Rotation Effects on the Target-Volume Margin Determination
  Rotational setup errors are usually neglected in most clinical centers. An analytical formula is developed to determine the extra margin between clinical target volume (CTV) and planning target volume (PTV) to account for setup errors. The proposed formula corrects for both translational and rotational setup errors and then incorporated into margin determination for PTV. 
Qinghui Zhang, Weijun Xiong, M. Chan, Y. Song, C. Burman
physics.med-ph