Combined tumour treatment by coupling conventional radiotherapy to an additional dose contribution from thermal neutrons

Aim: To employ the thermal neutron background in conventional X-rays radiotherapy treatments in order to add a localized neutron dose boost to the patient, enhancing the treatment effectiveness. Background: Conventional linear accelerators for radiotherapy produce fast secondary neutrons with a mean energy of about 1 MeV due to (γ, n) reaction. This neutron field, isotropically distributed, is considered as an extra unaccounted dose during the treatment. Moreover, considering the moderating effect of human body, a thermal neutron field is localized in the tumour area: this neutron background could be employed for Boron Neutron Capture Therapy (BNCT) by previously administering a boron (10B enriched) carrier to the patient, acting as a localized radiosensitizer. The thermal neutron absorption in the 10B enriched tissue will improve radiotherapy effectiveness. Materials and Methods: The feasibility of the proposed method was investigated by using simplified tissue-equivalent phantoms with cavities in correspondence of relevant tissues or organs, suited for dosimetric measurements. A 10 cm × 10 cm square photon field with different energies was delivered to the phantoms. Additional exposures were implemented, using a compact neutron photo-converter-moderator assembly, with the purpose of modifying the mixed photon-neutron field in the treatment region. Doses due to photons and neutrons were both measured by using radiochromic films and superheated bubble detectors, respectively, and simulated with Monte Carlo codes. Results: For a 10 cm × 10 cm square photon field with accelerating potentials 6 MV, 10 MV and 15 MV, the neutron dose equivalent in phantom was measured and its values was 0.07 mGy/Gy (neutron dose equivalent / photon absorbed dose at isocentre), 0.99 mGy/Gy and 2.22 mGy/Gy, respectively. For a 18 MV treatment, simulations and measurements quantified the thermal neutron field in the treatment zone in 1.55 × 107 cm−2 Gy−1. Assuming a BNCT- standard 10B concentration in tumour tissue, the calculated additional BNCT dose at 4 cm depth in phantom would be 1.5 mGy-eq/Gy. This ratio would reach 43 mGy- eq/Gy for an intensity modulated radiotherapy treatment (IMRT). When a specifically designed compact neutron photo-converter-moderator assembly is applied to the LINAC to enhance the thermal neutron field, the photon field is modified. Particularly, a 15 MV photon field produces a dose profile very similar to that would be produced by a 6 MV field in absence of the photo-converter-moderator assembly. As far as the thermal neutron field is concerned, more thermal neutrons are present, and thermal neutrons per photon increase of a factor 3 to 12 according to the depth in phantom and to different photoconverter geometries. By contrast, the photo-converter-moderator assembly was found to reduce fast neutrons of a factor 16 in the direction of the incident beam. Conclusions: The parasitic thermal neutron component during conventional high- energy radiotherapy could be exploited to produce additional therapeutic doses if the 10B-carrier was administered to the patient. This radiosensitization effect could be increased by modifying the treatment field by using the specifically designed neutron photo-converter-moderator assembly.