Fast-neutron dose evaluation in BNCT with Fricke gel layer detectors G. Gambarini a, b, * , G. Bartesaghi a, b , J. Burian c , M. Carrara d , M. Marek c , A. Negri a, b , L. Pirola a , L. Viererbl c a Università degli Studi di Milano, Department of Physics, Via Celoria 16, 20133 Milano, Italy b INFN Sezione di Milano, via Celoria 16, 20133 Milano, Italy c Department of Reactor Physics, Nuclear Research Institute  Re z, Husinec e  Re z 130, 250 68  Re z, Czech Republic d Medical Physics Unit, Fondazione IRCCS “Istituto Nazionale Tumori”, via Venezian 1, 20133 Milano, Italy article info Article history: Received 11 November 2009 Received in revised form 9 April 2010 Accepted 2 May 2010 Keywords: Gel dosimetry Dose imaging Neutron dosimetry BNCT abstract Boron neutron capture therapy (BNCT) is a cancer radiotherapy that uses epithermal and thermal neutron beams. The determination of the absorbed dose in healthy tissue, separating the various dose contributions having different radiobiological effectiveness (RBE) is of great importance for therapy planning. However, a standard code of practice has not yet been established because suitable methods for dosimetry in BNCT are still in progress. A study about the characterization of the epithermal column of the LVR-15 research reactor in  Re z (CZ) has been performed, in particular concerning the fast-neutron dose. This dose is not negligible and its determination is important owing to its high RBE. Fast-neutron and photon dose distributions in a water phantom have been measured by means of Fricke gel layer dosimeters. Even if gel layer dosimetry is not yet standardized, it is presently the only method for obtaining images of each dose contribution in BNCT neutron fields. The results were compared with values measured with thermoluminescence detectors, twin ionization chambers data taken from literature and Monte Carlo simulations. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Boron neutron capture therapy (BNCT) is a radiotherapy that targets cancer at the cellular level with high LET (linear energy transfer) radiation. BNCT experimental treatments carried out until now have been performed with neutron beams provided by nuclear research reactors. In particular, thermal neutrons, with their low penetration power, are useful only for shallow depth tumor treatments. For therapeutic benefits to deeper tissues, higher energies are neces- sary. For this reason, in the last two decades significant efforts have been devoted to develop epithermal neutron beams (0.5 eVe10 keV) capable of providing the required thermal neutron flux at depths of 8e10 cm. From the radiobiological point of view, epithermal neutron beams are very challenging. In fact, the absorbed dose in a patient is due to components of different quality and radiobiological effec- tiveness (RBE). In order to predict the treatment effects on human tissues, the evaluation of the total absorbed dose is meaningless, and it is important to characterize the beam quantifying each dose component in a tissue equivalent phantom (IAEA, 2001). The therapeutic dose is due to 10 B(n, a) 7 Li reactions. The maximum neutron fluence admitted for a treatment is established on the basis of the absorbed dose in healthy tissue, which is due to different contributions. The photon dose contribution is the sum of the gamma background coming from the reactor and of 2.2 MeV photons generated by slow neutrons through the 1 H(n,g) 2 H reac- tion. A low contribution comes from the thermal neutron reaction 14 N(n,p) 14 C with the nitrogen present in the biological tissues. Finally, epithermal neutron beams show an undesired component of fast neutrons (>10 keV). Such particles, mainly interacting with hydrogen nuclei, produce recoil protons which deposit their energy locally. The relative contribution of the different dose components varies from point to point inside a phantom. One of the great advantages of the gel dosimeters employed in the experimental method described in this paper, is the capability to achieve sepa- rated bi-dimensional distributions of the various dose components in a tissue equivalent material. This paper concerns the measurement of fast-neutron and photon doses. The nitrogen dose determination is not treated here. It is usually determined from thermal fluence values (obtained for example by means of activation foils) using kerma factors. Nitrogen dose images can be achieved with gel-dosimeter layers from the * Corresponding author. Università degli Studi di Milano, Dipartimento di Fisica, Via Celoria 16, 20133 Milano, Italy. Tel.: þ39 02 50317243; fax: þ39 02 50317630. E-mail addresses: grazia.gambarini@mi.infn.it (G. Gambarini), mauro.carrara@ istitutotumori.mi.it (M. Carrara). Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas 1350-4487/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.05.004 Radiation Measurements 45 (2010) 1398e1401