J Aubry

Patient dose considerations for routine megavoltage cone-beam CT imaging.

Megavoltage cone-beam CT (MVCBCT), the recent addition to the family of in-room CT imaging systems for image-guided radiation therapy (IGRT), uses a conventional treatment unit equipped with a flat panel detector to obtain a three-dimensional representation of the patient in treatment position. MVCBCT has been used for more than two years in our clinic for anatomy verification and to improve patient alignment prior to dose delivery. The objective of this research is to evaluate the image acquisition dose delivered to patients for MVCBCT and to develop a simple method to reduce the additional dose resulting from routine MVCBCT imaging. Conventional CT scans of phantoms and patients were imported into a commercial treatment planning system (TPS: Phillips, Pinnacle) and an arc treatment mimicking the MVCBCT acquisition process was generated to compute the delivered acquisition dose. To validate the dose obtained from the TPS, a simple water-equivalent cylindrical phantom with spaces for MOSFETs and an ion chamber was used to measure the MVCBCT image acquisition dose. Absolute dose distributions were obtained by simulating MVCBCTs of 9 and 5 monitor units (MU) on pelvis and head and neck patients, respectively. A compensation factor was introduced to generate composite plans of treatment and MVCBCT imaging dose. The article provides a simple equation to compute the compensation factor. The developed imaging compensation method was tested on routinely used clinical plans for prostate and head and neck patients. The quantitative comparison between the calculated dose by the TPS and measurement points on the cylindrical phantom were all within 3%. The dose percentage difference for the ion chamber placed in the center of the phantom was only 0.2%. For a typical MVCBCT, the dose delivered to patients forms a small anterior-posterior gradient ranging from 0.6 to 1.2 cGy per MVCBCT MU. MVCBCT acquisitions in the pelvis and head and neck areas deliver slightly more dose than current portal imaging but render soft tissue information for positioning. Overall, the additional dose from daily 9 MU MVCBCTs of prostate patients is small compared to the treatment dose (<4%). Dose-volume histograms of compensated plans for pelvis and head and neck patients imaged daily with MVCBCT showed no additional dose to the target and small increases at low doses. The results indicate that the dose delivered for MVCBCT imaging can be precisely calculated in the TPS and therefore included in the treatment plan. This allows simple plan compensations, such as slightly reducing the treatment dose, to minimize the total dose received by critical structures from daily positioning with MVCBCT. The proposed compensation factor reduces the number of MU per treatment beam per fraction. Both the number of fractions and the beam arrangement are kept unchanged. Reducing the imaging volume in the cranio-caudal direction can further reduce the dose delivered for MVCBCT. This is a useful feature to eliminate the imaging dose to the eyes or to focus on a specific region of interest for alignment.

DOSE RECALCULATION AND THE DOSE-GUIDED RADIATION THERAPY (DGRT) PROCESS USING MEGAVOLTAGE CONE-BEAM CT

PURPOSE At the University of California San Francisco, daily or weekly three-dimensional images of patients in treatment position are acquired for image-guided radiation therapy. These images can be used for calculating the actual dose delivered to the patient during treatment. In this article, we present the process of performing dose recalculation on megavoltage cone-beam computed tomography images and discuss possible strategies for dose-guided radiation therapy (DGRT). MATERIALS AND METHODS A dedicated workstation has been developed to incorporate the necessary elements of DGRT. Patient image correction (cupping, missing data artifacts), calibration, completion, recontouring, and dose recalculation are all implemented in the workstation. Tools for dose comparison are also included. Examples of image correction and dose analysis using 6 head-and-neck and 2 prostate patient datasets are presented to show possible tracking of interfraction dosimetric endpoint variation over the course of treatment. RESULTS Analysis of the head-and-neck datasets shows that interfraction treatment doses vary compared with the planning dose for the organs at risk, with the mean parotid dose and spinal cord D(1) increasing by as much as 52% and 10%, respectively. Variation of the coverage to the target volumes was small, with an average D(5) dose difference of 1%. The prostate patient datasets revealed accurate dose coverage to the targeted prostate and varying interfraction dose distributions to the organs at risk. CONCLUSIONS An effective workflow for the clinical implementation of DGRT has been established. With these techniques in place, future clinical developments in adaptive radiation therapy through daily or weekly dosimetric measurements of treatment day images are possible.

Physical performance and image optimization of megavoltage cone-beam CT.

Megavoltage cone-beam CT (MVCBCT) is the most recent addition to the in-room CT systems developed for image-guided radiation therapy. The first generation MVCBCT system consists of a 6 MV treatment x-ray beam produced by a conventional linear accelerator equipped with a flat panel amorphous silicon detector. The objective of this study was to evaluate the physical performance of MVCBCT in order to optimize the system acquisition and reconstruction parameters for image quality. MVCBCT acquisitions were performed with the clinical system but images were reconstructed and analyzed with a separate research workstation. The geometrical stability and the positioning accuracy of the system were evaluated by comparing geometrical calibrations routinely performed over a period of 12 months. The beam output and detector intensity stability during MVCBCT acquisition were also evaluated by analyzing in-air acquisitions acquired at different exposure levels. Several system parameters were varied to quantify their impact on image quality including the exposure (2.7, 4.5, 9.0, 18.0, and 54.0 MU), the craniocaudal imaging length (2, 5, 15, and 27.4 cm), the voxel size (0.5, 1, and 2 mm), the slice thickness (1, 3, and 5 mm), and the phantom size. For the reconstruction algorithm, the study investigated the effect of binning, averaging and diffusion filtering of raw projections as well as three different projection filters. A head-sized water cylinder was used to measure and improve the uniformity of MVCBCT images. Inserts of different electron densities were placed in a water cylinder to measure the contrast-to-noise ratio (CNR). The spatial resolution was obtained by measuring the point-spread function of the system using an iterative edge blurring technique. Our results showed that the geometric stability and accuracy of MVCBCT were better than 1 mm over a period of 12 months. Beam intensity variations per projection of up to 35.4% were observed for a 2.7 MU MVCBCT acquisition. These variations did not cause noticeable reduction in the image quality. The results on uniformity suggest that the cupping artifact occurring with MVCBCT is mostly due to off-axis response of the detector and not scattered radiation. Simple uniformity correction methods were developed to nearly eliminate this cupping artifact. The spatial resolution of the baseline MVCBCT reconstruction protocol was approximately 2 mm. An optimized reconstruction protocol was developed and showed an improvement of 75% in CNR with a penalty of only 8% in spatial resolution. Using this new reconstruction protocol, large adipose and muscular structures were differentiated at an exposure of 9 MU. A reduction of 36% in CNR was observed on a larger (pelvic-sized) phantom. This study demonstrates that soft-tissue visualization with MVCBCT can be substantially improved with proper system settings. Further improvement is expected from the next generation MVCBCT system with an optimized megavoltage imaging beamline.

Correction of megavoltage cone-beam CT images for dose calculation in the head and neck region

Megavoltage cone-beam computed tomography (MVCBCT) imaging systems are now available for image-guided radiation therapy delivery and verification. In order to use the three-dimensional anatomical information for dose calculation, the MVCBCT image must provide accurate electron density. This work proposes a new method that has been developed to correct for the cupping and missing data artifacts seen on MVCBCT images of the head and neck region. It uses a conventional kilovoltage CT (kVCT) image as a reference for electron density and rigid registration with a MVCBCT image to obtain correction factors. Dose calculations performed on MVCBCT images corrected with the proposed method agree with calculations done on kVCT images within +/- 1% on phantoms. With patients images the agreement is within +/- 13% above the shoulders and +/- 5% below the shoulder line. This level of dose calculation accuracy allows the use of MVCBCT images for dose verification purposes.

Mégavoltage cone-beam CT : récents développements et applications cliniques pour la radiothérapie conformationnelle avec modulation d'intensité

The Megavoltage cone-beam (MV CBCT) system consists of a new a-Si flat panel adapted for MV imaging and an integrated workflow application allowing the automatic acquisition of projection images, cone-beam CT image reconstruction, CT to CBCT image registration and couch position adjustment. This provides a 3D patient anatomy volume in the actual treatment position, relative to the treatment isocenter, moments before the dose delivery, that can be tightly aligned to the planning CT, allowing verification and correction of the patient position, detection of anatomical changes and dose calculation. In this paper, we present the main advantages and performance of this MV CBCT system and summarize the different clinical applications. Examples of the image-guided treatment process from the acquisition of the MV CBCT scan to the correction of the couch position and dose delivery will be presented for spinal and lung lesions and for head and neck, and prostate cancers.

Characteristics of megavoltage cone‐beam digital tomosynthesis

This article reports on the image characteristics of megavoltage cone-beam digital tomosynthesis (MVCB DT). MVCB DT is an in-room imaging technique, which enables the reconstruction of several two-dimensional slices from a set of projection images acquired over an arc of 20 degrees-40 degrees. The limited angular range reduces the acquisition time and the dose delivered to the patient, but affects the image quality of the reconstructed tomograms. Image characteristics (slice thickness, shape distortion, and contrast-to-noise ratio) are studied as a function of the angular range. Potential clinical applications include patient setup and the development of breath holding techniques for gated imaging.

Correction of megavoltage cone-beam CT images of the pelvic region based on phantom measurements for dose calculation purposes.

Megavoltage cone‐beam CT (MVCBCT) is an imaging technology that provides a 3D representation of the patient in treatment position. Because it is a form of x‐ray tomography, MVCBCT images give information about the attenuation coefficients of the imaged tissues, and thus could be used for dose calculation. However, the cupping and missing data artifacts seen on MVCBCT images can cause inaccuracies in dose calculations. To eliminate these inaccuracies, a correction method specific to pelvis imaging and based on phantom measurements has been devised. Pelvis‐shaped water phantoms of three different sizes were designed and imaged with MVCBCT. Three sets of correction factors were created from the artifacts observed in these MVCBCT images by dividing the measured CT number by the predefined CT number for water. Linear interpolation is performed between the sets of correction factors to take into account the varying size of different patients. To compensate for the missing anatomy due to the limited field of view of the MVCBCT system, the MVCBCT image is complemented with the kilovoltage CT (kVCT) image acquired for treatment planning. When the correction method is applied to an anthropomorphic pelvis phantom, the standard deviation between dose calculations performed with kVCT and MVCBCT images is 0.6%, with 98% of the dose points agreeing within ±3%. With uncorrected MVCBCT images this percentage falls to 75%. An example of dose calculation performed with a corrected clinical MVCBCT image of a prostate cancer patient shows that changes in anatomy of normal tissues result in variation of the dose distribution received by these tissues. This correction method enables MVCBCT images to be used for the verification of the daily dose distribution for patients treated in the pelvis region. PACS numbers: 87.57.Q‐ Computed tomography, 87.57.cp Artifacts and distortion

Investigation of geometric distortions on magnetic resonance and cone beam computed tomography images used for planning and verification of high–dose rate brachytherapy cervical cancer treatment

PURPOSE To measure the amount of geometric distortions present in the three-dimensional imaging modalities--cone beam computed tomography (CBCT) and magnetic resonance imaging (MRI)--used at University of California, San Francisco, CA, for gynecologic high dose rate brachytherapy. METHODS AND MATERIALS An MRI- and CT-compatible water phantom with two different sets of support structures was designed and built for this study. The support structures were used to precisely position catheters that were filled with either an MRI contrast agent or a string of radio-opaque markers. The first support structure without anatomy was built to test system-based distortions. A second structure included two types of gynecologic applicators as well as several anatomical structures, including bones and rectum to test object-induced distortions. Images were acquired with CT (for reference), kilovoltage CBCT, and MRI (1.5 T with T1- and T2-weighted images). The difference in catheter positions between the images and the CT images was analyzed. RESULTS For CBCT, the mean of the absolute deviations was below 1mm in all directions. The inherent uncertainty in the measurement of distortion was less than 0.5mm. MRI presented mean absolute system-based distortions between 0.6 and 1.1mm in the central region of the image and between 0.7 and 2.3mm in the outer region. Images with the applicator and anatomy in place created mean absolute distortions of 0.4, 0.8, and 0.8mm or less for CBCT, MR-T1, and MR-T2 images, respectively. CONCLUSIONS The distortions measured in the presence of applicators are small enough to validate the use of CBCT and 1.5 T MRI for GYN brachytherapy treatment planning and verification.

SU-FF-T-337: Multiobjective Inverse Planning Optimization: Adjustment of Dose Homogeneity and Urethra Protection in HDR-Brachytherapy of the Prostate

Purpose: Multiobjective optimizations are performed to evaluate the inverse planning (IP) ability to adjust the dose homogeneity and the urethra protection in HDR brachytherapy of the prostate. Materials and Methods: An IP is an anatomy‐based optimization guided by dose objectives specified for each organ extracted from medical imaging. It selects automatically the active dwell positions and optimizes the dwell times to fulfill the dose objectives. It is setup to maximize the prostate dose coverage while taking into account other clinical objectives like the dose homogeneity and the organs at risk protection. Multiobjective optimizations are performed using the IP for one small (23cc), one intermediate (35cc) and one large (80cc) prostate. 10 inverse plans were generated with different compromises between dose coverage and dose homogeneity. This was performed first with the prostate alone and then with all the organs at risk. In addition, 10 inverse plans were generated with different compromises between dose coverage and urethra protection. 90 DVH were generated and analyzed. Results: When only the prostate is included, the prostate V100 varies from 100% to 97% and the homogeneity index (HI) from 0.06 to 0.68. When all the organs at risk are included, the prostate V100 varies from 97% to 91% and the HI from 0.52 to 0.72. When the urethra protection is increased, the prostate V100 varies from 100% to 89%, the urethra V100 from 100% to 89%, the urethra V120 from 87% to 0% and the urethra V150 from 3% to 0%. Conclusion: For simple cases where only a target is defined, the dose homogeneity is adjustable with the IP. For complex cases where organs at risk are added, this anatomy‐based optimization automatically adjusts the dose homogeneity to protect the urethra. Additional organ protection can be achieved with specific penalty. This research was supported by Nucletron Corporation.

TH‐D‐L100J‐05: Quality Assurance of Megavoltage Cone‐Beam CT

Purpose: Megavoltage Cone‐Beam CT (MVCBCT) is now widely used in radiation therapy. The objective of this work was to evaluate the stability of MVCBCT and define a quality assurance protocol. Methods and Materials: Two MVCBCT systems were followed for a period of 4 months. The systems were fully calibrated (geometry, CT♯ scaling factor and flat panel corrections) and analyzed daily on the first week, weekly for a month and monthly thereafter. Images of a gold seed placed at the machine isocenter were used to track the positional accuracy and stability of the system. An image quality phantom was used to monitor the stability with respect to CT♯, contrast‐to‐noise ratio (CNR), spatial resolution, noise and uniformity. A graphical user interface was developed with Matlab to automatically analyze the image quality. For each day of analysis,images were reconstructed using the calibrations obtained that day and the oldest calibrations available to investigate how frequently calibration is needed. Results: The stability of all measurements over the 4‐month period was excellent. The reconstructedgold seed position relative to isocenter was better than 1 mm for a period of 4 months. Small variations in image quality were observed between the two systems. The inserts mean CNR were 12.4±0.7 and 12.2±0.7 for the two systems. Using the initial calibrations resulted in slightly more variability in the measurements. Based on the measurements and our experience of the last 5 years, we generated a practical list of possible artifacts occurring with MVCBCT. Conclusion: Monthly calibration of the MVCBCT system is largely sufficient. Imaging a small fiducial on a daily or weekly basis may also be warranted to detect geometric misalignments caused by sudden mechanical failures. Based on the measurements, a system performance baseline for MVCBCT has been defined. Conflict of Interest: Research sponsored by Siemens OCS.