Biomedical subjects
Publications and source records attributed to J Scrimger.
The separation of the two gantry angles in a pair of tangential fields required to obtain posterior beam edge coincidence is derived. This derivation enables one to calculate the second gantry angle at the time of simulation. Experience has shown that the setup angle is within 1 degree of the calculated value.
The dose due to Bremsstrahlung in stationary electron beams of nominal energies in the range 6-20 MeV is typically between 1-7% of the maximum dose and is usually not clinically significant. However, in treatments using rotational or multiple electron beams where the x-ray dose from several beams is added the x-ray dose will reach much higher proportions and will be of clinical significance. Moreover, this dose often is located in normal tissue beyond the target volume. Reduction of this x-ray dose is therefore desirable. In the present study a reduction of the x-ray component of electron beams produced by a Clinac 2100C accelerator by a change of the transmission ion chamber and scattering foils is reported. A reduction in Bremsstrahlung of up to 50% can be achieved.
Total body irradiation is used to prepare a patient for bone marrow transplantation. Traditional techniques often sacrifice dose uniformity for patient comfort and ease of treatment. A method has been developed using a translational bed under a Cobalt 60 photon beam. The bed and controller were designed and built on site. A bolused patient lying in the bed is moved at constant speed through the beam. Using this technique, dose homogeneity is optimized by the use of bolus, extended source-skin distance, adequate field size and use of anterior/posterior fields. The dose rate represents a compromise between a value high enough to keep treatment times tolerable by the patient and one that is sufficiently low to avoid treatment complications. The value of 50 cGy/min which was used meets these requirements. Extensive phantom measurements have shown that the dose homogeneity can be obtained to within an acceptable limit of +/- 5%.
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The magnitude and distribution of scattered radiation produced by scatterers inserted into megavoltage therapy beams, including the beams from an 8MV medical linear accelerator and a 60Co teletherapy unit, were investigated. The intensity distribution of scatter depends on the distance from the scatterer to the measurement plane (retraction distance) and also, to a lesser extent, on the atomic number of the scattering material. The effective energy of the scattered radiation was determined by depth dose measurements in tissue equivalent material using thermoluminescent dosimeters and was found to increase with photon beam energy.
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We have measured the radiation dose in simple heterogeneous phantoms and compared our results with those obtained by various methods of computation. Dose data were obtained both within and distal to simulated regions of lung in order to test the ratio of tissue-air ratios (TAR), Batho, and equivalent TAR methods. These procedures are used routinely in manual and computer-aided planning of radiation therapy, but have been validated primarily for cobalt-60 radiation. Tests performed with 6- and 15-MV x rays reveal that incorrect doses can be computed within or near to a low-density medium, particularly when the field size is small. In these cases, electronic equilibrium is not achieved in the lateral direction, thereby violating an implicit assumption of all the above calculation methods. We quantify the errors in dose calculation for simple slab phantoms, and support our interpretation with a Monte Carlo simulation in which the energy transported by charged particles away from sites of x-ray interactions is considered directly.
Film and thermoluminescent dosimetry (TLD) are investigated in heterogeneous phantoms irradiated by high-energy electron beams. Both film and TLD are practical dosimeters for multiple and moving beam radiotherapy. The accuracy and precision of these dosimeters for radiation dose measurements in homogeneous water-equivalent phantoms has been discussed in the literature. However, film and TLD are often used for dose measurements in heterogeneous phantoms. In those situations perturbations are produced which are related to the density and atomic number of the phantom material and the physical size and orientation of the dosimeter. In our experiments the relative dose measurements in homogeneous phantoms were the same regardless of dosimeter or dosimeter orientation. However, significant differences were observed between the dose measurements within the inhomogeneity. These differences were influenced by the type and orientation of the dosimeter in addition to the properties of the heterogeneity. These differences could be reproduced with Monte Carlo calculations and modeling of the experimental conditions.
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