Phantoms made of synthetic materials are commonly used in conformational radiotherapy to verify the proper operation of therapeutic medical equipment. The main objective of the paper is to design and build a prostate phantom to be used in the calibration of planning systems. The proposed phantom is made of nylon and acrylic. These inexpensive materials are readily available in Peru and have comparable density to soft tissue and bone. The phantom was used in the calibration of a planning system. Final results showed an average error of 5.49% between the measurements made on the phantom and planned values. These results are in the range of uncertainty recommended by the International Commission on Radiation Units and Measurements (ICRU).© 2012 IEEE.