RT-10 * DO WE NEED IMAGE-GUIDANCE OF GTV DEFINED BY FET-PET IN RADIOTHERAPY OF HIGH GRADE GLIOMA?

METHODS: The study population comprised of 54 pts with high grade glioma treated with RT and Temozolomide. CT and MRI images were fused in a radiation therapy treatment planning software. GTV was delineated as a contrast-enhancing tumor plus the operation cavity, defined by a radiologist blinded to the results of FET/PET. The CTV was generated by the geometric expansion of GTV plus 20 mm and the PTV was achieved by an additional margin to the CTV by 2 mm. The FET-PET volume was determined by SUV of 1.6. On corresponding axial slices, FET uptake was compared to contrast enhancement in T1-weighted MR. Intensity modulated arc therapy (IMAT) with steep dose-gradients was delivered using daily image-guidance and 6D patient positioning. Pts received 60 Gy in 30 fractions. RESULTS: Five pts had no postoperative FET-PET uptake. Of 49 pts, 5 pts were diagnosed with PET positive areas outside the GTV plus 20 mm. In 40% of the cases there was a similarity between GTV defined by MRI and FET/PET, respectively. CONCLUSION: In this retrospective study of 54 pts, pretreatment FET-PET appears to identify areas of tumor activity outside the MRI-GTV, with the majority placed in the CTV. With stereotactic radiotherapy and a steeper gradient it is important with better imaging technique to define the tumor. With an unchanged CTV margin and by including FET PET for GTV definition, the irradiated volume will tend to increase moderately for most patients, and quite substantially for a minority of patients. Prospective radiation therapy trials could determine if the GTV-to-CTV margin can be adjusted with FET-PET.