Evaluation of a commercial radiation oncology treatment planning system against Monte Carlo simulated dose distributions - Shaw, William (University of the Free State, 2007)
A method is described in this study whereby dose distributions calculated by a treatment planning system (TPS) were evaluated by using dose distributions calculated with Monte Carlo (MC) simulations. The MC calculated dose data were used as a benchmark. A generic Siemens MD 2 linear accelerator was simulated with the BEAMnrc MC code to obtain beam specific dynamic variables in a phase space file (PSF) related to particle fluence in a plane at a known distance from a water phantom. Dose distributions from various field sizes were produced by simulations with the DOSXYZnrc MC code. Two datasets were produced consisting of percentage depth dose (PDD), profiles and diagonal profile data for 6 and 15MV x-ray beams. The CadPlan TPS was commissioned with these datasets for both energies. Analyses of TPS calculated dose distributions were done in a water phantom and dose distributions for various clinical cases on patient CT data. Patient CT datasets were transformed into patient CT models that were suitable for dose calculations with DOSXYZnrc. These models consisted of various media with various densities for which interaction cross section data is available. Dose distributions for a number of clinical treatment plans could be devised on both the TPS and DOSXYZnrc. These included head and neck, breast, lung, prostate, oesophagus and brain plans. Calculations on the TPS were done for the Single Pencil Beam (SPB) and in some cases the Double Pencil Beam (DPB) convolution algorithms in combination with the Batho and ETAR (Equivalent Tissue-air ratio) inhomogeneity correction algorithms. Dose distributions were normalized to the depth of maximum dose (dmax) for single fields and to the ICRU reference point in full treatment plans. The location of these points was the same for the TPS and DOSXYZnrc distributions. PDD curves, beam profiles, dose-volume histograms (DVHs) and equivalent uniform doses (EUDs) were produced to aid in the evaluation of the TPS dose calculation accuracy. Results demonstrated that the assumptions in the convolution models used to produce beam penumbra regions, especially in blocked field cases, fail to account for scattered dose contributions outside the treatment field and overestimated the dose underneath small or thin shielding blocks. The PB algorithms in combination with the inhomogeneity corrections show total disregard for lateral and longitudinal electron transport through heterogeneous media. This effect is pronounced in regions where electronic equilibrium is not found, like low density lung. This region, in combination with high density bone nearby, proved even larger discrepancies as dose absorption decreases in low density media and increases in high density media. A small 15 MV field passing through lung tissue exhibited large dose calculation errors by the PB algorithms. The dataset produced here is flexible enough to be used as a benchmark for any TPS utilizing commissioning measurements in water. This method can address commissioning results as well as any clinical situation requiring dose calculation verification.

In hierdie studie word ‘n metode bespreek waardeur dosis distribusies wat met ‘n behandelingsbeplanning sisteem (TPS) bereken is, ge-evalueer kan word met dosis distribusies wat deur middel van Monte Carlo (MC) simulasies bereken word. Die MC berekende dosis data was as verwysings data gebruik. Die BEAMnrc MC kode was gebruik om ‘n Siemens MD2 lineêre versneller te simuleer sodat bundel spesifieke dinamiese veranderlikes gestoor kon word in ‘n faseruimte-lêer. Hierdie faseruimte-lêer was geskep op ‘n bekende afstand vanaf ‘n water fantoom. Dosis distribusies was bereken vir verskeie veld groottes met die DOSXYZnrc MC kode. Twee datastelle was geskep wat bestaan uit persentasie diepte dosis (PDD), bundel profiele, en diagonal profiel data vir 6 en 15MV x-straal bundels. Die CadPlan TPS was in gebruik gestel met hierdie datastelle vir beide energië. Die analiese van die TPS berekende dosis distribusies was op water fantoom data uitgevoer en die distribusies van verkeie kliniese gevalle was met behulp van rekenaartomografie (RT)- gebasseerde data uitgevoer. Die pasiënt RT beelddata was omgeskakel na pasiënt RT modelle wat geskik was om berekeninge met behulp van DOSXYZnrc uit te voer. Hierdie modelle het bestaan uit verskeie media met verskillende digthede waarvoor daar interaksie deursnit data beskikbaar is. Dosis verspreidings kon nou bereken word vir ‘n aantal kliniese behandelings gevalle met die TPS en DOSXYZnrc. Hierdie gevalle het bestaan uit ‘n kop en nek, bors, long, prostaat, esofagus en brein plan. Die berekeninge op die TPS was uitgevoer met behulp van die Enkel Dun Bundel (SPB) en Dubbel Dun Bundel (DPB) konvolusie algoritmes gekombineer met die Batho en ETAR (Ekwivalente Weefsel-lug verhouding) heterogeniteitskorreksie algoritmes. Die dosis verspreidings was genormaliseer by die diepte waar die maksimum dosis verkry word (dmax) vir enkel velde en by die ICRU verwysingspunt in geval van die gesommeerde dosis distribusies. PDD krommes, bundel profiele, dosis-volume histogramme (DVHe) en ekwivalente uniforme dosisse (EUDe) was geskep om die TPS se dosis berekening akkuraatheid mee te evalueer. Die resultate toon dat die aannames wat gemaak word in die konvolusie modelle om die bundel penumbra mee te skep, veral in die geval van afgeskermde (geblokte) velde, nie daarin slaag om vir verstrooide dosis bydraes buite die behandelings veld te korrigeer nie en oorskat ook die dosis onder klein en dun afskermings blokke. Dit bleik dat die PB algoritmes, gekombineer met die heterogeniteitskorreksies, geensins oorweging skenk aan die laterale en longitudinale elektron voortplanting binne heterogene media nie. Hierdie effek word veral beklemtoon in areas waar daar nie elektron ekwilibrium teenwoordig is nie, soos in die geval van lae digtheid long weefsel. Verskille was groter in sulke areas wat gekombineer is met nabygeleë hoë digtheid been aangesien dosis absorbsie afneem in lae digtheid media en toeneem in hoë digtheid media. ‘n Ondersoek na ‘n klein 15 MV veld wat deur long dring het getoon dat groot foute in dosis berkening deur die PB algoritmes gemaak word. Die datastelle wat tydens hierdie studie geskep was, is universeel genoeg om as verwysings data vir enige TPS gebruik te word wat van gemete water fantoom data gebruik maak tydens ingebruikneming. Hierdie metode kan resultate van sulke ingebruiknemings toetse aanspreek, asook die dosis verifikasie van enige kliniese gevalle. om ‘n kind op te voed. Geleenthede vir gemeenskappe wat bewus is van die uitdagings wat skole in die gesig staar, in terme van die implementering of effektiewe implementering van beleide soos die NSLP, moet in ag geneem word. Die gevolg is dat die oplossings tot hierdie uitdagings en die teweeg bring van verandering in die onderwysstelsel as maniere om leerders se akademiese prestasie te verbeter, deur hulle sal kom. Die formulering van strategieë, ten opsigte van die implementering van die NSLP as ‘n poging om leerders se akademiese prestasies te verbeter, is gebaseer op die literatuurstudie sowel as op die besprekings met die deelnemers. Vervolgens is die deelnemers ook aangespoor deur die woorde van Abraham Lincoln tydens sy 1862 toespraak aan die parlement: “The dogmas of the quiet past are inadequate to the stormy present. The occasion piled high with difficulty and we must rise to the occasion. As our case is new, so must we think anew” (Lincoln, 1953: 537).
Dissertation (M.Med.Sc. (Medical Physics))--University of the Free State, 2007
Keywords: Fluence; Electronic equilibrium; Water phantom; Inhomogeneity; Dose distributions; DOSXYZnrc; BEAMnrc; Monte Carlo method; Pencil beam algorithm; Treatment planning system; Radiation -- Dosage; Algorithms