While stereotactic radiosurgery (SRS) was originally conceived as a single-treatment technique [1], the case for re-irradiation in SRS has grown as a method to manage cases of disease progression after traditional large field treatments [2–4], and local failures [5–9], as well as incomplete response [10]. The high success rates of SRS have also led to attempts to treat larger tumors or tumors very close to sensitive normal tissue structures [11–15].