P09.11.A IMPACT OF FET-PET IMAGING IN SURVEILLANCE OF THERANOSTIC TREATMENT WITH 4-L-[131I]IODO-PHENYLALANINE ([131I]IPA) IN RELAPSED GLIOBLASTOMA PATIENTS

Abstract BACKGROUND Theranostic is an evolving field in oncology. IPAX1 and IPAX-Linz studies used the radioligand Iodine-131[131I] IPA in combination with external re-irradiation in glioblastoma (GBM) patients. Modality of choice for assessment after therapy is contrast-enhanced magnetic resonance imaging (CE-MRI). Further, O-(2-[18F]fluoroethyl-)-L-tyrosine (18F-FET) positron emission tomography (PET) has become a valuable tool for brain tumor surveillance by adding metabolic information. However, no real-world data exist in the joint application of RANO 2.0 and PET RANO 1.0 criteria in response assessment of GBM, especially in patients treated with a theranostic approach. MATERIAL AND METHODS 12 patients with histological diagnosis of recurrent GBM treated in our center within study protocols IPAX 1 and IPAX Linz were evaluated to compare MRI with FET-PET in follow up after treatment with theranostic therapy. PET RANO 1.0 criteria were used to evaluate the validity and predictive value of additional FET-PET imaging in treatment surveillance. Primary aim of the study was the response assessment and comparison of progression-free survival following RANO 2.0 criteria and PET RANO 1.0 criteria. RESULTS Mean PFS according to MRI criteria was 20.5 weeks (4.7 months). Mean OS was 48.5 weeks (11.2 months). No partial or complete responses were seen in MRI follow up. 7/12 (58.3 %) patients show progression at first FU MRI two months after baseline. In 2nd FU 11/12 (91.7 %) patients progressed. Evaluation based on PET RANO 1.0 criteria at first FET-PET FU investigation resulted in different outcomes depending on which PET parameter was used (volume, TBRmax, TBRmean). Mean PFS calculated by PET volume is 77 days (11 weeks/2.5 mo). Mean PFS based on TBRmax or TBRmean is not assessable because in the 2 follow up investigations most patients were stable or had a partial response in these two PET parameters. CONCLUSION This pilot study demonstrated different outcome in respect of PFS depending on whether MRI or PET RANO criteria have been used for analysis. Further, application of PET RANO criteria showed varying results in terms of response for each individual parameter (TBRmax, TBRmean and PETvolume). Based on our data, the value of an additional PET scan for a valid prediction of response in recurrent GBM patients must be discussed. Disclosure: IPAX-1 was sponsored by Telix Pharmaceuticals. IPAX Linz is an investigator-initiated trial; IMP was provided by Telix Pha