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PURPOSE: To study the image effects of the time-wise dynamic aspect of intravenous contrast agents to enable contrast-enhanced cone-beam CT (CE- CBCT) localization of liver lesions for stereotactic body radiation therapy (SBRT). METHODS: A model was developed to study dynamic IV contrast agents using static phantoms and to derive optimum parameters for CE- CBCT imaging. Ten samples containing iodine at 0-5 mg/mL were prepared in cylindrical tubes, corresponding roughly to 0-100 HU as measured by 120 kV helical CT imaging. Each sample was imaged separately in a tissue- equivalent phantom, yielding ten datasets (roughly 650 projections each) corresponding to these static CBCT images. To reconstruct images of dynamic contrast concentrations, the CBCT 2D projections were re- assembled to match the expected amount of contrast at different points in time. This model was applied to published hepatic contrast enhancement curves, and optimum imaging and contrast injection parameters were derived. RESULTS: A signal-to-noise ratio (SNR) decrease of 25%-75% in dynamic CE-CBCT images from ideal CT of samples with a 20-100 HU difference from water was observed in the un-optimized scans. This demonstrates the difficulty of CE-CBCT, and was noticed even in geometries that minimize or eliminate x-ray scatter, detector glare, and motion. Using our model, we found parameters for iodine injection, CBCT scanning, and injection/scanning timing which optimize contrast enhancement, and a 100% SNR increase with respect to the un-optimized scans was achieved. CONCLUSIONS: The effect of IV contrast is severely degraded in CBCT, and optimization of image and timing parameters is crucial for improved CE-CBCT imaging for target localization. CBCT has very low temporal resolution, and the pharmacokinetics of IV contrast must be carefully considered in order to apply this technique to localize liver lesions for SBRT. This model will be used to establish the feasibility of CE- CBCT for routine localization of liver lesions.
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PURPOSE: Four-dimensional cone-beam CT (4D-CBCT) is a novel imaging technique used to guide treatment setup for patients with pulmonary lesions by providing additional information about tumor motion at the time of treatment. This study aimed to evaluate the efficacy of the 4D-CBCT capability in ensuring accurate patient setup during SBRT. METHODS: Twelve patients with pulmonary lesions were imaged pre-treatment with Elekta XVI4.5 using the Symmetry protocol resulting in a respiratory correlated 4D-CBCT. Reconstruction produced 10 phased-based and one average 3DCT image set. Patient shifts were derived from contour-based(mask) registration driven by the weighted average of shifts from each phased CT(4D shifts). Physicians reviewed registration and manually adjusted shifts based on visual registration. We exported the average 3DCT to MIM Vista Software 5.1.1 in reference volume coordinates and manually fused to the reference CT. All manual fusions were contour-based registrations performed by a single observer. No rotations were permitted in manual fusion to mimic clinical procedure. Translational 3D shifts from manual fusion were compared to 4D(automatic registration) shifts and final physician-corrected shifts. RESULTS: Mean differences between 4D and 3D shifts in lateral, longitudinal, and vertical directions were 1.07mm, 5.92mm, and 1.43mm, respectively. Mean differences between physician-corrected and 3D shifts were 1.41mm, 4.83mm, and 1.61mm. Differences between 4D shifts and 3D shifts increased with increasing tumor motion. One patient had consistently large longitudinal differences between 4D and 3D shifts (mean=3.0cm). Further review revealed poor 4D registration(via mask and clipbox) on the XVI system which was corrected by physician adjustment prior to treatment. CONCLUSIONS: 4D-CBCT is a valuable imaging tool in patient setup. Physician review of contour-based registration is imperative in preventing a geometrical miss. Caution must be employed in tumors that exhibit a large degree of motion. Further research is necessary in determining functional limits of the 4D-CBCT system.
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PURPOSE: To determine whether the accuracy of CBCT based IGRT and ART lung SBRT treatments may require extra quality assurance (QA) steps. METHODS: During CBCT Rando phantom acquisition we detected an unexpected â¼2° image rotation when comparing the CW and CCW acquired scans. Misregistered angular coordinates may Result in a rotated reconstructed image and the target localization may lead to an under- or over-dosage of the target volume (TV) and organs at risk (OARs). The effect of image rotation on CBCT-guided lung SBRT was retrospectively examined in a group of six patients treated at our institution. Patient CT sets were rotated by 1,2, and 3°. Treatment plans were recalculated using these rotated images to examine changes of dose-volume histogram indicators for IGRT and ART guided treatments. C++ simulations were run to evaluate the effect of CBCT image rotation. RESULTS: We determined through mathematical analysis that the dose coverage of the TV is dependent on its shape, location and orientation relative to isocenter. Dosimetric evaluation of lung SBRT patients showed that even for 1< Ñ2 <3°, changes in D95 to the PTV were from 2.3 ± 2.1 to 11.5 ± 3.9% for IGRT and from 8.5 ± 8.4 to 16.6 ± 8.0% for ART. Significant changes were also detected at critical structure level. CONCLUSIONS: When IGRT and ART are employed for lung SBRT treatments, significant dosimetric changes may Result from the rotation of CBCT image data sets. The extent of alterations in dose indicators depends on both the shape of the TV and its relative location to isocenter. Based on our results, angular alignment of CBCT to <1° is essential in maintaining accurate dose delivery of IGRT and ART based lung SBRT treatments.
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PURPOSE: To develop and validate an EPID-based 4D patient dose reconstruction framework accounting for linac delivery uncertainties, interfractional and intrafractional motions, and interplay effect. METHODS: Patients with fiducial markers were scanned with 4D-CT for SBRT planning. Before treatment, in-room 4D-CT was performed. Both the MLC and the tumor movements were tracked by continuously acquiring EPID images during treatment. Instead of directly using the heterogeneous transit photon fluence measured by the EPID, this method reconstructed the incident beam fluence based on the MLC apertures measured by the EPID and the delivered MU recorded by the linac. To account for the time-dependent-geometry, the incident fluence distributions were sorted into their corresponding phases based on the tumor motion pattern detected by the EPID and accumulated as the incident fluence map for each phase. Together with 4D-CT, it was then used for Monte Carlo dose calculation. Deformable registration was performed to sum up the phase doses for treatment assessment. The feasibility of using the transit EPID images for incident fluence reconstruction was evaluated against EPID in-air measurements. The accuracy of 3D- and 4D-dose reconstruction was validated by a motordriven cylindrical diode array for six clinical SBRT plans. RESULTS: The average difference between the measured and reconstructed fluence maps is within 0.16%. The reconstructed 3D-dose shows 1.4% agreement in the CAX-dose and >98.5% gamma-passing-rate (2%/2mm) in the peripheral-dose. A distorted dose distribution is observed in the measurement for the moving ArcCheck-phantom. The comparison between the measured and the reconstructed 4D-dose without considering interplay fails the gammaevaluation (59%-88.9% gamma-passing-rate). In contrast, when the interplay is considered, the dose distortion phenomena is successfully represented in the reconstructed dose (>97.6% gamma-passing-rate). CONCLUSIONS: The experimental validation demonstrates that the proposed method provides a practical way to reconstruct the fractional 4D-doses received by the patient and enables adaptive SBRT strategy.