Single-Field Weighting of Arc-Rotation Technics

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Abstract
Arc rotation is a useful technic in radiation therapy. A single are, however, displaces the isodose contours toward the radiation source, away from the axis of rotation. One method for circumventing this difficulty is to oppose the arc with a single fixed field. The fixed-field weighting factor can be chosen so that any desired isodose contour becomes symmetrical about the axis. If the value of the isodose contour that is to be made symmetrical about the axis is b, and if the distance from the axis to this contour is x1, and if any opposed central ray dose distributions, normalized to the axis of rotation, are represented by f(x) and g(x), then it can be shown that Equation (1) is solved graphically for x1 and the weighting factor, A, is found from

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