An ultrasonic diagnostic imaging system has a thin two dimensional array transducer probe which is taped or belted to a patient to image a target region during radiotherapy. The radiotherapy procedure is conducted based upon planning done based on images of the target region acquired prior to the procedure. The array transducer is operated by an ultrasound system to produce three dimensional images of the target region by electronic beam steering either during or between fractions of the treatment procedure. The ultrasound images are used to adjust the treatment plan in response to any movement or displacement of the target anatomy during the treatment procedure.
