A technique for using low melting point alloy for individual patient shielding in radiotherapy

Individual patient shielding has been used in radiotherapy at a number of treatment centres for a number of years, particularly in the mantle treatment of Hodgkin's disease where accurate individual shielding is regarded as therapeutically significant (Edland and Hansen, 1969; Fordham, Hibbs and Hendrickson, 1969; Van Dorssen, Mellink and Thomas, 1970; Page, Gardner and Karzmark, 1970). Such individual shielding has generally been made in one of the following three ways: (a) Solid lead shielding blocks individually cast (i) or cut out (ii) for each patient (Fordham et al., 1969; Page et al., 1970). (b) Individual shielding made up from an aggregate of lead shot and a low melting point wax or thermoplastic (Edland and Hansen, 1969). (c) Individual shielding blocks cast in a low melting point alloy. All three schemes usually involve the preparation of individual cut-out moulds for each patient. At many centres, including Mount Vernon, a hot wire is used to cut the mould outlines in expanded polystyrene blocks (Edland and Hansen, 1969). The upper end of the cutting wire is secured so that the slope on each side of the resulting shielding block is parallel to the primary beam direction, so minimizing the penumbra cast by the shielding blocks during treatment. A cursor attached to the lower end of the wire is moved slowly around the block outline drawn on a diagnostic film taken on the simulator.