LEAD BEADS: THE USE AS THE IRREGULAR FIELD SHIELDING

Abstract

The use of lead beads in the irregular field shielding to replace the normal custom block is presented. By studying the shielding efficiency of two bead sizes (3.52±0.37 mm and 5.13±0.43 mm in diameter) with two packing methods, pressure and unpressure packing. From the study, it revealed that the small beads with pressure packing method (HVL=1.6 cm) has a little higher shielding efficiency depending on the dense of the beads. To provide the convenience in a practical use, the two perspex trays were designed to mounted with the collimator beam shaping rails (MURT 37/1) and table mounted beam shaping tray (MURT 37/2) to fit to the treatment couch of the Cobalt-60 machine for the larger field size. Test of accuracy and reproducibility in shielding position were accomplished by radiographic film. Comparison of shielding effect between the shielding blocks made by alloy and lead beads are also presented. Advantages in the use of lead beads are the ease and rapidity construction. Moreover, it is economical and no need of melting in the re-used. The only problem is that much care has to be taken for the correct position of the shielding block. It can be concluded from the study that the lead bead is practical for shielding irregu- lar shaped field, especially in an urgent situation in megavoltage therapy.