RTHP-22. EDEMA PROGRESSION DURING MRI-GUIDED GLIOBLASTOMA RADIOTHERAPY

Abstract

Abstract PURPOSE Despite the common finding of pseudoprogression or true progression one month after primary chemoradiotherapy for glioblastoma, there are few studies evaluating brain MRI changes that occur during concurrent chemotherapy and radiotherapy (RT). With the first generation combination MRI-RT device, daily predominantly T2-weighted MRIs are obtained of glioblastoma during RT. We quantified how many patients had significant MRI detectable volumetric changes through the six week course of primary chemoradiotherapy. This is of particular importance since glioblastoma RT is only planned at the beginning of therapy and not commonly re-planned for changes during therapy. METHODS We retrospectively reviewed the daily set-up imaging of 8 patients at our institution who received RT for glioblastoma using the Cobalt-60 MRI-RT system. Patients received standard chemoradiation at 60 Gy in 30 fractions with temozolomide per EORTC22981/26981. We contoured the abnormality on the initial ViewRay setup scan and the set-up scan for fraction 30. After rigid fusion of the contours of the initial setup MRI and fraction 30 MRI, the volumes were compared. RESULTS Of the 8 patients, 3 patients (37.5%) demonstrated edema expansion greater than 5 mm. The maximum distances of T2-weighted abnormality volume growth for these patients were 1.0 cm, 1.5 cm, and 4.1 cm. These findings were correlated with the post-treatment diagnostic MRIs at 3–4 weeks which demonstrated similar FLAIR abnormalities and expansion in T1 with gadolinium contrast volumes within these areas of the radiotherapy fields (pseudoprogression vs. true progression). CONCLUSION Review of MRIs obtained by daily MRI-RT for glioblastoma indicates that 3 of 8 patients had over 5 mm of change in T2-weighted dimensions from beginning to end of radiotherapy. Groups using limited CTV margins for treatment planning should be aware that MRI volumes could significantly increase during radiotherapy.