TREATMENT PLANNING WITH 2 MV X-RAYS: SOME TECHNICAL ASPECTS OF DOSIMETRY AND BEAM DIRECTION
Problems of dosimetry and beam direction encountered in using a 2-Mev Van de Graaff unit are discussed. Techniques axe examined for reducing errors that may occur when depth dose data, as published, are used for patient dosimetry. It was found practical to correct routinely for patient curvature with the tissue contour compensator method of Hall and Oliver. An aluminum compensating filter is used to correct irregular patient contours to an approximates plane surface at right angles to the central ray of the treatment beam. Transit dose corrections for tissue inhomogeneity are made by comparison of doses measured in a phantom with calculated doses that neglect the tissue inhomogeneity error and construction of a table of standard correction factors for standard treatment plans. With a focal-chamber distance of 300 cm the ratio of transmitted dose to air dose was measured for several field sizes with cylindrical water absorbers of varying diameter. When the fractional transmitted dose was plotted against field size, the slope of the curve, 0.0751 cm/sup -1/, is the transmission coefficient of the beam in water when the effect of scattered radiation has been eliminated. This transmission coefficient is called the zero area transmission coefficient. The new design gives a greater exposure rate but a lower halfvalue thickness (12.0 cm Cu) at 2 Mv. Clinical application of the transit dose method is described and examples of its use presented. The Du Sault system for the construction of standard summation isodose curves that are normalized to a predetermined source-to-target distance was extended to 2-Mev x-ray beams, Using basic isodose data, dose curves nomralized to a target distance of 100 cm were constructed and a library of fixed field summation isodose distributions was calculated. Typical standard isodose distributions are shown and applications of the distribution to a particular patient illustrated. Treatment techniques using wedge filters were explored. Two treatment fields that have been shaped by wedge filters can be applied at 90 deg separation to produce an almost ideal dose distribution for small lesions restricted to one quadrant in the head or neck. By use of ionization chambers in a Pressdwood phantom, isodose curves were plotted for the 2-Mev beam with 2 Cu wedge filters, 7 and 11 cm wide. It was found that the calculated and experimental curves had very nearly the same slope but that the depth dose values disagreed by as much as 8%. Final isodose curves were made in which the calculated curve shapes and the experimental depth dose values were used. The use of fields shaped with wedge filters for the treatment of a lesion in the maxillary sinus is illustrated. Comparison with the summation isodose curves that would have been obtained if the wedge filters had not been used showed the improvement in dose homogeneity within the target volume and the reduction of dose outside the target volume that can be obtained with the wedge filter technique. The accurate control of beam direction is considered with regard to two factors: the point and the angle of entry of the axis of the beam, A back-pointer developed for use with the Van de Graaff unit is shown. The pointer, located on the wall opposite the therapy unit, is raised or lowered until the reading on its ventical height scale corresponds to that on the treatment head. To ease the time demand on the Van de Graaff treatment facility, a treatment simulator, which is shown, is installed in another room. The beamlocalizing accessories for the simulator, including backpointer and pin and arc, are exactly the same and have the same position relative to the focal spot as those of the treatment unit, but the beam is provided by a 100-kv diagnostic tube. (BBB)
- Research Organization:
- Univ. of Pennsylvania, Philadelphia
- NSA Number:
- NSA-18-011925
- OSTI ID:
- 4117796
- Journal Information:
- Radiol. Clin. N. Am., Vol. Vol: 1; Other Information: Orig. Receipt Date: 31-DEC-64
- Country of Publication:
- Country unknown/Code not available
- Language:
- English
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