Imad Ali

COMPARISON OF KILOVOLTAGE CONE-BEAM COMPUTED TOMOGRAPHY WITH MEGAVOLTAGE PROJECTION PAIRS FOR PARASPINAL RADIOSURGERY PATIENT ALIGNMENT AND POSITION VERIFICATION

PURPOSE Implanted gold markers and megavoltage (MV) portal imaging are commonly used for setup verification of paraspinal tumors treated with high-dose, single-fraction radiotherapy. We investigated whether the use of kilovoltage cone-beam computed tomography (CBCT) imaging eliminates the need for marker implantation. METHODS AND MATERIALS Patients with paraspinal disease who were eligible for single-fraction stereotactic body radiotherapy were accrued to an institutional review board-approved protocol. Each of 16 patients underwent implantation of fiducial markers near the target. The markers were visible on the MV images. Three MV image pairs were acquired for each patient (initial, verification, and final) and were registered to the reference images. Every MV pair was complemented by a CBCT scan. CBCT image registration was performed automatically by maximizing the mutual information using a region of interest that excluded the markers. The corrections, as determined from the MV images, were compared with these from CBCT and were used for actual patient setup. RESULTS The mean and standard deviation of the absolute values of the differences between the CBCT and MV corrections were 1.0 +/- 0.7, 1.0 +/- 0.6, and 1.0 +/- 0.8 mm for the left-right, anteroposterior, and superoinferior directions, respectively. The absolute differences between the corresponding pre- and post-treatment kilovoltage CBCT image registration were 0.6 +/- 0.5, 0.6 +/- 0.5, and 1.0 +/- 0.8 mm. CONCLUSION The setup corrections found using CBCT without the use of implanted markers were consistent with the marker registration on MV projections. CBCT has additional advantages, including better positioning precision and robust automatic three-dimensional registration, as well as eliminating the need for invasive marker implantation. We have adopted CBCT for the setup of all single-fraction paraspinal patients. Our data have also demonstrated that target displacements during treatment are insignificant.

A motion algorithm to extract physical and motion parameters of mobile targets from cone-beam computed tomographic images

PURPOSE A motion algorithm has been developed to extract length, CT number level and motion amplitude of a mobile target from cone-beam CT (CBCT) images. MATERIALS AND METHODS The algorithm uses three measurable parameters: Apparent length and blurred CT number distribution of a mobile target obtained from CBCT images to determine length, CT-number value of the stationary target, and motion amplitude. The predictions of this algorithm are tested with mobile targets having different well-known sizes that are made from tissue-equivalent gel which is inserted into a thorax phantom. The phantom moves sinusoidally in one-direction to simulate respiratory motion using eight amplitudes ranging 0-20 mm. RESULTS Using this motion algorithm, three unknown parameters are extracted that include: Length of the target, CT number level, speed or motion amplitude for the mobile targets from CBCT images. The motion algorithm solves for the three unknown parameters using measured length, CT number level and gradient for a well-defined mobile target obtained from CBCT images. The motion model agrees with the measured lengths which are dependent on the target length and motion amplitude. The gradient of the CT number distribution of the mobile target is dependent on the stationary CT number level, the target length and motion amplitude. Motion frequency and phase do not affect the elongation and CT number distribution of the mobile target and could not be determined. CONCLUSION A motion algorithm has been developed to extract three parameters that include length, CT number level and motion amplitude or speed of mobile targets directly from reconstructed CBCT images without prior knowledge of the stationary target parameters. This algorithm provides alternative to 4D-CBCT without requirement of motion tracking and sorting of the images into different breathing phases. The motion model developed here works well for tumors that have simple shapes, high contrast relative to surrounding tissues and move nearly in regular motion pattern that can be approximated with a simple sinusoidal function. This algorithm has potential applications in diagnostic CT imaging and radiotherapy in terms of motion management.

Quantitative investigation of the effects of the scanning parameters in the digitization of EBT and EBT2 Gafchromic film dosimetry with flatbed scanners

With increasing popularity and complexity of intensity-modulated radiation therapy (IMRT) delivery modalities including regular and arc therapies, there is a growing challenge for validating the accuracy of dose distributions. Gafchromic films have superior characteristics for dose verification over other conventional dosimeters. In order to optimize the use of Gafchromic films in clinical IMRT quality assurance procedures, the scanning parameters of EBT and EBT2 films with a flatbed scanner were investigated. The effects of several parameters including scanning position, orientation, uniformity, film sensitivity and optical density (OD) growth after irradiation were quantified. The profiles of the EBT and EBT2 films had a noise level of 0.6% and 0.7%, respectively. Considerable orientation dependence was observed and the scanner value difference between landscape and portrait modes were about 12% and 10% for EBT and EBT2 films, respectively. The highest response sensitivity was observed using digitized red color images of the EBT2 film scanned with landscape mode. The total system non-uniformity composed of contributions from the film and the scanner was less than 1.7%. OD variations showed that EBT gray scale grew slower, however, reached higher growth values of 15% when compared with EBT2 gray scale which grew 12% after a long time (480 hours) post-irradiation. The EBT film using the red color channel showed the minimal growth where OD increased up to 3% within 3 days after irradiation, and took one week to stabilize.

SU‐FF‐J‐107: Extraction of Internal and External Marker 3D‐Motion in Liver Patients with Compression Belt Using KV Cone‐Beam Radiographic Projections

Purpose: To study correlation of internal implanted vs. external skin markers for tracking respiratory motion in liver patients using radiographic projections from on‐board kV cone‐beam scans. Material and Method: Cone‐beam projections were analyzed to extract 3D‐motion of internal and external markers for five liver patients receiving hypofractionated radiotherapy. Patients were immobilized in a stereotactic body frame and an abdominal compression belt was used to constrain respiratory motion. Marker motion was derived using a tracking algorithm and analysis of a sequence of 650 cone‐beam projections acquired during a 1 minute scan. Corrections were made for imager rotation and sag. Results: External and internal markers had the same frequency of respiratory motion, however, the amplitude of external marker motion is smaller. Internal and external marker motion was also out‐of‐phase in some patients. Internal marker motion is greater in the superior‐inferior direction than in anterior‐posterior and right‐left directions, which is due to compression belt constraining of respiratory motion in these direction. Two patients showed small or no motion of the external marker, whereas, internal marker motion was as large as 1.0 cm, which may be due to the proximity of the external marker to the compression belt. Conclusions: Although, the motions of internal and external markers are usually correlated and have similar motion frequency, the amplitude of marker motion may differ significantly and in some patients markers may move our‐of‐phase. The abdominal compression belt suppresses respiratory motion strongly normal to patient skin and may contribute to phase differences. The external markers motion for monitoring internal changes of respiration should be used with caution. Marker 3D‐motion from cone‐beam projections provides real time tumor trajectory that can be used to determine accuracy of PTV margins with no extra dose other than that used in CBCTimaging.Conflict of Interest: Supported by NCI Grant P01‐CA59017.

SU-FF-I-18: Quantifying the Geometric Accuracy of the On Board Imager Over a One-Year Period

Purpose: To quantify the geometric accuracy of the On Board Imager in both the kV radiographic and cone beam imaging modes. Method and Materials: The Winston‐Lutz test was performed to localize a 5mm tungsten sphere placed within +/− 0.25 mm of the radiation isocenter. The sphere was imaged with half fan cone beam scans, and kV radiographs at the 4 principal gantry angles. The displacement of the sphere from the ‘imaging isocenter’ (the actual position of a point object that the imaging system would find to be at isocenter) was determined for each imaging mode. This test has been repeated 18 times over a period of one year. Results: The average displacement of the sphere from the imaging isocenter using a half fan technique was found to be 0.9 mm Right, 0.9 mm Anterior, and 1.1 mm Inferior, assuming a head first supine orientation. These offsets are incorporated in image‐guided patient setup procedures. Small systematic errors as a function of gantry angle were also measured for the radiographs. A point at the radiation isocenter will appear about 1mm higher in a right lateral image than in a left lateral image. A similar left / right discrepancy exits for anterior and posterior images.Conclusion: The systematic geometric errors of the kV imaging equipment and associated techniques need to be measured and incorporated into the procedure of on‐line image‐guidedpatient treatment. For the On Board Imager, a geometric accuracy of better than 1mm can be achieved.

Correction of image artifacts from treatment couch in cone-beam CT from kV on-board imaging

PURPOSE To investigate image artifacts caused by a standard treatment couch on cone-beam CT (CBCT) images from a kV on-board imager and to develop an algorithm based on spatial domain filtering to remove image artifacts in CBCT induced by the treatment couch. METHODS Image artifacts in CBCT induced by the treatment couch were quantified by scanning a phantom used to quantify CT image performance. This was performed by scanning the phantom setup on a regular treatment couch and in air with the kV on-board imager. An algorithm was developed to filter image artifacts from the treatment couch by processing of cone-beam radiographic projections using two scans: one scan of the phantom and treatment couch and a second scan of the treatment couch only. This algorithm is based on a pixel-by-pixel removal of beam attenuation due to the treatment couch from each projection of the phantom and couch scan. The net couch-filtered projections were then used to reconstruct CBCT. RESULTS We found that the treatment couch causes considerable image artifacts: CT number uniformity is degraded and varies as much as 15%, and noise in CBCT scans with phantom plus couch (3.5%) is higher than for the phantom in air (1.5%). The spatial domain filtering technique reduces noise by more than 1.5%, improves uniformity by a factor of 2, and removes ringing and streaking artifacts related to the standard treatment couch in CBCT reconstructed from couch-filtered projections. This filtering technique was tested successfully to filter other hardware objects such as a patient immobilization body-fix frame. CONCLUSIONS The standard treatment couch causes image artifact in CBCT from kV on-board imaging systems. The spatial domain filtering technique developed in this work improves image quality of CBCT by preprocessing the projections prior to CBCT reconstruction. This technique might be useful to filter other hardware objects from CBCT which may contribute to the degradation of image quality.

Optimal densitometry wavelengths that maximize radiochromic film sensitivity while minimizing OD growth and temperature sensitivity artifacts.

It is well known that optical density (OD) of the radiochromic film (RCF) continues to grow after exposure at rates that have a complex dependence on dose, temperature, and densitometry wavelength. Dose rate and fractionation artifacts associated with variations in OD growth may limit the accuracy achievable by RCF dosimetry in brachytherapy and external beam applications, particularly at low doses (<5 Gy) and low dose rates (<10 cGy/h) where OD growth and sensitivity effects are large. To identify densitometry wavelengths that minimize OD growth artifacts and enhance RCF sensitivity at low doses, we have investigated Model MD-55-2 RCF response as a function of densitometry wavelength, irradiation-to-densitometry time interval, dose and temperature. Using a Perkin Elmer spectrophotometer, the absorption spectrum in the 500-700 nm range was measured for doses ranging from 1-100 Gy, over post-irradiation times from 1 h to 60 days. An empirical model with time-independent, fast and slow growth components was used to fit single exposure data and the dependence of the resulting best-fit parameters on dose and densitometry wavelength was investigated. RCF OD variation with temperature in the range 22-40 degrees was measured. Wavelengths in the 660-690 nm range were found to minimize the dose-dependence of OD post-exposure growth. Densitometry wavelengths in the range of 670-680 nm enhance RCF sensitivity and show small variations in OD with temperature in the range from 22-40 degrees. Compared to 633 nm light, 675 nm densitometry reduces OD growth at 1 Gy from 70% to 10% over a period of nearly 1174.0 h relative to the initial OD measured at 1.7 h post-irradiation. In addition, RCF sensitivity is nearly doubled at this wavelength for all dose levels.

Theoretical modeling of mobile target broadening in helical and axial computed tomographic imaging.

PURPOSE To investigate variations in mobile target length induced by sinusoidal motion in helical (HCT) and axial CT (ACT) imaging. A mathematical model was derived that predicts the measured broadening of the apparent lengths of mobile targets and its dependence on motion parameters, target size, and imaging couch speed in CT images. MATERIALS AND METHODS Three mobile targets of differing lengths and sizes were constructed of tissue-equivalent gel material and embedded into artificial lung phantom. Respiratory motion was mimicked with a mobile phantom that moves in one-dimension along the superior-inferior direction with sinusoidal motion patterns. A mathematical model was derived to predict quantitatively the variations of apparent lengths for mobile targets and its dependence on phantom and imaging couch motion parameters in HCT and ACT. The model predictions were verified by length measurements of the mobile phantom targets that were imaged with the different motion patterns using CT imaging. RESULTS The measured lengths of mobile targets enlarged or shrunk depending on the phantom motion parameters that include phantom speed, amplitude, frequency, phase and speed of the imaging couch. The target length variations were significant where some targets doubled lengths or shrunk to less than half of their actual length. The apparent lengths of mobile targets decreased if the target was moving in the same direction as the imaging couch motion and increased if the mobile target was moving opposed to imaging couch in both HCT and ACT. The model predicts well the variations in the mobile target apparent lengths and their dependence on the motion parameters. CONCLUSION The measured and model variations of apparent lengths of mobile targets are considerable and may affect the accuracy of tumor volumes obtained from HCT and ACT. This mathematical model provides a method to quantitatively assess the length variations of mobile targets and their dependence on motion parameters of the phantom and imaging system which may have potential applications in the fields of diagnostic imaging and radiotherapy.

TH‐D‐224C‐01: A Quality Assurance Procedure to Monitor Mechanical Stability and Image Quality of An On‐Board KV Cone‐Beam CT Imager

Purpose: To develop Quality Assurance (QA) procedures that monitor mechanical stability and image quality performance of a new kV on‐board‐imager (OBI) and cone beam CT(CBCT) system. Material and Methods: QA of mechanical stability includes measurements of the OBI kV and Linac MV isocenters, shifts resulting from gantry rotation, translational of imager, flexing of support arms, and reproducibility of couch shifts. Image quality QA includes measurement of noise,CT number uniformity, linearity, spatial and contrast resolutions. Results: Our system has a systematic shift between kV and MV isocenters of ∼1.4 mm. Translational motion of the OBI is accurate to ∼0.9 mm and rotational motion to ∼0.2 mm. Couch positioning accuracy has an error of ∼0.9 mm longitudinally. CT numbers are less uniform than for conventional CT with full fan (without filter) CBCT scans producing better results than other modes (CT number uniformity ∼2.5%). σ for CBCT is about 1% worse than for conventional CT and thus low‐contrast level objects such as supra‐and sub‐slice targets with < 0.5% nominal contrast in the contrast resolution module are not resolved in CBCT.CBCT numbers agree with simulator CT within 3% in the range −1000 to +1000 for scans in air. High‐contrast resolution of the OBI cone‐beam CT is comparable to the conventional CT simulator. Conclusions: Systematic shifts of the OBI isocenter from radiation isocenter must be considered for patient setup and IGRT procedures using CBCT. Systematic isocenter shifts caused by rotational and translational motions of couch and gantry must also be corrected for to achieve sub‐millimeter target localization accuracy. Image quality of kV cone‐beam CT is inferior to conventional simulator CT in terms of uniformity, and low‐contrast resolution, but has comparable CT number linearity and high‐contrast resolution. Conflict of Interest: This work is supported by NCI Grant P01‐CA59017.

Quantitative evaluation by measurement and modeling of the variations in dose distributions deposited in mobile targets

The objective of this study is to quantitatively evaluate variations of dose distributions deposited in mobile target by measurement and modeling. The effects of variation in dose distribution induced by motion on tumor dose coverage and sparing of normal tissues were investigated quantitatively. The dose distributions with motion artifacts were modeled considering different motion patterns that include (a) motion with constant speed and (b) sinusoidal motion. The model predictions of the dose distributions with motion artifacts were verified with measurement where the dose distributions from various plans that included three-dimensional conformal and intensity-modulated fields were measured with a multiple-diode-array detector (MapCheck2), which was mounted on a mobile platform that moves with adjustable motion parameters. For each plan, the dose distributions were then measured with MapCHECK2 using different motion amplitudes from 0-25 mm. In addition, mathematical modeling was developed to predict the variations in the dose distributions and their dependence on the motion parameters that included amplitude, frequency and phase for sinusoidal motions. The dose distributions varied with motion and depended on the motion pattern particularly the sinusoidal motion, which spread out along the direction of motion. Study results showed that in the dose region between isocenter and the 50% isodose line, the dose profile decreased with increase of the motion amplitude. As the range of motion became larger than the field length along the direction of motion, the dose profiles changes overall including the central axis dose and 50% isodose line. If the total dose was delivered over a time much longer than the periodic time of motion, variations in motion frequency and phase do not affect the dose profiles. As a result, the motion dose modeling developed in this study provided quantitative characterization of variation in the dose distributions induced by motion, which can be employed in radiation therapy to quantitatively determine the margins needed for treatment planning considering dose spillage to normal tissue.