An apparatus and method for determining a target-to-surface distance (TSD) between a target region in a body and an actual point of entry of a radiation beam into the body from a radiation source. The method may include determining an absorbed dose of radiation from the radiation beam at the target region using the TSD. The method may also include compensating for both the motion of the target region with respect to the radiation source, and the motion of surrounding tissue relative to the target region. The apparatus may include a sensor system to determine the actual point of entry of the radiation beam into the body.