1. A method of monitoring intrafraction target motion during radiation therapy, comprising: a. using a first simultaneous Mega-Voltage (MV) imaging process and Kilo-Voltage (KV) imaging process to determine an initial 3D target position, wherein said MV imaging process comprises a MV imager and said KV imaging process comprises a KV imager; b. monitoring a 2D target position using said MV imaging process during a radiation therapy treatment delivery, wherein said 2D target position monitoring is in combination with an online-updated characterization of target motion, wherein said 2D target position monitoring and said online-updated characterization of said target motion are disposed to estimate if said target has moved beyond a 3D threshold distance; and c. using another said simultaneous MV imaging process and KV imaging process to accurately determine a new 3D target position for intrafraction motion compensation and for further 2D imaging by said MV imaging process, wherein said another simultaneous MV imaging process and KV imaging process is initiated when said target has potentially moved beyond said threshold distance as measured by said MV imaging process, wherein said intrafraction target motion monitoring is provided at a cost of ultralow patient imaging dose.