Quality assurance of BNCT dosimetry

The Phase I clinical trials for boron neutron capture therapy (BNCT) started in May 1999 in Otaniemi, Espoo. For BNCT no uniform international guidance for the quality assurance of dosimetry exists, so far. Because of the complex dose distribution with several different dose components, the international recommendations on conventional radiotherapy dosimetry are not applicable in every part. Therefore, special guidance specifically for BNCT is needed. To obtain such guidelines a European collaboration project has been defined. The aim of the project is a generally accepted Code of Practice for use by all European BNCT centres. This code will introduce the traceability of the dosimetric methods to the international measurement system. It will also ensure the comparability of the results in various BNCT beams and form the basis for the comparison of the treatment results with the conventional radiotherapy or other treatment modalities. The quality assurance of the dosimetry in BNCT in Finland covers each step of the BNCT treatment, which include dose planning imaging, dose planning, boron infusion, boron kinetics, patient positioning, monitoring of the treatment beam, characterising the radiation spectrum, calibration of the beam model and the dosimetric measurements both in patients (in viva measurements) and in various phantoms. The dose planning images are obtained using a MR scanner with MRI sensitive markers and the dose distribution is computed with a dose planning software BNCTRtpe. The program and the treatment beam (DORT) model used have been verified with measurements and validated with MCNP calculations in phantom. Dosimetric intercomparison has been done with the Brookhaven BNCT beam (BMRR). Before every patient irradiation the relationship between the beam monitor pulse rate and neutron fluence rate in the beam is checked by activation measurements. Kinetic models used to estimate the time-behavior of the blood boron concentration have been verified using independent patient sample data. For the measurement of the absorbed doses in patients and phantoms, TL dosimeters and activation foils are used. At the moment the quality assurance of dosimetry in BNCT in Finland is according to the EU rules and planned recommendations and thus sufficient. Nevertheless microdosimetric measurement methods and gel dosimeters are planned to bring to use in addition to the dosimetric methods already utilised in order to strengthen the quality assurance (author)