Geometry of the phantom verification. The X-ray attenuation of mouthpiece materials (a) and the effect of dental impression materials on the PDD (b) were evaluated with a water-equivalent phantom. The polypropylene resin sheets were placed 5 mm deep and changed at 3 mm intervals up to 9 mm for each measurement. An ionization chamber was installed at 30 mm depth with the phantom at 50 mm depth for the measurement of the absorbed dose to water. Two types of dental impression materials were molded into 10 mm-thick plates and placed at 40 mm depth in the phantom.