OP-613 Role of 68Ga-DOTATOC PET/CT in Assessing Treatment Response in Re-Irradiated Recurrent Meningiomas: A Volumetric and Molecular Perspective Annual Congress of the European Association of Nuclear Medicine, October 4–8, 2025 | Barcelona, Spain

Aim/Introduction: Recurrent meningiomas remain a clinical challenge, particularly after prior surgery and radiation. Proton therapy re-irradiation represents an increasingly used strategy due to its dosimetric advantages. However, robust biomarkers for assessing treatment response remain under investigation. 68GaDOTATOC PET/CT, which targets somatostatin receptor subtype 2, offers high sensitivity for meningioma detection, but the utility of SUVmax and volumetric measures as reliable indicators of response remains unclear. The purpose of this study was to assess the value of SUVmax and tumor metabolic volume derived from 68Ga-DOTATOC PET/CT in evaluating treatment response following proton therapy re-irradiation in patients with recurrent meningioma. Materials and Methods: A retrospective study was conducted on 22 patients with histologically confirmed recurrent meningiomas (5 Grade I, 14 Grade II, 3 Grade III) treated with proton therapy re-irradiation between 2018 and 2022. All patients underwent PET/CT scans before and after treatment. Tumor regions were manually segmented to extract SUVmax standardized uptake volume ratio differences and (SRD) somatostatin receptor density values and volumes. Non-parametric statistical analyses were applied to evaluate differences across timepoints and tumor grades. Results: No statistically significant difference was observed between pre- and post-treatment, SUVmax values (mean: 17.01 vs. 18.29; p = 0.548). Similarly, no significant variation in SUVmax was noted across tumor grades. Conversely, a volumetric reduction of approximately 20% was observed post-treatment, with a 22% decrease in the Grade II subgroup. Conclusion: While SUVmax did not change significantly following re-irradiation, metabolic tumor volume demonstrated a consistent reduction. This discrepancy between metabolic activity and volume suggests potential biological and technical factors influencing tracer uptake, such as prior treatments, intratumoral fibrosis, or tumor dedifferentiation. These findings underscore the importance of volumetric PET metrics and support a multiparametric imaging approach combining PET and MRI for optimal evaluation of treatment response in recurrent meningiomas.