SU-E-J-05: Validation of an Iterative Tomosynthesis Algorithm for Low Dose on Board Cone Beam CT Patient Localization.

PURPOSE: Cone beam CT (CBCT) is a well established technique to localize patients using bone and soft tissue anatomy. Current protocols are limited to one weekly CBCT due to the considerable imaging dose delivered to the patient. The purpose of this project is to develop and validate a low dose CBCT algorithm to reduce dose and imaging time of current 3D imaging localization procedures using a novel iterative tomosynthesis algorithm to allow daily CBCT for patient positioning and target localization. METHODS: The algorithm is based on the combination of a tomosynthesis filtered back propagation (TFBP) acquisition geometry algorithm and a maximum likelihood expectation maximization (MLEM) iterative reconstruction. Circular or arc acquisition trajectory, projection number, and angular projection position are optimized according to the anatomical treatment site and region of interest. The TFBP method provides the first 3D image estimate, and the MLEM improves its quality. In this study, we focused on head and neck treatment localization imaging. RESULTS: We studied the performance of our tomosynthesis algorithm imaging resolution on an anthropomorphic head and neck phantom to determine image quality as a function of dose reduction techniques. Reconstructed anatomy shows that a 1/8 dose reduction provides similar image quality and resolution as current CBCT protocols. Seven iterations show an optimal compromise between image quality and reconstruction time. Tomosynthesis images provide digitally reconstructed radiographs with similar resolution and contrast as full CBCT. We verified that the iterative process eliminates phantom images originated by the acquired sparse angular data projections. CONCLUSIONS: We developed and validated an iterative algorithm for low dose cone beam CT based on circular or arc tomosynthesis geometries and iterative reconstruction techniques. The algorithm combines the strengths of both techniques to provide a novel low dose method to image patient anatomy for patient positioning and target localization.

SU-E-T-193: Using Truebeam's Research Mode to Automate Mechanical Quality Assurance.

PURPOSE: To determine the feasibility of automating mechanical quality assurance measurements on the Varian Truebeam LINAC. METHODS: Using the XML coding capability of the Varian Truebeam Research Mode, the LINAC was programmed to mimic the beams delivered for the following mechanical tests. These tests included: Field size accuracy, jaw positions for asymmetric fields, collimator rotation isocenter, and MLC positional-accuracy. Images for these beams were acquired with the EPID. The images were analyzed using an analysis code written in MATLAB. Tests for gantry and couch rotation isocenters and radiation and mechanical isocenter coincidence are being developed. RESULTS: For field-sizes ranging from 4×4cm2 to 15×15cm2 , the measured matched the nominal field sizes to within 1mm. The collimator rotation isocenter and the overall accuracy for asymmetric field matched to within 1mm. No positional error 〉1mm was seen in the 33 MLC pairs visible in the MLC positional-accuracy images. CONCLUSIONS: A large portion of the time required to make mechanical QA measurements using film is spent placing, processing, and scanning the film. Complete automation in performing these mechanical tests results in a significant time gain compared to film. A majority of the mechanical tests suggested by TG-142 have been performed using this technique, and an automated mechanical QA process has been established in our clinic.

Fundamental properties of the delivery of volumetric modulated arc therapy (VMAT) to static patient anatomy.

PURPOSE: The primary goal of this article is to formulate volumetric modulated arc therapy (VMAT) delivery problem and study interdependence between several parameters (beam dose rate, gantry angular speed, and MLC leaf speed) in the delivery of VMAT treatment plan. The secondary aim is to provide delivery solution and prove optimality (minimal beam on time) of the solution. An additional goal of this study is to investigate alternative delivery approaches to VMAT (like constant beam dose rate and constant gantry angular speed delivery). METHOD: The problem of the VMAT delivery is formulated as a control problem with machine constraints. The relationships between parameters of arc therapy delivery are derived under the constraint of treatment plan invariance and limitations on delivery parameters. The nonuniqueness of arc therapy delivery solutions is revealed from these relations. The most efficient delivery of arc therapy is then formulated as optimal control problem and solved by geometrical methods. A computer program is developed to find numerical solutions for deliveries of specific VMAT plan. RESULTS: Explicit examples of VMAT plan deliveries are computed and illustrated with graphical representations of the variability of delivery parameters. Comparison of delivery parameters with that of Varian's delivery are shown and discussed. Alternative delivery strategies such as constant gantry angular speed delivery and constant beam dose rate delivery are formulated and solutions are provided. The treatment times for all the delivery solutions are provided. CONCLUSION: The investigations derive and prove time optimal VMAT deliveries. The relationships between delivery parameters are determined. The optimal alternative delivery strategies are discussed.

Dosimetric variances anticipated from breathing- induced tumor motion during tomotherapy treatment delivery.

In their classic paper, Yu et al (1998 Phys. Med. Biol. 43 91) investigated the interplay between tumor motion caused by breathing and dynamically collimated, intensity-modulated radiation delivery. The paper's analytic model assumed an idealized, sinusoidal pattern of motion. In this work, we investigate the effect of tumor motion based on patients' breathing patterns for typical tomotherapy treatments with field widths of 1.0 and 2.5 cm. The measured breathing patterns of 52 lung- and upper-abdominal-cancer patients were used to model a one-dimensional motion. A convolution of the measured beam-dose profiles with the motion model was used to compute the dose-distribution errors, and the positive and negative dose errors were recorded for each simulation. The dose errors increased with increasing motion magnitude, until the motion was similar in magnitude to the field width. For the 1.0 cm and 2.5 cm field widths, the maximum dose-error magnitude exceeded 10% in some simulations, even with breathing-motion magnitudes as small as 5 mm and 10 mm, respectively. Dose errors also increased slightly with increasing couch speed. We propose that the errors were due to subtle drifts in the amplitude and frequency of breathing motion, as well as changes in baseline (exhalation) position, causing both over- and under-dosing of the target. The results of this study highlight potential breathing-motion-induced dose delivery errors in tomotherapy. However, for conventionally fractionated treatments, the dose delivery errors may not be co-located and may average out over many fractions, although this may not be true for hypofractionated treatments.