Title Experimental determination of field output factors and detector specific output correction factors in small fields of megavoltage radiotherapy beams Title (croatian) Eksperimentalno određivanje faktora polja i specifičnih korekcijskih faktora odziva detektora u malim poljima megavoltnih radioterapijskih snopova Author Božidar Casar Mentor Slaven Jurković (mentor)Mentor Robert Jeraj (komentor)305625654 Committee member Marija Majer (predsjednik povjerenstva)Committee member Mladen Kasabašić (član povjerenstva)Committee member Željka Knežević Medija (član povjerenstva)Granter University of Zagreb Defense date and country 2019-04-18, Croatia Scientific / art field, NATURAL SCIENCES Universal decimal classificationUDC ) 53 - Physics Abstract The goal of this work was to determine field output factors, Ω^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' using EBT3 radiochromic films and W1 plastic scintillator as reference detectors on two different linear accelerators. Secondly, we aimed to provide a large and consistent set of data for detector specific output correction factors, 𝑘^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' for small static fields for seven solid state detectors and seven small ionization chambers. In the case of
... More ionization chambers, 𝑘^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' were determined in two orientations – parallel and perpendicular, to test the hypothesis, that 𝑘^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' are lower and therefore advantageous for parallel orientation. Ω^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' were determined for 6 and 10 MV photon beams with and without flattening filter and for nine fields ranging from 0.5 x 0.5 cm2 to 10 x 10 cm2. Signal readings obtained with EBT3 radiochromic films and W1 plastic scintillator were fitted by an analytical function. 𝑘^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' were determined empirically for 14 clinically used small detectors, under the same experimental conditions. It has been found that, for a given linear accelerator, Ω^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' need to be determined for every combination of photon beam energy, filtration, and field size as the differences between them can be statistically significant (p < 0.05). Further, it has been demonstrated, that the parallel orientation of ionization chamber is advantageous over the perpendicular, which is an important contribution to the present international dosimetry protocol for small fields IAEA TRS-483. Use of two reference detectors for the determination of field output factors for small static fields in MV photon beams is a novel approach and has been justified in our thesis. Large set of 𝑘^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' data for seven solid-state detectors and seven ionization chambers in four MV beams determined on two linear accelerators by a single group of researchers can be considered a valuable supplement to the literature and the IAEA TRS-483 dataset. Recommendation on the orientation of ionization chambers is the last significant outcome of our work.The goal of this work was to determine field output factors, Ω^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' using EBT3 radiochromic films and W1 plastic scintillator as reference detectors on two different linear accelerators. Secondly, we aimed to provide a large and consistent set of data for detector specific output correction factors, 𝑘^^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' for small static fields for seven solid state detectors and seven small ionization chambers. In the case of ionization chambers, 𝑘^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' were determined in two orientations – parallel and perpendicular, to test the hypothesis, that 𝑘^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' are lower and therefore advantageous for parallel orientation. Ω^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' were determined for 6 and 10 MV photon beams with and without flattening filter and for nine fields ranging from 0.5 x 0.5 cm2 to 10 x 10 cm2. Signal readings obtained with EBT3 radiochromic films and W1 plastic scintillator were fitted by an analytical function. 𝑘^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' were determined empirically for 14 clinically used small detectors, under the same experimental conditions. It has been found that, for a given linear accelerator, Ω^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' need to be determined for every combination of photon beam energy, filtration, and field size as the differences between them can be statistically significant (p < 0.05). Further, it has been demonstrated, that the parallel orientation of ionization chamber is advantageous over the perpendicular, which is an important contribution to the present international dosimetry protocol for small fields IAEA TRS-483. Use of two reference detectors for the determination of field output factors for small static fields in MV photon beams is a novel approach and has been justified in our thesis. Large set of 𝑘^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' data for seven solid-state detectors and seven ionization chambers in four MV beams determined on two linear accelerators by a single group of researchers can be considered a valuable supplement to the literature and the IAEA TRS-483 dataset. Recommendation on the orientation of ionization chambers is the last significant outcome of our work. Less Abstract (croatian) Novi međunarodni protokol za dozimetriju uskih snopova fotona visokih energija (dalje mala polja) IAEA TRS-483 je objavljen prije nešto više od godinu dana. Njime su dane preporuke vezano za referentnu i relativnu dozimetriju malih polja, koje se temelje na vrlo opsežnim istraživanjima velikog broja istraživačkih grupa koje se bave dozimetrijom malih polja. Nedostatak izvornih podataka koji su objavljeni u protokolu je određena nekonzistentnost, odnosno njihova raspršenost, što je
... More posebice navedeno i u samom dozimetrijskom protokolu TRS-483. One se očituju u načinu određivanja specifične veličine polja (posebno veličine najmanjih polja), korištenoj radnoj udaljenosti, dubini u vodi na kojoj se mjeri ili računa, te mjestu definiranja veličine polja (na površini fantoma ili na referentnoj dubini). Unatoč tome što je značajan broj istraživanja proveden korištenjem različitih vrsta detektora (diode, male i mikro ionizacijske komore, dijamantni detektori, plastični scintilacijski detektori, alanin i radiokromski filmovi) u objavljenim podacima često nedostaje odgovarajuća procjena nesigurnosti svakog od koraka u određivanju, kako faktora polja, tako i korekcijskih faktora. U svrhu nadopune i poboljšanja protokola za dozimetriju malih polja postavljene su hipoteze doktorskog rada: Hipoteza 1: Na temelju mjerenja pomoću referentnih detektora (plastični scintilacijski detektor i radiokromski film) i određivanja faktora polja moguće je eksperimentalno odrediti ukupne korekcijske faktore za diode i ionizacijske komore u uskim snopovima X-zraka visokih energija proizvedenih linearnim akceleratorima. Hipoteza 2: Korekcijski faktori polja za male i mikro ionizacijske komore su manji ukoliko su određeni u tzv. paralelnoj orijentaciji ionizacijskih komora. Tako određeni faktori predstavljaju poboljšanje u odnosu na one određene u okomitoj orijentaciji ionizacijskih komora koja je preporučena u dozimetrijskom protokolu TRS-483. Primjena dva referentna detektora, EBT3 radiokromskog filma i W1 plastičnog scintilacijskog detektora, kao i primjena analitičke funkcije predstavljaju novi i potpuno opravdan pristup određivanju faktora polja uskih snopova X-zraka visokih energija, čime je potvrđena prva hipoteza. Nadalje, određen je značajan skup korekcijskih faktora 𝑘 ^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' za šest dioda, jedan dijamantni detektor te sedam ionizacijskih komora u po četiri snopa X-zraka različitih veličina polja zračenja na dva linearna akceleratora. To je napravljeno slijedeći stroge eksperimentalne zahtjeve pa se može smatrati vrijednom nadopunom recentnog dozimetrijskog protokola za dozimetriju malih polja TRS-483. Nadalje, rezultati provedenog istraživanja vezani za određivanje korekcijskih faktora 𝑘 ^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' ionizacijskih komora, unatoč tome što su u suprotnosti s preporukama danim u dozimetrijskom protokolu TRS-483, potvrđuju našu drugu hipotezu. Dakle, korekcijski faktori 𝑘 ^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓' su manji ukoliko se mjerenje ionizacijskom komorom provodi u paralelnoj orijentaciji. Zbog toga se mjerenje ionizacijskim komorama malog volumena u paralelnoj orijentaciji preporuča kao metoda izbora za određivanje korekcijskih faktora 𝑘 ^𝑓𝑐𝑙𝑖𝑛,𝑓𝑟𝑒𝑓 _𝑄𝑐𝑙𝑖𝑛,𝑄𝑟𝑒𝑓'. To također predstavlja važnu nadopunu dozimetrijskog protokola TRS-483 u kojem je preporučena isključivo okomita orijentacija ionizacijske komore za određivanje faktora polja. Less Keywords
W1 scintillator
EBT3 films
diodes
ionization chambers
small fields
field output factors
Keywords (croatian)
dozimetrija malih polja
W1 scintilator
EBT3 film
diode
ionizacijske komore
faktori polja
Language english URN:NBN urn:nbn:hr:217:291919 Study programme Title: Physics Type of resource Text Extent 128 str. File origin Born digital Access conditions Open access Terms of use Created on 2019-04-30 12:04:16
