Karakterizacija fotona i neutrona megavoltažnih terapijskih akceleratora

Abstract

Poznavanje spektra x-zračenja kao i drugih osobina je od izuzetnog značaja kako za planiranje radijacione terapije tako i sa aspekta zaštite pacijenta i osoblja. Takođe, poznavanje svojstava neutrona nastalih u interakciji fotona sa materijalima oko akceleratora, je od izuzetnog značaja sa dozimetrijskog aspekta. Poslednjih godina sve se više upotrebljavaju linearni akceleratori bez homogenizacionog filtera tzv. FFF (Flatteninig Filter Free), a kako su karakteristike snopa u toma načinu rada akceleratora još uvek nedovoljno ispitane, njima smo posvetili posebnu pažnju. Urađena su i detaljna dozimetrijska merenja fotona kako u radu sa homogenizacionim filterom (FF) tako i bez njega (FFF), kao i dozimetrijsko merenje neutrona. Direktno merenje samog spektra nekim od detektora (NaI ili HPGe) nije moguće zbog visokih intenziteta stoga smo u radu koristili indirektne metode i to aktivacionu analizu i atenuacionu analizu. Urađena su detaljna dozimetrijska merenja ekvivalentne doze neutrona u terapijskoj sobi sa dva zaokreta hodnika i na osnovu rezultata predložena je korekcija metode za izračunavanje neutronske doze na ulaznim vratima. Fotoaktivacionimmerenjima, gde su se u oba načina rada terapijskog akceleratora pratile aktivnosti izomera 115m In, ustanovljeno je da se kod dva tipa akceleratora (Elekta Versa HD, Varian TrueBeam) oblik spektra u visokoenergetskom delu znatno razlikuje, kao i da postoji razlika u obliku spektra na ivici polja i u njegovoj sredini. Atenuacionom analizom na osnovu standardnog metoda rekonstrukcije fotonskog spektra putem Laplasovih transforma, dobijene su izvesne razlike u oblicima spektara za dva režima rada. Takođe, rekonstrukcijom spektra putem odabira parametara korekcine funkcije koja bi teorijski oblik spektra (Šifova funkcija) trebao da prevede u realan spektar za oba režima rada dobijene su izvesne razlike u oblicima spektra. Dozimetrijska merenja fotonske doze na ulaznim vratima terapijske sobe sa jednim zaokretom, sa i bez homogenizacionog filtera za različite veličine polja i sa i bez vodenog kanistera koji simulira pacijenta u snopu, pokazuju da je izmerena doza na ulaznim vratima u FFF modu rada uvek niža od izmerene doze u slučaju da se koristi homogenizacioni filter.

Knoweledge of the x-ray spectrum as well as other characteristics of the beam is of great importance both for the planning of radiotherapy as well as for the protection of patients and staff. Also, the knowledge of the neutron properties generated in the interaction of photons with materials around the accelerator is of great importance from the dosimetric aspect. In recent years, linear accelerators are increasingly used without a flattening filter, so-called FFF (Flatteninig Filter Free), and as the characteristics of the beam in this mode of operation of the accelerator are still insufficiently investigated, we paid special attention to them. Detailed dosimetric measurements of photons were performed both with (FFF) and without the flattening filter (FF), as well as the dosimetric measurement of the neutron. Direct measurement of the spectrum itself by detectors (NaI or HPGe) is not possible due to high intensities, therefore, we used indirect methods in this work, both activation analysis and attenuation analysis. Detailed dosimetric measurements of the neutron equivalent dose in the therapeutic room with two band maze were made and based on the results, correction of the method for calculating the neutron dose at the entrance door was proposed. It was found by photoactivation measurements, where the activities of the isomer 115mIn were investigated, that in the two types of accelerators (Elekta Versa HD, Varian TrueBeam) the spectrum shape in the high-energy part is significantly different, and that there is a spectrum difference in the edge of the field and in the middle of the field. Attenuation analysis, based on the standard method of reconstructing the photon spectrum through Laplace transforms, showed some differences in the spectra of the two working modes of accelerators. Also, by reconstructing the spectrum by correction function that should transform the theoretical spectrum form (the Schiff function) into a real spectrum for both modes of operation, certain differences in spectrum shapes were obtained. Dosimetric measurements of the photon dose at the entrance door of treatment room with a single maze with and without a homogenizing filter for different field sizes and with and without a water canister which simulates the patient in a beam showed that the measured dose at the entrance door in the FFF mode is always lower than the measured doses in FF mode.