A three-film technique for reconstruction of radioactive seed implants

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Abstract
Computer dose calculations for interstitial implants of radioactive seeds [used in the treatment of localized tumors] require a knowledge of the spatial coordinates of each seed in the implant. These coordinates are usually constructed from the seed images on either an orthogonal or a stereo pair of radiographs. Such a procedure requires that each seed can be identified on each film. A computer algorithm was developed which computes the locations of the seeds from a set of 2 stereo and 1 [anteroposterior] AP radiographs, even when seed identities on all films are indeterminable. This is done by means of a ray tracing technique. Under ideal conditions the projections from the 3 films uniquely define all seed locations. Under clinical conditions where digitizing uncertainties and patient movement are inevitable, the algorithm must determine the most likely set of seed locations from the larger set of all possible seed locations. The criteria for making this selection, as well as clinical examples, are presented.

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