Biomedical subjects

P Reckwerdt

Publications and source records attributed to P Reckwerdt.

10 recordsLinked to original sources

PubMed1999

Tomotherapy.

Tomotherapy is delivery of intensity-modulated, rotational radiation therapy using a fan-beam delivery. The NOMOS (Sewickley, PA) Peacock system is an example of sequential (or serial) tomotherapy that uses a fast-moving, actuator-driven multileaf collimator attached to a conventional C-arm gantry to modulate the beam intensity. In helical tomotherapy, the patient is continuously translated through a ring gantry as the fan beam rotates. The beam delivery geometry is similar to that of helical computed tomography (CT) and requires the use of slip rings to transmit power and data. A ring gantry provides a stable and accurate platform to perform tomographic verification using an unmodulated megavoltage beam. Moreover, megavoltage tomograms have adequate tissue contrast and resolution to provide setup verification. Assuming only translational and rotational offset errors, it is also possible to determine the offsets directly from tomographic projections, avoiding the time-consuming image reconstruction operation. The offsets can be used to modify the leaf delivery pattern to match the beam to the patient's anatomy on each day of a course of treatment. If tomographic representations of the patient are generated, this information can also be used to perform dose reconstruction. In this way, the actual dose distribution delivered can be superimposed onto the tomographic representation of the patient obtained at the time of treatment. The results can be compared with the planned isodose on the planning CT. This comparison may be used as an accurate basis for adaptive radiotherapy whereby the optimized delivery is modified before subsequent fractions. The verification afforded tomotherapy allows more precise conformal therapy. It also enables conformal avoidance radiotherapy, the complement to conformal therapy, for cases in which the tumor volume is ill-defined, but the locations of sensitive structures are adequately determined. A clinical tomotherapy unit is under construction at the University of Wisconsin.

Dose Fractionation, Radiation↗

PubMed1999

Characterization of the output for helical delivery of intensity modulated slit beams.

The UW tomotherapy workbench utilizes a convolution/superposition based dose calculation and optimization program. It specifies the energy fluence that must be delivered from each leaf for each phantom projection angle. This requires that the spectrum of the radiation emitted from the one-dimensional MLC (multileaf collimator) attached to the linear accelerator be determined. The steps involved in that process are described. The spectrum along the central axis of the slit beam was determined, as well as the softening with off-axis position. Moreover, the magnitude of the energy-fluence output had to be quantified on a per MU (monitor unit) basis. This was done for a single leaf along the central axis of the beam. Factors, which modify that energy-fluence output, were investigated. The output increases with off-axis position due to the horns of the beam. The output for a leaf of interest will also increase if additional leaves are open due to the absence of the tongue-and-groove effect and penumbra blurring. The energy-fluence increase per leaf increase by 4.9% if an adjacent leaf is open. No other factors related to the state of additional leaves were found to significantly increase the energy-fluence output for an individual leaf.

Algorithms↗

PubMed1999

A simple model for examining issues in radiotherapy optimization.

Convolution/superposition software has been used to produce a library of photon pencil beam dose matrices. This library of pencil beams is designed to serve as a tool for both education and investigation in the field of radiotherapy optimization. The elegance of this pencil beam model stems from its cylindrical symmetry. Because of the symmetry, the dose distribution for a pencil beam from any arbitrary angle can be determined through a simple rotation of a pre-computed dose matrix. Rapid dose calculations can thus be performed while maintaining the accuracy of a convolution/superposition based dose computation. The pencil beam data sets have been made publicly available. It is hoped that the data sets will facilitate a comparison of a variety of optimization and delivery approaches. This paper will present a number of studies designed to demonstrate the usefulness of the pencil beam data sets. These studies include an examination of the extent to which a treatment plan can be improved through either an increase in the number of beam angles and/or a decrease in the collimator size. A few insights into the significance of heterogeneity corrections for treatment planning for intensity modulated radiotherapy will also be presented.

Computer Simulation↗

PubMed1997

An investigation of tomotherapy beam delivery.

Experimental simulations for tomotherapy beam delivery were performed using a computer-controlled phantom positioner, a cylindrical phantom, and a 6 MV x-ray slit beam. Both continuous helical beam and sequential segmented tomotherapy (SST) beam deliveries were evaluated. Beam junctioning problem due to couch indexing error or field width errors presented severe dose uniformity perturbations for SST, while the problem was minimized for helical beam delivery. Longitudinal breathing motions were experimentally simulated for helical and SST beam delivery. While motions reduced the dose uniformity perturbations for SST, small artifacts in dose uniformity can be introduced for helical beam delivery. With typical breath frequency and magnitude, for a slit beam of 2.0 cm width at 4 rpm, the dose uniformity perturbation was not significant. A running start/stop technique was implemented with helical beam delivery to sharpen the 20%-80% longitudinal dose fall-off from 1.5 to 0.5 cm. The latter was comparable to the corresponding dose penumbra of a conventional 6 MV 10 x 10 cm2 field. All together, helical beam delivery showed advantages over SST for tomotherapy beam delivery under similar delivery conditions.

Biophysical Phenomena↗

PubMed1996

Calculation of portal dose using the convolution/superposition method.

The convolution/superposition method was used to predict the dose throughout an extended volume, which includes a phantom and a portal imaging device. From the calculated dose volume, the dose delivered in the portal image plane was extracted and compared to a portal dose image. This comparison aids in verifying the beam configuration or patient setup after delivery of the radiation. The phantoms used to test the accuracy of this method include a solid water cube, a Nuclear Associates CT phantom, and an Alderson Rando thorax phantom. The dose distribution in the image plane was measured with film and an electronic portal imaging device in each case. The calculated portal dose images were within 4% of the measured images for most voxels in the central portion of the field for all of the extended volumes. The convolution/superposition method also enables the determination of the scatter and primary dose contributions using the particular dose deposition kernels for each contribution. The ratio of primary dose to total dose was used to extract the primary dose from the detected portal image, which enhances the megavoltage portal images by removing scatter blurring. By also predicting the primary energy fluence, we can find the ratio of computed primary energy fluence to total dose. Multiplying this ratio by the measured dose image estimates the relative primary energy fluence at the portal imager. The image of primary energy fluence possesses higher contrast and may be used for further quantitative image processing and dose modeling.

Biophysical Phenomena↗

PubMed1996

Modeling dose distributions from portal dose images using the convolution/superposition method.

Post-treatment dose verification refers to the process of reconstructing delivered dose distributions internal to a patient from information obtained during the treatment. The exit dose is commonly used to describe the dose beyond the exit surface of the patient from a megavoltage photon beam. Portal imaging provides a method of determining the dose in a plane distal to a patient from a megavoltage therapeutic beam. This exit dose enables reconstruction of the dose distribution from external beam radiation throughout the patient utilizing the convolution/superposition method and an extended phantom. An iterative convolution/superposition algorithm has been created to reconstruct dose distributions in patients from exit dose measurements during a radiotherapy treatment. The method is based on an extended phantom that includes the patient CT representation and an electronic portal imaging device (EPID). The convolution/superposition method computes the dose throughout the extended phantom, which allows the portal dose image to be predicted in the EPID. The process is then reversed to take the portal dose measurement and infer what the dose distribution must have been to produce the measured portal dose. The dose distribution is modeled without knowledge of the incident intensity distribution, and includes the effects of scatter in the computation. The iterative method begins by assuming that the primary energy fluence (PEF) at the portal image plane is equal to the portal dose image, the PEF is then back-projected through the extended phantom and convolved with the dose deposition kernel to determine a new prediction of the portal dose image. The image of the ratio of the computed PEF to the computed portal dose is then multiplied by the measured portal dose image to produce a better representation of the PEF. Successive iterations of this process then converge to the exiting PEF image that would produce the measured portal dose image. Once convergence is established, the dose distribution is determined by back-projecting the PEF and convolving with the dose deposition kernel. The method is accurate, provided the patient representation during treatment is known. The method was used on three phantoms with a photon energy of 6 MV to verify convergence and accuracy of the algorithm. The reconstructed dose volumes agree to within 3% of the forward computation dose volumes. Furthermore, this technique assumes no prior knowledge of the incident fluence and therefore may better represent the dose actually delivered.

Algorithms↗

PubMed1995

An iterative filtered backprojection inverse treatment planning algorithm for tomotherapy.

PURPOSE: An inverse treatment planning algorithm for tomotherapy is described. METHODS AND MATERIALS: The algorithm iteratively computes a set of nonnegative beam intensity profiles that minimizes the least-square residual dose defined in the target and selected normal tissue regions of interest. At each iteration the residual dose distribution is transformed into a set of residual beam profiles using an inversion method derived from filtered backprojection image reconstruction theory. These "residual" profiles are used to correct the current beam profile estimates resulting in new profile estimates. Adaptive filtering is incorporated into the inversion model so that the gross structure of the dose distribution is optimized during initial iterations of the algorithm, and the fine structure corresponding to edges is obtained at later iterations. A three dimensional, kernel based, convolutions/superposition dose model is used to compute dose during each iteration. RESULTS: Two clinically relevant treatment planning examples are presented illustrating the use of the algorithm for planning conformal radiotherapy of the breast and the prostate. Solutions are generally achieved in 10-20 iterations requiring about 20 h of CPU time using a midrange workstation. The majority of the calculation time is spent on the three-dimensional dose calculation. CONCLUSIONS: The inverse treatment planning algorithm is a useful research tool for exploring the potential of tomotherapy for conformal radiotherapy. Further work is needed to (a) achieve clinically acceptable computation times; (b) verify the algorithm using multileaf collimator technology; and (c) extend the method to biological objectives.

Algorithms↗

PubMed1991

A unified approach to the optimization of brachytherapy and external beam dosimetry.

Semi-automated optimization of dose distributions is possible using techniques borrowed from imaging science. The ideal distribution of dose is first deconvolved by a convolution kernel yielding an ideal weighting distribution in the patient. The weighting distribution describes the total energy released per unit mass of the irradiated medium. For internal and external radiation sources, this is directly related to the amount and distribution of radioactivity and energy fluence in the medium, respectively. For external sources, the exponential Radon transform is used to obtain ideal fluence projections incident on the patient. In both instances negative values are produced, which when set to zero result in perturbed dose distributions. This may necessitate iterative techniques to reduce the 'residual dose' produced by the zeroing process. Application of the approach is presented for the optimization of 2-dimensional dose distributions in external beam therapy, radioimmunotherapy, and brachytherapy sources.

Brachytherapy↗

PubMedDate not supplied

Tomotherapy: a new concept for the delivery of dynamic conformal radiotherapy.

Tomotherapy, literally "slice therapy," is a proposal for the delivery of radiation therapy with intensity-modulated strips of radiation. The proposed method employs a linear accelerator, or another radiation-emitting device, which would be mounted on a ring gantry like a CT scanner. The patient would move through the bore of the gantry simultaneously with gantry rotation. The intensity modulation would be performed by temporally modulated multiple independent leaves that open and close across the slit opening. At any given time, any leaf would be (1) closed, covering a portion of the slit, (2) open, allowing radiation through, or (3) changing between these states. This method would result in the delivery of highly conformal radiation. Overall treatment times should be comparable with contemporary treatment delivery times. The ring gantry would make it convenient to mount a narrow multisegmented megavoltage detector system for beam verification and a CT scanner on the treatment unit. Such a treatment unit could become a powerful tool for treatment planning, conformal treatment, and verification using tomographic images. The physical properties of this treatment delivery are evaluated and the fundamental design specifications are justified.

Humans↗

PubMedDate not supplied

Investigation of the convolution method for polyenergetic spectra.

The distribution of absolute dose per unit fluence from polyenergetic photon beams impinging upon a water phantom was calculated using two convolution approaches that properly account for beam hardening effects. Dose deposition kernels calculated previously using the EGS4 Monte Carlo code are convolved with the primary terma to give the dose for monoenergetic photon beams of energies ranging from 100 kev to 50 MeV. A polyenergetic dose distribution is composed of separately calculated monoenergetic components, which are appropriately weighted with the fluence spectrum to yield the polyenergetic dose distribution. Alternatively, a single convolution for the polyenergetic beam is considered, where a composite polyenergetic kernel is convolved with the respective polyenergetic terma. The effects of the polyenergetic kernel variance due to beam hardening as well as the effect of tilting the kernels for a diverging beam geometry were also examined. The depth dose data produced using the two proposed methods were compared with measured data and Monte Carlo simulations and showed good agreement.

Biophysical Phenomena↗