The self-complementary technique has been employed to widen the range of operating frequencies for the planar rectangular monopole antenna, which has been designed in a previous work with two configurations according to its feeding position. The complementary structure enables the designed antenna to be used in medical imaging application for detecting a cancerous tumor in a simulated human breast. Two breast phantom models have been simulated in the CST software environment. The detection technique is mainly depended on mapping the Specific Absorption Rate (SAR) values. The coordinates of maximum SAR value have been identified to detect the positions of the tumor precisely within the breast. Simulation results shown that the tumor inside the breast can be detected, accurately