A NEW TECHNIQUE FOR POSITIONING TANGENTIAL FIELDS

Purpose: A technique that eliminates the use of a mechanical ''breast- bridge'' for positioning tangential fields for treatment of the intact breast or chest wall has been developed. Methods and Materials: Treat ment set-up parameters are determined using measuring capabilities (ga ntry angles and source-skin distances) available on a standard simulat or unit. A programmable scientific calculator is used to determine fie ld geometry from polar coordinates for various points on the patient's skin. The calculator program determines the field size, a depth and l ateral shift from a skin reference point to the isocenter for the tang ential fields, and the gantry angles. The program provides additional information which facilitates the simulation process: First, the coord inates of the isocenter for the tangential fields are expressed relati ve to couch coordinates for an initial arbitrary isocenter so that the ''auto go to'' capability available on some simulators can be used. S econd, the coordinates of the medial and lateral entry points can be e dited when the first set of tangents are not accepted. This part of th e program allows quick and efficient adjustment of the fields to obtai n adequate treatment volume coverage and a minimum of irradiated lung or heart. Results: Simulation of more than 300 patients has shown the technique to be a practical and efficient method for positioning tange ntial fields for breast or chest wall irradiation. Conclusion: The tec hnique described here takes full advantage of the capabilities of the new generation of computer controlled simulators, and offers an altern ative to previous methods employing a mechanical ''breast-bridge.''