Volume 35, Issue 6, June 2008
Index of content:
- Joint Therapy/History Scientific Session: Room 351
- The John S. Laughlin Science Council Research Symposium
35(2008); http://dx.doi.org/10.1118/1.2962708View Description Hide Description
Purpose: To investigate the development of light activated gold nanoshells as mediators for highly conformal photothermal therapy of soft‐tissue tumors with an emphasis on illustrating how results from MR‐guidance in phantoms, in vivo small and large animals have helped demonstrate feasibility and shape the therapeutic approach. Method and Materials: Experiments were performed on a 1.5T MRI.Temperature was measured in real‐time using MR thermography based on the proton resonance frequency shift. Nanoshell phantoms were employed to evaluate effects of concentration and size on distribution of heat when activated by an 808‐nm laser and quantitatively correlated to a 3D finite element model. Passive (enhanced permeability and retention) and active (EGFR receptor) targeting was studied in tumor xenografts. Feasibility of interstitial therapy delivery was investigated in a large animalmodel of braincancer using a canine sarcomamodel.Results: Numerical modeling of heating of gold‐silica nanoshells in phantom correlated excellently with observed MR measured heating patterns. SEM results indicate that passive accumulation of nanoshells (∼140 nm) results in large concentrations near tumor microvasculature (corroborated by MR heating and dynamic enhancement patterns) and this results in statistically significant increase in temperature (21±4°C) over controls after 3 minutes at 4 W/cm2 in a PC3 xenograph. Intravenous injection of these nanoshells into a dog model of cancer resulted in selective heating of the target tumor observable on MRI. EGFR receptor targeting allowed increased uptake of smaller nanoshells (<30 nm) versus non‐targeted nanoshells. Addition of iron‐oxide cores facilitated MR imaging of nanoshells in addition to heating. Conclusion:Gold nanoshells effect a more conformal thermal therapy delivery, as confirmed in a large animalmodel of braincancer. MR temperature imaging and guidance is an invaluable tool in the investigation and development of this emerging thermal therapy technique as it moves from Petri dish to patient.
WE‐C‐351‐02: On‐Line Perfusion Measurement with Dynamic Contrast Enhanced Cone‐Bean CT in Radiation Therapy35(2008); http://dx.doi.org/10.1118/1.2962709View Description Hide Description
Purpose: On‐line functional imaging might play an important role in the process of biological adaptive radiation therapy. This project is to develop a novel on‐line system for perfusion measurement using dynamic contrast enhanced (DEC) cone‐beam CT(CBCT) at the treatment unit. Method and Materials: This novel technique involves a single baseline CBCT followed by a singlecontrast enhanced CBCT synchronized with contrast injection. It makes use of a mathematical expression to parameterize the wash‐in and wash‐out behavior of contrast uptake in each voxel. These parameters in each voxel were optimized using the projection data in the two CBCT scans. Three rabbits implanted with VX2 tumor at the upper limb were used for validating the method. The subjects were scanned with DCE‐CBCT and DCE‐CT at 2, 3 and 4 weeks after tumor implantation with the latter to be the gold standard. The time intensity curves of contrast uptake in tumors, normal tissues and blood vessels obtained from DCE‐CBCT were compared with those from DCE‐CT. Results: The contrast enhancement in different tissue types estimated with the DCE‐CBCT method had the general shape compared to the gold standard. The correlation between the initial slopes (a surrogate of perfusion) for different tissues obtained from DCE‐CBCT and those from DCE‐CT was found to be significant with R=0.97 (p<0.001). The estimated temporal contrast enhancement on the CBCTimages appeared to be in excellent agreement with that on the DCE‐CT images.Conclusion: The proposed method has been shown to be able to estimate the temporal contrast enhancement using a baseline and a contrast enhanced CBCT in the rabbit model. By providing on‐line perfusion measurement, this novel method is potentially useful in the process of adaptive radiation therapy.
WE‐C‐351‐03: Association of Texture Features to Structure and Function in a Simple Tumor Model and Potential Application to Treatment Response Monitoring Using FDG‐PET35(2008); http://dx.doi.org/10.1118/1.2962710View Description Hide Description
Purpose: PET images carry information about structure and function of a tumor. When acquired longitudinally this information becomes invaluable for response monitoring and possibly adaptation of therapy. In this work, texture features are used to characterize structural and functional information in PET images and their change over time. Method and Materials: Haralick texture features based on second‐order image statistics were computed for PET images of a longitudinal treatment response monitoring study of non‐Hodgkin lymphoma patients. The performance of 4 features was investigated: two features characterizing spatial homogeneity within a region of interest (energy and homogeneity) and two features characterizing local variations (contrast and correlation). Observed changes of these features over time were compared against those of a computer‐generated tumormodel consisting of spheroids of well‐defined stochastic structural and functional properties. Results: The texture features allow to well differentiate between irregular structures arising from random functional properties (spheroid activity) and irregular structures caused by spatialrandomness (spheroid displacements). All investigated features were fairly insensitive to changes in spatialrandomness, however, they strongly indicate changes in activity patterns. The features of the Non‐Hodgkin lymphoma over time are mostly dominated by spatialtumor shrinkage and to a lesser extent by changes in uptake distribution. Conclusion: The use of simulated structures proves very valuable in gaining understanding of the various texture features.
35(2008); http://dx.doi.org/10.1118/1.2962711View Description Hide Description
Purpose: To develop a comprehensive high accuracy procedure for computing respiratory motion from 4D‐CT image datasets. The estimated motion field is applied to model and predict respiratory behavior at different breathing states. Method and Materials: The 4D‐CT image data consisted of 16‐slice CT volume segments acquired in cine mode. We used the optical flow deformable image registration algorithm to register the image segments to a common reference 3D‐CT volume. Before registration, the segments were optimally aligned to the reference volume according to the measured tidal volumes and the positions of the diaphragm apex in the inhaled and exhaled phases. Image registration was applied using a multiresolution approach and a feature‐preserving image down‐sampling filter, the max filter, to achieve faster computation speed and better registration accuracy. Computed backward motion fields were inverted to obtain the forward motion fields. Registration accuracy was validated using anatomical landmarks and a digital phantom. The motion fields were fit using 5D (spatial 3D + tidal volume + air flow rate) motion models independently applied to the forward and inverse motion vector maps. The forward model characterized how the tissue moved, and the inverse model characterized how the CTimage density changed with respect to the tidal volume and the air flow rate. Target modeling errors (TME), modeling errors (ME) and modeling prediction errors (MPE) were computed to evaluate these 5D models. Results: The registration error was less than 1.1±0.8 mm for lung tissue. For the forward model, TME = 0.6±0.4 mm, ME and MPE were both 0.3±0.5 mm. For the inverse model, TME = 1.1±0.6 mm, ME = 0.5±0.4 mm and MPE = 0.4±0.5 mm. Conclusion: Our procedure computes 4D respiration motion with high accuracy. Both 5D models could be utilized in multiple radiation oncology applications, including tumor motion tracking and 4D treatment planning.
35(2008); http://dx.doi.org/10.1118/1.2962712View Description Hide Description
Purpose: To develop an effective method of improving low‐dose kV CBCTimage quality using statistics‐based sinogram smoothing so that the patient's imagingdose can be greatly reduced through the use of low‐mAs protocol. Method and Materials: Based on the sinogramnoise properties, a penalized weighted least‐squares (PWLS) objective function was constructed, and the ideal sinogram was then estimated by minimizing the PWLS objection function. The variance of sinogram data was chosen as the weight the PWLS objective function and it determined the contribution of each measurement. To preserve edge information during noise reduction, we proposed an anisotropic quadratic form penalty. The quadratic form penalty encourages equivalence between neighbors and the anisotropic penalty provides the mechanism to control the influence of different neighbors according to its corresponding gradient. The proposed anisotropic penalty tends to discourage equivalence between neighbors if the gradient is large, thus edge information will be preserved in the smoothed sinogram. Two experimental phantom studies were performed to demonstrate the effectiveness of the presented algorithm. CBCT projection images were acquired by the ExactArms (kV source/detector arms) of a Varian Trilogy™ treatment system. Results:Noise in the reconstructedCBCTimage acquired with the low‐mAs protocol was greatly suppressed after the proposed sinogramimage processing, without noticeable sacrifice of the spatial resolution. For both phantoms, image quality of the low‐dose (100 mAs/projection) CBCTreconstructed from the sinogram processed by the PWLS criterion is comparable to its corresponding normal‐dose (800 mAs/projection) images in terms of contrast‐to‐noise ratio and fine structures preservation. Conclusion: The presented PWLS sinogram smoothing algorithm reduces CBCT radiation dose by a factor of 1/8 without compromising the quality of reconstructed images.
WE‐C‐351‐06: Tracking Multiple Moving Fiducials During Treatment Based On Simultaneous Onboard KV and Treatment MV Imaging35(2008); http://dx.doi.org/10.1118/1.2962713View Description Hide Description
Purpose: Intra‐fractional organ motion can limit the advantage of highly conformal dose techniques as IMRT due to target position uncertainty. A new strategy has been proposed and investigated to track implanted fiducial markers during treatment on a LINAC with an onboard kV imaging system by using simultaneous kV and treatment MV beam imaging.Method and Materials: A pelvic phantom implanted with three gold cylindrical seeds was tested on a moving platform on a Varian Trilogy. Two phantom verification plans for 3DCRT and IMRTtreatments from real patients were delivered on this phantom while the platform was moving at different speeds. A new pattern matching algorithm has been developed to locate multiple cylindrical fiducials on MV and kV images. Their 3D positions were then calculated from simultaneous orthogonal MV and kV imaging results. Results: Four issues were studied towards the clinical application of this method. 1) Multiple methods were adopted to predict marker positions and reduce search regions to achieve an analysis speed of 10 frame/second. 2) The kV and MV images were acquired while the phantom was moving at various speeds and fiducials could be successfully detected at a linear speed of 1.6 cm/s or less. 3) Varying MV field size and kV source to imager distance indicated that the detection of fiducials on kV images was not affected by the scattering from simultaneous treatment MV beams. 4) Prediction based on relative positions of fiducials was used to closely locate positions of fiducials blocked by a multileaf collimator.Conclusion: These four studies pave the way to automatically track nearly real‐time 3D positions of multiple moving fiducial markers during treatment based on simultaneous kV and MV imaging. It does not require extra hardware such as stereo kV imaging systems and utilizes the treatment beam to reduce dose.
WE‐C‐351‐07: Tetrahedron Beam Computed Tomography: A New Design of Online Imaging System for Image Guided Radiotherapy35(2008); http://dx.doi.org/10.1118/1.2962714View Description Hide Description
Purpose: Cone‐beam computed tomography(CBCT) is an important online imaging modality for image‐guidedradiotherapy(IGRT) as well as other forms of image guided interventions. However, current CBCTimage quality is inferior to that of the diagnostic fan beam CT. We have designed a novel Tetrahedron Beam Computed Tomography (TBCT) imagingsystem that may achieve the same diagnostic quality as helical CT scanners. Method and Materials: The TBCT imagingsystem is comprised of a linear scan x‐ray source and a linear discrete x‐ray detector array. The axis of linear x‐ray tube and the detector array are aligned perpendicular to and within the rotation plane, respectively. The x‐ray beams are narrowly collimated into fan beams and focused to the linear detector.Detector and x‐ray tube rotate slowly while the fan beams scan quickly along the axis. The TBCT reconstruction geometry is similar to CBCT. Approximate and exact reconstruction algorithms can be modified for TBCT reconstruction. Results: TBCT will produce diagnostic quality online images due to its scatter rejection mechanism and the use of high‐performance discrete x‐ray detectors. TBCT also has several other advantages such as larger clearance, ease of performing dynamic field size and mAs controls, etc. Conclusion: TBCT will significantly improve online image quality. Clinical implementation of TBCT would be of importance in IGRT as well as other forms of image guided interventions.
This research is partially supported by NCI grant under award number 1R21CA130330‐01A1.