Volume 36, Issue 6, June 2009
Index of content:
- Joint Imaging/Therapy Scientific Session: Room 303A
- Modeling of Intrafraction Motion
TU‐C‐303A‐01: Evaluating the Accuracy of Four‐Dimensional Photon Dose Calculations with Phantom Measurements36(2009); http://dx.doi.org/10.1118/1.3182338View Description Hide Description
Purpose: Respiratory motion can cause deviations between the intended and delivered dose distributions. Recent work has focused on developing four‐dimensional (4D) dose calculation algorithms, which explicitly account for respiration in the dose calculation process. Before these dose calculations methods can be used clinically, it is necessary to verify their accuracy. The purpose of this study was to evaluate the accuracy of 4D dose calculations with phantom measurements. Methods: Measurements were made using two anthropomorphic phantoms: a rigid moving phantom and a deformable phantom. Two motion patterns were designed to drive both phantoms: a sinusoidal motion pattern and an irregular motion pattern that was extracted from a patient breathing profile. Three plans were generated on each phantom: a single‐beam, a multiple‐beam, and an IMRT plan. Doses were calculated using the 4D dose calculation capabilities of a commercial radiation treatment planning system. Each plan was used to irradiate the phantoms, and doses were measured using TLD and radiochromic film. The measured doses were compared to the 4D‐calculated doses using a measured‐to‐calculated TLD ratio and a gamma analysis. For the TLD and film, relevant passing criteria (5% for TLD and 5%/3mm for gamma) were applied to determine if the 4D dose calculations were accurate to within clinically acceptable standards. Results: All of the TLD measurements met the passing criteria. 42 out of the 48 evaluated films passed the gamma criteria. The films that did not pass the gamma criteria were from the irregular moving rigid phantom. Conclusions: In controlled conditions, 4D dose calculations are accurate to within clinically acceptable standards. In clinical terms, this means that if patient breathing is reproducible, 4D dose calculations will produce accurate dose distributions. Conversely, irregular breathing can produce inaccurately calculated 4D dose distributions. Conflict of Interest: This work is partially supported through an SRA with Philips Healthcare.
TU‐C‐303A‐02: Intrafraction Prostate Motion Monitoring with Cine‐MV and Minimal As‐Needed Onboard KV Imaging36(2009); http://dx.doi.org/10.1118/1.3182339View Description Hide Description
Purpose: To examine the feasibility and performance of using treatment MV beam imaging with prior knowledge to estimate 3D prostate intrafraction motion and to instantaneously reposition based on information from minimal usage of on‐board kV imaging during IMRT.Methods and Materials: In contrast with current motion monitoring techniques which seek to accurately and continuously localize a moving target, we attempted for the first step only to detect potential motion beyond a pre‐defined threshold using MV images and in the second step through combined MV‐kV imaging (by turning on the kV imager) to confirm the over‐threshold event as well as obtaining accurate position information which could be used for instantaneously repositioning. EPIDimages were used to measure 2D prostate displacement in the imaging plane. By taking into account the strong correlation between prostate SI and AP motion, small displacement in the LR direction, and data from a previous IMRT session (different gantry angle), we estimated in‐line prostate motion as well which in turn allowed us to detect potential 3D over‐threshold motion. To minimize the influence from training data, no statistical motionprobability distribution information was used. Simulation has been done using 536 patient‐measured trajectories from 17 patients. Experiments were performed on a Varian Trilogy linac using a motion phantom programmed for selected typical trajectories, and the results were compared with simulations. Results: Prostate displacement beyond a set threshold (3mm) was detected for over 99% of the time at the cost of negligible kV dose (< 3 images/fraction on average). The position information required for repositioning was found to have sub‐millimeter accuracy using combined MV‐kV data. Conclusion: Significant reduction of adverse effects of intrafraction prostate motion is achievable. The technique can be readily implemented in clinics and incurs minimal imaging dose to the patient as compared with other stereoscopic imaging techniques.
TU‐C‐303A‐03: Real‐Time Profiling of Respiratory Motion and Its Application to Continuous Horizon Prediction36(2009); http://dx.doi.org/10.1118/1.3182340View Description Hide Description
Purpose: Respiration‐induced tumor motions are semi‐periodic and exhibit different variations that have distinct clinical implications. We develop and investigate a system that is capable of estimating baseline drift, frequency variation and fundamental change (oscillatory amplitude and shape) in real time and utilize such information to predict abdominal or thoracic tumor positions. Method and Materials: The observation is modeled as a periodic fundamental pattern modulated in both frequency and baseline, which is subsequently corrupted by independent additive noise. To jointly estimate these coupled components online, we utilize three key techniques : (1) augment state to capture system dynamics and hysteresis; (2) create a low order elliptical shape model to characterize semi‐periodicity; (3) perform Poincare sectioning to automatically identify iso‐phase instances. Linear interpolation/extrapolation from iso‐phase points provides a continuous phase warping function. The baseline drift is obtained by projecting the elliptical center onto the original time‐displacement axis. A least squared error (LSE) estimate for the fundamental pattern is obtained by inversely phase‐ warping the observed trajectory and compensating for the mean drift. The proposed method is applied to simulated data with known “ground‐truth”. In a preliminary prediction test with RPM data, each component (baseline, phase and fundamental pattern) is extrapolated respectively, and then reassembled to provide a “predicted trajectory”. Results: The proposed method provides the first unsupervised system that achieves robust real‐time estimation of mean displacement, phase and fundamental pattern. Application of the online profiling system to prediction yields continuous horizon prediction of 3∼5 seconds, with similar behavior to what is expected from human observers.
Conclusion: We have proposed an online profiling paradigm for describing and characterizing respiratory motions. Tests on simulation data and RPM signals have demonstrated the efficacy of the proposed method.
This work was partially sponsored by NIH P01‐CA59827.
36(2009); http://dx.doi.org/10.1118/1.3182341View Description Hide Description
Purpose: Four‐dimensional digital tomosynthesis (4D‐DTS) was recently introduced as a novel on‐board 4D imaging technique. The purpose of this study is to develop and implement a slow gantry rotation acquisition protocol for 4D‐DTS. A formalism for determining appropriate acquisition parameters based on patients' respiratory characteristics was derived and tested. Methods & Materials: Equations were derived to explain the relationships between slow gantry acquisition parameters (scan speed, frame rate and scan angle) and respiratory characteristics (average period and regularity). Additional equations relate acquisition parameters with resultant scan times, projection numbers and the distribution of projections within phase bins once sorting and binning are complete. Phantom studies were conducted to test the new protocol. A body phantom was mounted on a moving platform to simulate short and long respiratory cycles (3 and 6‐s) and scanned using the slow gantry protocol with a 1.1‐deg/sec scan speed and a 7.8‐fps frame rate. Projections were binned and sorted into 10 phases with 10% phase windows. Resultant distributions of projections within phase bins were compared with expected distributions based on the derived formalism. Reconstructions were completed to investigate dependence on respiratory period. Results: The maximum angular intervals between projections within phase bins were estimated by the formalism to be 3.5 and 6.8° for the 3 and 6‐s profiles, respectively. Actual values ranged 3.4 to 3.6° for the 3‐s profile and from 6.8 to 7.16° for the 6‐s profile. Reconstructions demonstrated the importance of adjusting scan speed according to respiratory cycle length. Conclusion: A 4D‐DTS slow gantry rotation acquisition protocol was developed, implemented, and tested. The derived formalism has proven useful for determining appropriate acquisition parameters based on respiratory characteristics and will be used as a framework for further optimization and testing of the protocol.
Conflict of Interest: Partially supported by a Varian research grant.
TU‐C‐303A‐05: On the Need for Multiple External Markers to Predict Tumor Displacement Due to Respiration36(2009); http://dx.doi.org/10.1118/1.3182342View Description Hide Description
Purpose: To study the correlation between external markers and tumor displacement, investigate the need for multiple markers to independently predict tumor displacement and perform a stratified analysis by disease site and tumor location. Methods: Data was acquired from 91 patients undergoing stereotactic body radiation therapy using the Cyberknife Synchrony™ system. The data consisted of three dimensions corresponding to each of three external markers and the tumor. Univariate analysis was performed to determine the correlation between the displacement of individual external markers and the tumor.Multivariate analysis was performed to determine which external marker dimensions (and the number) independently predicted tumor displacement. A stratified univariate and multivariate analysis based on tumor site and location was also performed. Results: Univariate analysis showed that more than one external marker and more than one external marker dimension had a correlation coefficient > 0.5 with the tumor position. Multivariate analysis showed that 2.6 external markers and 4.7 external marker dimensions were needed to independently predict tumor displacement. Each external marker dimension was independently predictive of tumor displacement in an equal number of cases. Conclusions: Univariate shows that more than one external marker and more than one external marker dimension had a strong correlation with the tumor position. Multivariate analysis showed that 2–3 external markers and 4–5 external marker dimensions were independently predictive of tumor displacement. The resulting RMS error between the predicted and actual tumor displacement from the multivariate regression model was approximately 1 mm.
36(2009); http://dx.doi.org/10.1118/1.3182343View Description Hide Description
Purpose: To determine the quiet respiration breathing motion model parameters for lungcancer and non‐lung cancer patients. Method and Materials: 49 free‐breathing patient 4DCT imagedata sets (25 scans, ciné mode) were collected with simultaneous quantitative spirometry. A cross‐correlation registration technique was employed to track the lungtissue motion between scans. The registration results were fed back to a lung‐motion model:, where x is the position of a piece of tissue located at reference position x 0. α is a parameter which characterizes the motion due to local air filling (motion as a function of tidal volume) and β is the parameter that accounts for the motion due to the imbalance of dynamical stress distributions during inspiration and exhalation which cause lung motion hysteresis (motion as a function of airflow). The parameters α and β together provide a quantitative characterization of breathing motion that inherently includes the complex hysteresis interplay. The α and β distributions were examined for each patient to determine overall general patterns and intra‐patient pattern variations. Results: For 44 patients, the greatest value of |α| was observed in the inferior and posterior lungs. In three patients, |α| reached its maximum in the anterior lung, while for two patients; |α| was greatest in the lateral lung. The hysteresis motion β had greater variability, but for the majority of patients, |β| was largest in the lateral lungs.Conclusion: This is the first report of the 3‐dimensional breathing motion model parameter for a large cohort of patients. The overall α and β maps varied smoothly as expected. The majority of patients exhibited consistent α maps, and the β maps showed greater intra‐patient variability. The motion parameter intra‐patient variability will inform our need for custom radiation therapy motion models.
This work supported in part by NIHR01CA096679 and NIHR01CA116712
TU‐C‐303A‐07: Pediatric Organ Motion Evaluated by Respiratory‐Correlated CT with a Pressure‐Sensitive Abdominal Belt36(2009); http://dx.doi.org/10.1118/1.3182344View Description Hide Description
Purpose: We present the first report of pediatric thoracic and abdominal organ motion extent measured using respiratory‐correlated CT with an abdominal sensor belt. The knowledge is essential for defining internal target volumes and planning organ at risk volumes. It is also critical for safe employment of treatment techniques producing high dose gradient such as IMRT and intensity‐modulated proton therapy.Methods and Materials: Seventeen pediatric patients have undergone radiotherapy simulation including a respiratory‐correlated 4D CT scan in supine position for evaluation of target and internal organ motion. A pressure‐sensitive belt was wrapped around the abdomen just below the diaphragm. The pressure change was used as a respiratory surrogate. Each respiration cycle was divided into eight phases. Liver, left and right kidneys, and the representative 3rd, 7th, and 11th rib pairs were contoured on each of the eight CT datasets. Results: The center‐of‐mass displacements of kidneys in the lateral and anterior/posterior directions due to respiration were small (0–2mm). The superior/inferior displacement was larger (0.6–4.6mm). Younger pediatric patients tend to have smaller kidney motion than their older counterparts. Diaphragm motion (4–10mm) was larger than kidney motion. The magnitude of diaphragm motion did not predict the magnitude of kidney motion. The rib motion ranged from 0.5 to 2mm maximum displacement in the lateral direction and 2 to 4mm in the anterior/posterior and superior/inferior directions. The 11th pair of ribs had smaller respiratory motion than the 3rd and the 7th pair. Conclusion: Pediatric internal organ motion was smaller compared to adult data reported in the literature. Conventional large planning margin of 5–10mm may be reduced for tumors neighboring these organs such as neuroblastoma or chest wall sarcoma if treated with 3D conformal technique and verified with on‐treatment imaging. Whether motion management is required for proton beam scanning in children needs further investigation.
TU‐C‐303A‐08: A Hybrid Strategy Using Discriminant Analysis for Prostate Intrafraction Motion Management36(2009); http://dx.doi.org/10.1118/1.3182345View Description Hide Description
Purpose: Prostate intrafraction motion has been shown to be patient specific; however most current compensation strategies are population wide. We evaluate the feasibility of applying margins based on a subpopulation of patients with similar intrafraction motion characteristics and of predicting group membership within the initial treatment fractions. Method and Materials: 22 prostate patients from a hypofractionated radiotherapy protocol with online CBCTimage guidance and kV fluoroscopy measurements of intrafraction motion were divided into small and large motion groups using k‐means clustering of the 90th percentile of vector displacement during treatment delivery. Group margins were computed containing 90% of measured prostate displacements at treatment delivery. Descriptive statistics from the intrafraction motion in the AP and SI‐axes and rectal filling status were used to predict group membership after 2–10 fractions using a perceptron linear discriminant function. Results: ∼75% of patients (17/22) were categorized as having relatively small intrafraction motion, with ∼25% (5/22) having large motion. Population margins were 1.7, 4.0, and 3.9 mm in the RL, AP, and SI axis. When classified into groups, margins for the small motion group reduced to 1.6, 2.9, and 3.0 mm and for the large motion group increased to 2.0, 6.4, and 6.0 mm in the RL, AP, and SI‐axes respectively. The percentage of patients correctly classified after 2, 3, 4, 5, 6, and 10 fractions was 65%, 85%, 90%, 90%, 95%, and 95% when both motion characteristics and rectal filling status were used. Omitting rectal filling status decreased the ability to correctly classify patients with large motion. Conclusion: Discriminant analysis may be used to incorporate intrafraction motion measurements from initial fractions into prostate margin calculation with reasonable accuracy, leading to margin reductions in the AP and SI‐axes for ∼75% of patients and increased coverage for ∼25%.
Conflict of Interest: Partially supported by NIH Grant CA118037.
36(2009); http://dx.doi.org/10.1118/1.3182346View Description Hide Description
Purpose: Total lung volume (TLV) varies during respiration. The TLV dose‐volume criteria currently used for treatment planning of lung irradiation are derived based on 3DCT. We investigate the variations of TLV and lung dose during respiration and determine if the current TLV dose‐volume criteria can be safely used for planning respiratory‐correlated treatment. Methods and Materials: The data of 4DCT and 3DCT acquired with free breathing during the same session for nine patients were analyzed. Each 4DCT set consisted of 10 phases. TLVs delineated using an automatic tool on 3DCT, 4DCT individual phases, and the average intensity projection (Ave‐IP) were compared. For each patient, a representative plan designed based on the 3DCT was applied to each phase of the 4DCT and the Ave‐IP CT. The dose volume parameters of V20 (volume receiving 20 Gy), V10, V5, and mean lung dose (MLD) from the above 12 plans for each case were compared. Results: TVL varied by 20% during respiration as compared with the TLV from 3DCT or Ave‐IP. On average, the TLV at 30% (mid‐exhalation) or 70% (mid‐inhalation) phases agreed within 2% with the TLV from 3DCTs for the nine cases studied. The average TLV from the Ave‐IP CTs was consistent within 2% with the average TLV of 20% or 80% phase. For all cases studied, the average MLD, V20, and V10 varied by 2% while the average V5 varied by 3% during respiration. The maximum variation for these dose‐volume parameters was 7%. Conclusion: TLVs for 20%‐ or 30%‐ phase, Ave‐IP, and 3DCT are comparable, and the 20%–30% phase CT may be used as the base CT for non‐gated ITV‐based treatment planning. For treatment planning of a gated delivery at maximum exhalation or inhalation, a margin of 3% should be considered when using the current lung dose‐volume criteria.
36(2009); http://dx.doi.org/10.1118/1.3182347View Description Hide Description
Purpose: To develop a Hidden Markov Model (HMM) of tumor motion behavior for use in adaptive image‐guidedradiation therapy(IGRT) to overcome the beam delivery system's inherent mechanical and imaging‐rate latency. As input to the HMM we investigated clinically defined parameters and tumor motion characteristics. Method and Materials: Motion data from 43 lungtumors were collected by tracking an implanted fiducial using a fluoroscopic real‐time tracking system. Data on a total of 1297 radiotherapy fractions were collected and for 637 fractions a convex hull was created over the data points for three consecutive breathing cycles. Statistical analysis led to the removal of outlier points, then the volumes of the hulls were calculated and their shapes visually examined. Tumor location in the lung as defined by bronchial segments was related to the volume and shape of the tumor movement envelope. Results: Outlier points were removed based on data density and tumor velocity limits. It was found that tumors located in the upper apex had smaller volume of movement envelope (<50 mm≈3), whereas tumors located near the chest wall or diaphragm were larger (>70 mm≈3). Tumors attached to fixed anatomical structures had a small volume of movement envelope (<30 mm≈3). Three general shapes described the tumor motion envelopes. Envelope volume and shape was inter‐fractionally consistent. Fifty percent of tumors exhibited largely 1D oscillation; Thirty‐eight percent of tumors had motion enclosed by an ellipsoid envelop with few data points in the center region, six percent of tumors moved in an arc‐like defining a concave shaped movement envelope, and six percent defined a movement envelope that was of hybrid shape. Conclusions: The location‐space correlation and the inter‐fractional consistency of the movement envelope shapes will, in part, inform the development of a HMM to predict lungtumor motion for real‐time beam adjustments in IGRT.
- Innovations and Frontiers in Medical Physics
WE‐D‐303A‐01: Feasibility Study of Microbeam Radiation Therapy Using a Carbon Nanotube Field Emission Based Electron Microbeam Irradiator36(2009); http://dx.doi.org/10.1118/1.3182528View Description Hide Description
Purpose: Microbeam radiation therapy(MRT) is an innovative radiation therapy treatment method. It has unique capability of eliminating tumor while sparing normal tissue. Currently all the MRT studies are carried out using synchrotron based radiation facility, which in general has limited accessibility. We have recently developed a carbon nanotube(CNT)field emission based electron microbeam irradiation system. It has the potential of providing extremely high radiationdose rate (>100 Gy/sec) with high spatial and temporal beam resolution. Method and Materials: The electron microbeam irradiator is based on carbon nanotubeelectron field emission technology. The system consists of a triode type CNTfield emissioncathode sealed in vacuum. The cathode current is controlled by an adjustable gate electrode, while a high voltage power supply is used to create an acceleration potential between the cathode and anode. A laser drilled aperture is used as the electron beam exit window and also provides required electron beam collimation. Results: The CNT microbeam irradiator is capable of delivering electron radiation at extremely high dose rate (>100 Gy/sec). The beam profile was recorded using Gafchromic film (HD‐810) and showed beam FWHM (full‐width‐at‐half‐maximum) about 62 μm. The measured PVDR (peak‐to‐valley dose ratio) is about 15:1, which is very reasonable for MRT application and can be further improved with better beam collimation. SW480 colorectal cancer cells were also irradiated using the same device for demonstration purpose. Conclusion: We have demonstrated the feasibility of generating high dose rate electron microbeam radiation using a carbon nanotubefield emission based electron microbeam irradiator. The compact size and low cost of the device will make the device more accessible for cancer research community. The preliminary study using the as‐developed electron microbeam system showed its promising future for potential MRT related research, especially at cellular level.
WE‐D‐303A‐02: Deoxyglucose Labeled Gold Nanoparticles as X‐Ray Computed Tomography Contrast Agents for Cancer Imaging36(2009); http://dx.doi.org/10.1118/1.3182529View Description Hide Description
Purpose: To study the feasibility of using 2‐Deoxy‐D‐Glucose (2‐DG) labeled gold nanoparticle (AuNP‐DG) as a metabolic functional CTcontrast agent through in vitro experiments. Method and Materials: The goldnanoparticles (AuNP) were fabricated using a citrate acid reduction method. The size of the AuNP was determined from Transmission Electron Microscopy images to be 4 nm in diameter. The conjugation of the 2‐DG with the AuNP core was accomplished using mercapto group in the 2‐carbon position by condensation reaction of 2‐amino‐deoxyglucose with mercaptosuccinic acid. The human alveolar epithelial cancer cell line, A‐549, was chosen for the in vitro cellular uptake assay. Two groups of cell samples (∼1 × 105 cells per sample) were incubated with the AuNP‐DG and the unlabeled AuNP, respectively, for 30 minutes (37°C, 7% CO2). Following the incubation, the cells were washed with sterile PBS six times to remove the excess goldnanoparticles. The cells were then spun to cell pellets using a centrifuge. The cell pellets were imaged using a microCT scanner immediately after the centrifuging (75 kVp, 135 μA, 1184 × 1120 matrix size, 360 views, averaging 5 frames per view). The reconstructedCTimages were analyzed using a commercial software package. Results: Significant contrast enhancement in the cell samples incubated with the AuNP‐DG with respect to the cell samples incubated with the unlabeled AuNP was observed in multiple CT slices. Quantitative analysis of the image data showed that (45.6 ± 14.2)% of the cells that were incubated with the AuNP‐DG exhibit enhanced contrast compared to the cells in the control group (incubated with AuNP). Conclusion: Results from these experiments strongly suggest enhanced uptake of the AuNP‐DG over the unlabeled AuNP by the highly glycolytic cancer cells in vitro and indicate that AuNP‐DG could served as a metabolic functional CTcontrast agent with tumor‐specific targeting capability.
WE‐D‐303A‐03: Magnetic Resonance Temperature Imaging Guided Laser‐Induced Thermal Therapy with Multi‐Walled Carbon Nanotubes36(2009); http://dx.doi.org/10.1118/1.3182530View Description Hide Description
Purpose: Feasibility has been studied for combining Multi‐Walled Carbon Nanotubes(MWNTs), as a near‐infrared (NIR) laser absorber and heat generator, with ProtonResonance Frequency (PRF) based Magnetic Resonance Temperature Imaging (MRTI), to improve the safety and efficacy of laser‐induced thermal therapy. Method and Materials: MWNTs‐laser‐induced therapy was evaluated using 3 tissue equivalent gel phantoms: alginate‐only (sodium alginate 3g/L), MWNTs‐instilled (0.5mg/ml) and MWNTs‐implanted (sub‐surface, simulating a subcutaneous tumor containing MWNTs).In vivo experiments used 4 RENCA kidneytumor bearing mice in their right flank, thermally treated using an external laser beam after direct MWNTs injection (100ug), and monitored by MRTI throughout the treatment. MRTI‐guided laser‐induced thermal therapy for phantom and in vivo experiments was performed using MR‐compatible laser systems (fiber‐optic and external laser beam) and a 7T MRI small animal scanner (Bruker Biospin). The 3‐D MRTI in vivo protocol has a standard deviation of < 1°C, temporal resolution of 4.2s and a high spatial resolution of 0.25mm. Results: With minimum‐invasive fiber‐optic laser heating (ø 0.6mm, 1 min @ 0.1W), phantom results show that the MWNTs—instilled phantom heated preferentially (from 20°C to 47°C; Δt=+27°C), compared to the alginated‐only phantom (from 20°C to 25°C; Δt=+5°C). With non‐invasive laser heating (ø 10mm, 0.5 min @ 1.8W), the implanted region of the MWNTs‐implanted phantom showed significantly elevated temperatures compared to the nearby alginate‐only medium (Δt=+15°C). Implanted RENCA kidney flank tumors in mice injected with MWNTs were heated to 77°C (Δt=+51°C) after a single 30s 3W non‐invasive laser irradiation compared to 44°C in the laser‐only tumors (no MWNTs Δt=+18°C). In two weeks post‐treatment study, all control tumors (no MWNT no laser) and laser‐only tumors keep growing while MWNTs‐tumors begin to shrink. Conclusion: This study indicates the significant improvement in small animal laser‐induced thermal therapy and may be applicable for superficial tumors in humans.
WE‐D‐303A‐04: Mircometer‐Sized Iron Oxide Particles (MPIO) Enhanced MRI with Granulocyte‐Colony Stimulating Factor (GCSF) Modulation in Murine Myocardial Infarction Model36(2009); http://dx.doi.org/10.1118/1.3182531View Description Hide Description
Purpose: To monitor the MPIO and enhanced green fluorescenceprotein (eGFP) labeled mesenchymal stem cells (MSCs) infiltration into the myocardial infarction (MI) site using ‐weighted MRI; To monitor the MRI contrast around the MI site post‐GCSF modulation. Methods: C57Bl/6 male mice (6–8 weeks old) were irradiated with an 8‐Gy dose. The labeled MSCs (3–7×105) were transplanted into the tibial medullary space 2 days post‐irradiation. The mice were divided into: 1) a sham‐operated group (Sham, n=7); 2) a MI group without GCSF injection (MI‐GCSF, n=7); and 3) a MI group with GCSF treatment (MI+GCSF, n=3). At 14 days post‐labeled MSCs transplantation, the two MI groups underwent surgery via permanent ligation of the left anterior descending coronary artery while the Sham group underwent open‐chest operation without perturbing the heart. The MI+GCSF group received subcutaneous GCSF injection 1 day post‐MI to enhance MSC mobilization. ‐weighted short‐axis cardiac MRI was performed at baseline, 3, 7 and 14 days (D14) post‐surgery. Results: The MRI signal at the MI site was temporally attenuated for both MI groups, with more attenuated for MI+GCSF group (SNR 18.17±6.06 vs 11.37±1.01 at D14, p<0.05), but not for Sham group (30.63±5.69). The MI+GCSF group showed a trend of cardiac function improvement relative to MI‐GCSF group (left ventricular ejection function 45.55±7.52% vs 40.80±16.69% at D14), but it is insignificant possibly due to the small sample number. Dual‐labeled cells were fluorescently detected around the infarction site. Conclusions: Migration of MPIO‐labeled MSCs from bone marrow into the injured heart can be temporally monitored by MRI and additional signal attenuation caused by GCSF treatment can be differentiated. Results of this study suggest a potential approach in cell therapy to noninvasively monitor migration of labeled cells as well as the mobilization modulation produced by pharmaceuticals in the MI related events.
WE‐D‐303A‐05: How to Measure Fluoroscopic “dose Efficiency”: The Spatio‐Temporal Detective Quantum Efficiency36(2009); http://dx.doi.org/10.1118/1.3182532View Description Hide Description
Purpose: To measure the “dose efficiency” of fluoroscopic systems on a quantitative and absolute scale by developing a spatio‐temporal detective quantum efficiency (DQE) metric of performance. Method and Materials: We developed the first comprehensive spatio‐temporal approach to fluoroscopic system performance through the development of a spatio‐temporal DQE. It is defined in terms of the presampling spatio‐temporal modulation transfer function(MTF), describing resolution, and spatio‐temporal noise power spectrum (NPS), describing noise. We developed what we call the “semi‐transparent moving slanted‐edge method” to measure the temporal component of the MTF, which uses a small‐signal approach to temporal resolution. This method overcomes temporal non‐linearity problems associated with differences between lag build‐up and decay responses of CsI‐based systems. A three‐dimensional (3D) approach to fluoroscopic noise, in which 3D spatio‐temporal ROIs are selected in space and time, was used to calculate the spatio‐temporal NPS. Spatio‐temporal DQE values of a bench‐top x‐ray image intensifier fluoroscopic system were calculated, illustrating use of the metric and gaining insight into fluoroscopic temporal dose efficiency. Results: Non‐ideal temporal performance was noted in the test system at temporal frequencies > 5 Hz, and there was a ∼50% drop in DQE values by 15 Hz, analogous to decreases in DQE values commonly seen at large spatial frequencies. This is the first time that fluoroscopic temporal performance has been quantized using a DQE metric, and the non‐ideal system temporal performance may be due to temporal noise aliasing and the statistical nature of lag. Conclusion: The spatio‐temporal DQE describes fluoroscopic system performance and dose efficiency in both space and time, providing temporal information that can not be obtained using lag‐corrected or other spatial approaches. The ∼50% decrease in performance by 15 Hz may suggest the presence of temporal quantum sinks.
36(2009); http://dx.doi.org/10.1118/1.3182533View Description Hide Description
Purpose: Accurate dosimetry is critical for effective management of persons exposed to radiation; in many cases this can only be achieved by biodosimetry, measuring the change in a biological parameter correlated to dose received. This study evaluates the suitability of FLT PETimaging as a biodosimeter by investigating the correlation between absorbed dose and FLT uptake in bone marrow in patients undergoing fractionated radiotherapy.Methods and Materials: Patients received a series of two PET/CT scans usingFLT (a cellularproliferation marker), one prior to the start of radiotherapy and another later in the treatment course (2–3 weeks). The ratio of the bone marrow SUVs of the later scan to the earlier scan was calculated, and the result was used as the surviving fraction in the linear quadratic formalism. A constant α/β ratio of 10 Gy was assumed. Alpha values were calculated for individual bones on a voxel‐by‐voxel basis, using dose information from the treatment plan. Inter‐patient and intra‐patient comparisons of the results were made, and the relationship between α and received dose was investigated. Results: High FLT uptake in bone marrow was observed for all patients in pre‐treatment scans, and a negative correlation was observed between dose received and change in bone marrow FLT uptake. Interestingly alpha was observed to decrease with dose. The decrease in α occurs exponentially, with the extrapolation value at zero of 0.15Gy−1 and the exponential dose decay constant of 0.5 Gy−1. Intra‐patient variability suggests that the calculation is sensitive to the image registration techniques employed. Conclusions: FLT‐PET imaging shows great potential for use as a biodosimeter; these results suggest that a correlation exists between change in FLT uptake and absorbed dose. Further study is needed to assess the sensitivity of these calculations both to setup errors and to different image processing techniques.
WE‐D‐303A‐07: A 3D System for Spatial Localization of Scanned Proton Beams: Characterization and Experimental Validation36(2009); http://dx.doi.org/10.1118/1.3182534View Description Hide Description
Purpose: Intensity‐modulated proton therapy (IMPT) using scanned proton beamlets relies on the delivery of a large numbers of beamlets to shape the dose distribution in a highly conformal manner. We have developed a 3D system based on liquid scintillator to evaluate the spatial localization, intensity and depth of penetration (energy) of the proton beamlets in near real‐time. Method and Materials: The liquid scintillation (LS) detectorsystem consists of a volume of Liquid Scintillator (20×20×23 cc) in a light tight enclosure connected to a CCDcamera. This camera has a field of view of 25.7 cm by 19.3 cm and a pixel size of 0.4 mm. While the LS is irradiated, the camera continuously acquire images of the light distribution produced inside the LS. Irradiations were conducted with proton pencil beams produced with a spot‐scanning nozzle. Pencil beams with nominal range in water between 9.5 cm and 17.6 cm were irradiated in a square area 10 cm wide. Image frames where acquired at 50 ms per frame. Results: The signal to noise ratio of a typical Bragg peak was about 170. Proton range measured from the light distribution produced in the LS was accurate within 0.3 mm on average. The largest deviation seen between the nominal and measured range was 0.6 mm. Lateral position of the measured pencil beam was accurate within 0.4 mm on average. The largest deviation seen between the nominal and measured lateral position was 0.8 mm. However, the accuracy of the measurement of lateral beam position could be improved by correcting light scattering artifacts. Conclusion: Our LS detectorsystem has been shown to be capable of fast, sub‐millimeter spatial localization of proton spots delivered in a 3D volume. This system could be used for quality assurance of IMPT.
Supported partly by the NCI (1R01CA120198‐01A2) and (2P01CA021239‐29A1)
36(2009); http://dx.doi.org/10.1118/1.3182535View Description Hide Description
Purpose: The most commonly used NTCP and TCP models are acknowledged to be over simplified. The typical approach to determining if a model ‘fits’ the data is to look at residuals of predicted rates, but this is not straightforward in multi‐dimensional modeling problems because data are sparse. It is therefore crucial to develop methods that investigate the goodness of fit of the models over the dataspace. The goal of this work is to investigate a supervised machine‐learning tool for producing contour plots of outcome probability, as a reference against simpler models.Method and Materials: For demonstrative purposes, we used an institutional dataset consisting of 281 patients treated for non‐small cell lungcancer. Two endpoints were investigated: radiation‐induced pneumonitis (n=219), and local control (n=56). Clinical and dosimetric variables were extracted using CERR and logistic regression model building was performed in the DREES software system. Kernel‐based support vector machines with radial basis functions (SVM‐RBF) were used to produce corresponding contour maps of risk levels. Results: The SVM projection plots identified four possible regions based on the risk group and the confidence level: (i) a region of low risk patients with high confidence prediction level; (ii) a region of low risk patients with lower confidence prediction level; (iii) a region of high risk patients with lower confidence prediction level, and (iv) a region of high risk patients with lower confidence prediction level. The SVM‐RBF produces nonlinear contours that differ significantly from the logistic regression contours, with some agreement where the data is most dense. Conclusion: SVM models provide a powerful, automated, tool to help understand NTCP and TCP model fits. We plan to integrate SVM methods within the open‐source DREES modeling system.
Partially supported by K25 CA 128809 and R01 CA85181.
36(2009); http://dx.doi.org/10.1118/1.3182536View Description Hide Description
Purpose: To develop a compact and powerful FPGA‐based dose calculation system to meet the performance requirement of emerging radiation therapy technologies. Method and Materials: One challenge for advanced radiation therapy technologies such as intensity‐modulated arc therapy (IMAT) is the speed of dose calculation since a large number of beams are used to approximate continuous rotational delivery, resulting in significantly increased dose calculation workload. To accelerate this process, we built a multi‐FPGA (field‐programmable gate array) accelerator with 8 independent computing channels, which can be plug into a PCI slot of any computer to serve as a coprocessor to the CPU and turn it into a powerful dose calculation workstation. The algorithm we implemented is the popular collapsed‐cone convolution/superposition (CCCS) algorithm. After receiving the total energy released per unit mass (TERMA), density and necessary geometrical information from the PCI bus, the accelerator will work on its own to do transport‐line generation, raytracing, kernel tilting, and dose deposition. Results: With the support of memory‐rich multi‐FPGA platform, our floating‐point‐based design working at 90Mhz provides a speedup of 20X over the commercial multi‐threaded software on a state‐of‐the‐art quad‐core system. The FPGA architecture was optimized in design to achieve the best performance specifically for the CCCS algorithm with the minimum resources as compared with a general purpose FPGA board. Our modular approach can also be easily expanded to achieve even greater speedup for cases where the computation time is still an issue by inserting additional such FPGA boards into the computer.Conclusion: We developed a complete dose calculation system powered by a multi‐FPGA plug‐in board, which is capable of delivering very fast dose calculation based on the collapsed‐cone convolution/superposition algorithm. Experimental studies demonstrated the potentials of our new approach to dynamic radiation delivery technologies such as IMAT that demand tremendous computation power for dose calculation.
- Target Localization
TH‐C‐303A‐01: Initial Clinical Experience with Electromagnetic Localization and Tracking for External Beam Partial Breast Irradiation36(2009); http://dx.doi.org/10.1118/1.3182625View Description Hide Description
Purpose: The Calypso® 4D Localization System™ (Calypso Medical) uses non‐ionizing ACelectromagnetic radiation to localize and track small wireless devices (called Beacon® transponders) implanted in or near a patient's tumor. We report on the first clinical experience with the use of the system for localizing and tracking the lumpectomy cavity during external‐beam accelerated partial breast irradiation (EB APBI). Method and Materials: The study included patients treated receiving EB APBI on an IRB approved protocol. Thirteen patients were implanted with both gold markers (GM) and beacon® transponders and two patients were implanted with beacon® tranponders alone. For patients in whom MRI follow‐up was anticipated, two removable interstitial breast catheters were inserted and afterloaded with gold markers and transponders. The catheters were removed post radiation therapy. Initial alignment was performed using lasers. For patients with gold markers, orthogonal images were used to obtain the necessary shift. The shift values were compared to the shift predicted under electromagnetic guidance. During treatment, Calypso was used to track the target motion. Results: Fifteen patients have been studied, and 93 treatment fractions were analyzed. The catheters and transponders overall showed good stability with inter‐transponder distance changes of less than 2 mm. Calypso based setup can be performed in less than 2 minutes. An average residual setup error of 10.29 mm was determined using gold markers. For the 63 fractions analyzed, the difference between the residual setup error determined by the GM and the Calypso system on average was 1.5 mm. Tracking showed regular motion in the range of 2–3 mm with occasional deeper breaths exceeding 4–5 mm. Conclusion: Results show excellent agreement between gold markers and electromagnetic guidance in EB APBI with electromagnetic guidance providing a more rapid setup and real time tracking during delivery. Research sponsored by Calypso Medical.