Dosimetry of a kilovoltage radiotherapy x-ray machine

We present the percent depth doses, half-value thicknesses, exposure rates in air, dose uniformity, backscatter factors, isodose curves, and penumbra widths for 75-kVp, 100-kVp, 150-kVp, 200-kVp, and 250-kVp beams for rectangular cones (4 cm x 6 cm, 6 cm x 8 cm, and 8 cm x 10 cm) and exposure rates in air and backscatter factors for cylindrical cones (diameters 2 cm, 3 cm, and 3.5 cm) with flat and bevelled ends for the Philips RT-250 x-ray machine. Published dosimetry data specific to this machine are limited to percent depth doses and beam qualities. Rectangular cone percentage depth dose curves and beam quality agreed with published values. While measured backscatter factors for large rectangular fields generally agree with calculated and published values, smaller field backscatter factors were less than those reported by others. Monte Carlo calculations for small field backscatter factors appear more accurate than those measured.