A radiation therapy system, comprising: a plurality of fiducials (22, 90) locatable internal to a subject; an ultrasound imaging unit which includes a transducer configured to emit ultrasonic sound waves to image in real-time an anatomic portion of a subject in a real-time coordinate system; a radiation planning unit configured to use at least one radiation treatment planning image to identify planned treatment locations and planned amounts of therapeutic radiation to be delivered to anatomic portions of the subject in a planning coordinate system; an image fusion unit configured to register the real-time coordinate system to the planning coordinate system based on the locations of the internal fiducials and to track the locations of the fiducials in real-time; and a delivery evaluation unit configured to identify the locations of the fiducials and the amounts of delivered therapeutic radiation relative to the imaged real-time anatomic portion of the subject, characterised in that: each of the plurality of fiducials includes: a wireless transmitter configured to transmit at least a self-identity; a sensor connected to the transmitter which includes a piezoelectric element activated in response to receiving the emitted ultrasonic sound waves and which powers the transmitter; and a capsule constructed of biocompatible material which encapsulates the wireless transmitter and the sensor; and wherein the delivery evaluation unit (3 8) is configured to identify the locations of the fiducials based on the transmitted self-identities and the amount of radiation based on the radiation delivery mechanism, and wherein the image fusion unit is configured to determine the locations of the fiducials relative to the ultrasound transducer based on a time between emission and receipt of the ultrasonic sound waves, and a direction of one or more emitted ultrasonic sound waves and the amplitude of the received one or more emitted ultrasonic sound waves; and wherein the radiation therapy system further comprises at least one antenna located external to the subject and configured to receive the transmitted self-identities. a plurality of fiducials (22, 90) locatable internal to a subject; an ultrasound imaging unit which includes a transducer configured to emit ultrasonic sound waves to image in real-time an anatomic portion of a subject in a real-time coordinate system; a radiation planning unit configured to use at least one radiation treatment planning image to identify planned treatment locations and planned amounts of therapeutic radiation to be delivered to anatomic portions of the subject in a planning coordinate system; an image fusion unit configured to register the real-time coordinate system to the planning coordinate system based on the locations of the internal fiducials and to track the locations of the fiducials in real-time; and a delivery evaluation unit configured to identify the locations of the fiducials and the amounts of delivered therapeutic radiation relative to the imaged real-time anatomic portion of the subject, each of the plurality of fiducials includes: a wireless transmitter configured to transmit at least a self-identity; a sensor connected to the transmitter which includes a piezoelectric element activated in response to receiving the emitted ultrasonic sound waves and which powers the transmitter; and a capsule constructed of biocompatible material which encapsulates the wireless transmitter and the sensor; and a wireless transmitter configured to transmit at least a self-identity; a sensor connected to the transmitter which includes a piezoelectric element activated in response to receiving the emitted ultrasonic sound waves and which powers the transmitter; and a capsule constructed of biocompatible material which encapsulates the wireless transmitter and the sensor; and