Teatment planning figures of merit in thermal and epithermal boron neutron capture therapy of brain tumours.

ABSTRACT

The boron neutron capture therapy (BNCT) figures of merit of advantage depth, therapeutic depth, modified advantage depth and maximum therapeutic depth have been studied as functions of 10B tumour to blood ratios and absolute levels. These relationships were examined using the Monte Carlo neutron photon transport code, MCNP, with an ideal 18.4 cm diameter neutron beam incident laterally upon all ellipsoidal neutron photon brain-equivalent model. Mono-energetic beams of 0.025 eV (thermal) and 35 eV (epithermal) were simulated. Increasing the tumour to blood 10B ratio predictably increases all figures of merit. concentration was also shown to have a strong bearing on the figures of merit when low levels were present in the system. This is the result of a non-10B dependent background dose. At higher levels however, the concentration of 10B has a diminishing influence. For boron sulphydryl (BSH), little advantage is gained by extending the blood 10B level beyond 30 ppm, whilst for D,L,-p-boronophenylalanine (BPA) this limit is 10 ppm. To achieve a therapeutic depth of 6 cm (brain mid-line from brain surface) using the thermal beam, a tumour to blood ratio of 25 with 10 ppm 10B in the blood is required for BPA. Similarly, a tumour to blood ratio of 8.5 with 30 ppm blood 10B is required for the maximum therapeutic depth of BSH to reach the brain mid-line. These requirements are five times above current values for these compounds in humans. Applying the epithermal beam under identical conditions, the therapeutic depth reaches the brain mid-line with a tumour to blood 10B ratio of only 5.7 for BPA. For BSH, the maximum therapeutic depth reaches the brain mid-line with a tumour to blood ratio of only 1.9 with 30 ppm in the blood. Human data for these compounds are very close to these requirements.