Kalibracija radiokromskih filmova za dozimetriju kliničkih fotonskih snopova

master's thesis

• Title (english): Radiochromic film calibration for clinical photon beam dosimetry
• Author: Giovanni Ursi
• Mentor: Tomislav Bokulić (mentor)
• Committee member: Tomislav Bokulić (predsjednik povjerenstva)
• Committee member: Matko Milin (član povjerenstva)
• Committee member: Nils Paar (član povjerenstva)
• Committee member: Tamara Nikšić (član povjerenstva)
• Committee member: Damir Bosnar (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Physics); Zagreb
• Defense date and country: 2022-09-23, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Physics

Abstract

Radiokromski film standardni je dvodimenzijski detektor ionizirajućeg zračenja koji se u kliničke svrhe u radioterapiji koristi za relativnu dozimetriju uglavnom fotonskih i elektronskih snopova. Izlaganje ionizirajućem zračenju inducira proces polimerizacije aktivnog sloja filma uzrokujući promjenu optičkih svojstava, prije svega optičke transmitivnosti u vidljivom dijelu spektra. Očitanje odziva filma vrši se standardnim fotografskim skenerom. Odziv filma u ovisnosti o apsorbiranoj dozi je nelinearan i opisan kalibracijskom krivuljom. U ovom radu radiokromski filmovi EBT3 zračeni su fotonskim snopovima medicinskog linarnog akceleratora nominalnog ubrzavajućeg napona 6 MV i 10 MV te Leksellovog gama noža (60^Co, 1.25 MeV) u rasponu apsorbiranih doza 0.4 Gy - 8.0 Gy. Na podatke ovisnosti apsorbirane doze o optičkoj gustoći prilagođene su u literaturi najčešće korištene analitičke kalibracijske funkcije za sva tri promatrana optička kanala (crveni, zeleni, plavi). Procijenjene su nepouzdanosti apsorbirane doze određene iz ozračenih filmova, a izračunati su i izlazni faktori navedenih fotonskih snopova linearnog akceleratora za veličine polja 1 cm x 1 cm do 7 cm x 7 cm kojima su filmovi bili ozračeni. Analiza nepouzdanosti apsorbirane doze određene filmom pokazala je da crveni optički kanal i dvoparametarska eksponencijalna kalibracijska funkcija fiksnog eksponenta rezultiraju najmanjom nepouzdanosti od približno 4%. Omjer signala i šuma promatranih filmova sugerira da se prilikom kalibracije za postizanje manje standardne devijacije nepouzdanosti apsorbirane doze treba koristiti i neozračeni film. Trokanalna metoda filmske dozimetrije poboljšala je integralnu uniformnost u slikama apsorbirane doze u crvenom kanalu i nepouzdanost apsorbiranih doza u filmovima zračenim dozama do 2 Gy. U području promatranih kvaliteta fotonskih snopova nije primijećena ovisnost odziva filma o energiji ili apsorbiranoj dozi. Rezultati pokazuju veća odstupanja za manje vrijednosti polja u odnosu na rezultate dobivene mjerenjem dijamantnim i poluvodičkim detektorom te veći interval nepouzdanosti za veća polja. Izlazni faktori gama noža, određeni za kolimatore veličine 4 mm i 8 mm u slaganju su s vrijednostima Monte Carlo simulacija.

Abstract (english)

Radiochromic film is a standard two-dimensional detector of ionizing radiation used in clinical radiotherapy for relative dosimetry of photon and electron beams. Upon exposure to ionizing radiation, the active film layer begins a process of polymerization causing a change in optical properties, particularly optical transmittance in the visible spectrum. A standard flatbed scanner is used for digitising the film. Film response has a non-linear relationship with the absorbed dose which is described by the calibration curve. In this thesis, EBT3 radiochromic films were irradiated using clinical photon beams generated by a linear accelerator with a nominal accelerating potential of 6 MV and 10 MV as well as Leksell gamma knife beams (60^Co, 1.25 MeV). Absorbed doses delivered to the film were between 0.4 Gy and 8.0 Gy. The most used analytical calibration functions were fitted to optical density data obtained in all three analysed colour channels (red, green, blue). The uncertainties of the absorbed dose determined from irradiated films were estimated and the linear accelerator beam output factors were calculated for field sizes in a range from 1 x 1 cm2 to 7 x 7 cm2. The absorbed dose uncertainty analysis showed that a red optical channel data and the two-parameter exponential calibration function with a fixed exponent exhibit the lowest uncertainty of about 4%. The calculated signal-to-noise ratio showed that to minimize the absorbed dose uncertainty, a pre-irradiation film scan should be used in the calculation of the film net optical density. The triple-channel calibration approach reduced dose integral uniformity and dose uncertainty in the red channel on films irradiated with a dose up to 2 Gy. Film response was found not to be energy and absorbed dose-dependent in examined beam qualities and dose range. Film determined output factors for linear accelerator beams showed larger deviations for smaller fields compared to measurements with diamond and semiconductor detectors. For larger field sizes, larger uncertainties were observed. Output factors measured for gamma knife collimators with radii of 4 mm and 8 mm agreed well with Monte Carlo simulations.

• Keywords: radiokromski film; EBT3; filmska dozimetrija; izlazni faktori
• Keywords (english): radiochromic film; EBT3; film dosimetry; output factors
• Language: croatian
• URN:NBN: urn:nbn:hr:217:485665
• Study programme: Title: Physics; specializations in: Research; Course: Research; Study programme type: university; Study level: integrated undergraduate and graduate; Academic / professional title: magistar/magistra fizike (magistar/magistra fizike)
• Type of resource: Text
• File origin: Born digital
• Access conditions: Open access
• Created on: 2022-10-19 11:57:42