We report a fiber-optic-based ultrafast time-stretch laser detection and ranging (Lidar) sensor with 10 MHz speed and 10 μm accuracy with 30 mm dynamic range for head motion detection under the thermoplastic mask during image-guided radiotherapy procedures. The sensor (1) is miniaturized and fits under the mask, (2) is small enough not to cause attenuation in radiation beams, (3) has a spatial resolution of a tenth of a millimeter, (4) is real-time, and (5) is immune to electromagnetic radiation.