Disclosed is a system for detecting an internal object in a body. The system includes at least one antenna pair including two antennas which are adapted to be symmetrically positioned around the body in relation to a line of symmetry. The system is adapted to transmit microwave signal into the body which are reflected and/or scattered from the internal object. The system is adapted to receive the reflected and/or scattered microwave signals, and to compare the received microwave signals at the symmetrically positioned antennas. The system is adapted to detect the internal object or a change in an already detected internal object when there is a difference between the received microwave signals at symmetric antenna pairs. The difference is related to the different dielectric properties between the internal object and the body.