The trends and significance of SSTR PET/CT added to MRI in follow-up imaging of low-grade meningioma treated with fractionated proton therapy

Purpose Overexpression of the somatostatin receptor (SSTR) has led to adoption of SSTR PET/CT for diagnosis and radiotherapy planning in meningioma, but data on SSTR expression during follow-up remain scarce. We investigated PET/CT quantifiers of SSTR tracers in WHO grade I meningioma following fractionated proton beam therapy (PBT) compared to standard response assessment with MRI. Methods Twenty-two patients diagnosed with low-grade meningioma treated by PBT were included. Follow-up included clinical visits, MRI, and [ 68 Ga]Ga-DOTATOC PET/CT scans. Radiologic tumor response, MRI and PET volume (V MRI and V PET ), maximum and mean standardied uptake value (SUVmax/SUVmean), total lesion activity (TLA), and heterogeneity index (HI) were evaluated. Results Median follow-up was 35.3 months (range: 6.4–47.9). Nineteen patients (86.4%, p = 0.0009) showed a decrease of SUVmax between baseline and first follow-up PET/CT (median: −24%, range: −53% to +89%) and in 81.8% of all cases, the SUVmax, SUVmean, and TLA at last follow-up were eventually lower than at baseline ( p = 0.0043). Ambiguous trends without significance between the timepoints analyzed were observed for V PET . HI increased between baseline and last follow-up in 75% of cases ( p = 0.024). All patients remained radiologically and clinically stable. Median V MRI decreased by −9.3% (range 0–32.5%, p < 0.0001) between baseline and last follow-up. Conclusion PET/CT in follow-up of irradiated meningioma showed an early trend towards decreased binding of SSTR-specific tracers following radiation and MRI demonstrated consistently stable or decreasing tumor volume. Translational research is needed to clarify the underlying biology of the subsequent increase in SSTR PET quantifiers.