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Biomedical subjects

J Welleweerd

Publications and source records attributed to J Welleweerd.

9 recordsLinked to original sources
PubMed2004

A six-bank multi-leaf system for high precision shaping of large fields.

In this study, we present the design for an alternative MLC system that allows high precision shaping of large fields. The MLC system consists of three layers of two opposing leaf banks. The layers are rotated 60 degrees relative to each other. The leaves in each bank have a standard width of 1 cm projected at the isocentre. Because of the symmetry of the collimator set-up it is expected that collimator rotation will not be required, thus simplifying the construction considerably. A 3D ray tracing computer program was developed in order to simulate the fluence profile for a given collimator and used to optimize the design and investigate its performance. The simulations show that a six-bank collimator will afford field shaping of fields of about 40 cm diameter with a precision comparable to that of existing mini MLCs with a leaf width of 4 mm.

Computer Simulation↗
PubMed2002

A Monte Carlo study on internal wedges using BEAM.

To do calculations for wedged photon beams with the NRC Monte Carlo simulation package BEAM, a new Component Module for wedges called WEDGE has been designed and built. After an initial series of benchmarks using monoenergetic photon beams as well as realistic 6 MV and 10 MV beams, it was found, that the new CM did work fine for the large wedge (maximum field size 30 x 40 cm2) of the Elekta SL-linac. The next step was to calculate dose distributions and output factors for a range of wedged fields with field size from 3 x 3 cm2 to 30 x 30 cm2. Results from these simulations have been compared to measurements. Calculated values for the reference wedge transmission factor and the relative wedge transmission factors were within 1.5% from the measured data. Dose distributions showed an identical behavior; both depth-dose curves as well as cross profiles were within 1.5% from measured data, usually even better. Despite the increased mean energy, there was no indication that, as a result, the phantom scatter output factors will change for a 10 MV photon beam. It was found that by adding a wedge the contributions for the different sources of head scatter changed considerably as compared to the open fields, apart from the additional scatter from the wedge. Another consequence of inserting a wedge was an increase in the mean energy of both primary and scattered radiation with 0.3 MV and 0.7 MV, respectively, for all wedged fields with respect to the corresponding open fields. Despite the statistical uncertainty in the calculated data, which is in the same order of magnitude as the effect to be determined, it was possible to derive reliable data for the beam hardening from the calculated dose distributions. Only for the smallest field (field size 3 x 3 cm2) a large difference between the measured and calculated beam hardening factor was found due to the relative large voxel size of 1 x 1 x 1 cm3 compared to the field size. For a description of the influence of a wedge on a photon beam, the results of this study strongly support the use of a reference wedge transmission factor (determined under reference conditions) in combination with a relative wedge transmission factor. The product of these variables should replace the collimator scatter output factor used in open fields. The influence on the dose distribution should be incorporated by using the (field size dependent) beam hardening. The ultimate solution will be to make this beam hardening depending on the actual position in the radiation field, as the photon energy varies over the field (holds also for open fields).

Algorithms↗
PubMed1999

Calculating photon beam characteristics with Monte Carlo techniques.

This study describes the results from a simulation of a 10 MV photon beam from a medical linac using the BEAM code. To check the quality of the generated photon beam, the characteristics of this beam (depth dose curve, cross profiles, and output factors) have been calculated and compared to measured data. By splitting up the radiation head in two parts, the target section and the collimator section calculation times were long, but acceptable when aiming at phase space files containing some 5 million particles. Given the number of particles evaluated, the accuracy of all data was around 2%. Analysis of the phase space files for different field size supports results from previous studies about contaminant particles and sources for scattered radiation for photon beams from medical linacs. The total scatter output factor Scp as well as the collimator scatter output factor Sc have been calculated within 2% of measurements. Also, the ratio between dose at a reference point for the full scatter situation and the no-scatter situation has been calculated correctly. All depth dose curves and cross profiles have also been calculated correctly, although with only moderate statistics. Improvements are possible by increasing the number of particles in the simulations (up to 50 million for the largest field size) at least 4-8 times, although calculation times will increase with the same factor. Nevertheless, the method proved itself as reliable. Still, the accuracy should be improved to 1% or better. This is necessary as we plan to use Monte Carlo simulations to benchmark three-dimensional radiotherapy planning systems. By increasing the number of particles in the phase space files and subsequently increasing the number of particles in each simulation, this 1% accuracy will be achieved. The easy way to increase the number of particles in a simulation by increasing the number of times phase space files, which were already recycled ten times, are reused from ten times (this study) to 40 times or more will not work, as it introduces artifacts, especially in the cross profiles.

Biophysical Phenomena↗
PubMed1999

Finding mechanisms responsible for the spectral distribution of electron beams produced by a linear accelerator.

The measured electron spectra of linear accelerators from several manufacturers differ in comparison to the spectral form and width. As part of our investigations of linac performance and stability, we analyzed the electron spectra of our linacs. After building a spectrometer the electron spectra were measured. The measured spectral widths were comparable with the results published in the literature. It appeared that the phase of the recycled radio pulse in combination with the limited bandwidth of the bending magnet are responsible for unusual (multipeak) electron spectra. The scatter filters only have a relatively small widening effect on the spectrum. There was no indication that multipeak or wide spectra are related to linac instabilities.

Electrons↗
PubMed1992

Responses of cultured mammalian cells to prolonged low dose rate gamma irradiation.

Experiments were performed to verify the finding of others that prolonged irradiation caused sudden death of cells. Continuous irradiation (dose rate 0.09 Gy/h) was applied to the R-1, RUC-2, V79 and T-1g cells for up to 140 days. End points were: population growth rate, plating efficiency and radiosensitivity. The population doubling times were increased and plating efficiencies decreased; cell survival after single doses at acute dose rate were lower than that of control cells. Part of the cell parameters returned to control values within three weeks after prolonged irradiation had been terminated. Sudden death after prolonged exposure was not observed.

Animals↗
PubMed1984

Flow cytometric determination of radiation-induced chromosome damage and its correlation with cell survival.

Chinese hamster M3-1 cells were irradiated with several doses of X rays or alpha particles from 238Pu. Propidium iodide-stained chromosome suspensions were prepared at different times after irradiation; cells were also assayed for survival. The DNA histograms of these chromosomes showed increased background counts with increased doses of radiation. This increase in background was cell-cycle dependent and was correlated with cell survival. The correlation between radiation-induced chromosome damage and cell survival was the same for X rays and alpha particles. Data are presented which indicate that flow cytometric analysis of chromosomes of irradiated cell populations can be a useful adjunct to classical cytogenic analysis of irradiation-induced chromosomal damage by virtue of its ability to express and measure chromosomal damage not seen by classical cytogenic methods.

Alpha Particles↗
PubMed1982

Detection of lung cancer with 55Co-bleomycin using a positron camera. A comparison with 57Co-bleomycin and 55Co-bleomycin single photon scintigraphy.

57-Co-bleomycin is useful in the detection and staging of lung cancer, but the long half-life of 57Co (270 days) has discouraged its widespread acceptance. We investigated the shorter living positron emitting 55Co (half-life 18.2 h) as a level for bleomycin. In eleven patients with proven lung cancer scintigraphy with 55Co-bleomycin, using a positron camera, demonstrated the tumor in ten cases. Tumor to lung ratios were calculated. The results were superior to those obtained with 55Co-bleomycin single photon imaging but inferior to those obtained with 57Co-bleomycin scintigraphy.

Aged↗