Monte Carlo simulations of spectral properties of clinical photon beams with Geant4
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2019Copyright
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Tutkielma tehtiin yhteistyössä Varian Medical Systems Finland -yrityksen kanssa. Tutkielmassa keskityttiin ulkoiseen alle 15 MeV energian säteilylähteseen, jolla tarkoitetaan yleensä sitä, että säteilykeila on tuotettu lineaarikiihdyttimellä. Tutkielman tavoitteena oli tutkia fotonilähteen puolivarjoa vesifantomissa ja kohtion avulla synnytettyä fotonikeilaa sekä sen spektriä. Simulaatiot pyrkivät kuvaamaan fysikaalista tilannetta todenmukaisesti ja niistä saatava tieto on käytännössä hyvin vaikea saada kliinisen ympäristön ja ilmaisimien rajoitteissa. Informaatio on tärkeää sädehoidon laadunvalvonnalle ja sen suunnittelun kehitykselle.
Tutkielmassa käytettiin apuna Monte Carlo -menetelmään perustuvaa Geant4 simulaatio- ja ohjelmointikirjastoa. Geant4-applikaation eri parametrejä tutkittiin ulkoisen fotonilähteen puolivarjon määritelmän avulla. Kolmea eri fysiikkakirjastoa verrattiin ja tulokset olivat samankaltaiset, mutta niissä näkyi myös eroja.
Parametrien validoinnin jälkeen tutkittiin kolmen eri kohtiomateriaalin (volframi, lyijy sekä näiden yhdistelmä) hiukkas- ja energiaspektrejä. Spektreissä näkyi alkuaineille ominaiset karakteristisen säteilyn piikit sekä voimakas annihilaatiopiikki. Tulokset olivat samankaltaiset Varian Medical System -yrityksen tarjoaman datan kanssa. Energiaspektrien kulmariippuvuus havaittiin volframi ja lyijy kohtioille selkeästi kolmella eri kulmalla 0°, 5.7° ja 11.3°. Kulman kasvaessa muutos spektrissä näkyi huipun siirtymisenä matalemmille energioille ja hännän kurvikkuuden pienentymisenä. Kyseiset tulokset vaativat lisätutkimuksia.
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This Master's Thesis was made in cooperation with Varian Medical Systems Finland. The main focus of the thesis is on the external photon beam with energy below 15 MeV. The aim of this thesis is to obtain information about the beam edge and the spectrum of the photon beam, which are difficult to measure in practice because of the clinical limitations.
The information was obtained by Monte Carlo simulations with a Geant4 application. The effects of user-editable parameters of Geant4 were studied by looking at the behavior of the penumbra region of an external beam. There were three physics lists compared and calculated results showed similarity but rather big differences.
After the validation of parameters, particle and energy spectra with three different targets (tungsten, lead, and a combination of these two) at the isocenter plane were studied. There were the characteristic peaks of the corresponding element and the annihilation peak shown in the spectra. These results were consistent with the older data provided by Varian Medical Systems. The angle dependence of spectra with tungsten and lead targets was shown to be significant with different angles 0°, 5.7° and 11.3°. The shape of the spectra detected to shift and curvaceousness of the tail decrease with an increasing angle. Further studies are needed to evaluate these results.
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