Biomedical subjects

R A Siochi

Publications and source records attributed to R A Siochi.

4 recordsLinked to original sources

PubMed2001

Leaf sequencing techniques for MLC-based IMRT.

The nonuniform fields required by intensity-modulation radiation therapy (IMRT) can be delivered using conventional multileaf collimators (MLC) as beam modulators. In MLC-based IMRT, the nonuniform field is initially converted into an intensity map represented as a matrix of beam intensities. The intensity map is then decomposed into a series of subfields or segments of uniform intensities. Although there are many ways of segmenting the beam intensity matrix, a resulting subfield is only deliverable if it satisfies the constraints imposed by the MLC. These constraints exist as a result of the design of the MLC. The simplest constraint of the MLC is that its pairs of leaves can only move in and out in one dimension. Additional constraints include collision of opposing leaves and the need to match the tongue-and-groove to reduce interleaf leakage. The practical aspect of MLC-based IMRT requires that an optimized algorithm decomposes the nonuniform field into the least number of segments and therefore reduces the delivery time. This paper examines the static use and the dynamic use of MLCs to perform MLC-based IMRT.

Radiotherapy, Conformal↗

PubMed2000

Virtual micro-intensity modulated radiation therapy.

Virtual micro-intensity modulated radiation therapy (VMIMRT) combines a 10 x 5 mm2 intensity map with a 5 x 10 mm2 intensity map, delivered at orthogonal collimator settings. The superposition of these component maps (CM) yields a 5 x 5 mm2 virtual micro-intensity map (VMIM) that can be delivered with a 1 cm leaf width MLC. A pair of CMs with optimal delivery efficiency and quality must be chosen, since a given VMIM can be delivered using several different pairs. This is possible since, for each group of four VMIM cells that can be covered by an MLC leaf in either collimator orientation, the minimum intensity can be delivered from either collimator setting. By varying the proportions of the minimum values that go into each CM, one can simultaneously minimize the number of potential junction effects and the number of segments required to deliver the VMIM. The minimization is achieved by reducing high leaf direction gradients in the CMs. Several pseudoclinical and random VMIMs were studied to determine the applicability of this new technique. A nine level boost map was also studied to investigate dosimetric and spatial resolution issues. Finally, clinical issues for this technique are discussed.

Brain Neoplasms↗

PubMed1999

Minimizing static intensity modulation delivery time using an intensity solid paradigm.

PURPOSE: A leaf sequencing optimization algorithm that minimizes the delivery time for a static intensity modulated field is presented. METHODS AND MATERIALS: Sets of segments are created by intensity map operations subject to leaf collision constraints and tongue and groove effects. Each set's delivery time is evaluated as a function of leaf travel, beam on time, and the verify and record (V&R) overhead. The configuration with the minimum delivery time is chosen. As a test, optimization was done on three clinical cases of varying complexity. RESULTS: Assuming 10 x 10-cm fields with an average of 17 intensity levels, the optimization technique reduced delivery times by 27% and 45%, when compared to rod pushing and power of two extraction, respectively. The treatment time for the optimal case with a V&R overhead of 4 s would be 11.5 min for 9 coplanar ports. Tongue-and-groove underdosages are removed, and the worst case leakage is 2% of the peak dose. CONCLUSION: Compared to previously reported leaf sequencing methods, the new optimization algorithm described here reduces treatment times for complex static intensity modulated fields. Additionally, leakage is minimal and no tongue-and-groove underdosage occurs.

Algorithms↗

PubMed1997

A self-collimating convolution backprojection algorithm for optimizing dose distributions of I-125 prostate implants.

The algorithm presented here for optimizing brachytherapy dose distributions is based on the idea that the seed distribution can be modeled as an activity distribution determined analogously to gamma camera imaging. The peripheral dose to the tumor is converted to a set of uncollimated projection data that are then filtered and backprojected to produce an initial seed distribution. The actual doses resulting from the seed placement are used to correct the initial projection data for attenuation, scatter, and lack of collimation. The corrected projection data are backprojected a second time to yield the optimized but unconstrained seed distribution. Clinical constraints such as the number of different seed activities, the maximum seed activity, the minimum peripheral tumor dose, and the minimum percentage of the volume which receives less than a specified dose are then applied to the unconstrained solution. Through the entire process, the dose calculations are functions of source anisotropy, scatter, and attenuation. When applied to a set of elliptical contours, the algorithm produces elliptical peripheral dose isodose contours and reasonable dose volume histograms for a constrained solution. The results for actual patient prostate contours were not as good, primarily because of the difficulties encountered in dealing with the irregular geometry of the prostate. However, the algorithm shows promise for further research.

Algorithms↗