Poster - Wed Eve-51: Geometric Plan Adaptation for Rotational Radiation Therapy

Abstract

Inter-fractional position uncertainty of patient anatomy limits the radiation dose we can deliver, requiring us to compromise the margins of the planning target volume (PTV) and/or the prescription dose to maintain a low risk of severe complications. Spinal cord is often the critical organ and the spine is difficult to position consistently even with vac-loc bags. A difference in curvature of the spine between planning and each treatment can lead to geometric changes that cannot be corrected by couch movements, requiring an adaptation of the aperture shapes. Intensity Modulated Arc Therapy (IMAT) has stronger geometrical influences than IMRT, and therefore, potentially more amenable for geometric adaptive strategies and has not been studied. We propose to use online imaging to locate the spinal canal and use geometric and algebraic adaptive algorithms to adapt an IMAT plan in real time to better shield the spinal cord while preserving the required dose to the PTV. Test results on a C-shaped phantom demonstrated that the maximum dose to the cord (originally 114cGy per fraction) became 119cGy and 141cGy with and without plan adaptation. A new plan created on the shifted cord geometry had a maximum cord dose of 134 cGy. The maximum and minimum PTV doses respectively were 210 cGy and 149 cGy in the initial plan, 214 cGy and 109 cGy after adaptation, while the new plan was 210 cGy and 157 cGy. In summary, geometric and algebraic adaptive algorithms maintains the original plan better than a de-novo plan based on shifted geometries.