On-board Cone-beam CT Research Articles

Impact of flattening-filter-free techniques on delivery time for lung stereotactic volumetric modulated arc therapy and image quality of concurrent kilovoltage cone-beam computed tomography: a preliminary phantom study

To the Editor, Volumetric modulated arc therapy (VMAT) serves as a means for stereotactic hypofractionated treatment of lung tumors [1]. The present authors proposed an efficient VMAT sequence by restricting leaf speeds of a multi-leaf collimator per gantry rotation angle to below 1 mm/degree, thereby reducing the dose delivery time down to 210 s for a D95 prescription dose of 50 Gy in four fractions [2, 3]. The authors also suggested further reduction of the dose delivery time to below 100 s using flattening-filter-free (FFF) techniques [3]. Recently we evaluated a non-clinical FFF research configuration by adding an FFF filter to our linac system, a Synergy with an MLCi multi-leaf collimator (Elekta AB, Stockholm, Sweden). The FFF filter is made of stainless steel and has a constant thickness of 2 mm for 6- and 10-MV beams. Furthermore, we also tested a non-clinical on-board cone-beam CT (CBCT) system, XVI 5.0 research version, that allows concurrent 4D CBCT imaging during VMAT delivery. It is anticipated that the reduced VMAT delivery time may degrade CBCT image quality due to a much lower number of projection images. The purposes of this study were: (i) to quantify the stereotactic lung VMAT delivery time with FFF, and (ii) to visually inspect CBCT image quality with FFF.

Accuracy of Off-Line Bioluminescence Imaging to Localize Targets in Preclinical Radiation Research

In this study, we investigated the accuracy of using off-line bioluminescence imaging (BLI) and tomography (BLT) to guide irradiation of small soft tissue targets on a small animal radiation research platform (SARRP) with on-board cone beam CT (CBCT) capability. A small glass bulb containing BL cells was implanted as a BL source in the abdomen of 11 mouse carcasses. Bioluminescence imaging and tomography were acquired for each carcass. Six carcasses were setup visually without immobilization and 5 were restrained in position with tape. All carcasses were setup in treatment position on the SARRP where the centroid position of the bulb on CBCT was taken as "truth". In the 2D visual setup, the carcass was setup by aligning the point of brightest luminescence with the vertical beam axis. In the CBCT assisted setup, the pose of the carcass on CBCT was aligned with that on the 2D BL image for setup. For both 2D setup methods, the offset of the bulb centroid on CBCT from the vertical beam axis was measured. In the BLT-CBCT fusion method, the 3D torso on BLT and CBCT was registered and the 3D offset of the respective source centroids was calculated. The setup results were independent of the carcass being immobilized or not due to the onset of rigor mortis. The 2D offset of the perceived BL source position from the CBCT bulb position was 2.3 mm ± 1.3 mm. The 3D offset between BLT and CBCT was 1.5 mm ± 0.9 mm. Given the rigidity of the carcasses, the setup results represent the best that can be achieved with off-line 2D BLI and 3D BLT. The setup uncertainty would require the use of undesirably large margin of 4-5 mm. The results compel the implementation of on-board BLT capability on the SARRP to eliminate setup error and to improve BLT accuracy.

Accuracy of Off-Line Bioluminescence Imaging to Localize Targets in Preclinical Radiation Research.

In this study, we investigated the accuracy of using off-line bioluminescence imaging (BLI) and tomography (BLT) to guide irradiation of small soft tissue targets on a small animal radiation research platform (SARRP) with on-board cone beam CT (CBCT) capability. A small glass bulb containing BL cells was implanted as a BL source in the abdomen of 11 mouse carcasses. Bioluminescence imaging and tomography were acquired for each carcass. Six carcasses were setup visually without immobilization and 5 were restrained in position with tape. All carcasses were setup in treatment position on the SARRP where the centroid position of the bulb on CBCT was taken as "truth". In the 2D visual setup, the carcass was setup by aligning the point of brightest luminescence with the vertical beam axis. In the CBCT assisted setup, the pose of the carcass on CBCT was aligned with that on the 2D BL image for setup. For both 2D setup methods, the offset of the bulb centroid on CBCT from the vertical beam axis was measured. In the BLT-CBCT fusion method, the 3D torso on BLT and CBCT was registered and the 3D offset of the respective source centroids was calculated. The setup results were independent of the carcass being immobilized or not due to the onset of rigor mortis. The 2D offset of the perceived BL source position from the CBCT bulb position was 2.3 mm ± 1.3 mm. The 3D offset between BLT and CBCT was 1.5 mm ± 0.9 mm. Given the rigidity of the carcasses, the setup results represent the best that can be achieved with off-line 2D BLI and 3D BLT. The setup uncertainty would require the use of undesirably large margin of 4-5 mm. The results compel the implementation of on-board BLT capability on the SARRP to eliminate setup error and to improve BLT accuracy.

Accelerated barrier optimization compressed sensing (ABOCS) reconstruction for cone-beam CT: phantom studies.

Recent advances in compressed sensing (CS) enable accurate CT image reconstruction from highly undersampled and noisy projection measurements, due to the sparsifiable feature of most CT images using total variation (TV). These novel reconstruction methods have demonstrated advantages in clinical applications where radiation dose reduction is critical, such as onboard cone-beam CT (CBCT) imaging in radiation therapy. The image reconstruction using CS is formulated as either a constrained problem to minimize the TV objective within a small and fixed data fidelity error, or an unconstrained problem to minimize the data fidelity error with TV regularization. However, the conventional solutions to the above two formulations are either computationally inefficient or involved with inconsistent regularization parameter tuning, which significantly limit the clinical use of CS-based iterative reconstruction. In this paper, we propose an optimization algorithm for CS reconstruction which overcomes the above two drawbacks. The data fidelity tolerance of CS reconstruction can be well estimated based on the measured data, as most of the projection errors are from Poisson noise after effective data correction for scatter and beam-hardening effects. We therefore adopt the TV optimization framework with a data fidelity constraint. To accelerate the convergence, we first convert such a constrained optimization using a logarithmic barrier method into a form similar to that of the conventional TV regularization based reconstruction but with an automatically adjusted penalty weight. The problem is then solved efficiently by gradient projection with an adaptive Barzilai-Borwein step-size selection scheme. The proposed algorithm is referred to as accelerated barrier optimization for CS (ABOCS), and evaluated using both digital and physical phantom studies. ABOCS directly estimates the data fidelity tolerance from the raw projection data. Therefore, as demonstrated in both digital Shepp-Logan and physical head phantom studies, consistent reconstruction performances are achieved using the same algorithm parameters on scans with different noise levels and∕or on different objects. On the contrary, the penalty weight in a TV regularization based method needs to be fine-tuned in a large range (up to seven times) to maintain the reconstructed image quality. The improvement of ABOCS on computational efficiency is demonstrated in the comparisons with adaptive-steepest-descent-projection-onto-convex-sets (ASD-POCS), an existing CS reconstruction algorithm also using constrained optimization. ASD-POCS alternatively minimizes the TV objective using adaptive steepest descent (ASD) and the data fidelity error using projection onto convex sets (POCS). For similar image qualities of the Shepp-Logan phantom, ABOCS requires less computation time than ASD-POCS in MATLAB by more than one order of magnitude. We propose ABOCS for CBCT reconstruction. As compared to other published CS-based algorithms, our method has attractive features of fast convergence and consistent parameter settings for different datasets. These advantages have been demonstrated on phantom studies.

Technical report. Accuracy of intensity-modulated radiation therapy dose calculations: verification based on-board conebeam CT imaging

Abstract Background: Patient’s conebeam computer tomography (CBCT) images have suggested a possibility for adaptive radiotherapy although the dose delivery is of structural complexity. It is of practical importance to verify and test the intensity-modulated radiation (IMRT) planning system for radiation therapy. Objective: Verify accuracy of dose calculations based on CBCT imaging. Materials and methods: Electron density calibration curve was generated for planning CT and CBCT data set using two CT phantoms (Gammex RMI® and Catphan® 600). Anthropomorphic head and neck phantom images were acquired from planning CT and CBCT. The routine IMRT technique was generated on the planning CT, which was applied to the CBCT. Dose distributions were computed. All LiF TLD-100 dosimeters were calibrated with gamma-ray. Forty-eight TLD measuring points were chosen in five different slices of the phantom. Measurements were repeated four times, and the average dose was compared to the reading doses on both CT and CBCT plans. Dose volume histograms (DVH) of various structures were generated, and dose statistics were analyzed. Results: Hounsfield unit obtained from Catphan phantom was similar between planning CT and CBCT. IMRT dose calculations based on the planning CT and CBCT agreed well with reading doses at 48 points. Statistical point doses by DVH calculation on CBCT were about 3% lower than those by the conventional CT. Dose ratios calculated over measured ones ranged from 0.82 to 1.09. Conclusion: Point doses and DVH calculations based on the planning CT and on-board CBCT were in acceptable agreement. CT phantom specifically designed for CBCT is recommended to improve accuracy of IMRT dose calculation on CBCT images.

Accuracy of intensity-modulated radiation therapy dose calculations: verification based on-board conebeam CT imaging

Background: Patient’s conebeam computer tomography (CBCT) images have suggested a possibility for adaptive radiotherapy although the dose delivery is of structural complexity. It is of practical importance to verify and test the intensity-modulated radiation (IMRT) planning system for radiation therapy. Objective: Verify accuracy of dose calculations based on CBCT imaging. Materials and methods: Electron density calibration curve was generated for planning CT and CBCT data set using two CT phantoms (Gammex RMI® and Catphan® 600). Anthropomorphic head and neck phantom images were acquired from planning CT and CBCT. The routine IMRT technique was generated on the planning CT, which was applied to the CBCT. Dose distributions were computed. All LiF TLD-100 dosimeters were calibrated with gamma-ray. Forty-eight TLD measuring points were chosen in five different slices of the phantom. Measurements were repeated four times, and the average dose was compared to the reading doses on both CT and CBCT plans. Dose volume histograms (DVH) of various structures were generated, and dose statistics were analyzed. Results: Hounsfield unit obtained from Catphan phantom was similar between planning CT and CBCT. IMRT dose calculations based on the planning CT and CBCT agreed well with reading doses at 48 points. Statistical point doses by DVH calculation on CBCT were about 3% lower than those by the conventional CT. Dose ratios calculated over measured ones ranged from 0.82 to 1.09. Conclusion: Point doses and DVH calculations based on the planning CT and on-board CBCT were in acceptable agreement. CT phantom specifically designed for CBCT is recommended to improve accuracy of IMRT dose calculation on CBCT images. Keywords: Cone-beam CT, dose verification, IMRT, on-board imager

Reduction of motion artefacts in on-board cone beam CT by warping of projection images

We describe the development and testing of a motion correction method for flat panel imager-based cone beam CT (CBCT) based on warping of projection images. Markers within or on the surface of the patient were tracked and their mean three-dimensional (3D) position calculated. The two-dimensional (2D) cone beam projection images were then warped before reconstruction to place each marker at the projection from its mean 3D position. The motion correction method was tested using simulated cone beam projection images of a deforming virtual phantom, real CBCT images of a moving breast phantom and clinical CBCT images of a patient with breast cancer and another with pancreatic cancer undergoing radiotherapy. In phantom studies, the method was shown to greatly reduce motion artefacts in the locality of the radiotherapy target and allowed the true surface shape to be accurately recovered. The breast phantom motion-compensated surface was within 1 mm of the true surface shape for 90% of surface points and greater than 2 mm from the true surface at only 2% of points. Clinical CBCT images showed improved image quality in the locality of the radiotherapy target after motion correction. The proposed method is effective in reducing motion artefacts in CBCT images.

A novel off‐axis scanning method for an enlarged ellipse cone‐beam computed tomography field of view

Current on-board imaging systems commonly used by modern linear accelerators (LINACs) have a limited field of view (FOV) for a cone-beam CT (CBCT) scan, which is typically less than 50 cm. Consequently, truncation artifacts often occur for large patients. The goal of this work is to investigate a novel method to increase the FOV for current on-board CBCT systems. When a large patient is scanned with CBCT, any region outside the FOV is only partially sampled within a short range of projection angles, and at other angles no x-ray beams may pass through that region. To increase the sampling rate for the region outside the FOV, we have designed a new source trajectory by shifting the center of rotation during a CBCT scan. This resulted in a reduced sampling rate at the central area and increased sampling rate at the edges. The tradeoff led to a more balanced sampling for an enlarged FOV. An iterative algorithm was also developed to reconstruct the CT image under the new sampling scheme using a compressed sensing technique. The method was validated by numerical simulations mimicking a Varian Trilogy CBCT system, and it was found that artifact-free images could be obtained with the FOV as large as 80 cm. The new CT scanning trajectory can be easily realized under current clinical setup with little modification of the control system, and this can be useful for treating obese patients.

Implementation of Back-projection Filtering Algorithm for Image Reconstruction of On-board Cone-Beam CT

Implementation of Back-projection Filtering Algorithm for Image Reconstruction of On-board Cone-Beam CT

Monte Carlo evaluation of scatter mitigation strategies in cone‐beam CT

To investigate the efficacy of two widely used scatter mitigation methods: antiscatter grids (ASGs) and beam modulating with bowtie filters (BTFs), in combination with subtractive scatter correction or zeroth order normalization phantom calibration, for improving image noise, contrast, contrast-to-noise ratio (CNR), and image uniformity for on-board cone-beam CT (CBCT) imaging systems used for image-guided radiation therapy. PTRAN Monte Carlo CBCT x-ray projections of head and pelvic phantoms were calculated for combinations of beam-modulation and scatter rejection methods and images were reconstructed by in-house developed software. In addition, a simple one-dimensional analytic model was developed to predict scatter-to-primary ratio (SPR) and CNR as a function of cylindrical phantom thickness, ASG transmission, and beam modulation with bow-tie filters. ASGs were found to have slightly negative or no effect on head phantom image CNR and to modestly improve CNR (10%-20%) in pelvic phantom images. However, scatter subtraction and norm-phantom calibration perform better when applied on data acquired with ASGs. Scatter subtraction improves CT number accuracy, but increases noise, and in high SPR/low primary-photon transmission scenarios can dramatically reduce CNR and introduce streaking artifacts. The BTF is found to reduce SPR and image noise, resulting in a better trade-off between CNR and imaging dose, but introduces a circular band artifact. Our study shows that ASGs have a modest positive impact in pelvic scans and negative in head scans, scatter subtraction improves the HU accuracy but reduces CNR, while BTF has a clearly positive effect.

Image Quality of On-board Cone-beam CT Acquired during VMAT Delivery

Image Quality of On-board Cone-beam CT Acquired during VMAT Delivery

Assessment of Patient Setup Error in Prostate Radiation Therapy using Fiducial-based Image Guided Radiation Therapy with kV Onboard Imaging and Conebeam CT

Assessment of Patient Setup Error in Prostate Radiation Therapy using Fiducial-based Image Guided Radiation Therapy with kV Onboard Imaging and Conebeam CT

SU-FF-T-560: Evaluation of ExacTrac and CBCT Patient Positioning On the Novalis TX

Purpose: To evaluate the precision and accuracy of patient localization using a cone-beam CT (CBCT) and an orthogonal x-ray pair with infrared markers on the Novalis Tx treatment unit. Method and Materials: Twenty stereotactic radiosurgery/therapy patients with a total of twenty-five lesions over ninety sessions (ranging from 1−5 fractions per patient) were localized daily using both ExacTrac (ETX, BrainLab) and on-board CBCT (Varian) coupled to the Novalis Tx treatment unit. Each patient was first positioned using the ETX system accounting for variances in all six dimensions using a robotic couch top. Following these shifts, a CBCT was performed and further translations were made (x, y, z, table rotation) based on image fusion between the CBCT and simulation CT. A phantom study was also performed, mimicking the patient set-up method to assess the reproducibility of each system and to determine any systematic differences between the ETX and CBCT localization approaches. Results: Patient positioning between ETX and CBCT was consistent in all four dimensions within 1.1mm and 0.1°. The average discrepancy between each system across all sessions was 1.1±1.2mm A/P, 1.0±1.2mm S/I, 0.1±1.4mm M/L, and 0.1°±0.5° couch rotation. Phantom testing showed that both systems were reproducible within 1.5mm and 0.5° in all dimensions. A systematic discrepancy of 0.3mm A/P, 1.2mm S/I, 0.8mm M/L, and 0.6° rotation was found between the two systems; however, this difference was deemed to be within the calibration tolerance of both systems. Conclusions: The ETX and on-board CBCT systems were found to agree on tumor localization within 1.1mm (all dimensions) and 0.1° (couch rotation). Phantom studies showed the reproducibility of each system to be acceptable for stereotactic treatments. The Novalis Tx treatment unit incorporates both fiducial marker-based, and volume-based localization for reproducible and accurate SRS/SBRT patient treatments.

Scatter correction for cone‐beam CT in radiation therapy

Cone-beam CT (CBCT) is being increasingly used in modern radiation therapy for patient setup and adaptive replanning. However, due to the large volume of x-ray illumination, scatter becomes a rather serious problem and is considered as one of the fundamental limitations of CBCT image quality. Many scatter correction algorithms have been proposed in literature, while a standard practical solution still remains elusive. In radiation therapy, the same patient is scanned repetitively during a course of treatment, a natural question to ask is whether one can obtain the scatter distribution on the first day of treatment and then use the data for scatter correction in the subsequent scans on different days. To realize this scatter removal scheme, two technical pieces must be in place: (i) A strategy to obtain the scatter distribution in on-board CBCT imaging and (ii) a method to spatially match a prior scatter distribution with the on-treatment CBCT projection data for scatter subtraction. In this work, simple solutions to the two problems are provided. A partially blocked CBCT is used to extract the scatter distribution. The x-ray beam blocker has a strip pattern, such that partial volume can still be accurately reconstructed and the whole-field scatter distribution can be estimated from the detected signals in the shadow regions using interpolation/extrapolation. In the subsequent scans, the patient transformation is determined using a rigid registration of the conventional CBCT and the prior partial CBCT. From the derived patient transformation, the measured scatter is then modified to adapt the new on-treatment patient geometry for scatter correction. The proposed method is evaluated using physical experiments on a clinical CBCT system. On the Catphan 600 phantom, the errors in Hounsfield unit (HU) in the selected regions of interest are reduced from about 350 to below 50 HU; on an anthropomorphic phantom, the error is reduced from 15.7% to 5.4%. The proposed method is attractive in applications where a high CBCT image quality is critical, for example, dose calculation in adaptive radiation therapy.

Image-guided small animal radiation research platform: calibration of treatment beam alignment

Small animal research allows detailed study of biological processes, disease progression and response to therapy with the potential to provide a natural bridge to the clinical environment. The small animal radiation research platform (SARRP) is a portable system for precision irradiation with beam sizes down to approximately 0.5 mm and optimally planned radiation with on-board cone-beam CT (CBCT) guidance. This paper focuses on the geometric calibration of the system for high-precision irradiation. A novel technique for the calibration of the treatment beam is presented, which employs an x-ray camera whose precise positioning need not be known. Using the camera system we acquired a digitally reconstructed 3D ‘star shot’ for gantry calibration and then developed a technique to align each beam to a common isocenter with the robotic animal positioning stages. The calibration incorporates localization by cone-beam CT guidance. Uncorrected offsets of the beams with respect to the calibration origin ranged from 0.4 mm to 5.2 mm. With corrections, these alignment errors can be reduced to the sub-millimeter range. The calibration technique was used to deliver a stereotactic-like arc treatment to a phantom constructed with EBT Gafchromic films. All beams were shown to intersect at a common isocenter with a measured beam (FWHM) of approximately 1.07 mm using the 0.5 mm collimated beam. The desired positioning accuracy of the SARRP is 0.25 mm and the results indicate an accuracy of 0.2 mm. To fully realize the radiation localization capabilities of the SARRP, precise geometric calibration is required, as with any such system. The x-ray camera-based technique presented here provides a straightforward and semi-automatic method for system calibration.

Clinical Implementation of On-Board CBCT-Based Adaptive IMRT for Head and Neck Cancer

Onboard cone beam CT (CBCT) allows one to acquire volumetric anatomical information of a patient prior to treatment on a routine basis and makes it possible to adaptively modify the patient treatment plan with consideration of organ deformation as well as previously delivered doses. The aim of this work is to develop a clinically practical offline and eventually online adaptive therapy procedure and evaluate the potential impact of the CBCT-based adaptive head and neck IMRT treatment.

Evaluation of a CT to Cone-Beam CT Deformable Registration Algorithm

The advent of on-board cone-beam CT (CBCT) imagers for radiation therapy delivery devices offers the potential for daily dosimetry and adaptive therapy. Heretofore this potential has been limited by several artifacts in the CBCT volumes. This obstacle may require surrogates with reliable density values to be used for recalculation of dosimetry, such as a CT volume deformed to the treatment space of the CBCT. Additionally, automatically accumulating dose to critical structures and target volumes requires structures defined on the original planning CT to be deformed into the treatment space identified by the CBCT.

Patient Positioning Using a Fast Robust 2D-3D Registration Strategy With a Registration Quality Evaluator

Implement 2D-3D registration on linacs with electronic portal imaging devices (EPIDs) to yield accuracy similar to 3D-3D registration using linac-based conebeam CT (CBCT) for registrations based on bony anatomy. Our specific methodology overcomes 1) long computation times due to multiple digitally reconstructed radiograph (DRR) generation requirement, and 2) potential positioning errors due to local minima trapping. It offers high precision 3D patient positioning without requiring onboard CBCT on each treatment linac. 2D-3D registration uses multiple 2D projection images to achieve 3D positioning by computing the best alignment based on bony anatomy of the 2D portal images with DRRs at incrementally different poses. Challenge 1) can be overcome with specialized code that assigns DRR generation to the graphics processing unit (GPU) rather than CPU but this requires dedicated software/hardware for each EPID. Challenge 2) can be overcome by requiring human oversight to check the validity of the registration but this limits the automation level needed for large patient throughput. We present a 2D-3D registration methodology that can be implemented either on 1) a single workstation equipped with a NVIDIA GeForce 8800 GPU and a DRR server software, which is networked to multiple standard EPID viewing stations via TCP/IP, hence minimizing hardware requirements, or 2) each viewing workstation containing the GPU and DRR software. A registration quality evaluator (RQE) is used to avoid local optima associated with 2D-3D registrations. RQE is an algorithm based pattern classifier that identifies local optima trapping of an optimization, which can lead to incorrect patient positioning. We implemented a 2D-3D registration using two strategies: 1) GPU on a server and a remote client PC connected through intranet, which achieved an accuracy of 1 mm and 1° for phantom and clinical imaging with computation times <30 sec; 2) both the server and the client in a single workstation, in which computation time <10 sec. The use of RQE eliminated any local optima trapping. RQE training yielded a sensitivity and a specificity of 0.9804 (0.8955–0.9995) and 0.9388 (0.8313–0.9872), respectively, at 95% confidence interval. Using test dataset from phantom imaging, the sensitivity and the specificity of RQE were 0.939 and 0.937, respectively. Currently CBCT generally has insufficient image quality for accurate deformable registration based on soft tissue, and 3D-3D registrations are largely driven by bony anatomy. Our 2D-3D registration provides accuracy comparable to 3D-3D registration using CBCT for registrations based on bony anatomy. The proposed proof-of-concept system offers a simple and inexpensive solution for those radiotherapy patients requiring precise 3D patient positioning based on bony anatomy without invasive fiducial markers that can be implemented on any linac with EPID imaging.

A study on adaptive IMRT treatment planning using kV cone-beam CT

Background and purpose Changes in tumor size during the course of radiotherapy warrant performing adaptive radiotherapy (ART). This work investigates the feasibility and usefulness of acquiring on-board cone-beam CT (CBCT) for ART for patients with bulky head and neck tumors treated with IMRT and for prostate patients with potentially significant target position variations during the treatment course. Materials and methods A phantom designed for CT quality assurance was used to compare the dosimetric and geometric accuracy between conventional CT and CBCT from a linear accelerator’s on-board imager. Patient planning CT and CBCT images were acquired before treatment and at mid-course. The IMRT plans made on the CT were applied to the CBCT and dose–volume histograms were calculated. Results In both phantom and patient studies, the dose–volume histograms (DVHs) based on CBCT images were in excellent agreement with DVHs based on planning CT images. Minimum, maximum and mean doses agreed very well. In a patient study, doses for targets and normal tissues from the same IMRT plans calculated on CBCT images agreed within 1–3% with those calculated on planning CT images. Conclusions CBCT images can be used to accurately predict dosimetric results. It is feasible to use CBCT to determine dosimetric consequences resulting from tumor shrinkage and patient geometry changes. An additional planning CT may be necessary to perform IMRT re-planning at present in order to accurately delineate tumor and organs. The CBCT has potential to become a very useful tool for on-line ART.

SU-DD-A3-04: On-Line Re-Optimization of Prostate IMRT Plans for Image Guided Adaptive Radiation Therapy

Purpose: This study proposes a novel on-line adaptive planning technique for the prostate treatment by directly re-optimizing the IMRT plan based on both the “anatomy-of-the-day” and the original plan information. Method & Materials: Using a commercial treatment planning system, an initial IMRT plan for prostate cancer treatment was optimized using seven coplanar 15-MV beams based on the structures of interest (SOIs) delineated in the planning CT. Then, the optimized plan with both intensity fluence maps and dose distributions was exported to an in-house adaptive planning platform. To correct for target variations, a set of on-board cone-beam CT (CBCT) images were acquired prior to treatment and the daily SOIs (“anatomy-of-the-day”) were identified. The original planning SOIs and daily SOIs were registered using a thin-plate-spline based deformable registration algorithm. The optimal dose distribution was then deformed to match the “anatomy-of-the-day” and served as the “prescription dose distribution” for the re-optimization process. Linear programming algorithm was used for re-optimization and could reach a solution in seconds. The algorithm also used the original IMRT plan information as the initial solution. Both hard and soft constraints were used to differentiate the priorities of meeting the prescription dose distribution. A proof-of-concept case using an IMRT prostate plan was presented. The dose-volume histograms from the original, the uncorrected and re-optimized plans were compared. Results: The proposed re-optimization technique reached solutions within 1minute on a desktop PC. The D95(min/max) doses to the GTV were 99.3%(90.0%/104.1%), 83%(57.6%/104.1%) and 98.3%(90.0%/103.8%), for the original, uncorrected and re-optimized plans, respectively. The median doses to the rectum and bladder were comparable for the original and re-optimized plans (41.0% vs. 45.0%, and 14.0% vs. 16.4%, respectively). Conclusion: It is technically feasible to perform on-line re-optimization based on the “anatomy-of-the-day” and to achieve results similar to the original dose distribution.