 0.05), and perfusion index (r = 458, p < 0.05). Cerebral blood flow was correlated with rise time (r = − 0.589, p < 0.01) and time to peak (r = − 0.543, p < 0.05). Conclusions: MRI can be used for the assessment of radiotherapy treatment response and CEUS with AUC as a new technique and can also be one of the assessment methods for early response to radiation in glioma.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2014

Projections of Future Summertime Ozone over the U.S.

This study uses a regional fully coupled chemistry-transport model to assess changes in surface ozone over the summertime U.S. between present and a 2050 future time period at high spatial resolution (12 km grid spacing) under the SRES A2 climate and RCP8.5 anthropogenic pre-cursor emission scenario. The impact of predicted changes in climate and global background ozone is estimated to increase surface ozone over most of the U.S; the 5th - 95th percentile range for daily 8-hour maximum surface ozone increases from 31-79 ppbV to 30-87 ppbV between the present and future time periods. The analysis of a set of meteorological drivers suggests that these mostly will add to increasing ozone, but the set of simulations conducted does not allow to separate this effect from that through enhanced global background ozone. Statistically the most robust positive feedbacks are through increased temperature, biogenic emissions and solar radiation. Stringent emission controls can counteract these feedbacks and if considered, we estimate large reductions in surface ozone with the 5th-95th percentile reduced to 27-55 ppbV. A comparison of the high-resolution projections to global model projections shows that even though the global model is biased high in surface ozone compared to the regional model and compared to observations, both the global and the regional model predict similar changes in ozone between the present and future time periods. However, on smaller spatial scales, the regional predictions show more pronounced changes between urban and rural regimes that cannot be resolved at the coarse resolution of global model. In addition, the sign of the changes in overall ozone mixing ratios can be different between the global and the regional predictions in certain regions, such as the Western U.S. This study confirms the key role of emission control strategies in future air quality predictions and demonstrates the need for considering degradation of air quality with future climate change in emission policy making. It also illustrates the need for high resolution modeling when the objective is to address regional and local air quality or establish links to human health and society.

58 GEOSCIENCES↗

DOE OSTI2013

4D tumor centroid tracking using orthogonal 2D dynamic MRI: Implications for radiotherapy planning

Purpose: Current pretreatment, 4D imaging techniques are suboptimal in that they sample breathing motion over a very limited “snapshot” in time. Heretofore, long-duration, 4D motion characterization for radiotherapy planning, margin optimization, and validation have been impractical for safety reasons, requiring invasive markers imaged under x-ray fluoroscopy. To characterize 3D tumor motion and associated variability over durations more consistent with treatments, the authors have developed a practical dynamic MRI (dMRI) technique employing two orthogonal planes acquired in a continuous, interleaved fashion.Methods: 2D balanced steady-state free precession MRI was acquired continuously over 9–14 min at approximately 4 Hz in three healthy volunteers using a commercial 1.5 T system; alternating orthogonal imaging planes (sagittal, coronal, sagittal, etc.) were employed. The 2D in-plane pixel resolution was 2 × 2 mm{sup 2} with a 5 mm slice profile. Simultaneous with image acquisition, the authors monitored a 1D surrogate respiratory signal using a device available with the MRI system. 2D template matching-based anatomic feature registration, or tracking, was performed independently in each orientation. 4D feature tracking at the raw frame rate was derived using spline interpolation.Results: Tracking vascular features in the lung for two volunteers and pancreatic features in one volunteer, the authors have successfully demonstrated this method. Registration error, defined here as the difference between the sagittal and coronal tracking result in the SI direction, ranged from 0.7 to 1.6 mm (1σ) which was less than the acquired image resolution. Although the healthy volunteers were instructed to relax and breathe normally, significantly variable respiration was observed. To demonstrate potential applications of this technique, the authors subsequently explored the intrafraction stability of hypothetical tumoral internal target volumes and 3D spatial probability distribution functions. The surrogate respiratory information allowed the authors to show how this technique can be used to study correlations between internal and external (surrogate) information over these prolonged durations. However, compared against the gold standard of the time stamps in the dMRI frames, the temporal synchronization of the surrogate 1D respiratory information was shown to be likely unreliable.Conclusions: The authors have established viability of a novel and practical pretreatment, 4D tumor centroid tracking method employing a commercially available dynamic MRI sequence. Further developments from the vendor are likely needed to provide a reliably synchronized surrogate 1D respiratory signal, which will likely broaden the utility of this method in the pretreatment radiotherapy planning context.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2013

Late Toxicity and Patient Self-Assessment of Breast Appearance/Satisfaction on RTOG 0319: A Phase 2 Trial of 3-Dimensional Conformal Radiation Therapy-Accelerated Partial Breast Irradiation Following Lumpectomy for Stages I and II Breast Cancer

Purpose: Late toxicities and cosmetic analyses of patients treated with accelerated partial breast irradiation (APBI) on RTOG 0319 are presented. Methods and Materials: Patients with stages I to II breast cancer ≤3 cm, negative margins, and ≤3 positive nodes were eligible. Patients received three-dimensional conformal external beam radiation therapy (3D-CRT; 38.5 Gy in 10 fractions twice daily over 5 days). Toxicity and cosmesis were assessed by the patient (P), the radiation oncologist (RO), and the surgical oncologist (SO) at 3, 6, and 12 months from the completion of treatment and then annually. National Cancer Institute Common Terminology Criteria for Adverse Events, version 3.0, was used to grade toxicity. Results: Fifty-two patients were evaluable. Median follow-up was 5.3 years (range, 1.7-6.4 years). Eighty-two percent of patients rated their cosmesis as good/excellent at 1 year, with rates of 64% at 3 years. At 3 years, 31 patients were satisfied with the treatment, 5 were not satisfied but would choose 3D-CRT again, and none would choose standard radiation therapy. The worst adverse event (AE) per patient reported as definitely, probably, or possibly related to radiation therapy was 36.5% grade 1, 50% grade 2, and 5.8% grade 3 events. Grade 3 AEs were all skin or musculoskeletal-related. Treatment-related factors were evaluated to potentially establish an association with observed toxicity. Surgical bed volume, target volume, the number of beams used, and the use of bolus were not associated with late cosmesis. Conclusions: Most patients enrolled in RTOG 0319 were satisfied with their treatment, and all would choose to have the 3D-CRT APBI again.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2013

Anti-PD-1 Blockade and Stereotactic Radiation Produce Long-Term Survival in Mice With Intracranial Gliomas

Purpose: Glioblastoma multiforme (GBM) is the most common primary brain tumor in adults, and radiation is one of the main treatment modalities. However, cure rates remain low despite best available therapies. Immunotherapy is a promising modality that could work synergistically with radiation, which has been shown to increase antigen presentation and promote a proinflammatory tumor microenvironment. Programmed-death-1 (PD-1) is a surface receptor expressed on activated and exhausted T cells, which mediate T cell inhibition upon binding with its ligand PD-L1, expressed on many tumor types including human GBMs. We tested the combination of anti-PD-1 immunotherapy with stereotactic radiosurgery in a mouse orthotopic GBM model. Methods and Materials: We performed intracranial implantation of mouse glioma cell line GL261 transfected with luciferase into C57BL/6 mice. Mice were stratified into 4 treatment groups: (1) control; (2) radiation only; (3) anti-PD-1 antibody only; and (4) radiation plus anti-PD-1 antibody. Overall survival was quantified. The mice were killed on day 21 after implantation to assess immunologic parameters in the brain/tumor, cervical lymph nodes, and spleen. Results: Improved survival was demonstrated with combination anti-PD-1 therapy plus radiation compared with either modality alone: median survival was 25 days in the control arm, 27 days in the anti-PD-1 antibody arm, 28 days in the radiation arm, and 53 days in the radiation plus anti-PD-1 therapy arm (P<.05 by log-rank Mantle-Cox). Long-term survival was seen only in the combined treatment arm, with a fraction (15%-40%) of animals alive at day 180+ after treatment. Immunologic data on day 21 after implantation showed increased tumor infiltration by cytotoxic T cells (CD8+/interferon-γ+/tumor necrosis factor-α+) and decreased regulatory T cells (CD4+/FOXP3) in the combined treatment group compared with the single modality arms. Conclusions: The combination of PD-1 blockade and localized radiation therapy results in long-term survival in mice with orthotopic brain tumors. These studies provide strong preclinical evidence to support combination trials in patients with GBM.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2013

Hedgehog Pathway Inhibition Radiosensitizes Non-Small Cell Lung Cancers

Purpose: Despite improvements in chemoradiation, local control remains a major clinical problem in locally advanced non-small cell lung cancer. The Hedgehog pathway has been implicated in tumor recurrence by promoting survival of tumorigenic precursors and through effects on tumor-associated stroma. Whether Hedgehog inhibition can affect radiation efficacy in vivo has not been reported. Methods and Materials: We evaluated the effects of a targeted Hedgehog inhibitor (HhAntag) and radiation on clonogenic survival of human non-small cell lung cancer lines in vitro. Using an A549 cell line xenograft model, we examined tumor growth, proliferation, apoptosis, and gene expression changes after concomitant HhAntag and radiation. In a transgenic mouse model of Kras{sup G12D}-induced and Twist1-induced lung adenocarcinoma, we assessed tumor response to radiation and HhAntag by serial micro-computed tomography (CT) scanning. Results: In 4 human lung cancer lines in vitro, HhAntag showed little or no effect on radiosensitivity. By contrast, in both the human tumor xenograft and murine inducible transgenic models, HhAntag enhanced radiation efficacy and delayed tumor growth. By use of the human xenograft model to differentiate tumor and stromal effects, mouse stromal cells, but not human tumor cells, showed significant and consistent downregulation of Hedgehog pathway gene expression. This was associated with increased tumor cell apoptosis. Conclusions: Targeted Hedgehog pathway inhibition can increase in vivo radiation efficacy in lung cancer preclinical models. This effect is associated with pathway suppression in tumor-associated stroma. These data support clinical testing of Hedgehog inhibitors as a component of multimodality therapy for locally advanced non-small cell lung cancer.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2012

Evaluation of Microscopic Disease in Oral Tongue Cancer Using Whole-Mount Histopathologic Techniques: Implications for the Management of Head-and-Neck Cancers

Purpose: To map the distribution of microscopic disease (MD) in head-and-neck cancer by analyzing digital images of whole-mounted serial sections of tongue cancer specimens. Methods and Materials: Ten T1-3 oral tongue cancer specimens were evaluated. The specimens were sliced into 3-mm blocks from which one or more 4-{mu}m slides were taken and digitized to create whole-mounted serial sections. Gross tumor and microscopic disease were digitally contoured on each slide. Lines perpendicular to the gross tumor volume (GTV) edge were created at 0.05-mm intervals and the distance between GTV and MD measured. Results: Of 88 slides assessed, 44 (50%) had evidence of MD. Of the 63,809 perpendicular lines drawn along the GTV edges, 2320 (3.6%) encountered microscopic disease along their path. The majority of MD abutted the GTV, and only 26.7% was noncontiguous with the GTV edge. The maximum distance from the border was 7.8 mm. Ninety-nine percent of all MD was within 4.75 mm and 95% was within 3.95 mm of the GTV. Conclusion: In this study we were able to assess the distribution of MD more accurately than has been possible with routine pathologic techniques. The results indicate that when the GTV is correctly identified, there is very little MD to be found outside this volume. This has implications for the volume of tissue resected at surgery and the volume included in the clinical target volume in conformal radiotherapy planning.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2012

A method for deriving a 4D-interpolated balanced planning target for mobile tumor radiotherapy

Purpose: Tumor control and normal tissue toxicity are strongly correlated to the tumor and normal tissue volumes receiving high prescribed dose levels in the course of radiotherapy. Planning target definition is, therefore, crucial to ensure favorable clinical outcomes. This is especially important for stereotactic body radiation therapy of lung cancers, characterized by high fractional doses and steep dose gradients. The shift in recent years from population-based to patient-specific treatment margins, as facilitated by the emergence of 4D medical imaging capabilities, is a major improvement. The commonly used motion-encompassing, or internal-target volume (ITV), target definition approach provides a high likelihood of coverage for the mobile tumor but inevitably exposes healthy tissue to high prescribed dose levels. The goal of this work was to generate an interpolated balanced planning target that takes into account both tumor coverage and normal tissue sparing from high prescribed dose levels, thereby improving on the ITV approach. Methods: For each 4DCT dataset, 4D deformable image registration was used to derive two bounding targets, namely, a 4D-intersection and a 4D-composite target which minimized normal tissue exposure to high prescribed dose levels and maximized tumor coverage, respectively. Through definition of an ''effective overlap volume histogram'' the authors derived an ''interpolated balanced planning target'' intended to balance normal tissue sparing from prescribed doses with tumor coverage. To demonstrate the dosimetric efficacy of the interpolated balanced planning target, the authors performed 4D treatment planning based on deformable image registration of 4D-CT data for five previously treated lung cancer patients. Two 4D plans were generated per patient, one based on the interpolated balanced planning target and the other based on the conventional ITV target. Plans were compared for tumor coverage and the degree of normal tissue sparing resulting from the new approach was quantified. Results: Analysis of the 4D dose distributions from all five patients showed that while achieving tumor coverage comparable to the ITV approach, the new planning target definition resulted in reductions of lung V{sub 10}, V{sub 20}, and V{sub 30} of 6.3% {+-} 1.7%, 10.6% {+-} 3.9%, and 12.9% {+-} 5.5%, respectively, as well as reductions in mean lung dose, mean dose to the GTV-ring and mean heart dose of 8.8% {+-} 2.5%, 7.2% {+-} 2.5%, and 10.6% {+-} 3.6%, respectively. Conclusions: The authors have developed a simple and systematic approach to generate a 4D-interpolated balanced planning target volume that implicitly incorporates the dynamics of respiratory-organ motion without requiring 4D-dose computation or optimization. Preliminary results based on 4D-CT data of five previously treated lung patients showed that this new planning target approach may improve normal tissue sparing without sacrificing tumor coverage.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2011

SmartArc-Based Volumetric Modulated Arc Therapy for Oropharyngeal Cancer: A Dosimetric Comparison With Both Intensity-Modulated Radiation Therapy and Helical Tomotherapy

Purpose: To investigate the roles of volumetric modulated arc therapy with SmartArc (VMAT-S), intensity-modulated radiation therapy (IMRT), and helical tomotherapy (HT) for oropharyngeal cancer using a simultaneous integrated boost (SIB) approach. Methods and Materials: Eight patients treated with IMRT were selected at random. Plans were computed for both IMRT and VMAT-S (using Pinnacle TPS for an Elekta Infinity linac) along with HT. A three-dose level prescription was used to deliver 70 Gy, 63 Gy, and 58.1 Gy to regions of macroscopic, microscopic high-risk, and microscopic low-risk disease, respectively. All doses were given in 35 fractions. Comparisons were performed on dose-volume histogram data, monitor units per fraction (MU/fx), and delivery time. Results: VMAT-S target coverage was close to that achieved by IMRT, but inferior to HT. The conformity and homogeneity within the PTV were improved for HT over all strategies. Sparing of the organs at risk (OAR) was achieved with all modalities. VMAT-S (along with HT) shortened delivery time (mean, -38%) and reduced MU/fx (mean, -28%) compared with IMRT. Conclusion: VMAT-S represents an attractive solution because of the shorter delivery time and the lower number of MU/fx compared with IMRT. However, in this complex clinical setting, current VMAT-S does not appear to provide any distinct advantage compared with helical tomotherapy.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2011

Volumetric Change of Selected Organs at Risk During IMRT for Oropharyngeal Cancer

Purpose: To assess volumetric changes of selected organs at risk (OAR) during intensity-modulated radiotherapy (IMRT) for oropharyngeal carcinoma. Materials and Methods: Twenty-six consecutive patients that were treated with definitive IMRT {+-} chemotherapy between November 2007 and November 2008 were selected for the present study. As part of an internal quality assurances program, a repeat kilovolt (KV) computed tomography was planned weekly during the 7-week treatment course. On each available scan, a single observer contoured the parotid submandibular, and thyroid glands (PG/SMG/TG), larynx (L), and constrictor, masticatory, and sternocleidomastoid muscles (CM/MM/SCM) as appropriate. The volume at each scan was compared with the one at planning CT in a pair-wise fashion. p values <0.05 after correction for multiple testing were considered significant. Results: A total of 159 scans was obtained during treatment for a total of 185 scans, including the baseline imaging. All OARs showed statistically significant changes over baseline by week 5. At week 7, the PG showed the largest absolute change with an average reduction of {approx}10 mL followed by both the SCM and MM ({approx}-5 mL). The largest ({approx}-30%) relative change was observed for the salivary glands. L and CM showed a {approx}15% increase in volume during treatment. Conclusion: All selected OAR undergo significant volumetric changes during a course of IMRT for oropharyngeal squamous cell carcinoma.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2011

Real-time dose computation: GPU-accelerated source modeling and superposition/convolution

Purpose: To accelerate dose calculation to interactive rates using highly parallel graphics processing units (GPUs). Methods: The authors have extended their prior work in GPU-accelerated superposition/convolution with a modern dual-source model and have enhanced performance. The primary source algorithm supports both focused leaf ends and asymmetric rounded leaf ends. The extra-focal algorithm uses a discretized, isotropic area source and models multileaf collimator leaf height effects. The spectral and attenuation effects of static beam modifiers were integrated into each source's spectral function. The authors introduce the concepts of arc superposition and delta superposition. Arc superposition utilizes separate angular sampling for the total energy released per unit mass (TERMA) and superposition computations to increase accuracy and performance. Delta superposition allows single beamlet changes to be computed efficiently. The authors extended their concept of multi-resolution superposition to include kernel tilting. Multi-resolution superposition approximates solid angle ray-tracing, improving performance and scalability with a minor loss in accuracy. Superposition/convolution was implemented using the inverse cumulative-cumulative kernel and exact radiological path ray-tracing. The accuracy analyses were performed using multiple kernel ray samplings, both with and without kernel tilting and multi-resolution superposition. Results: Source model performance was <9 ms (data dependent) for a high resolution (400{sup 2}) field using an NVIDIA (Santa Clara, CA) GeForce GTX 280. Computation of the physically correct multispectral TERMA attenuation was improved by a material centric approach, which increased performance by over 80%. Superposition performance was improved by {approx}24% to 0.058 and 0.94 s for 64{sup 3} and 128{sup 3} water phantoms; a speed-up of 101-144x over the highly optimized Pinnacle{sup 3} (Philips, Madison, WI) implementation. Pinnacle{sup 3} times were 8.3 and 94 s, respectively, on an AMD (Sunnyvale, CA) Opteron 254 (two cores, 2.8 GHz). Conclusions: The authors have completed a comprehensive, GPU-accelerated dose engine in order to provide a substantial performance gain over CPU based implementations. Real-time dose computation is feasible with the accuracy levels of the superposition/convolution algorithm.

60 APPLIED LIFE SCIENCES↗