Monte Carlo evaluation of the dose perturbation effect of various hip prostheses during pelvic megavoltage photon radiotherapy
| dc.contributor.advisor | Du Plessis, F. C. P. | |
| dc.contributor.author | Mahuvava, Courage | |
| dc.date.accessioned | 2017-01-23T09:56:07Z | |
| dc.date.available | 2017-01-23T09:56:07Z | |
| dc.date.issued | 2016-11 | |
| dc.description.abstract | English: Introduction: Hip prostheses (HPs) are routinely used in hip augmentation surgery to replace painful or dysfunctional hip joints, especially in the elderly population. A number of patients with HPs are undergoing pelvic radiotherapy (RT) for localised prostate cancer. However, radiographic discrepancies between high-density and high-atomic-number (Z) inserts and surrounding tissue may cause considerable dose perturbations within the target volume and in regions where tissues interface with the prosthetic device. Furthermore, conventional treatment planning systems (TPSs) do not accurately predict dose effects incurred around metallic implants. Therefore, concerns regarding dose inhomogeneities near the prosthesis always arise, especially in patients with bilateral hip prostheses (bHPs) who require teletherapy of prostate cancer, where the tumour typically lies between the prostheses. The aim of this study was to evaluate the dosimetric effect of various HPs during 3D conformal prostate RT using Monte Carlo (MC) simulations. Materials and methods: The MC radiation transport simulation user-code BEAMnrc was used to simulate an Elekta Precise linear accelerator (linac) head, based on the manufacturer’s specifications. The MC linac model was validated by comparing dosimetric features including depth dose and dose profile data simulated in a cubic water tank (WT) with measured values. DOSXYZnrc was used to calculate 3D absorbed dose distributions in a CT based phantom (patient model) with and without HPs. Simulations were performed for 6, 10, 15 and 20 MV conformal photon beams using different beam arrangements. Three treatment plans were generated by XiO TPS and incorporated into MC simulations: a four–field (4F) box plan, a five–field (5F) plan and a six–field (6F) plan. The planning target volume (PTV) was generated by a 1 cm expansion of the prostate alone. The HP materials used were stainless steel (SS316L), titanium (Ti6Al4V) and ultra-high-molecular-weight-polyethylene (UHMWPE). These prosthetic models were manually drawn into the CT dataset from actual CT images of the patient pelvis using MCSHOW graphical user interface (GUI). The prosthesis was made part of the patient using a locally-developed Interactive Data Language (IDL) code that converts the density of the drawn volume into the desired HP material density. Both unilateral and bilateral models were considered in the simulations and dose perturbation factors (DPFs) were calculated on the proximal and distal interfaces of the implant. The dose reduction in the PTV as well as the dose to critical organs was also evaluated. Results: Results indicated that the central axis depth dose within and beyond the inhomogeneity drops significantly due to beam attenuation. For patients with bHPs, the dose contribution from lateral ports at 6 MV was attenuated by up to 23% and 17% for SS316L and Ti6Al4V, respectively. For a unilateral HP (uHP), the respective dose attenuations were 19% and 12%. The dose perturbation was always < 1% for a patient fitted with UHMWPE. Up to 38% dose increase was found at the proximal bone–HP interface due to backscattered electrons from the metal implant. There was a weak dependence of dose distribution on beam energy at the target isocenter, with the maximum dose reduction ranging only from 22.8 to 16.9% from 6 to 20 MV in a patient with bilateral steel HPs. However, interface effects were more pronounced at higher beam energies. However, increasing the number of treatment beams improved the plan quality. The greatest PTV dose perturbation was observed in a 4F box and lowest in a 6F plan. Production of scatter radiation was found to be larger for backscatter compared to forward scatter in this study. Conclusions: The dose perturbation effect of metallic HPs is significant and must be taken into account during treatment planning. UHMWPE poses no significant dose perturbation in the shadow of the implant and on the interface with tissue or bone. The use of MC–based TPSs is recommended for treatments using beam portals passing through HPs. MCSHOW allows the addition of HP contours in the virtual phantom from CT dataset of a patient without a HP. This allows one to carry out MC calculations for several implant models without metal artefacts. Results also highlight the significant influence of the implant’s composition and the beam position relative to the HP as well as beam energy on the dose distribution. Increasing the beam energy may help overcome the attenuation effects of metallic HPs and to improve target coverage. Therefore this study recommends plans with a larger number of beams that would allow avoiding the hip inhomogeneity in order to effectively compensate for dose attenuated in fields passing through HPs. 1It is also evident from the results that the shadowing effect is density-dependent, and its maximum value is for the SS316L HP. A more sophisticated, non-coplanar beam orientation may be necessary to avoid the HPs whilst sparing organs at risk (OARs) and giving sufficient target coverage. | en_ZA |
| dc.description.abstract | Afrikaans: Inleiding: Heup prostese (HPS) word gereeld gebruik in heupvervanging chirurgie om pynlike of abnormale heupgewrigte te vervang, veral in die bejaarde bevolking. 'n Aantal pasiënte met HPS ondergaan bekken bestraling (RT) vir gelokaliseerde prostaat of servikskanker. Dit kan egter radiografiese verskille veroorsaak tussen ‘n hoë-digtheid materiaal met 'n hoë atoomgetal (Z) en omliggende weefsel. Die effek veroorsaak aansienlike dosis versteurings binne die teiken volume. Konvensionele behandeling beplanning stelsels (BPSs) bereken dosis effekte nie akkuraat rondom metaal inplantings nie. Daarom bestaan daar kommer oor dosis nie-homogeniteite naby die prostese, veral in pasiënte met bilaterale heup prostese (bHPs) wat teleterapie van prostaatkanker ontvang, waar die gewas gewoonlik tussen die prostese geleë is. Die doel van hierdie studie is om die dosimetriese effek van verskillende HPS tydens 3D konforme prostaat bestraling met behulp van Monte Carlo (MC) simulasies te evalueer. Materiale en metodes: Die MC bestralingstransport simulasie kode, BEAMnrc, is gebruik om 'n Elekta Precise lineêre versneller (linac) se kop na te boots, en is gebaseer op die vervaardiger se spesifikasies. Die MC linac model is getoets deur dit te vergelyk met diepte dosis en dosis profiel data soos gesimuleer in 'n kubiese waterbad (WB) en gemete waardes. DOSXYZnrc is gebruik om 3D geabsorbeerde dosis te bereken in 'n RT-gebaseerde fantoom (pasiënt model) met en sonder HPS. Simulasies is uitgevoer vir 6, 10, 15 en 20 MV foton velde met verskillende kofigurasies. Drie behandelingsplanne is opgestel deur ‘n Xio BPS naamlik 'n vier-veld (4F) kubiese plan, 'n vyf-veld (5F) plan en 'n ses-veld (6F) plan. Die beplannings parameters vanaf XiO is in die MC simulasies gebruik vir die studie. Die beplannings teiken volume (PTV) is gegenereer deur 'n 1 cm uitbreiding van die prostaatrand. Die HPS materiaal was vlekvrye staal (SS316L), titanium (Ti6Al4V) en ultra-hoë-molekulêre gewig-poliëtileen (UHMWPE). Hierdie prostetiese modelle is ingetrek in die RT beelde vanaf werklike RT beelde van die pasiënt pelvis, met behulp van die MCSHOW grafiese gebruikerskoppelvlak (GGK). Die prostese is deel van die pasiënt gemaak met behulp van 'n plaaslik ontwikkelde Interaktiewe Data vertaler kode (IDL) wat die digtheid van die gemete volume in die gewenste HPS materiaal digtheid omskakel. Beide eensydige en bilaterale modelle is oorweeg in die simulasies en dosis versteuringsfaktore (DVFs) is bereken op grond van die proksimale en distale koppelvlakke van die inplanting. Die vermindering in dosis in die PTV sowel as die dosis om kritieke organe is ook geëvalueer. Resultate: Die resultate dui daarop dat die diepte dosis binne en buite die nie-homogeniteit beduidend val weens bundel attenuasie. Vir pasiënte met bHPs, is die dosis bydrae deur laterale velde op 6 MV verswak met tot 23% en 17% vir SS316L en Ti6Al4V, onderskeidelik. Vir 'n enkele HPS, was die dosis verswakking 19% en 12% onderskeidelik. Die dosis versteuring was < 1% vir 'n pasiënt wat toegerus is met UHMWPE HPS. Tot ‘n 38% verhoogde dosis is gevind by die proksimale been–HPS–koppelvlak te danke aan terug verstrooide elektrone van die metaal inplanting. Daar was 'n geringe afhanklikheid van die verspreiding van dosis op m.b.t bundel energie by die beplannings teiken geleë by die iso-senter, met die maksimum dosis verlaging wat wissel tussen 22,8 tot 16,9% van 6 tot 20 MV in 'n pasiënt met bilaterale staal HPS. Koppelvlak effekte was egter meer opgemerk teen hoër straal energie. ‘n Toename van die aantal behandelings velde het die plan gehalte verbeter. Die grootste PTV dosis versteurings is waargeneem in 'n 4F kubus en laagste in 'n 6F plan. Die produksie van verstrooide strale was geneig om meer te wees a.g.v die prostese soos vergelyk met fotone wat voorwaarts verstrooi. Gevolgtrekkings: Die dosis versteurings effek van metaal HPs is betekenisvol en moet tydens die behandeling beplanning in ag geneem word. UHMWPE het geen beduidende dosis versteuring in die skaduwee van die inplanting en op die koppelvlak met weefsel of been nie. Die gebruik van MC-gebaseerde BHP word aanbeveel waar stralings behandelings gebruik word wat deur HPS beweeg. MCSHOW stel ons instaat om HP kontoere op die RT data van die pasient toe tevoeg. Dit stel ons in staat om MC berekeninge vir 'n paar inplanting modelle uit te voer sonder metaal artefakte. Resultate beklemtoon die belangrike invloed van die inplanting se samestelling en die veld posisie relatief tot die HPS asook bundel energie op die verspreiding dosis. Die verhoging van die bundel energie kan ons help om die attenuasie effek van metaal HPS te oorkom en om teikendosis te verbeter. Hierdie studie beveel aan planne met meer velde om nie-homogeniteite te omseil wat vir bundel attenuasie verantwoordelik is soos HPS. Dit is ook duidelik uit die resultate dat die grootste effek die digtheidsafhanklik is en dat sy maksimum waarde is vir die SS316L HP. 'n Meer gesofistikeerde, nie-saamvlakkige dosis oriëentasie mag nodig wees om die HPS vermy, terwyl organe bespaar word. | af |
| dc.identifier.uri | http://hdl.handle.net/11660/5388 | |
| dc.language.iso | en | en_ZA |
| dc.publisher | University of the Free State | en_ZA |
| dc.rights.holder | University of the Free State | en_ZA |
| dc.subject | Dissertation (M.Med.Sc. (Medical Physics))--University of the Free State, 2016 | en_ZA |
| dc.subject | Prostate cancer | en_ZA |
| dc.subject | Hip prosthesis | en_ZA |
| dc.subject | Pelvic radiotherapy | en_ZA |
| dc.subject | Dose perturbation | en_ZA |
| dc.subject | Treatment planning | en_ZA |
| dc.subject | Photon emission | en_ZA |
| dc.subject | Monte Carlo method | en_ZA |
| dc.title | Monte Carlo evaluation of the dose perturbation effect of various hip prostheses during pelvic megavoltage photon radiotherapy | en_ZA |
| dc.type | Dissertation | en_ZA |