A medical apparatus (100, 300, 400, 800) includes a magnetic resonance imaging system (104), a radiation therapy device (102) having a gantry (106) and a radiation source (110). A radiation detection system (102) measures radiation detection data (174) descriptive of a path and intensity of a radiation beam at an intersection of the radiation beam with at least one surface (144, 144′, 144″) surrounding the subject using at least one radiation detector (144, 144′, 144″). Execution of machine readable instructions causes a processor controlling the medical apparatus to: receive (200) a treatment plan (168), acquire (202) magnetic resonance data (164) from the imaging zone using the magnetic resonance imaging system, generate (204) radiation therapy device control commands (172) using the magnetic resonance data and the treatment plan, irradiate (206) the target zone by controlling the radiation therapy device using the radiation therapy device control commands, measure (208) the radiation detection data during irradiation, and determine a time dependent radiation beam path (176) and a time dependent radiation beam intensity (178) using the radiation detection data.