A medical apparatus (100, 300, 400) comprising: - a processor for controlling the medical apparatus, - a memory containing machine executable instructions (120, 122, 124, 370, 372, 374, 376, 378, 380, 382, 384, 386) for execution by the processor, wherein execution of the machine executable instructions causes the processor to: - receive a treatment plan descriptive of an ionizing radiation beam treatment of a target zone within a subject using a radiation beam therapy system, wherein the radiation beam therapy system comprises a radiation source and a gantry for rotating the radiation source about a rotational axis, wherein the radiation source is operable for irradiating a radiation beam path, wherein the radiation source is operable for aiming the radiation beam path at the rotational axis; - receive a planning thermal distribution descriptive of ultrasound heating of the subject with a high intensity focused ultrasound system, wherein the planning thermal distribution is spatially dependent, wherein the planning thermal distribution is descriptive of a heating zone and an ultrasound beam path within the subject, wherein the target zone is within the heating zone; and - generate radiation control command data for controlling the radiation beam therapy system to irradiate the target zone using the planning thermal distribution and the treatment plan, wherein 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. - a processor for controlling the medical apparatus, - a memory containing machine executable instructions (120, 122, 124, 370, 372, 374, 376, 378, 380, 382, 384, 386) for execution by the processor, wherein execution of the machine executable instructions causes the processor to: - receive a treatment plan descriptive of an ionizing radiation beam treatment of a target zone within a subject using a radiation beam therapy system, wherein the radiation beam therapy system comprises a radiation source and a gantry for rotating the radiation source about a rotational axis, wherein the radiation source is operable for irradiating a radiation beam path, wherein the radiation source is operable for aiming the radiation beam path at the rotational axis; - receive a planning thermal distribution descriptive of ultrasound heating of the subject with a high intensity focused ultrasound system, wherein the planning thermal distribution is spatially dependent, wherein the planning thermal distribution is descriptive of a heating zone and an ultrasound beam path within the subject, wherein the target zone is within the heating zone; and - generate radiation control command data for controlling the radiation beam therapy system to irradiate the target zone using the planning thermal distribution and the treatment plan, wherein 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. - receive a treatment plan descriptive of an ionizing radiation beam treatment of a target zone within a subject using a radiation beam therapy system, wherein the radiation beam therapy system comprises a radiation source and a gantry for rotating the radiation source about a rotational axis, wherein the radiation source is operable for irradiating a radiation beam path, wherein the radiation source is operable for aiming the radiation beam path at the rotational axis; - receive a planning thermal distribution descriptive of ultrasound heating of the subject with a high intensity focused ultrasound system, wherein the planning thermal distribution is spatially dependent, wherein the planning thermal distribution is descriptive of a heating zone and an ultrasound beam path within the subject, wherein the target zone is within the heating zone; and - generate radiation control command data for controlling the radiation beam therapy system to irradiate the target zone using the planning thermal distribution and the treatment plan, wherein 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.