An adaptive imaging method of monitoring intrafraction target motion during radiation therapy is provided that includes using a simultaneous Mega-Voltage (MV) imaging process and Kilo-Voltage (KV) imaging process to determine an initial 3D target position. 2D target position is monitored using the MV imaging process during a radiation therapy treatment delivery, and is in combination with an online-updated characterization of target motion that are disposed to estimate if the target has moved beyond a 3D threshold distance. The simultaneous MV imaging and KV imaging processes are for accurately determining a new 3D target position for intrafraction motion compensation and for further 2D imaging by the MV imaging process, where another simultaneous MV and KV imaging process is initiated when the target has potentially moved beyond the threshold distance as measured by the MV imaging process. The intrafraction target motion monitoring is achieved at the cost of ultralow patient imaging dose.
