MRI and ¹⁸F-BPA PET-guided targeting in boron neutron capture therapy

In modern Boron neutron capture therapy (BNCT) treatment planning, ¹⁸F-BPA (¹⁸F-boronophenylalanine) PET (positron emission tomography) imaging is used to assess boron uptake and guide accurate dose delivery. This study evaluates the geometric and dosimetric differences between target volumes defined by MRI (magnetic resonance imaging) and PET images in accelerator-based BNCT using the NeuPex system. The GTV (gross tumor volume) was defined based on MRI (GTV_MRI) and PET images with SUV thresholds of 2.0 (GTV_PET2.0) and 2.5 (GTV_PET2.5) in patients with head-and-neck cancer (HNC) and malignant glioma (MG). GTV_PET2.5 was approximately 50% smaller, while GTV_PET2.0 showed closer volumetric agreement, with ∆V% of 23.52 ± 17.44 (HNC) and 28.71 ± 12.57 (MG), and corresponding Dice similarity coefficients of 0.71 ± 0.14 and 0.86 ± 0.17. Dosimetric analysis revealed that GTV_PET2.0 achieved higher dose coverage than GTV_MRI, with mean doses of 24.07 ± 6.42 Gy-Eq (HNC) and 26.36 ± 9.77 Gy-Eq (MG), and D₈₀ values of 19.01 ± 6.90 Gy-Eq and 21.00 ± 7.51 Gy-Eq, respectively. Tumor-to-blood ratio (TBR) and tumor-to-normal ratio (TNR) analyses further indicated substantial heterogeneity in boron uptake, with mean TBR and TNR of 2.71 ± 0.53 and 2.97 ± 0.79 for GTV_PET2.0. These results suggest that GTV_PET2.0 provides a balanced and biologically relevant target for BNCT planning. A TBR threshold of ≥ 2.0 is proposed as a practical criterion for patient selection. While this study focuses on geometric and dosimetric endpoints, future work correlating these findings with clinical outcomes is warranted. Overall, this study supports the integration of functional PET imaging with anatomical MRI to improve the precision and efficacy of BNCT treatment planning.