Investigation of CT Dose Distributions with Cylindrical PMMA Phantoms

The purpose of this study is to investigate the effect of patient size on the distribution and amount of CT dose based on standard cylindrical polymethyl methacrylate (PMMA) phantoms. In this study two dimensional film dosimetry method is represented by using radiographic films, and the effect of patient size on the distribution and the received dose amount is investigated using cylindrical phantoms with 10, 16, 24 and 32cm diameters. The cylindrical phantoms were made of PMMA sheets. By proper calibration, two dimensional distribution of the dose is measured using film dosimetry in these phantoms in a single axial rotation. The dose distribution in the cylindrical phantoms was not symmetric particularly in closer points to the surface. The received dose in the cross section of phantom increases with beam width. The received dose in the central part of phantom is decreased more than its surface by the increment of phantom diameter. Specially, when using a cylindrical phantom with 16cm diameter, the received dose in the central part of the phantom is decreased to 65% of its surface's dose, whereas this percentage for cylindrical phantoms with 24cm and 32cm diameters is 45% and 35% respectively. The dose distribution in the cross section of the pediatric patients is more uniform. It is highly dependent on the couch attenuation and overscan, leading to the dose distribution asymmetry. The received dose in the pediatric patients will be more than adults for the same mAs.