Microdosimetric calculations of boron-drug compound factors for boron neutron capture therapy

The relative biological effect (RBE) is not a useful term to describe 10B capture events because dose is not a greatly useful term for boron neutron capture therapy (BNCT), and each drug compound exhibits unique pharmacokinetics, resulting in differences in 10B spatial distributions and tremendous variation in biological effect. These considerations have led to the suggestion that a quantity called the 'compound factor' (CF) be defined. The CF can replace RBE for BNCT terminology and avoid some misinterpretations caused by quoting RBE values. A study was performed to estimate CFs in normal brain and brain-tumor tissues for borocaptate sodium [Na2B12H11SH (BSH)] and para-boronophenylalanine (p-BPA). A Monte Carlo computer model of the capillaries was used to determine charged-particle energy deposition patterns following 10B capture events in various regions within and around the endothelial cell. A spherical model of tumor cells was also used to determine energy transport in the tumor. The tumor cells were assumed to be 10 μm in diameter with 5-μm-diam nuclei, and extracellular space occupied 15% of the volume. From these Monte Carlo results, transmission factors were obtained for energy deposition to the nucleus due to 10B capture events in any region within and external to the cells