Biomedical subjects

H Rashid

Publications and source records attributed to H Rashid.

41 records · Page 3Linked to original sources

PubMedDate not supplied

Small-field electron dosimetry for the Philips SL25 linear accelerator.

Electron-beam characteristics of a Philips SL25 linear accelerator have been studied. Central-axis percentage depth doses, cross-beam profiles and beam output factors of 6-, 10-, and 20-MeV beams, selected from the available energy range of 4 to 22 MeV, are reported in this paper. The main thrust of this work is to determine the systematic variation of beam characteristics, especially the output factor, with standard cone sizes and cerrobend beam-shaping cutouts down to a field size of 2 X 2 cm Output factors for the standard cones (open field) are energy dependent in a complex manner, increasing with the cone size for the 6-MeV beam whereas decreasing for 10- and 20-MeV beams. The output factor falls below unity at lower energies (6 and 10 MeV) for fields with at least one side smaller than 6 cm, and stays nearly constant for the 20-MeV beam. Measured output factors of small fields are least squares fitted by a second-order polynomial function. Output factors for small rectangular fields have been derived from the one-dimensional and square-root formulas, and the equivalent-square method. Only the one-dimensional formula predicts the measured output factors of highly elongated fields to within +/- 1% experimental uncertainties. Different cones with the same size electron cutout show a varied dose response, primarily due to variation in scattered electron contamination from the cones.

Electrons↗

PubMedDate not supplied

Energy constancy checking for electron beams using a wedge-shaped solid phantom combined with a beam profile scanner.

An energy constancy checking method is presented which involves a specially designed wedge-shaped solid phantom in combination with a multiple channel ionization chamber array known as the Thebes device. Once the phantom/beam scanner combination is set up, measurements for all electron energies can be made and evaluated without re-entering the treatment room. This is also valid for the readjustment of beam energies which are found to deviate from required settings. The immediate presentation of the measurements is in the form of crossplots which resemble depth dose profiles. The evaluation of the measured data can be performed using a hand-held calculator, but processing of the measured signals through a PC-type computer is advisable. The method is insensitive to usual fluctuations in beam flatness. The sensitivity and reproducibility of the method are more than adequate. The method may also be used in modified form for photon beams.

Electrons↗

PubMedDate not supplied

A simple method of producing depth ionization data for electron energy constancy check.

A simple method has been developed to reproduce depth ionization data of electron beams for energy determination. The method utilizes a simple set of equipment, a combination of a specially designed wedge-shaped polystyrene phantom and a linear array of detectors, to collect the necessary data. The wedge-shaped phantom provides varying depths to various detectors in the array. The ionization readings received from the detectors were corrected for off-axis ratio and plotted against corresponding ray-line depths to produce depth ionization curves. The instrument setup was fast and simple. The relevant data, for a high-energy linear accelerator with multiple electron energies, were collected in minutes. The depths of 80% and 50% ionization determined by this method were found to differ by 2 and 3 mm, respectively, at the most, with those determined by a conventional method.

Biophysical Phenomena↗

PubMedDate not supplied

Dosimetry of 125I sources in a low-density material using scaling.

One may apply O'Connor's scaling theorem to dose measurements with brachytherapy sources in order to overcome the difficulties associated with the need for high spatial accuracy. This possibility has been evaluated by measuring the dose distribution around 125I sources in a low-density styrofoam phantom and comparing it with the dose distributions in water and solid water. Some generalization of the scaling theorem is proposed to allow for the minor differences in atomic composition between styrofoam and water, and the distances are scaled according to the ratio of the linear attenuation coefficients, instead of the physical densities, of the two media. The validity of this application of the scaling theorem has also been tested using Monte Carlo calculations. The results indicate that the scaling of the styrofoam measurements to water is a useful approximation in brachytherapy dosimetry.

Brachytherapy↗

PubMedDate not supplied

"Zero-field" dose data for 60Co and other high-energy photon beams in water.

A procedure of separating the primary- and scatter-dose components in therapeutic photon beams is examined. It is based on the observation that the scatter-dose component is proportional to the variable z = rd/(r+d), where r and d are beam radius and depth in phantom, respectively. It is, therefore, possible to express an absorbed dose in the form of a linear equation D(r) = P+Nz, where at a fixed depth d, both primary dose P and coefficient N are constant. A method of linear extrapolation of an absorbed dose D(r) to "zero-field" size, i.e., r = 0, is utilized. Since Monte Carlo technique is capable of scoring separately the primary- and scatter-dose components, it is used to evaluate the accuracy of the linear extrapolation method within the range of 60Co-15-MV nominal photon energies. The results demonstrate that this method is sufficiently accurate to obtain the primary dose component in photon beams. For 60Co gamma radiation in water, tabulated sets of measured depth-dose data are analyzed by the linear extrapolation method to review "zero-field" dose values [percentage depth dose (PDD) and tissue-air ratio (TAR) tables of the British Journal of Radiology, Suppl. 17]. The "zero-field" PDD data are found to be accurate within limits of experimental uncertainties. Inconsistencies in the TAR table are illustrated and discussed. 60Co tables of relative doses, D(r,d)/P(dmax), including "zero-field" values for both fixed SSD and isocentric geometries, are generated. Dose calculation in irregular fields is considered. The linear extrapolation method is recommended as a standard procedure for separating primary dose from depth-dose data in high-energy photon beams.

Biophysical Phenomena↗