Determination of the relative angle between orthogonal films for radioactive implants

Dose calculations for radioactive seed implants require knowledge of the three-dimensional source coordinates. These are usually determined from a pair of orthogonal radiographs. However, occasionally localization films are taken with portable, and/or nonisocentric x-ray units, and no assurances can be made as to whether or not the films were truly exposed in an orthogonal geometry. A three-dimensional magnification ring is described (consisting of three mutually orthogonal rings) which permits the treatment planner to accurately establish the exact source rotation angle used when exposing the films. This angle can in turn be used when reconstructing source coordinates, to insure accuracy in treatment planning.