Comparison of dose distribution with and without reflecting heterogeneous boron distribution using 18F-BPA positron emission tomography in boron neutron capture therapy

This study investigated the dosimetric impact of implementing heterogeneous boron distribution into dose evaluations on tumors in BNCT. The study involved 27 patients who underwent ¹⁸F-BPA PET/CT scans. Dose evaluations were performed using various methods reflecting boron concentrations on CT images. The conventional dose evaluations, which reflected uniform boron concentration in blood of 25 ppm and a constant tumor-to-blood (T/B) ratio of 3.5, were compared with individual dose evaluations, which reflected the blood boron concentration and T/B ratio calculated from ¹⁸F-BPA in each patient. The heterogeneous tumoral dose distribution was also compared, revealing the dosimetric impact of the boron distribution calculated from each voxel of ¹⁸F-BPA. The spatial correspondence between ¹⁸F-BPA and dose distribution was compared using metabolic tumor volume (MTV) from ¹⁸F-BPA and isodose volume from the heterogeneous dose distribution. Results showed that the median blood boron concentration and T/B ratio calculated from ¹⁸F-BPA were 25.57 (23.90-27.84) ppm and 3.75 (2.54-4.59), respectively, comparable to those in the conventional dose evaluations. All dose indices in the heterogeneous tumoral dose evaluations were significantly lower than those in the conventional dose evaluations (p < 0.01). However, the spatial correspondence between the ¹⁸F-BPA and the dose distribution was not observed in the dice similarity coefficients of both MTV40-40% isodose volume and MTV50-50% isodose volume. In conclusion, the study confirmed the validity of applying the boron concentration calculated from ¹⁸F-BPA to the dose evaluation for a patient in BNCT. The differences might be associated with non-inter-patient variations of ¹⁸F-BPA, but the conventional dose evaluations mainly focused on the high boron concentration area within the tumor. Furthermore, the discrepancies in the patients were also observed between the ¹⁸F-BPA distribution and the heterogeneous dose distribution. Therefore, this study suggested that the indications for BNCT should consider not only ¹⁸F-BPA but also the dose distributions, which could reflect the heterogeneous tumoral boron distribution.