Figure 10: Percentage depth dose curves of a 6-MV, 40 × 40-cm2 photon beam measured using a Markus parallel plate ionization chamber inserted in a 20 cm thick solid water phantom on the floor. A 1 cm thick acrylic beam spoiler was placed above the floor at a distance of 30 cm. The gantry angle was set at (a) 0° without a spoiler, (b) 0° with a spoiler, and (c) 11° with a spoiler.