In vivo dosimetry for boron neutron capture therapy

The Phase I trials for boron neutron capture therapy (BNCT) has started at May 1999 at the Finnish BNCT facility in Espoo, Otaniemi. In patient studies, it is necessary to monitor the absorbed doses to healthy tissues of the patients; to the head and to the body. For clinical practise, when verifying the absorbed doses in vivo, the dosimeters used must be placed on the skin of the patient or in accessible cavities. The neutron fluence is measured with activation detectors, foils and wires. The activation detectors used are diluted Mn-Al foils (1 %, diameter 12 mm x 0.2 mm), obtained from ECN Petten, and wires (2.4 %). The detectors are measured with a Ge detector and the saturation activities are calculated with the SAMPO program. The determination of the neutron fluence is based on the reaction 55Mn(n,γ)56Mn (T1/2=2.6 h). The same method has successfully been used for both the phantom and healthy tissue tolerance study at the Finnish BNCT facility. Thermoluminescent (TL) detector type MCP-7s (7LiF:Mg,Cu,P) is used for the determination of absorbed doses due to gamma radiation. In order to get the absorbed gamma dose, the additional dose, i.e. the excess TL signal in the net readout value, caused by thermal neutron sensitivity of the MCP-7s detectors used, have to be subtracted: Dg = D - Dn ≡ D - fth φ where factor fth represents thermal neutron sensitivity of MCP-7s detectors and Φ is thermal neutron fluence measured with activation detectors at the measurement point. The absorbed gamma doses and thermal neutron fluences are measured at various points in head in order to verify the dose planning. The calculated (individual plans) and measured values correlate well within the uncertainties. In addition, body doses due to gamma radiation are also measured in four different measurement points; those representing thyroid, sternum and vertebra C7, and in umbilicus. Despite the different treatment cases, the body doses have been of same magnitude for all the patients, e.g. at umbilicus 40 ± 10 mGy and at the point representing thyroid 250 ± 70 mGy (author)