A system (1010, 510, 510', 610, 610") for localization of a marker (540, 640) within a target tissue region within a patient's body, comprising: a probe (1020, 930) comprising one or more antennas (532, 950, 560t, 560r, 560', 660, 660') for transmitting a radar signal (1034T, 570, 522a', 622) into a patient's body towards a marker (40, 140) and receiving a reflected signal (1034R, 574) from the marker, and a light source for delivering infrared light pulses into the patient's body to cause the marker to change its reflective properties; a signal generator (620, 520, 520') for generating a substantially continuous wave; characterised in that the system further comprises a divider coupled to the signal generator for dividing the wave into first (522a, 622a) and second (522b, 622b) signals, the first signal delivered to the one or more antennas to transmit a substantially continuous transmit signal; a mixer (526, 526', 626I, 626Q, 626I', 626Q') coupled to the divider for receiving the second signal and coupled to the one or more antennas for receiving the reflected signal, the mixer mixing the second signal and the reflected signal (574, 674) to produce an intermediate frequency signal comprising components associated with modulation of amplitude and phase of the reflected signal caused by the light pulses changing the reflective properties of the marker; and a processor (660, 530, 530', 630, 630") coupled to the mixer comprising a synchronous modulation detector that processes the IF signal to provide an output indicative of range from the one or more antennas to the marker based at least in part on the modulation of amplitude and phase synchronous with the light pulses delivered by the light source. a probe (1020, 930) comprising one or more antennas (532, 950, 560t, 560r, 560', 660, 660') for transmitting a radar signal (1034T, 570, 522a', 622) into a patient's body towards a marker (40, 140) and receiving a reflected signal (1034R, 574) from the marker, and a light source for delivering infrared light pulses into the patient's body to cause the marker to change its reflective properties; a signal generator (620, 520, 520') for generating a substantially continuous wave; characterised in that the system further comprises a divider coupled to the signal generator for dividing the wave into first (522a, 622a) and second (522b, 622b) signals, the first signal delivered to the one or more antennas to transmit a substantially continuous transmit signal; a mixer (526, 526', 626I, 626Q, 626I', 626Q') coupled to the divider for receiving the second signal and coupled to the one or more antennas for receiving the reflected signal, the mixer mixing the second signal and the reflected signal (574, 674) to produce an intermediate frequency signal comprising components associated with modulation of amplitude and phase of the reflected signal caused by the light pulses changing the reflective properties of the marker; and a processor (660, 530, 530', 630, 630") coupled to the mixer comprising a synchronous modulation detector that processes the IF signal to provide an output indicative of range from the one or more antennas to the marker based at least in part on the modulation of amplitude and phase synchronous with the light pulses delivered by the light source.