Disclosed herein are systems and methods for real-time monitoring of patient position and/or location during a radiation treatment session. Images acquired of a patient during a treatment session can be used to calculate the patient's position and/or location with respect to the components of the radiation therapy system. One variation of a radiation therapy system includes a circular gantry with a rotatable ring coupled to a stationary frame, a therapeutic radiation source mounted on the rotatable ring, and a patient-monitoring imaging system mounted on the rotatable ring. The patient-monitoring system may have one or more image sensors or cameras disposed on the rotatable ring within a bore region of the radiation therapy system, and may be configured to acquire image data as the ring rotates.