TI detectors in BNCT dosimetry

The main detectors for characterising and controlling of BNCT beams are activation foils and paired ionisation chambers. Thermoluminescent (TL) dosimeters are also of interest because of their following advanta: i) small physical size, ii) need for high voltage or cables, i.e. stand alone character, and iii) suitability for large scale measurements; with TL dosimeters it is possible to measure depth dose curves and profiles at the same time, with one irradiation. Also, TL dosimeters may be possible detectors for in vivo use. At the Finnish BNCT facility, a TL detector MTS-Ns of TLD Niewiadomski & Co. (Krakow, Poland) with an ultra-thin active Lif:Mg,Ti layer for small self-shielding of thermal neutrons was selected for use as a neutron sensitive dosimeter. A TL detector MCP-7s (7Lif:Mg,Cu,P) of the same manufacturer was used for gamma detection because of its high sensitivity to gamma radiation compared to that to high LET radiation. The gamma dose and neutron fluence distributions have been measured in PMMA, water and brain substitute liquid phantoms at the BNCT beam. Gamma dose and neutron fluence profiles measured with TL detectors correlate with those calculated using DORT (Two Dimensional Discrete Ordinates Transport Code) and measured with ionisation chambers. MTS-Ns TL detectors were found to measure accurately (8 %, 1 S.D.) the relative neutron fluence, and therfore to be a useful addition to the activation foils in bnct neutron dosimetry. Due to the high uncertainty of the thermal neutron sensitivity of the MCP-7s TL detectors, the absorbed gamma doses can be measured with MCP-7s detectors within 20% in the mixed neutron-gamma field of BNCT. The treatments of glioma patients at the Finnish BNCT facility will start in the spring 1999. The doses to the target volume and sensitive organs, i.e. brain, will be calculated individually in the dose planning. Since it is also necessary to monitor the absorbed doses to the head and to the body, in vivo dosimeters are used. For clinical practice, when verifying the absorbed doses in vivo the used TL and activation foil dosimeters must be placed on the skin of the patient of in accessible cavities. The TL detector MCP-7s will be used in in vivo gamma dosimetry. The correlations for the thermal neutron sensitivity of the MCP-7s TL detectors will be made based on the neutron fluence measured with activation foils. The accuracy of approximately 10% can be achieved in those measurement points, in which thermal neutron fluence is negligible and, therefore, no correlation for thermal neutron sensitivity is required. This applies to those measurement points in the body, i.e. total body dose.