The invention provides for a medical apparatus (100, 300, 400) comprising a processor (104). Execution instructions (120, 122, 124, 370, 372, 374, 376, 378, 380, 382, 384, 386) causes the processor to receive (200) a treatment plan (120) descriptive of an ionizing radiation beam treatment of a target zone (340) within a subject (320) using a radiation beam therapy system (301) with a radiation source (354) and a gantry (350). The radiation source is operable for aiming a radiation beam path (358) at a rotational axis (352) of the gantry. The instructions further cause the processor to receive (202) a planning thermal distribution (122) descriptive of ultrasound heating of the subject with a high intensity focused ultrasound system (304) of a heating zone (338) and an ultrasound beam path (332) within the subject. The target zone is within the heating zone. The instructions further cause the processor to generate (204) radiation control command data (124) using the planning thermal distribution and the treatment plan. The radiation control command data is executable to cause the radiation source to reduce irradiation of the target zone when more than a predetermined volume of the radiation beam path intersects the ultrasound beam path.
