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J Llacer

Publications and source records attributed to J Llacer.

14 recordsLinked to original sources
PubMed2001

Comparative behaviour of the dynamically penalized likelihood algorithm in inverse radiation therapy planning.

This paper presents a description of tests carried out to compare the behaviour of five algorithms in inverse radiation therapy planning: (1) The Dynamically Penalized Likelihood (DPL), an algorithm based on statistical estimation theory; (2) an accelerated version of the same algorithm: (3) a new fast adaptive simulated annealing (ASA) algorithm; (4) a conjugate gradient method; and (5) a Newton gradient method. A three-dimensional mathematical phantom and two clinical cases have been studied in detail. The phantom consisted of a U-shaped tumour with a partially enclosed 'spinal cord'. The clinical examples were a cavernous sinus meningioma and a prostate case. The algorithms have been tested in carefully selected and controlled conditions so as to ensure fairness in the assessment of results. It has been found that all five methods can yield relatively similar optimizations, except when a very demanding optimization is carried out. For the easier cases. the differences are principally in robustness, ease of use and optimization speed. In the more demanding case, there are significant differences in the resulting dose distributions. The accelerated DPL emerges as possibly the algorithm of choice for clinical practice. An appendix describes the differences in behaviour between the new ASA method and the one based on a patent by the Nomos Corporation.

Algorithms↗
PubMed2000

ROC and LROC analyses of the effects of lesion contrast, size, and signal-to-noise ratio on detectability in PET images.

UNLABELLED: Image quality in PET is typically assessed using measures such as contrast recovery, noise variation, and signal-to-noise ratio (SNR). However, these criteria do not directly reflect performance in the clinical use of the images. Lesion detection is a critical task in the clinical interpretation of many PET studies. A receiver operating characteristic (ROC) study is an accepted method for quantitatively evaluating detection performance with respect to factors that influence image quality. ROC and localization ROC (LROC) analyses were conducted to investigate the effects of lesion contrast, SNR, and size on detectability of hot lesions in PET images. METHODS: A thorax phantom was imaged with spheres of 3 sizes simulating lesions (0.45, 1.0, and 1.9 mL). The relative activity in the lesions and the total number of counts acquired were each varied by factors of 2 to ascertain the effects of contrast and SNR, respectively. Measured attenuation correction and a standard reconstruction protocol were used. Three nuclear medicine physicians and 6 medical physicists participated as readers, rating each image and indicating the suspected lesion location. The area under the calculated ROC and LROC curves (Az and Az,LROC) were used as measures of detection performance. RESULTS: Detection performance was shown to increase from virtually random (Az approximately 0.5, Az,LROC approximately 0.2) to superior (Az > 0.9, Az,LROC > 0.9) as lesion contrast was increased by 50% and as lesion SNR was doubled. Detection performance was not seen to vary when comparison was made using image-based measures alone. CONCLUSION: This study quantitatively shows that moderate increases in the image-based measures of lesion contrast and SNR give a relatively large increase in the task-based measure of lesion detection as measured by ROC and LROC analyses. Thus, techniques that give modest increases in lesion contrast or SNR are expected to improve detection. Results will be useful in evaluating improvement in detection for various reconstruction, acquisition, and data analysis methods that enhance contrast or noise performance.

Heart↗
PubMed1999

ROC and localization ROC analyses of lesion detection in whole-body FDG PET: effects of acquisition mode, attenuation correction and reconstruction algorithm.

UNLABELLED: Receiver operating characteristic (ROC) and localization ROC (LROC) studies were performed to compare lesion detection at the borderline of detectability on images reconstructed with two-dimensional filtered backprojection (FBP) without attenuation correction (a common clinical protocol), three-dimensional FBP without attenuation correction, two-dimensional FBP with segmented attenuation correction and a two-dimensional iterative maximum a posteriori (MAP) algorithm using attenuation correction. Lung cancer was the model for the study because of the prominent role of 18F-fluorodeoxyglucose PET in the staging of lung cancer and the importance of lesion detection for staging. METHODS: Simulated lung cancer lesions were added to two-dimensional and three-dimensional PET data from healthy volunteers. Data were reconstructed using the four methods. Four nuclear medicine physicians evaluated the images. Detection performance with each method was compared using ROC and LROC analysis. Jackknife analysis provided estimates of statistical significance for differences across all readers for the ROC results. RESULTS: ROC and LROC results indicated statistically significant degradation in detection performance with three-dimensional acquisition (average area under ROC curves [Az] 0.51; average area under LROC curves [A(z,LROC)] 0.13) and segmented attenuation correction (average Az 0.59; average Az,LROC 0.29) compared with two-dimensional FBP without attenuation correction (average Az 0.79; average A(z,LROC) 0.54). ROC and LROC results indicated an improvement in detection performance with iterative MAP reconstruction (average Az 0.83; average A(z,LROC) 0.64) compared with two-dimensional FBP reconstruction; this improvement was not statistically significant. CONCLUSION: Use of segmented attenuation correction or three-dimensional acquisition with FBP reconstruction is not expected to improve detection of lung lesions on whole-body PET images compared with images with two-dimensional FBP without attenuation correction. The potential improvement in detection obtained with an iterative MAP reconstruction method is small compared with that obtained with two-dimensional FBP without attenuation correction.

Algorithms↗
PubMed1997

Inverse radiation treatment planning using the Dynamically Penalized Likelihood method.

In this paper we present a new method of solving the inverse radiation treatment planning problem. The method is based on a Maximum Likelihood Estimator with dynamically changing penalization terms. The resulting Dynamically Penalized Likelihood (DPL) algorithm achieves a dose distribution of excellent uniformity in a tumor volume and a much lower dose in regions containing sensitive volumes. A simple model of a patient and of energy deposition has been used for the initial results presented: a two-dimensional computer generated phantom and monochromatic x rays, without scattering. Three two-dimensional problems are solved with the DPL algorithm, corresponding to different size and spatial relationships between the tumor and sensitive tissue volumes. The results show that the DPL algorithm is robust and flexible; it only requires moderate computation times and leads to promising solutions, even in rather difficult problems. The results encourage the extension of the present work to more realistic therapy situations.

Algorithms↗
PubMed1993

Results of a clinical receiver operating characteristic study comparing filtered backprojection and maximum likelihood estimator images in FDG PET studies.

The results of a receiver operator characteristic (ROC) study comparing maximum likelihood estimator (MLE) reconstructions of human FDG PET brain scan data to filtered backprojection reconstructions of the same data are reported. The purpose of the study was to determine whether MLE reconstructions would result in higher detectability of small focal lesions introduced artificially into otherwise normal scan data. One physician assisted in defining the location and intensity of the lesions and five physicians read the final images. Data from 90 datasets were used for the study. Of those, 42 were left in their original "normal" condition and 48 were modified by added lesions. All datasets were reconstructed by the two methods and submitted to the five physicians for evaluation. The results show an increase in the area under the ROC curve from approximately 0.65 for filtered backprojection to approximately 0.71 for the maximum likelihood reconstructions for four of the five observers with good statistical significance.

Brain↗
PubMed1985

A comparison of water equivalent thickness measurements: CT method vs. heavy ion beam technique.

The purpose of this study was to evaluate existing X-ray CT methods for determining water-equivalent path length from body surface to a tumour site. In the method, the CT numbers are used to obtain linear attenuation coefficients which provide a measure of the electron density. These numbers are averaged over the energy spectrum of the diagnostic X-ray beam and other parameters which have a dependence on energy. From range measurements with heavy charged particles, it is also possible to obtain an independent and direct measure of electron density along the beam path. In the results reported here, the beam path electron density or water-equivalent path length was measured with charged particle beams, using radiation sensitive diodes as target markers. To minimise error which would be introduced by motion of the target volume, a frozen dog cadaver was used. Comparison was made between the water-equivalent path length measured with high energy particle beams, and the water-equivalent path length estimated from an X-ray CT image of the same target volume by the methods presently used in charged particle therapy (Chen et al. 1979). There was good agreement between the values determined directly with neon or helium ion beams, but when these values were compared with estimated path lengths derived from X-ray CT data, it was observed that the CT range could be in error by as much as 11% for adverse conditions of marked inhomogeneity and the presence of high atomic number bone. Under the best conditions of moderate inhomogeneity and absence of bone, the derived CT range values agreed reasonably well with the direct measurements.

Animals↗
PubMedDate not supplied

On-line characterization of heavy-ion beams with semiconductor detectors.

Heavy-ion beams used in biomedical studies suffer a substantial amount of nuclear reactions (fragmentation) as they traverse matter. Since it has been demonstrated that dose and linear energy transfer (LET) are not a sufficient description of a beam for the purpose of understanding its biological effects, it is necessary to be able to separate the components of a complex beam so that their individual effects can be analyzed. A simple and small assembly consisting of a thin silicon LET detector, in time coincidence with a thick germanium residual energy detector has been used in measurements of the components of Ne-20 and Si-28 high-energy ion beams. The detector system can be placed at any experimental area without difficulty and it can carry out a beam analysis in a few minutes, making it very appropriate for fast on-line measurements and verification of beam characteristics. LET values measured by the silicon detector agree well with results of the Bethe stopping-power calculations, and the dose measured for the beam components can be used to obtain Bragg curves that are in good agreement with those obtained by ionization chamber measurements on the same beams. The numbers and LET distribution of primaries and fragments at different positions of the Bragg curves, as well as fractional dose contributed by the different components are determined directly from the experimental data. Particle velocity distributions can be obtained for the higher Z fragments. Limitations and advantages of the simple measurement technique are discussed.

Energy Transfer↗
PubMedDate not supplied

Characterization of fragmented heavy-ion beams using a three-stage telescope detector: measurements of 670-MeV/amu 20Ne beams.

Measurements of a 670-MeV/amu 20Ne beam at the Lawrence Berkeley Laboratory Bevalac heavy-ion accelerator with various thicknesses of water absorber were obtained with the BERKLET. The BERKLET, a simple three-stage solid-state telescope detector, has been described previously. This instrument measures the linear energy transfer (LET) and residual energy of particles, allows the identification of the particle's charge, and provides a means of obtaining LET and energy statistics for the beam, separated by particle charge. The track and dose averaged LET dependence on the amount of water absorber was determined for each species of fragment in the beam. Large numbers of low-LET particles in the fragmented beam were detected. The results of the analysis are presented followed by a discussion of the effects of multiple scattering and secondary fragmentation on the measurements. A brief discussion of the implications of the BERKLET measurements for radiobiology is also presented.

Energy Transfer↗
PubMedDate not supplied

Characterization of fragmented heavy-ion beams using a three-stage telescope detector: detector configuration and instrumentation.

Accelerated heavy-ion beams used in biological and medical research are often utilized in conjunction with absorbers which lead to the fragmentation of the beam. The BERKLET, initially a two-stage solid-state telescope detector, was designed to make rapid, on-line energy and linear energy transfer (LET) measurements of individual particles in a heavy-ion beam, thus allowing characterization of fragmented beams. From data collected with the BERKLET, one is able to determine a number of important parameters. These include: residual energy and LET histograms for the full beam and for the individual Z components, relative number of particles with a given Z, and dose and track average LET's for the full beam and for the individual Z's. Improvements to the BERKLET design and changes in data analysis are discussed and contrasted with the results of an earlier BERKLET configuration. The most notable improvements are the addition of a thin scintillation detector for improved LET measurement, a tenfold improvement in the dynamic range of the event discriminator, reported here as 1:2000, and dual high-and low-gain amplification of the LET signals, permitting the identification of particles with Z's ranging from 12 down to 1.

Energy Transfer↗
PubMedDate not supplied

Association of chromosome 7, chromosome 10 and EGFR gene amplification in glioblastoma multiforme.

Glioblastoma multiforme (GBM) is characterized by intratumoral heterogeneity in both histomorphological and genetic changes, displaying a wide variety of numerical chromosome aberrations, the most common of which are trisomy 7 and monosomy 10. The amplification of the epidermal growth factor receptor (EGFR) gene is the most frequently reported genetic abnormality. The associations between these parameters and their implication in the tumoral progression are poorly understood. We performed simultaneous fluorescence in situ hybridization (FISH) with centromeric DNA probes for chromosomes 7 and 10 in smear preparations, and EGFR gene amplification by PCR from 25 cases of GBM. Trisomy/ polysomy for chromosome 7 was present in 76% of cases and monosomy 10 in 68%. Both alterations were associated in 56% of cases. The EGFR gene was amplified in 52% of tumors; in 44% associated with trisomy/ polysomy 7, and in 36% with monosomy 10. The three parameters were associated together in 28% of cases. Kaplan-Meier survival rate analysis demonstrated lower survival rates in patients with monosomy 10, trisomy 7, and monosomy associated with trisomy 7. The other combinations were not different in frequency in relation to survival. In the present study, trisomy/polysomy 7 and monosomy 10 have been found to be frequently associated. The combination of both anomalies is probably important in the tumorigenesis of glioblastoma. Moreover, this association is apparently independent of EGFR gene amplification, which could be a later event in this process.

Adult↗