The effect of compensator-induced scatter on external beam dose calculations

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Date
2003-11
Authors
Du Plessis, Frederik Carl Philippus
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University of the Free State
Abstract
English: In this study the effect of compensator-induced scatter on external beam dose calculations were studied for compensators of wax, aluminum, brass, copper and lead for 6, 8 and 15 MV parallel x-ray beams. An outline is given of the necessity for the inclusion of compensator induced scatter in the design of compensators for their use in IMRT applications. A method is described for deriving effective attenuation coefficients (EACs), as calculated by the DOSXYZ Monte Carlo (MC) code. Various properties of the EACs were studied, among which their dependence on small beam (beamlet) sizes as well as their depth dependence in water. These EACs are used for the initial approximate design of a compensator. In conjuction with these EACs, scatter and beam hardening is included in the compensator modeling process. Compensator-induced scatter and beam hardening properties were studied in some detail. The EGSnrc based DOSRZnrc MC code was used to study the evolution of a pencil beam (PB) as it traverses different thicknesses of a compensator material. It was found that the relative dose profiles of the PB could be adjusted for scatter and beam hardening using empirically derived functions, and that these adjustments were proportional to the thickness of the compensator material. A compensator planning system (CPS) is described, used in the design of a compensator. Dose calculations are performed with this CPS using the superposition method for cartesian PBs. An algorithm is described that transforms the cylindrical PB as obtained with the DOSRZnrc MC code, into a cartesian PB. The CPS was tested for a step wedge shaped compensator over square field sizes with side lengths of 5, 10 and 20 cm. A correction function was introduced to account for side penetration in the compensator. It was found that the relative dose profiles calculated with the CPS at a depth of 10 cm in water was within 1.5 percent of similar dose profile data derived from DOSXYZ MC dose calculations for a 5x5 cm2 field. For the 20x20 cm2 field, the accuracy was within 3 percent in most cases and beam energies. It is also demonstrated how an aluminum compensator can be designed by an iterative method with the CPS to yield a dose profile that conforms accurately to a pre-determined dose profile, such as would be produced by an inverse planning system for IMRT treatments.
Afrikaans: In hierdie studie is die effek van kompensator-geïnduseerde verstrooïng op eksterne bundel dosis berekeninge vir was, aluminium, brass, koper en lood kompensators ondersoek, vir paralelle x-straal bundels met energieë van 6, 8 and 15 MV. 'n Oorsig asook die noodsaaklikheid van die studie word uiteengesit t.o.v. kompensatorgeinduseerde verstrooïng en die gebruik daarvan in IMRT toepassings. 'n Metode vir die berekening van effektiewe attenuasie koefisiënte (BAKs), met die DOSXYZ Monte Carlo (MC) program, word beskryf. Verskeie eienskappe van die EAKs is bestudeer waaronder die afhanklikheid t.o.v. klein subvelde en diepte in water ondersoek is. Hierdie EAKs word gebruik om 'n voorlopige benaderde kompensator te kan ontwerp. Saam met hierdie EAKs word x-straal bundel verstrooïng en verharding dan ingesluit in die kompensator ontwerp proses, soos bestudeer. Die EGSnrc gebasseerde DOSRZnrc MC program is gebruik om die evolusie van 'n dunbundel (DB), soos dit deur verskillende diktes kompensator materiaal beweeg, te ondersoek. Daar is gevind dat die relatiewe dosis profiele van die DBs aangepas kan word vir verstrooïng en verharding deur die gebruik van empiries- afgeleide funksies, en dat hierdie aanpassings eweredig aan die dikte van die kompensator materiaal is. 'n Kompensator beplanningstelsel (KBS) word beskryf wat dan gebuik word in die ontwerp van 'n kompensator. Dosis berekeninge is met die KBS gedoen en werk op die beginsel van die superponering van kartesiese DBs. 'n Algoritme wat 'n silindriese DB (soos bereken met die DOSRZnrc program) na 'n kartesiese DB omskakel, is ontwikkel. Die KBS is getoets vir 'n stapwig vormige kompensator model vir vierkantige velde met sylengtes van 5, 10 en 20 cm. 'n Korreksie funksie is ingevoer om vir sydelingse stralingspenetrasie in die kompensator te kan korrigeer. Daar was gevind dat dosisprofiele, soos bereken met die KBS by 'n diepte van 10 cm in water binne 1.5 persent ooreengestem het van 'n soortgelyke dosis profiel wat met die DOSXYZ MC program bereken is vir 'n veld grootte van 5x5cm2. Vir 'n 20x20cm2 veld was die akkuraatheid binne 3 persent vir die meeste materiale en bundel energieë. Daar word ook gedemonstreer hoe 'n aluminium kompensator ontwerp kan word met 'n iteratiewe metode met die KBS om 'n dosis profiel te lewer wat akkuraat ooreenstem met 'n voorafbepaalde dosis profiel, soos wat deur 'n inverse beplanningstelsel vir IMRT toepassings, geproduseer sal word.
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Keywords
Compensator, Monte Carlo, DOSRZnrc, Beam hardening, Scatter, IMRT, Effective attenuation, Pencil beam, Radiation dosimetry, Electron beams -- Therapeutic use, Thesis (Ph.D. (Medical Physics))--University of the Free State, 2003
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