Dose distributions in phantoms with different dimensions exposed to BNCT epithermal neutron beam: measurements and calculations

摘要

Boron neutron capture therapy (BNCT) is a cancer treatment still in development that is considered with increasing interest It consists in irradiating with thermal or epithermal neutrons patients to whom a specific tumor-seeking Boron carrier has been administrated. The high cross section of the ~(10)B(n,α)~7Li reaction for thermal neutrons and the high radiobiological effectiveness are exploited in order to damage the tumour cells without afflicting the surrounding healthy tissues. Nowadays, adequate neutron fluxes for BNCT treatments can be provided by nuclear reactors. For each neutron source, it is important to measure the dose distribution in tissue equivalent phantoms in order to characterize the radiation field and to acquire useful data exploited by the treatment planning system to calculate the dose distribution in the patient. In the epithermal column of the LVR-1S reactor at NRI Rez (CZ) a series of measurements were performed with FriXy-gel dosimeters inserted in a standard parallelepiped (40 × 40 × 20 cm~3) water phantom and in a cylindrical (d = 16cm, h = 14cm) water-equivalent phantom. FriXy gels are based on ferrous sulphate solution (Fricke solution) containing Xylenol-Orange infused in a porcine skin 3 mm-thick matrix. The dosimeters are imaged with a CCD camera system before and after irradiation, and the measured difference of optical density at 585 nm wavelength is proportional to the absorbed dose. In particular, boron dose distributions have been evaluated using 16 × 14 × 0.3 cm~3 dosimeters placed along the beam axis. In the two phantoms, similar boron capture distributions have been found on the beam axis; on the opposite hand, more relevant differences have been detected off-axis, showing a not negligible difference of distribution of the thermal neutron flux in the standard phantom and in a volume with head dimensions. In-depth measurements and off-axis profiles were compared with Monte Carlo simulations performed with MCNP5, showing good consistency.