Three-dimensional dose distribution in cobalt-60 teletherapy of the head and neck.
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Cobalt Isotopes↗
Biomedical subjects
N Suntharalingam
Publications and source records attributed to N Suntharalingam.
A comparison of TLD and film for personnel dosimetry.
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Environmental Exposure↗
Treatment of cancer of the pancreas with high energy photons and electrons.
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Carcinoma, Intraductal, Noninfiltrating↗
High-energy-photon dose measurements using exposure-calibrated ionization chambers.
The current method for determining absorbed dose from high-energy photons is to use in water a Cobalt-60 exposure-calibrated ionization chamber and Clambda. Recently questions have arisen about the correctness of currently recommended Clambda values and whether or not the Cobalt-60 buildup cap should be used for in-water dose measurements. The effect of the buidup cap on Clambda was obtained by measuring Farmer chamber response in water with and without the buildup cap. Results show no measurable change at Cobalt-60 or 4 MV but an increase of 1%-2% above 20 MV. The effects of the wall materials and the displacement factors on Clambda are also considered. It is shown that at high energies errors of 3%-5% can be made in determining dose in water using commercially available Farmer or Farmer-like chambers and currently recommended Clambda. The exact size of the error depends upon the materials and thicknesses of the wall and cap, the chamber cavity volume, and whether or not the buildup cap is used.
Elementary Particles↗
Physical characterization of 45-MV photon beams for use in treatment planning.
Central-axis percentage depth doses and tissue-maximum ratios (TMR) for 45-MV photon beams from a betatron have been measured in water. Also the influence of field size and collimator scatter on the dose in the buildup region have been investigated. The maximum dose for TMR has been shown to occur at a point about 2 cm deeper than the maximum dose for percentage depth dose. This difference is significant in the characterization of the photon beam at this high energy. The measured physical data have been computerized for use in routine treatment planning. Computer-gererated beams have been found to be in close agreement with measured isodose curves. Computer-generated isodose distributions for typical clinical irradiation techniques have been verified using RP/V film in an Alderson phantom.
Computers↗
Clarification of the AAPM Task Group 21 protocol.
In light of recent questions and comments from the physics community, a review is made of the AAPM protocol for high-energy x-ray and electron beam dosimetry.
Biophysical Phenomena↗
Coordinate transformations and calculation of the angular and depth parameters for a stereotactic system.
Stereotactic systems have been used to assist in the precise implantation of radioactive sources in selected brain tumors. Use of such systems requires an algorithm that transforms spatial points in computed tomography coordinates into stereotactic frame coordinates. A simple algorithm performing the coordinate transformations, intended for inclusion in treatment-planning software packages for interstitial brain implants, has been developed. This algorithm was formulated using the geometrical configurations of the Brown-Roberts-Wells (BRW) stereotactic system. After the transformations, the BRW angular coordinates and depth specifying the probe direction, defined from the entry point to the target point, are determined from their respective cartesian coordinates. These angular coordinates and depth on the BRW stereotactic system allow accurate neurosurgical implantations of catheters into the brain, and thereafter the insertion of radioactive sources.
Brachytherapy↗
Field size dependence of wedge factors.
The radiation output in the presence of wedge filters is characterized by the wedge transmission factor and open beam field size factors. Conventionally, the wedge factor for high-energy photons is measured in a water phantom at depth of maximum dose for a reference field size. Experimental measurements on different wedges indicate that the wedge factors are a function of field size. An analysis of these data show that this is primarily caused by the change in scattered radiation from the treatment head in the presence of wedge filters. The change in phantom scatter and radiation backscattered to the monitor chamber are minimal. For 4- or 6-MV x rays with a 60 degrees wedge, the use of a single wedge factor measured for 10 cm X 10 cm field introduces errors of up to 3.5%, for a 16-cm-wide field. For a 20-cm-wide field with this wedge, the error is 7%. Thinner wedges exhibit less differences.
Humans↗
Reference dose rates for single- and double-plane 192Ir implants.
The proper selection of the reference dose rate is critical in the irradiation of a tumor or tumor bed using interstitial implant. The selected reference dose rate should result in the delivery of highly homogeneous dose throughout the target volume, adequately cover the target volume, and minimize irradiation of the surrounding normal tissues. In this paper, the influence of the reference dose rate on the adequacy of the irradiation of idealized target volumes using single- and double-plane 192Ir implants was studied in terms of three volumetric irradiation indices. The results show that maximum relative dose homogeneity can be attained if the proper reference dose rate is chosen. This isodose rate contour exhibits a width larger than the thickness of the target volume in the central plane. The lowest dose rate within the idealized target volume is not recommended as the reference dose rate to avoid a large volume of the surrounding tissues from receiving dose rates equal to or greater than this dose rate, and also the inhomogeneous irradiation of the target volume.
Brachytherapy↗
Optimization of parameters for fitting linear accelerator photon beams using a modified CBEAM model.
Measured beam profiles and central-axis depth-dose data for 6- and 25-MV photon beams are used to generate a dose matrix which represents the full beam. A corresponding dose matrix is also calculated using the modified CBEAM model. The calculational model uses the usual set of three parameters to define the intensity at beam edges and the parameter that accounts for collimator transmission. An additional set of three parameters is used for the primary profile factor, expressed as a function of distance from the central axis. An optimization program has been adapted to automatically adjust these parameters to minimize the chi 2 between the measured and calculated data. The average values of the parameters for small (6 X 6 cm2), medium (10 X 10 cm2), and large (20 X 20 cm2) field sizes are found to represent the beam adequately for all field sizes. The calculated and the measured doses at any point agree to within 2% for any field size in the range 4 X 4 to 40 X 40 cm2.
Humans↗
Electron beam characteristics on a Philips SL25.
Dosimetry measurements at nominal electron energies of 4, 6, 8, 10, 12, 15, 17, 20, and 22 MeV were made for different sized, open-sided applicators on two Philips SL25 linear accelerators. Measurements include beam flatness, percentage depth dose, surface dose, isodose curves, field size dependence, output at extended distances, virtual source position, and required low melting point alloy thickness for field shaping. These measurements are presented to document the characteristics of electron beams with a new type of applicator design on this series of Philips accelerators.
Electrons↗
Characteristics of photon beams from Philips SL25 linear accelerators.
The Philips SL25 accelerator is a multimodality machine offering asymmetric collimator jaws and a new type of beam bending and transport system. It produces photon beams, nominally at 6 and 25 MV, and a scattered electron beam with nine selectable energies between 4 and 22 MeV. Dosimetric characteristics for the 6- and 25-MV photon beams are presented with respect to field flatness, surface and depth dose characteristics, isodose distribution, field size factors for both open and wedged fields, and narrow beam transmission data in different materials.
Humans↗
Experimental verification of a three-dimensional dose calculation algorithm using a specially designed heterogeneous phantom.
A solid heterogeneous phantom made up of 25- and 50-mm cubes of materials with different electron densities was used to verify the accuracy of a three-dimensional (3-D) dose calculation algorithm. This algorithm uses 3-D information obtained from contiguous CT (computed tomography) slices, spaced 5 mm apart. Primary and scatter doses at a point are calculated by using information from ray-tracing CT voxels. The algorithm was developed on a Stardent model 1500 Supergraphic workstation. Cubes of materials with different electron densities were stacked up to simulate finite heterogeneities in three dimensions. This design allows verification of the algorithm for surface contour corrections and finite heterogeneities in the treatment field. Thermoluminescent lithium fluoride chips were placed in grooves milled on the cubes for dose measurement at various points. Different experiments were performed to investigate both the accuracy of the dose calculation algorithm and the utility of the versatile test phantom.
Algorithms↗