Mixed Beam Dosimetry - From Reactor Core to BNCT Patient

Detta är en avhandling från Per Munck af Rosenschöld, Dept. of Radiation Physics, Klinikg. 7, 22185 LUND, SWEDEN

Sammanfattning: Accurate radiation dosimetry is of paramount importance in order to ensure safe delivery of radiotherapy, and for the possibility of meaningful evaluation of clinical trials, as the radiation absorbed dose is correlated to tissue response. The aim of the studies described in this thesis was to investigate and devise novel approaches to mixed beam dosimetry in boron neutron capture therapy (BNCT). The computer model of the epithermal neutron beam at the Studsvik BNCT facility was described and experimentally verified. Good agreement was generally obtained in phantom (<5%) for the photon and fast neutron absorbed dose, and thermal neutron fluence, which can be seen as verification of the computer model. However, the calculated in-air photon kerma (photon contamination) of the beam was about 39% lower than the measured value. The effective energy of this photon contamination was investigated using photon kerma transmission through bismuth. The calculated photon spectrum consisted mainly of 478 keV gamma-rays from boron neutron capture in the collimator, while the measurements indicated a higher effective energy; an actual photon spectrum comprising of 53% and 47% relative 478 keV and 2.22 MeV fluence free in air, respectively, reproduced the measured transmission within 3%. The validated computer model of the mixed beam was subsequently used to calculate beam-dependent correction factors for the detectors used for dosimetry under reference conditions. Efforts were made to adhere to the (kQ) formalism and practice used in conventional radiotherapy, rather than that previously used in neutron radiotherapy. The calculations showed that commonly used ionization chambers could be employed in the epithermal neutron beam at Studsvik with kQ factors ranging between 1.02 and 1.10. The methodology employed ensured measurements with uncertainties of the thermal neutron fluence (1.4%, 1 SD), and the photon absorbed dose (2.5%, 1 SD) comparable to conventional radiotherapy standards. The measurement of the fast neutron absorbed dose, however, was associated with substantial uncertainties (24%, 1 SD). Clinical trials ensued, and the treatment procedure, as well as aspects of the clinical dosimetry, at the Studsvik facility is discussed in this thesis. Novel methods for verification of the clinical dosimetry in BNCT, which are presented in this thesis, ensure therapeutic safety. The methods presented include verification of the clinical dosimetry both prior to and following therapy. The verification employed prior to therapy was shown to have an uncertainty of 6.0% (1 SD), while the in vivo dosimetry method utilized in a post-therapy analysis benefits from improvements, as the uncertainty of 11.2% (1 SD) was estimated. This thesis presents a comprehensive discourse on the mixed beam dosimetry of epithermal neutron beams designed for BNCT.

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