The Value of SSTR2 Receptor-Targeted PET/CT in Proton Irradiation of Grade I Meningioma

SIMPLE SUMMARY: Meningiomas are the most frequent tumors of the central nervous system. Their treatment outcomes depend on the histologic grade. Grade I tumors, even bulky and unresectable, are well curable with radiation; therefore, most precise and conservative techniques such as proton therapy are utilized. To better distinguish between tumor extent and normal tissues, PET/CT using tracers that bind specifically to somatostatin receptor type 2 (SSTR2), commonly expressed by these tumors, can be used in addition to MRI. In an experimental blinded study of 30 pre-operated WHO grade I meningiomas independently delineated by four radiation oncologists, we confirmed that the overwhelming majority of meningiomas do express SSTR2 and that the addition of receptor-targeted PET/CT helps visualize lesions not unanimous in MRI and improves the homogeneity of tumor volume definition between observers. SSTR2-targeted PET/CT should be a standard in the planning of curative radiation in pre-operated meningioma. ABSTRACT: Grade I meningioma is the most common intracranial tumor in adults. The standard imaging for its radiation treatment planning is MRI, and [(68)Ga]1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-conjugated PET/CT can further improve delineation. We investigated the impact of PET/CT on interobserver variability in identifying the tumor in 30 anonymized patients. Four radiation oncologists independently contoured residual tumor volume, first using only MRI and subsequently with the addition of PET/CT. Conformity indices (CIs) were calculated between common volumes, observer pairs and compared to the volumes previously used. Overall, 29/30 tumors (96.6%) showed [(68)Ga]Ga-DOTA avidity. With help of PET/CT, the participants identified six cases with new lesions not recognized in MRI, including two where new findings would critically alter the target volume used for radiation. The PET/CT-aided series demonstrated superior conformity, as compared to MRI-only between observer pairs (median CI = 0.58 vs. 0.49; p = 0.002), common volumes (CI = 0.34; vs. 0.29; p = 0.002) and matched better the reference volumes actually used for patient treatment (CI = 0.55 vs. 0.39; p = 0.008). Cis in the PET/CT-aided series were lower for meningiomas outside of the skull base (0.2 vs. 0.44; p = 0.03). We conclude that SSTR2 receptor-targeted PET/CT is a valuable tool for planning particle therapy of incompletely resected meningioma. It serves both as a workup procedure and an aid for delineation process that reduces the likelihood of marginal misses.