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Biomedical subjects

W Laub

Publications and source records attributed to W Laub.

6 recordsLinked to original sources
PubMed2002

Investigation of photon beam output factors for conformal radiation therapy--Monte Carlo simulations and measurements.

The purpose of this study was to investigate beam output factors (OFs) for conformal radiation therapy and to compare the OFs measured with different detectors with those simulated with Monte Carlo methods. Four different detectors (diode, diamond, pinpoint and ionization chamber) were used to measure photon beam OFs in a water phantom at a depth of 10 cm with a source-surface distance (SSD) of 100 cm. Square fields with widths ranging from 1 cm to 15 cm were observed; the OF for the different field sizes was normalized to that measured at a 5 cm x 5 cm field size at a depth of 10 cm. The BEAM/EGS4 program was used to simulate the exact geometry of a 6 MV photon beam generated by the linear accelerator, and the DOSXYZ-code was implemented to calculate the OFs for all field sizes. Two resolutions (0.1 cm and 0.5 cm voxel size) were chosen here. In addition, to model the detector four kinds of material, water, air, graphite or silicon, were placed in the corresponding voxels. Profiles and depth dose distributions resulting from the simulation show good agreement with the measurements. Deviations of less than 2% can be observed. The OF measured with different detectors in water vary by more than 35% for 1 cm x 1 cm fields. This result can also be found for the simulated OF with different voxel sizes and materials. For field sizes of at least 2 cm x 2 cm the deviations between all measurements and simulations are below 3%. This demonstrates that very small fields have a bad effect on dosimetric accuracy and precision. Finally, Monte Carlo methods can be significant in determining the OF for small fields.

Humans↗
PubMed2001

[Monte Carlo simulation of a dynamic multileaf collimator:implementation and applications].

A model for the simulation of the accelerator heads of two identical linear accelerators was designed at the University Hospital of Tübingen, using the BEAM program developed at the National Research Council of Canada. Both linear accelerators are equipped with multileaf collimators (MLCs) and backup jaws (y-direction) with curved leaf-ends. The accelerator models were divided into two parts. The first part consisted of target, primary collimator, flattening filter, monitor chamber, and mirror. After the Monte Carlo simulation of these parts, the phase-space characteristics below the mirror were stored in a file and used as source for the second part of the accelerator head (jaw, MLC). The electron source was assumed to deliver a gaussian energy spectrum, with parallel direction to the beam axis. With this electron source, there was good agreement between the measured and simulated depth dose curves in water, with difference < 2%. A new module was created for the BEAM program to simulate backup jaws, while the standard MLCQ module from BEAM was used to simulate a MLC with curved leaf-ends. As a result, MLCs and backup jaws with curved leaf-ends make the shoulder of the y-profile higher than the straight-end MLCs.

Computer Simulation↗
PubMed2000

Monte Carlo dose computation for IMRT optimization.

A method which combines the accuracy of Monte Carlo dose calculation with a finite size pencil-beam based intensity modulation optimization is presented. The pencil-beam algorithm is employed to compute the fluence element updates for a converging sequence of Monte Carlo dose distributions. The combination is shown to improve results over the pencil-beam based optimization in a lung tumour case and a head and neck case. Inhomogeneity effects like a broader penumbra and dose build-up regions can be compensated for by intensity modulation.

Algorithms↗
PubMed1999

Experimental investigation of a fast Monte Carlo photon beam dose calculation algorithm.

An experimental verification of the recently developed XVMC code, a fast Monte Carlo algorithm to calculate dose distributions of photon beams in treatment planning, is presented. The treatment head is modelled by a point source with energy distribution (primary photons) and an additional head scatter contribution. Utility software is presented, allowing the determination of the parameters for this model using a single measured depth dose curve in water. The simple beam model is considered to be a starting point for more complex models being planned for future versions of the code. This paper is mainly focused on the influence of the different techniques on variance reduction and material property determination for dose distributions. It is demonstrated that XVMC and the simple beam model reproduce measured (by a diamond detector) relative dose distributions with an accuracy of better than +/-2% in various homogeneous and inhomogeneous phantoms. Furthermore, relative dose distributions in solid state phantoms have been measured by film. Also for these cases, measured and calculated dose distributions agree within experimental uncertainty. The short calculation time (depending on voxel resolution, statistical accuracy, field size and energy, a span of 1 min to 1 h using a present-day personal computer) and an interface to a commercial planning system will allow the implementation of the code for routine treatment planning of clinical electron and photon beams.

Algorithms↗
PubMed1982

Rat mammary tumor classification: notes on comparative aspects.

Mammary tumors (170 spontaneous and 1,613 induced with 7,12-dimethylbenz[a]anthracene) in inbred SD rats were classified histologically. The neoplasms were divided into three main categories: fibroepithelial, epithelial, and connective tissue. In the fibroepithelial category, compound tumors showing a wide range of histologic structures with variation in the arrangement of both epithelial and connective tissue elements that differ among lobules occurred in 44 (25.9%) spontaneous and 1,027 (63.7%) induced neoplasms, whereas fibroadenomas occurred in 41 (24.1%) untreated and 175 (10.9%) treated rats. Uncommon fissured tumors were found in 1 (0.7%) untreated and 6 (0.4%) treated animals, whereas carcinosarcomas were found in only 10 untreated animals. In the epithelial category, tubular adenomas occurred in 48 (28.2%) untreated and 164 (10.2%) treated animals. Cystadenomas, duct papillomas, intraductal carcinomas, and anaplastic carcinomas were found less frequently. Adenoacanthomas occurred in only 12 (0.8%) of the treated animals. Among tumors with a fibrous component only, fibromas occurred in 22 (13%) untreated and 97 (6.0%) treated animals, Fibrosarcomas were less frequent, occurring in 4 untreated (2.4%) and 54 (3.4%) treated animals.

Adenocarcinoma↗