Radiation therapy is a central modality in the treatment of glioblastoma multiforme (GBM). Integral to adequate radiation therapy delivery is the appropriate determination of tumor volume and extent at the time treatment is being delivered. As a matter of routine practice, radiation therapy treatment fields are designed based on tumor volumes evident on pre-operative or immediate post-operative MRIs; another MRI is generally not obtained for planning boost fields. In some instances the time interval from surgery to radiotherapy initiation is up to 5 weeks and the boost or "cone-down phase" commences 4-5 weeks later. The contrast enhanced T1 MRI may not be a totally reliable indicator of active tumor, especially in regions where such blood-brain barrier breakdown has not occurred. Moreover, these volumes may change during the course of treatment. This may lead to a geographic miss when mid-treatment boost volumes are designed based on a pre-radiotherapy MRI. The goal of this study is to examine how a mid-treatment MRI impacts the delineation and definition of the boost volume in GBM patients in comparison to the pre-treatment MRI scan, particularly when the tumor-specific agent Motexafin-Gadolinium is used.