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The effects of annular and conical inhomogeneities upon the central-axis scatter dose in a cobalt 60 photon beam

University of British Columbia

Tissue inhomogeneities can significantly alter the radiation dose delivered to a tumor. In the past, the accuracy of dose calculations has been limited by a lack of precise knowledge of the inhomogeneities within the patient's anatomy. Such anatomical information is now available in much greater detail due to the widespread use of computed tomography (CT) in radiotherapy treatment planning. Consequently, the potential for more accurate dosimetry has grown. The difficulty now resides iii being able to efficiently handle this CT data. The effect of an inhomogeneity upon the primary dose is well known. That upon the scatter is not. Therefore, we have limited our study to that of the alteration in the scatter dose. The perturbation is a complicated function of the density and position of the inhomogeneity, information which is available from CT data. A method, known as the differential Batho method, has been developed to estimate the scatter dose change. It provides a calculation of the effects due to an inhomogeneity in the shape of an annulus, or a cone's frustum, concentric about the central axis. This method allows the effects of multiple inhomogeneities to be estimated. Several series of experiments were performed to measure the effects of such inhomogeneities, in a ⁶⁰Co beam, as functions of density and geometry. It was found that for many of the geometries tested, the differential Batho method gave a good account of the changes measured. The deviations that occurred were readily accounted for. The differential Batho method provides a good measure of the scatter effects for simple symmetric geometries. In this thesis, it is recommended that further studies be made with phantoms simulating the human anatomy to test the method for specific treatment techniques.

Text

2010-03-26

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http://hdl.handle.net/2429/22570

Physics

University of British Columbia

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