The present application relates to a radiotherapy apparatus for delivering radiation to a subject. The apparatus comprises a source of radiation configured to rotate about an isocenter and emit radiation in a radiation plane containing said isocentre. The apparatus also comprises a subject support surface configured such that a portion of the subject support surface can be located substantially at the isocenter. The apparatus also comprises a subject support surface rotation mechanism configured to rotate the subject support surface about an axis of rotation that passes through the isocenter, wherein the subject support surface rotation mechanism is located outside the radiation plane.

Described herein are variations of a radiotherapy patient motion detection system. For example, a patient motion detection system may include a sensor coupled to a patient support platform for use in measuring patient motion during a radiotherapy treatment session. The motion sensor may transmit measurements to a controller for processing and subsequent use in timing delivery of radiation to a treatment region of interest within a patient. Also described herein are methods of use of a patient motion detection system.