Radiation therapy systems and methods using an external signal

A method for estimating the positon of a target in a body of a subject, comprising:

• receiving an external signal that is related to motion of the target;
• using a model of a correlation between the external signal and the motion of the target to estimate the position of the target, wherein said position estimation includes both an estimate of three dimensional location of the target and orientation of the target at the three dimensional location, whereby position is estimated for six degrees of freedom of movement of the target including both translational movement and rotational movement;
• periodically receiving a 2-dimensional projection of the target;
• updating the model of correlation between the external signal and the motion of the target based on a comparison of the estimated position of the target and the 2-dimensional projection of the target.