During the implementation of clinical trials NCT00950001 and NCCTG/N107C/CEC.3 on post-operative stereotactic radiosurgery into clinic, it brought us some thinking of fundamental concept in science that the local control rate and survival rate rely on the treatment of marginal region more than resection cavity. Marginal region might still contain residual cancer cell while the resection cavity contains only water fluid most time. Radiation treatment should focus more on the margin rather than the cavity, thus treating cancer rather than water.