Temporospatial tumor dynamic changes in glioblastoma during radiotherapy

Glioblastoma (GBM) management post maximal-safe resection consists of concurrent chemoradiation (CRT) and adjuvant chemotherapy. MRIs are historically performed post-operatively and/or at treatment planning. Continuous interfractional changes during CRT have not been adequately characterized. MR-guided radiation therapy (MRgRT) allows for detailed imaging of tumor volumes during the course of treatment. This is a preliminary initial report evaluating temporal and spatial changes that occur in GBM, in order to model tumor dynamics.

Five GBM patients enrolled onto an institutional biorepository registry underwent treatment with our 0.35T MRgRT workflow. Target volumes were delineated based on T2/FLAIR (GTV_46Gy) and T1 gadolinium-enhanced MR (GTV_14Gy) sequences. Weekly post-contrast MRIs were performed during CRT with the 0.35T magnet to monitor target volume dynamics.

Thirty-five MR scans were evaluated. The median time from surgery to CRT was 32 days (range: 28-40), with a median of 13 days (range: 12-14) from simulation to CRT. We found median volume reductions of 40.0% (range: 8.3-86.5%), and 37.1% (range: 15.0-67.5%) for GTV_46Gy and GTV_14Gy, respectively. The bulk of these changes occurred early, within the first 3 weeks of the 6-week treatment, with significant reductions observed between baseline and week 1 -32.6% for GTV_46Gy and 17.9% for GTV_14Gy. Separately, statistically significant volume reductions for the cavity volume (F = 59.43, p < 0.05) were observed. Compared to baseline, centroid migrations of the target volumes were also noted: the median GTV_46Gy centroid migration was 7.4 mm (range: 2.0-10.8 mm) and the median GTV_14Gy centroid migration was 3.6 mm (range: 1.3-8.8 mm).

Our pilot study suggests that weekly MRgRT imaging for GBM patients undergoing long course CRT reveals significant GTV reductions and centroid migrations, especially during the first 3 weeks of treatment. A more detailed understanding of which patients are at highest risk for tumor change and migration is needed to best apply these imaging parameters to clinical practice.