The main objective of this work is to design a prostate phantom to calibrate the planning system for treatment using conformation radiotherapy. The phantom is made of acrylic and nylon. These materials have densities similar to soft tissues and bone, and are readily available in Peru with low cost. The phantom is imaged using a calibrated CT scanner. The CT images are used for the calculation of the absorbed dose using a three-dimensional planning system (WINPLT-3D). This calculation is compared to the experimentally measured data in the phantom by means of the clinical linear accelerator Varian 2100 C/D. Preliminary results show that the planned dose and the measured dose differ in less than ± 3.0%, which suggests that the three-dimensional planning system has been correctly calibrated. © Springer-Verlag Berlin Heidelberg 2007.