Comparison of [¹⁸F]FAPI-42 and [¹¹C]methionine PET/MRI in defining biological tumor volume with conventional gross tumor volume: consistency and differences.

Abstract

High-grade gliomas are highly aggressive brain tumors, and precise target volume delineation is crucial for effective treatment. This study aimed to evaluate the concordance and discrepancy between biological tumor volume (BTV) delineated using union of [¹⁸F]FAPI-42 and [¹¹C]methionine and conventional gross tumor volume (GTV) in gliomas. Nineteen glioma patients were retrospectively enrolled, who underwent [¹⁸F]FAPI-42 and [¹¹C]methionine PET/MRI before radiation therapy. The optimal [¹⁸F]FAPI-42 threshold for delineation was determined from five predefined thresholds based on volumetric consistency. Target volumes, including BTV based on [¹⁸F]FAPI-42 PET (BTV-FAPI), [¹¹C]methionine (BTV-MET), their union (BTV-UNION), GTV on contrast-enhanced T1-weighted MRI (GTV-ce-T1w), clinical target volume (CTV)-UNION, and CTV-ce-T1w were delineated, and geometric comparisons were made between BTV-FAPI and BTV-MET, BTV-UNION and GTV-ce-T1w, and CTV-UNION and CTV-ce-T1w. Dice Similarity Coefficient (DSC), Jaccard similarity coefficient (JSC), and overlap volumes (OV) parameters were calculated for comparison among the different target volumes. The 20% standardized uptake value maximum (SUVmax) threshold was chosen as the optimal value for [¹⁸F]FAPI-42 delineation. No significant volume difference was found between BTV-FAPI and BTV-MET (12.64 vs. 9.67 cc; P = 0.562), with low DSC (0.28), JSC (0.17), and OV (0.39). BTV-UNION showed no significant volume difference compared to GTV-ce-T1w (18.41 vs. 8.67 cc; P = 0.073), but had higher OV (0.82) and moderate improvements in DSC (0.50) and JSC (0.33). CTV-UNION was significantly larger than CTV-ce-T1w (198.03 vs. 118.58 cc; P = 0.011), with significant increases in DSC (0.82), JSC (0.70), and OV (0.96). The union of [¹¹C]methionine and [¹⁸F]FAPI-42 PET for target delineation offers complementary value, enabling the identification of a broader tumor-infiltrated region compared to that delineated by ce-T1w MRI, with potential for reduced glioma recurrence risk.