Dosimetry of narrow photon beams in stereotactic gamma knife neuroradiosurgery

U ovom doktorskom radu prikazani su rezultati eksperimentalnog određivanja prostornih konvolucijskih jezgri za više detektora koji se koriste za dozimetriju uskih fotonskih snopova u neuroradiokirurgiji gama nožem (PinPoint i Semiflex ionizacijske komore, p-tip poluvodički detektori i radiokromski film). Istraživanje odziva detektora provedeno je u uvjetima volumnog usrednjavanja signala i narušenja lateralne elektronske ravnoteže (engl. lateral charged particle equilibrium – LCPE) koje je svojstveno snopovima gama noža. Konvolucijske jezgre određivane su analitičkim rješavanjem integralne konvolucijske jednadžbe na sferičnom modelu doze za profile doza određene kolimatorima gama noža, uz primjenu konvolucijskog teorema za Gaussovu konvolucijsku jezgru. Zbog rasporeda snopova u kutnom rasponu 54° - 84° istražena je i kutna ovisnost odziva detektora. U konačnici je dekonvoluiran prostorni utjecaj konvolucijskih jezgri na izlazne faktore (engl. output factors – OPF) i profile doza gama noža te je kompenziran i utjecaj kutne ovisnosti detektora na OPF faktore. Za 18, 14 i 8 mm-ski kolimator, nakon korekcije profila i OPF faktora za volumni učinak usrednjavanja, komore daju rezultate koji su unutar ±0.5% od referentnih. Za 4 mm-ski kolimator, korekcijski faktori za komore Semiflex i PinPoint ne mogu kompenzirati značajni gubitak signala koji je posljedica narušenja LCPE u malim fotonskim snopovima. Za detektore Diode P i Diode E učinak usrednjavanja je minimalan te korekcija daje rezultate unutar ±0.1% od referentnih. Kutna ovisnost odziva iznosila je maksimalno 9% za poluvodičke detektore i 15% za komore. Poluvodički detektori, zbog svoje velike prostorne razlučivosti te zanemarivog učinka volumnog usrednjavanja, predstavljaju prvi izbor za dozimetriju snopova gama noža.In this doctoral thesis the results of experimental determination of spatial convolution kernels are presented for the response of several types of detectors (PinPoint and Semiflex ionization chambers, p-type semiconductor detector and radiochromic film) which are used in stereotactic Leksell Gamma Knife neuroradiosurgery for dosimetry in the narrow photon beams. Investigation of the spatial response of detectors was conducted under conditions of volumetric averaging of the measured signal and the lack of lateral charged particle equilibrium (LCPE) which are inherent to the narrow photon beams of the Gamma Knife. In this work the deconvolution method of the detector spatial influence was used for correction of measured signal for the volumetric averaging effect. The convolution kernels were determined by solving integral convolution equation on the spherical dose model for the dose profiles defined by different Gamma Knife collimators with the application of convolution theorem for the Gaussian convolution kernel. The dose profiles were measured in lateral-lateral (LL) and superior-inferior directions (SI). Analytical deconvolution of measured dose profiles allows compensation of the volumetric averaging effect, except for PinPoint and Semiflex ionization chambers for dosimetry of 4 mm photon beams which are characterized with significant loss of lateral charged particle equilibrium in the central part of dose distribution. Spatial convolution kernels of cylindrical detectors and the radiochromic film-scanner system were determined analytically and they enable correction of dose profiles for 18 mm and 14 mm collimator helmet while for 8 mm and 4 mm collimator helmet reliable correction is possible only with the use of semiconductor detector Diode P which has small active volume. Convolution kernels of cylindrical detectors tend to compensate the loss of measured signal resulting from the lack of lateral electronic equilibrium in 8 and 4 mm photon beams. This leads to conclusion that for a small photon beams convolution kernels of detectors depend on the conditions of electronic equilibrium in central part of dose distribution