Dosimetric characteristics of wedges mounted beyond the blocking tray

In a beam accessory configuration for a linear accelerator using a prototype multileaf collimator, newly designed wedges were mounted beyond the blocking tray. The isodose curves, depth of maximum dose, surface dose, and wedge transmission factors were measured for the wedges designed for this unique configuration. The same set of wedges was used for both 6- and 18-MV x rays. The shape of the wedged isodose curves was essentially unchanged from those produced by the conventional wedges located above the blocking tray. The isodose curves exhibited the desired wedge angles over the range of field sizes from 5 x 5 to 15 x 40 cm. In the 10 x 10-cm field, the average difference between the observed wedge angle and the desired wedge angle was 3.8 degrees. The surface doses ranged from 18% to 35% for the wedged 10 x 10-cm fields as compared with about 15% for the same open field. Dosimetrically the wedges were acceptable for clinical use.