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The determination of effective thicknesses from transmission measurements with gamma rays Goulet, Pierre

Abstract

The usual starting-point for clinical dosimetry is the dose distribution due to a radiation beam in a homogeneous medium like water. The modification of the standard distribution for the case of an inho-mogeneous medium (patient) requires detailed information about internal structures. For heavy charged particle beams and for photon beams of high enough energy, the single parameter which most nearly determines the penetration of the radiation is the electron density (electrons/ cm³). The integrated electron density along a path (electrons/cm²) is uniquely related to the narrow-beam γ-ray transmission along that path. Hence, a measurement of γ-ray transmission should provide useful information. This work describes a new technique for performing these measurements. We have used a ⁶⁰Co therapy source with a rectangular field long enough in one direction to span the width of a human cross section and narrow in the other direction to minimize the effects of scattered radiation. Rather than collimate our detectors, we have devised a simple mathematical procedure to calculate the scatter contribution at each point of interest along the width of the radiation field. Narrow-beam transmission values can then be evaluated from the original measurements by simple substraction of this scatter contribution. An ionization chamber, because of its energy independence and accuracy, was used to acquire the basic transmission data. X-ray film and silicon diodes, because of their superior resolution and response speed, were selected for the measurements performed with an inhomogeneous phantom. The energy dependence of the latter dosimeters has not been a drawback when used in the experimental conditions described here. The method thus permits that rapid transmission measurements be performed along various paths of a transverse cross section. When tested with an inhomogeneous phantom, the accuracy obtained compares favourably with published values using collimated detectors. Information obtained by this method is directly applicable to the dosimetry of treatments performed with photon beams of energy greater than 0.6 MeV and heavy charged particle beams like protons and π⁻ mesons.

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