Biomedical subjects

J Unkelbach

Publications and source records attributed to J Unkelbach.

5 recordsLinked to original sources

PubMed2006

Relating two techniques for handling uncertainties in IMRT optimization.

Radiotherapy treatment planning is associated with uncertainties. Examples are uncertainties in the tumour location due to organ movement or the inter/intra observer variability in target definition. Different approaches to incorporate uncertainties into IMRT optimization have been proposed. In this note, we point out a relation between two previously published methods: the coverage probability approach and the concept of optimizing the expectation value of an objective function that depends on a set of random variables. Both concepts are generally different, but turn out to be equivalent in special cases.

Biophysical Phenomena↗

PubMed2006

Simulation and visualization of dose uncertainties due to interfractional organ motion.

In this paper, we deal with the effects of interfractional organ motion during radiation therapy. We consider two problems: first, treatment plan evaluation in the presence of motion, and second, the incorporation of organ motion into IMRT optimization. Concerning treatment plan evaluation, we face the problem that the delivered dose cannot be predicted with certainty at the time of treatment planning but is associated with uncertainties. We present a method to simulate stochastic properties of the dose distribution. This provides the treatment planner with information about motion-related risks of different plans and may support the decision for or against a treatment plan. This information includes the display of probabilities of individual voxels to receive doses from a therapeutical interval or above critical levels, as well as a diagram that shows the variability of the dose volume histogram. Concerning the incorporation of organ motion into IMRT planning, we further analyse the approach of inverse planning based on probability distributions of possible patient geometries. We consider three different sources of uncertainty, namely uncertainty about the amplitude of motion, a systematic error and a random error. We analyse the impact of these sources of uncertainty on the optimized treatment plans for prostate cancer.

Artifacts↗

PubMed2005

Incorporating organ movements in inverse planning: assessing dose uncertainties by Bayesian inference.

We present a method to calculate dose uncertainties due to inter-fraction organ movements in fractionated radiotherapy, i.e. in addition to the expectation value of the dose distribution a variance distribution is calculated. To calculate the expectation value of the dose distribution in the presence of organ movements, one estimates a probability distribution of possible patient geometries. The respective variance of the expected dose distribution arises for two reasons: first, the patient is irradiated with a finite number of fractions only and second, the probability distribution of patient geometries has to be estimated from a small number of images and is therefore not exactly known. To quantify the total dose variance, we propose a method that is based on the principle of Bayesian inference. The method is of particular interest when organ motion is incorporated in inverse IMRT planning by means of inverse planning performed on a probability distribution of patient geometries. In order to make this a robust approach, it turns out that the dose variance should be considered (and minimized) in the optimization process. As an application of the presented concept of Bayesian inference, we compare three approaches to inverse planning based on probability distributions that account for an increasing degree of uncertainty. The Bayes theorem further provides a concept to interpolate between patient specific data and population-based knowledge on organ motion which is relevant since the number of CT images of a patient is typically small.

Algorithms↗

PubMed2004

Inclusion of organ movements in IMRT treatment planning via inverse planning based on probability distributions.

In this paper, we investigate an off-line strategy to incorporate inter-fraction organ motion in IMRT treatment planning. It was suggested that inverse planning could be based on a probability distribution of patient geometries instead of a single snap shot. However, this concept is connected to two intrinsic problems: first, this probability distribution has to be estimated from only a few images; and second, the distribution is only sparsely sampled over the treatment course due to a finite number of fractions. In the current work, we develop new concepts of inverse planning which account for these two problems.

Humans↗

PubMed2003

Time-delay autosynchronization of the spatiotemporal dynamics in resonant tunneling diodes.

The double barrier resonant tunneling diode exhibits complex spatiotemporal patterns including low-dimensional chaos when operated in an active external circuit. We demonstrate how autosynchronization by time-delayed feedback control can be used to select and stabilize specific current density patterns in a noninvasive way. We compare the efficiency of different control schemes involving feedback in either local spatial or global degrees of freedom. The numerically obtained Floquet exponents are explained by analytical results from linear stability analysis.

Journal Article↗