Biomedical subjects

M A Earl

Publications and source records attributed to M A Earl.

5 recordsLinked to original sources

PubMed2005

An examination of the number of required apertures for step-and-shoot IMRT.

We have examined the degree to which step-and-shoot IMRT treatment plans can be simplified (using a small number of apertures) without sacrificing the dosimetric quality of the plans. A key element of this study was the use of direct aperture optimization (DAO), an inverse planning technique where all of the multi-leaf collimator constraints are incorporated into the optimization. For seven cases (1 phantom, 1 prostate, 3 head-and-neck and 2 lung), DAO was used to perform a series of optimizations where the number of apertures per beam direction varied from 1 to 15. In this work, we attempt to provide general guidelines for how many apertures per beam direction are sufficient for various clinical cases using DAO. Analysis of the optimized treatment plans reveals that for most cases, only modest improvements in the objective function and the corresponding DVHs are seen beyond 5 apertures per beam direction. However, for more complex cases, some dosimetric gain can be achieved by increasing the number of apertures per beam direction beyond 5. Even in these cases, however, only modest improvements are observed beyond 9 apertures per beam direction. In our clinical experience, 38 out of the first 40 patients treated using IMRT plans produced using DAO were treated with 9 or fewer apertures per beam direction. The results indicate that many step-and-shoot IMRT treatment plans delivered today are more complex than necessary and can be simplified without sacrificing plan quality.

Algorithms↗

PubMed2003

Inverse planning for intensity-modulated arc therapy using direct aperture optimization.

Intensity-modulated arc therapy (IMAT) is a radiation therapy delivery technique that combines gantry rotation with dynamic multi-leaf collimation (MLC). With IMAT, the benefits of rotational IMRT can be realized using a conventional linear accelerator and a conventional MLC. Thus far, the advantages of IMAT have gone largely unrealized due to the lack of robust automated planning tools capable of producing efficient IMAT treatment plans. This work describes an inverse treatment planning algorithm, called 'direct aperture optimization' (DAO) that can be used to generate inverse treatment plans for IMAT. In contrast to traditional inverse planning techniques where the relative weights of a series of pencil beams are optimized, DAO optimizes the leaf positions and weights of the apertures in the plan. This technique allows any delivery constraints to be enforced during the optimization, eliminating the need for a leaf-sequencing step. It is this feature that enables DAO to easily create inverse plans for IMAT. To illustrate the feasibility of DAO applied to IMAT, several cases are presented, including a cylindrical phantom, a head and neck patient and a prostate patient.

Algorithms↗

PubMed2003

A toolbox for intensity modulated radiation therapy optimization.

We have designed a toolbox that provides an environment for testing radiotherapy optimization techniques, objective functions, and constraints. A set of three-dimensional (3D) pencil beam dose distributions have been computed for a cylindrical phantom. The 6 MV pencil beams were computed using a superposition-based dose engine commissioned for an Elekta SL20 linear accelerator. Due to the cylindrical symmetry of the phantom, the pencil beam dose distributions for any arbitrary beam angle can be determined by simply rotating the pencil beam data sets. Thus, the full accuracy is maintained without the need for additional dose calculations or large data storage requirements. In addition to the pencil beam data sets, tools are included for (1) rotating the pencil beams, (2) calculating the beam's eye view, (3) drawing structures, (4) writing the pencil beam dose data out to the optimizer, and (5) visualizing the optimized results. The pencil beam data sets and the corresponding tools are available for download at http://medschool.umaryland.edu/departments/radiationoncology/pencilbeam/. With this toolbox, researchers will have the ability to rapidly test new optimization techniques and formulations for intensity modulated radiation therapy and 3D conformal radiotherapy.

Algorithms↗

PubMed2002

Depth dose enhancement of electron beams subject to external uniform longitudinal magnetic fields: a Monte Carlo study.

We studied the dose distributions from electron beams subjected to a longitudinal magnetic field applied to them before they reach the phantom. We found that specific combinations of the length and intensity of the magnetic field produced enhancement of the peaks of the central-axis depth-dose distributions. The EGS4 Monte Carlo system was used in this study. In the simulations, a uniform axial magnetic field parallel to the electron beam direction was applied to the air gap between the collimation and the phantom. We extensively studied the simplified case of an 18 MeV electron beam point source. Dose deposition was calculated for various magnetic field strengths, distances through which the magnetic field was applied, collimation sizes, and source to collimation distances. The magnetic field strengths varied from 0 to 3 T, the source-to-collimation distances studied were 50 and 95 cm, the collimation sizes studied were 10 x 10 and 20 x 20 cm2, and the distance through which the field was applied ranged from 10 to 20 cm. Specific combinations of these variables resulted in as much as a 70% enhancement of the peak dose relative to the surface dose. Finally, to determine how the geometry of a real accelerator affects the resulting dose distribution, we performed a full simulation of an Elekta SL20 linear accelerator and compared the results with the ideal case.

Electromagnetic Fields↗

PubMed2002

Direct aperture optimization: a turnkey solution for step-and-shoot IMRT.

IMRT treatment plans for step-and-shoot delivery have traditionally been produced through the optimization of intensity distributions (or maps) for each beam angle. The optimization step is followed by the application of a leaf-sequencing algorithm that translates each intensity map into a set of deliverable aperture shapes. In this article, we introduce an automated planning system in which we bypass the traditional intensity optimization, and instead directly optimize the shapes and the weights of the apertures. We call this approach "direct aperture optimization." This technique allows the user to specify the maximum number of apertures per beam direction, and hence provides significant control over the complexity of the treatment delivery. This is possible because the machine dependent delivery constraints imposed by the MLC are enforced within the aperture optimization algorithm rather than in a separate leaf-sequencing step. The leaf settings and the aperture intensities are optimized simultaneously using a simulated annealing algorithm. We have tested direct aperture optimization on a variety of patient cases using the EGS4/BEAM Monte Carlo package for our dose calculation engine. The results demonstrate that direct aperture optimization can produce highly conformal step-and-shoot treatment plans using only three to five apertures per beam direction. As compared with traditional optimization strategies, our studies demonstrate that direct aperture optimization can result in a significant reduction in both the number of beam segments and the number of monitor units. Direct aperture optimization therefore produces highly efficient treatment deliveries that maintain the full dosimetric benefits of IMRT.

Algorithms↗