Purpose/Objective(s) IDH-wildtype glioblastoma (GBM) has an exceptionally poor prognosis due to its rapid growth. Delaying post-operative radiation therapy (RT) more than 6 weeks after surgery negatively impacts prognosis. Post-operative magnetic resonance imaging (MRI) is commonly used to assess the extent of surgical resection followed by a delayed MRI for RT planning. In a heterogeneous population of high-grade gliomas treated in the pre-molecular era, significant resolution of edema and a trend towards increase in surgical cavity sizes were observed. Post-operative volumetric changes in IDH-wildtype GBM have not been described and we suspect these tumors may demonstrate higher rates of recurrence or regrowth during the latent period before the initiation of RT. Materials/Methods 30 patients with IDH-wildtype GBM treated with post-operative RT from our institution were identified. Patients with RT planning MRIs obtained between 3-6 weeks after biopsy or surgical resection were included. RT target volumes were contoured and compared between post-operative and RT planning MRIs. Volumetric changes in GTV2 (surgical cavity + residual enhancing disease) and GTV1 (edema + GTV2) were calculated. Progression-free survival (PFS) and overall survival (OS) were measured. Results The mean time between post-operative and RT planning MRI was 29.4 days (range: 21-42 days). When comparing post-operative and RT planning MRIs, the mean decrease in GTV1 was 25.8cc or 14.7% (P = 0.011) and the mean increase in GTV2 was 10.21cc or 39.6% (P = 0.0013). GTV1 decreased at an average rate of 0.5% per day and GTV2 increased at an average rate of 1.4% per day. The proportional growth was similar to a rate of historical controls of mixed high-grade gliomas. Median PFS and OS in our group were 5.6 and 11.2 months, respectively. A delay of more than one month between post-operative and planning MRI was associated with worse median OS of 8.0 months compared with 11.5 months (P = 0.044). Conclusion To our knowledge, this is the largest volumetric study of the impact of post-operative MRIs in a uniform group of IDH-wildtype GBM. RT planning MRIs performed 3-6 weeks after biopsy or surgical resection of IDH-wildtype GBM demonstrate significant volumetric changes. Similar to previously published data on a heterogenous population of high-grade gliomas, decreases in GTV1 signify a resolution of edema, however, increases in GTV2 imply recurrence or regrowth of disease over a relatively short time. While this was not statistically significant in a previously published analysis of a heterogeneous population of high-grade gliomas, our analysis of a homogeneous population of IDH-wildtype GBM demonstrates a statistically significant increase in GTV2. Delaying planning MRI more than one month post-operatively was associated with worse OS. These findings support the need for RT planning MRIs and confirm the association between delayed treatment and worse survival. Radiomic analysis of cavity regression vs. tumor regrowth is forthcoming. Author Disclosure J.B. Bell: None. B.J. Rich: None. I.B. Mihaylov: Research Grant; Philips Radiation Oncology Systems, NIH.C. Benjamin: None. E.A. Mellon: None. M.C. Abramowitz: None. G. Azzam: None. M. Guillermo Prieto Ei: None. M. De La Fuente: None. M. Butkus: None. T. Diwanji: None.
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