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A Monte Carlo study of accelerator head scatter

The production of off-focus x rays in the head of a 6 MV linac has been investigated using the EGS4 Monte Carlo code. The purpose of the study was to identify the sources of off-focus radiation and the relative contribution for each source. Even though a particular energy and linac were modeled, the broad conclusions are expected to be general since the effects of head scatter are similar for most conventional head designs, regardless of manufacturer, energy, and model. The head components that were modeled include the exit window of the accelerating structure, target, beam stopper, flattening filter, monitor chamber, primary and secondary collimators, and air. Monoenergetic 6 MeV electrons were followed through the exit window, target, and beam stopper until all energy was expended. Primary- and higher-order x rays produced throughout the head were followed until they were either absorbed or passed through a plane at the isocenter. Sites of off-focus radiation were found to be diffusely distributed throughout the head, with the most intense sources being the primary collimator, flattening filter, and beam stopper. Data analysis shows that the collimator effect is determined primarily by the volume of the extended head-scatter source that is exposed to the point of measurement through the collimating system. The results of this study provide a rationale for developing extended source models to calculate the collimator factor for fields defined by arbitrary collimation. An additional advantage is an improvement in the agreement between measured and calculated isodose distributions.

Beam orientation selection for intensity-modulated radiation therapy based on target equivalent uniform dose maximization

PURPOSE To develop an automated beam-orientation selection procedure for intensity-modulated radiotherapy (IMRT), and to determine if a small number of beams picked by this automated procedure can yield results comparable to a large number of manually placed orientations. METHODS AND MATERIALS The automated beam selection procedure maximizes an unconstrained objective function composed of target equivalent uniform dose (EUD) and critical structure dose-volume histogram (DVH) constraints. Beam orientations are selected from a large feasible set of directions through a series of alternating fluence optimization and orientation alteration steps, until convergence to a stable orientation set. The fluence optimization step adjusts fluences to maximize the objective function. The orientation alteration step substitutes beams in the orientation set currently under consideration with beams of the parent set in the immediate angular vicinity; the altered orientation set is deemed current if it produces a higher objective function value in the fluence optimization step. RESULTS AND CONCLUSIONS It is demonstrated, for prostate IMRT planning, that a modest number of appropriately selected beam orientations (3 or 5) can provide dose distributions as satisfactory as those produced by a large number of unselected equispaced orientations. Such selected beam orientations can reduce overall treatment time, thus making IMRT more clinically practical.

Compensators: An alternative IMRT delivery technique

Seven years of experience in compensator intensity‐modulated radiotherapy (IMRT) clinical implementation are presented. An inverse planning dose optimization algorithm was used to generate intensity modulation maps, which were delivered via either the compensator or segmental multileaf collimator (MLC) IMRT techniques. The in‐house developed compensator‐IMRT technique is presented with the focus on several design issues. The dosimetry of the delivery techniques was analyzed for several clinical cases. The treatment time for both delivery techniques on Siemens accelerators was retrospectively analyzed based on the electronic treatment record in LANTIS for 95 patients. We found that the compensator technique consistently took noticeably less time for treatment of equal numbers of fields compared to the segmental technique. The typical time needed to fabricate a compensator was 13 min, 3 min of which was manual processing. More than 80% of the approximately 700 compensators evaluated had a maximum deviation of less than 5% from the calculation in intensity profile. Seventy‐two percent of the patient treatment dosimetry measurements for 340 patients have an error of no more than 5%. The pros and cons of different IMRT compensator materials are also discussed. Our experience shows that the compensator‐IMRT technique offers robustness, excellent intensity modulation resolution, high treatment delivery efficiency, simple fabrication and quality assurance (QA) procedures, and the flexibility to be used in any teletherapy unit. PACS numbers: 87.53Mr, 87.53Tf

The tetrad and hexad: Maximum beam separation as a starting point for noncoplanar 3D treatment planning: Prostate cancer as a test case

PURPOSE In contrast to computer optimized three-dimensional (3D) treatment planning, we have used maximally separated, noncoplanar beams as the starting point for 3D treatment planning of prostate cancer to maximize the rate of dose fall off from the target volume and minimize dose to surrounding tissues. MATERIALS AND METHODS A planar four-field plan, a planar six-field plan, a tetrad plan, and a hexad plan are analyzed using a 3D treatment planning system which is capable of displaying real-time 3D dose distributions within volume reconstructed data sets (VISTAnet--an extension of the virtual simulator). The tetrad plan is based on the methane molecule and the hexad plan has a minimum separation of 58 degrees on beam entrance. All fields are conformal. The irradiated volume equals the clinical target volume plus a 1 cm margin. Competing plans are compared using cumulative dose-volume histograms and normal tissue complication probabilities. RESULTS The crossover point, the isodose surface that conforms more to the beams than the target, is introduced and described. The hexad and tetrad plans result in tighter dose distributions when compared to the planar plans with the same number of beams. The tetrad plan treats a volume less than or equal to the planar six-field plan at isodose surfaces above 18% except between 37% and 44% where the tetrad volume is slightly larger. As expected from integral dose considerations, the amount of normal tissue receiving some radiation increases, but the amount receiving clinically significant amounts of radiation decreases as the number of beams increase. The plan involving the largest number of noncoplanar beams results in the tightest isodose distribution. Analysis of rectal and bladder cumulative dose volume histograms does not reveal a clearly superior plan based on normal tissue complication probabilities. CONCLUSIONS Using basic principles of solid geometry, maximally separated beams without significant overlap on exit or entrance can be designed which minimize clinically significant dose to surrounding tissues and tighten the isodose distribution around the target volume. The emphasis of this treatment plan optimization is geometric in contrast to methods using computer optimization or artificial intelligence.

Toward Interactive Object Definition in 3D Scalar Images

We present a method of object definition designed to allow fast interactive definition of object regions in 2D and 3D image data by a human user based on an automatically computed image description of sensible image regions. The image description is a quasi-hierarchy of ridges (or courses) and subridges (or subcourses) in the intensity surface corresponding to the image. Two methods of ridge computation are presented, one based on the intensity axis of symmetry and another based on flow lines in the intensity surface. A system for interactive object definition using this approach is described, and the use of this approach on a variety of medical images is evaluated. Generalizations of these descriptions and the interactive object definition tool to 3D are discussed.

Digitally reconstructed fluoroscopy and other interactive volume visualizations in 3-D treatment planning

PURPOSE Add radiographic context to the beams-eye-view used in 3-dimensional treatment planning. Improve methods for interactive visualization of anatomy and dose distributions. METHODS AND MATERIALS Most 3-dimensional treatment planning systems feature a beams-eye view that includes only graphical representations of patient anatomy. With input devices such as a mouse or trackball, the user interactively shapes the treatment field using the graphical models to provide geometric information. Radiographic context provides additional geometric information important for determining field shape. We have implemented digitally reconstructed fluoroscopy in the beams-eye view by increasing the efficiency for computing digitally reconstructed radiographs. In addition we have improved algorithms for real-time surface and volume rendering for anatomy and doses using an experimental graphics supercomputer. RESULTS Without radiographic context in the beams-eye-view, field shapes were sometimes changed after simulation or portal images were obtained. Digitally reconstructed fluoroscopy has essentially eliminated these changes. Higher quality interactive three-dimensional displays improve the comprehension, confidence and efficiency of the user. Our improvements have already been implemented on one model of a new generation of commercial graphics workstations. CONCLUSION Addition of radiographic context to the beams-eye-view is recommended. Incorporation of higher quality interactive graphics is rapidly becoming practical and is encouraged.

A portable software tool for computing digitally reconstructed radiographs

PURPOSE To develop a portable software tool for fast computation of digitally reconstructed radiographs (DRR) with a friendly user interface and versatile image format and display options. To provide a means for interfacing with commercial and custom three-dimensional (3D) treatment planning systems. To make the tool freely available to the Radiation Oncology community. METHODS AND MATERIALS A computer program for computing DRRs was enhanced with new features and rewritten to increase computational efficiency. A graphical user interface was added to improve ease of data input and DRR display. Installer, programmer, and user manuals were written, and installation test data sets were developed. The code conforms to the specifications of the Cooperative Working Group (CWG) of the National Cancer Institute (NCI) Contract on Radiotherapy Treatment Planning Tools. RESULTS The interface allows the user to select DRR input data and image formats primarily by point-and-click mouse operations. Digitally reconstructed radiograph formats are predefined by configuration files that specify 19 calculation parameters. Enhancements include improved contrast resolution for visualizing surgical clips, an extended source model to stimulate the penumbra region in a computed port film, and the ability to easily modify the CT numbers of objects contoured on the planning computed tomography (CT) scans. CONCLUSIONS The DRR tool can be used with 3D planning systems that lack this functionality, or perhaps improve the quality and functionality of existing DRR software. The tool can be interfaced to 3D planning systems that run on most modern graphics workstations, and can also function as a stand-alone program.

Interactive object definition in medical images using multiscale, geometric image descriptions

The medical image object definition method, which involves automatic computation of a region-based image description, along with a region containment directed acyclic graph (RCDAG) induced from the description by multiscale analysis of image structures, is described. The information resulting from this computation provides the basis for interactive object definition, during which the human user inserts semantics into the image description through additions to and alteration of the automatically computed RCDAG. A tool for interactive object definition, the Image Hierarchy Editor (IHE) is also described. Design criteria and resulting design decisions for this tool are presented, followed by a discussion of preliminary image segmentation and object definition results. For comparison papers see ibid., p.94-101 and ibid., p.102-7.<<ETX>>

Multiscale, Geometric Image Descriptions for Interactive Object Definition

A means is described of analyzing two- and three-dimensional images into a directed acyclic graph of visually sensible, coherent regions and of using this DAG as the basis for interactive object definition. The image analysis is in terms of the geometry of the intensity surface via a multiscale approach with a focus on symmetry properties about ridges. The image analysis method, a system for interactive object definition, and results of their use on two-dimensional images are reported.

SU-FF-T-362: PLanUNC as An Open-Source Radiotherapy Planning System for Research and Education

Purpose: PLanUNC is a radiotherapy planning software package that has been under development and clinical use at the University of North Carolina for approximately 20 years. Under a joint grant from the NCRR and NCI (R01 RR 018615), PLanUNC has been documented, commented, and prepared for distribution as a freely available open‐source treatment planning tool for use as an adaptable and common platform for radiotherapy research. Method and Materials: The software and source code have been made available to qualifying users through a web portal located at http://planunc.radonc.unc.edu. Licenses for PLanUNC are available without fee to institutions, departments, and other facilities engaged in research and education involving radiation therapy.Results: Recent research milestones demonstrating the extensibility and increasing utility of PLanUNC include tools for 4D planning, interfaces with ITK segmentation and registration tools, daily correction of patient positioning, and interfaces with a variety of Monte Carlo dose engines. PLanUNC is currently supported for Linux and Windows operating systems, but has been successfully compiled on Alpha, Macintosh, Solaris, and other platforms. Conclusion: Licensed users will have access to PLanUNC source code, user and development documentation, annual training workshops, and limited support from UNC and the PLanUNC research community. PLanUNC is distributed as source code, making it customizable and extensible to meet the needs of a diverse range of research applications.