For megavoltage electron beams, current dosimetry protocols assume well-guarded plane-parallel ionization chambers have electron perturbation correction factors of unity. Papers by other researchers have shown the contrary. The National Physical Laboratory (NPL), Teddington UK, has an electron calibration service that requires knowledge of the ratio of perturbation factors in water to graphite for their NACP-02 chamber. Using the Monte Carlo code EGSnrcMP, perturbation correction factors at depth zref in graphite for the NPL's Radiation Dynamics linac 4 – 19 MeV were calculated. These results combined with those in water calculated by Zakikhani (2006) showed that only the ratio of perturbation factors in water to graphite for the 4 MeV and 12 MeV beams, at 1.0080 ± 0.27 % and 1.0071 ± 0.28 %, were significantly different from unity. Perturbation correction factors in water and graphite for the Varian clinical linacs 4 – 18 MeV were also investigated. Important issues that arose requiring further study included the discrepancy in results caused by 1 keV vs. 10 keV transport cutoffs and whether the depth of the effective point of measurement should account for the phantom equivalent thickness of the chamber window. / Pour les faisceaux d'électrons de haute énergie (MV), les protocoles de dosimétrie actuels supposent que les chambres d'ionisation à plaques parallèles ont des facteurs de correction de perturbation électronique égaux à l'unité. Plusieurs articles d'autres auteurs ont démontré le contraire. Le National Physical Laboratory (NPL), Teddington Royaume-Uni, possède un service d'étalonnage qui requiert de savoir les facteurs de perturbation de l'eau par rapport au graphite pour leur chambre NACP-02. A l'aide du code Monte Carlo EGSnrcMP, les facteurs de correction de perturbation à profondeur zref dans le graphite ont été calculés pour le linac Radiation Dynamics du NPL à des énergies de 4-19 MeV. Ces résultats, combinés à ceux dans l'eau calculés par Zakikhani (2006), ont montré que seulement les facteurs de perturbation de l'eau par rapport au graphite des faisceaux de 4 MeV et 12 MeV, à 1,0080 ± 0,27 % et 1,0071 ± 0,28 % respectivement, étaient significativement différents de l'unité. Les facteurs de correction de perturbation dans l'eau et dans le graphite ont aussi été étudiés pour les linacs cliniques de Varian à 4-18 MeV. D'importants problèmes ont alors été soulevés, notamment des différences dans les résultats causés par des énergies de coupure de 1 keV par rapport à 10 keV ainsi que la question de savoir si la profondeur du point de mesure devrait prendre en compte l'épaisseur effective de la fenêtre d'entrée. Ces problèmes requièrent davantage d'études.

This project is to develop methods to retrospectively determine the size of alpha-emitting particles that have been collected on personal air samplers. The alpha radiation from such particles produces a cluster of tracks on the surface of an etched nuclear track detector, CR-39. The number of tracks in a cluster, as well the diameter of the cluster, are dependent on several factors: the diameter of the hot particle, the distance between the particle and the CR-39 surface, the composition of the particle, and the alpha-particle energy. / The dependence of the alpha-emitting particle size and the number of registered tracks were revealed, and produced predictions of the track density distribution observed on the CR-39 plastic. There is a good fit between the simulation of track density observed on the CR-39 and the tracks arising from uranium oxide and plutonium oxide particles.

The problem of charge creation by x-rays in amorphous selenium (a-Se) is studied. A quantitative theory is developed which includes collective and single electron-hole pair excitations by a passing electron. This theory is incorporated into a Monte Carlo code to calculate track structures in a-Se. The initial positions of the electron-hole pairs along these tracks are used to study the fraction of pairs which recombine versus incident x-ray energy and applied electric field. The experimentally-observed energy dependence of recombination is attributed to a spur size which is dependent on the velocity of the ionizing electrons. The theory and simulations agree with available experimental data in the energy range from 20 keV to 10 MeV. / The use of an a-Se based direct-detection active matrix flat-panel imager (AMFPI) is explored at megavoltage energies for use in the verification of radiotherapy treatments. As with most other megavoltage detectors, a metal front plate is used to reduce patient scatter and to act as a buildup layer. The Modulation Transfer Function (MTF), Noise Power Spectrum (NPS), and Detective Quantum Efficiency (DQE) are measured. The DQE for the direct detection AMFPI is compared with the published DQE of an indirect detection AMFPI for portal imaging. The direct detector has a lower DQE at zero frequency, but there is a cross-over at approximately 0.3 cycles/mm after which it has a higher DQE. / A theoretical expression for the DQE of medical imaging detectors with non-elementary cascade stages is derived. This formalism can be used in conjunction with Monte Carlo techniques to evaluate the DQE of megavoltage imaging detectors. The predictions of the theory agree with the experimental DQE results for the direct-detection AMFPI and also for published results for the DQE of both a metal/phosphor detector and an indirect-detection AMFPI. / The effect of scatter on image quality is modeled in terms of the scatter fraction (SF) and scatter-to-primary ratio (SPR) using Monte Carlo techniques. To validate these simulations, the SF is measured experimentally using a prototype a-Se detector which uses an electrostatic probe to measure the a-Se surface potential. The simulations are used, along with the DQE simulations, to study the effect of metal/a-Se or metal/phosphor thicknesses on image quality in direct and indirect AMFPIs at megavoltage energies. It is found that for a-Se or phosphor thicknesses less than about 300 mum, a front plate of about 1 mm copper is optimal whereas for larger a-Se/phosphor thicknesses a front plate of about 0.4 mm may in some situations lead to better image quality.

A phantom-embedded extrapolation chamber (PEEC) made of Solid Water(TM) was used for studying the buildup region of megavoltage radiation beams. We investigated the polarity effect produced in the PEEC and found that radiation induced currents, called the Compton current, were the dominant cause of the polarity effect in the PEEC. In the dose build-up region of megavoltage photon beams, the collecting electrode is the primary Compton current source and the magnitude of the current depends on the measurement depth, field size, and photon beam quality. The connecting cable acts as a secondary Compton current source that produces a very small Compton current which depends on the field size and the photon beam quality, becoming the dominant source when the PEEC is placed at depths greater than the depth of maximum dose. A study of the dose build-up region of megavoltage photon beams showed that the percentage depth ionizations obtained from measurements are higher than the percentage depth doses obtained with Monte Carlo (MC) techniques. To validate the MC-calculated percentage depth doses, the design of the PEEC was incorporated in the simulations. While the MC-calculated and measured percentage depth ionizations in the dose build-up region agreed with one another for the 6 MV, a non-negligible difference is observed for the 18 MV x-ray beam. A number of experiments and theoretical studies of various possible effects which could be the source of this discrepancy is investigated. We show that the contribution of contaminating neutrons and protons to the doses in the 18 MV x-ray beam is negligible. Moreover, the MC calculations using the XCOM photon cross sections database and the NIST bremsstrahlung differential cross sections do not explain the discrepancy between the MC calculations and measurement in the dose build-up region for the 18 MV and this discrepancy is yet to be further investigated.

The integration of FDG-PET functional data into conventional anatomically-based GTV delineation may lead to optimization of dose to biological target volumes in radiotherapy. We are proposing a method for uncovering tumor sub-volumes using functional data. For a cohort of 27 histo-pathologically proven non-small cell lung carcinoma patients, background uptake values were sampled over slices containing tumor within contra-lateral healthy lung and then scaled by the ratio of tissue densities between healthy lung and tumor. Signal-to-background uptake values within volumes of interest encompassing the tumor were scored from which differential uptake volume histograms were constructed. These were subsequently decomposed into the minimum number of analytical functions that yielded acceptable net fits, as assessed by chi^2 values. For our patient population, at least four sub-volumes over the sampled volume of interest consistently evolved. It is possible that sub-volume thresholds may be extracted from custom-made differential uptake volume histograms for individual patients and then subsequently used for delineation of biological target volumes. At this point, however, we can only make a hypothesis that crossing point of the second and third fitted peak of the uptake volume histogram may help in singling out necrotic tissue, while the crossing point between third and fourth fitting function may help in identification of the glycolytic sub-volume within the tumor. Extensive pre-clinical studies are needed to establish correlation between these sub-volumes and their true underlying physiology. / En radiothérapie, l'intégration en données fonctionnelles de la PET au FDG au processus conventionnel de délimitation anatomique du GTV peut conduire à l'optimisation de la dose aux volumes biologiques cibles. Nous proposons une méthode pour découvrir des sous volumes tumoreaux, en utilisant des données fonctionnelles. Pour une cohorte de vingt-sept patients histo-pathologique ateints de carcinomes pulmonaires (à cellules non petites) prouvées histologiquement, les valeurs absorption de fond ont été prélevés sur les tranches contenant la tumeur dans le poumon sain controlatéral, puis amplifiée par le rapport des densités entre des tissus pulmonaires sains et tumoraux. Les valeurs d'absorption du signal de fond dans les volumes d'intérêt englobant la tumeur ont été choisies et utilisés pour calculer des histogrammes de volumes différentiels d'absorption. Ils furent ensuite décomposés en un nombre minimum de fonctions requises pour produire un ajustement analytique acceptable, tel qu'évalué par les valeurs du chi ^ 2. Pour notre population de patients, au moins quatre sous-volumes sur le volume de l'échantillon d'intérêt ont constamment évolué. Il est possible que les seuils de sous volume peuvent être extraits à partir d'histogrammes différentiels d'absorption spécifiques à chaque patient et par la suite utilisés pour la délimitation des volumes biologiques cibles. À ce stade, cependant, nous ne pouvons faire l'hypothèse que le point de passage du deuxième pic au troisième de l'histogramme de volume absorption peut aider à l'identification les tissus nécrosés, alors que le point de passage entre la fonction de montage de niveau trois à quatre peut aider à l'identification du sous volume glycolytique de la tumeur. Des études préliminaires cliniques approfondies seront nécessaires pour établir une corrélation entre ces sous-volumes et leur vraie physiologie sous-jacente.

This thesis aimed to validate the Monte Carlo (MC) algorithm in BrainLab's iPlan treatment planning system, used in conjunction with stereotactic radiosurgery (SRS) mode of the Varian Novalis TX linear accelerator for clinical use. Specifically, the iPlan algorithm was "benchmarked" by comparing results obtained with a BEAMnrc model developed for the Novalis TX's SRS mode. The BEAMnrc model was obtained by modifying an existing model for a Varian linac to include the different SRS flattening filter and the high definition 120 leaf multi-leaf collimator (HD120 MLC) of the Novalis TX. Characterization of the source model used a recently published procedure to fit beam energy, source size and angular spread, and an existing BEAMnrc Monte Carlo component module (DYNVMLC) was reprogrammed to model the HD120 MLC of the Novalis TX linac. For the latter, the interleaf air gap and leaf density were adjusted such that simulations matched interleaf leakage profiles measured with film. Validation of the iPlan MC algorithm was accomplished through comparisons between both MC codes and film measurements for MLC defined fields, depth dose curves of square fields incident on heterogeneous slab phantoms, and more clinically realistic plans incident on a Lucy® stereotactic QA phantom and a Rando® head phantom. The source characterization procedure and the modeling of the HD120 MLC were successful, with subsequent simulations performing well compared to measurements of output factors, profiles in water and dose planes of MLC defined fields. Some discrepancies were observed between either MC code and film measurements, but calculations with iPlan MC and EGSnrc MC codes agreed well with each other in all cases. These results suggest that the iPlan Monte Carlo dose calculation algorithm is capable of accurately predicting radiation dose for complex fields in heterogeneous media. / Ce mémoire visait à valider l'algorithme Monte Carlo (MC) dans le cadre du système de planification de traitement iPlan de BrainLab où il est conjointement utilisé avec le mode radiochirurgical stéréostatique (SRS) de l'accélérateur linéaire Novalis TX de Varian. Plus particulièrement, l'algorithme iplan a été validé en comparant les résultats obtenus avec un modèle BEAMnrc du mode SRS de Novalis TX. Le modèle BEAMnrc a été créé en modifiant un modèle existant d'accélérateur Varian afin d'y inclure le filtre compensateur SRS et le collimateur multilames de haute définition de Novalis TX (HD120MLC). La caractérisation de la source a utilisé une procédure récente pour ajuster l'énergie, la taille et l'ouverture angulaire de la source. Par ailleurs, un module multilames de BEAMnrc existant (DYNVMLC) a été reprogrammé pour simuler le collimateur multilames de haute définition (HD120MLC). Pour ce dernier, l'écart entre les lames et la densité des lames ont été ajustés de sorte que les simulations correspondent aux profils de fuites interlames mesurées par films. La validation de l'algorithme iPlan a été réalisée par comparaisons entre les deux codes MC et des mesures de films pour des champs définis par le collimateur multilames, pour des courbes de la dose en profondeur de champs carrés administrés sur des fantômes hétérogènes et de plans plus réalistes du point de vue clinique administrés sur un fantôme stéréostatique Lucy® et un fantôme de tête Rando®. La procédure de caractérisation de la source et la modélisation du collimateur multilames (HD120MLC) ont été réussies ainsi que les simulations ultérieures correspondaient bien aux mesures des facteurs d'ouverture, des profiles dans l'eau et des distributions de dose des champs définis par le collimateur multilames. Des différences ont été observées s entre les codes MC et les mesures de films, mais les calculs avec les codes IPlan MC et EGSnc correspondaient bien dans tous les cas. Ces résultats suggèrent que l'algorithme MC de Iplan peut prédire précisément les doses de rayonnement pour des champs complexes dans des medias hétérogènes.

A limitation of current dose calculation algorithms employed in radiotherapy treatment planning is the assumption that the patient's anatomy is static throughout the imaging, planning and delivery. 4D dose calculation methods employ non-linear image registration to determine the cumulative dose received in a deforming anatomy. In this work, we developed a 4D Monte Carlo dose calculation code, designated defDOSXYZ, which determines the dose received in a deforming voxel grid. Voxel deformations were determined from deformation vectors resulting from non-linear image registration between images of the reference and target states. The ANIMAL non-linear image registration algorithm was implemented for registration of thoracic 4D CT images. Modifications were performed to ANIMAL to minimize deformation vector discontinuities. A method for correcting artifacts in 4D CT images was developed which uses non-linear image registration to interpolate voxel intensities from temporally adjacent artifact-free images. Dose calculations in deforming phantoms and 4D CT patient data using defDOSXYZ were compared to conventional center-of-mass (COM) and trilinear (TL) dose remapping methods. defDOSXYZ calculations were determined to be accurate to within 1% by comparison with DOSXYZ calculations and internal consistency checks. Conventional dose remapping methods were found to underestimate the dose by 29% and 8%, on average, when remapping dose from Exhale to Inhale within simple deforming phantoms with voxel sizes of 1 cm and 0.5 cm, respectively. These discrepancies were reduced to 0.2% for voxel sizes of 0.25 cm and smaller, however dose errors of 20-30% still existed in regions of steep dose gradients. The accuracy of non-linear image registration between inhale and exhale images for 5 lung patients was found to be within 2 mm which was deemed acceptable for clinical dose calculations. Temporal interpolation using ANIMAL was demonstrated to improve image quality in 4D data se / L'hypothèse que l'anatomie du patient est statique tout au long de l'imagerie, de la planification et du traitement est une limite des algorithmes actuels de calcul de dose en planification de traitement radiothérapeutique. Des méthodes de calcul de dose en 4D utilisent le recalage d'image non linéaire pour déterminer la dose cumulée reçue par une anatomie déformable. Dans cette thèse, nous avons développé un code Monte-Carlo de calcul de dose en 4D, appelé defDOSXYZ, qui détermine la dose reçue dans une grille de voxels déformables. La déformation des voxels a été établie à partir de la variation des vecteurs résultant du recalage d'image non linéaire entre les états de référence et les états cibles. L'algorithme de recalage d'image non linéaire ANIMAL a été appliqué au recalage d'images thoraciques obtenues en tomodensitométrie 4D. Des modifications ont été apportées à ANIMAL afin de minimiser les discontinuités dans la variation des vecteurs. Une méthode a été développée pour corriger les images de tomodensitométrie 4D ; cette méthode utilise le recalage d'image non linéaire pour interpoler l'intensité dans les voxels à partir d'images sans artéfacts consécutives dans le temps. Les calculs de dose avec defDOSXYZ dans les fantômes déformables et sur des images tomodensitométriques 4D de patients ont été comparés aux méthodes conventionnelles de redistribution de dose, l'une dite « du centre de mass » (COM), l'autre trilinéaire (TL). Les calculs avec defDOSXYZ ont été mesurés et sont exacts à mieux que 1 % en comparaison des calculs de DOSXYZ et de tests d'autocohérence. Nous avons trouvé que les méthodes conventionelles de redistribution de dose sous-estiment la dose, en moyen, par 29% et 8% dans de simples fantômes déformables avec des tailles de voxel de 1.0 cm et 0.5 cm, respectivement. Les divergences étaient réduites jusqu'à 0.2% pour des tailles de voxel de 2.5 mm et plus petits, mal

The aim of external beam radiotherapy is to deliver a prescribed dose to a target volume accurately and uniformly while sparing the surrounding healthy tissue. In radiation dosimetry calculations, many functions are employed to achieve this goal, and the Peak Scatter Factor (PSF) is one of the fundamental functions used in dosimetry. / A brief background of some of the basic physics employed in external beam radiotherapy is given, illustrating some of the applications of the PSF in dosimetry. Also, the evolution of the definition of the PSF is discussed by presenting the PSF definitions quoted in several dosimetric references. In addition, concerns debated among physicists regarding the consistency of tabulated values of the PSF in dosimetric references with the definition of the PSF are presented. / A practical method for measuring the PSF for megavoltage photon beams is developed. The method is applied to Co-60, 6 MV, 10 MV, and 18 MV photon beams using water, polytyrene, and solid water phantoms. The measured PSFs are compared to tabulated PSF and Normalized Peak Scatter Factors (NPSF) published in the British Journal of Radiology (BJR), supplement 25.

We investigated the feasibility of constructing compensators for intensity modulated beams. The average uncertainty in the constructed compensator thickness was +/-0.3 mm. It was found that the beam hardening effect was significant, and can lead to an error of 6.2% in the transmission, for 6 cm of lead alloy in the beam. The maximum scatter contribution to the measured fluence was 19.8% for a 20 x 20 cm2 field size, and 6 cm of lead alloy in the beam. The compensators were constructed using a simple attenuation model. For a simple wedge-step compensator there was a maximum deviation of 6% between the measured and our predicted fluence profile. For simple compensators this deviation can be attributed to scatter according to the scatter analysis introduced. The maximum deviation between the measured and predicted fluence, for fluence files derived from a commercial inverse treatment planning system was slightly higher at 7%. This is because other factors such as penumbra and scatter from neighboring modulations must be considered for each compensator individually.

The purpose of this thesis is to measure the peak scatter factors (PSFs) for a 10X10 cm2 field size in 6 MV and 18 MV photon beams using a Solid WaterTM phantom and build-up caps made of LuciteTM, aluminum, brass, and copper. The PSF for the 10X10 cm2 field size was first determined by extrapolating the measured normalized peak scatter factor (NPSF) to 0X0 cm2 field size. The extrapolated NPSF was the reciprocal of the PSF(10, hv). The measured PSF(10, hv) was then compared to the PSF determined with Monte Carlo methods. We used Monte Carlo methods to investigate the dependence of the measured signal on the build-up cap material. This was carried out by calculating the primary and scatter dose contributions to the measured signal in the air cavity of the ionization chamber. Based on Monte Carlo studies, a factor was calculated to obtain the PSF from the measurements. / L'objectif de cette thèse est d'obtenir le peak scatter factor (PSF) de faisceaux de photons de 6 et 18 MV. Les champs utilisés étaient de 10x10 cm^2 et les mesures furent effectuées à l'aide d'un fantôme de Solid Water tm couplé à des capuchons d'accumulation fait de Lucite tm, d'aluminium, de laiton et de cuivre. Le PSF du champ de 10x10cm2 a été déterminé en extrapolant le peak scatter factor normalisé (NPSF) d'un champ de 0x0cm2. Le NPSF extrapolé correspond à la réciproque du PSF(10,hv). Le PSF(10, hv) fut ensuite comparé à celui déterminé à l'aide de méthodes Monte Carlo. Les méthodes Monte Carlo furent utilisées pour étudier la dépendance entre le signal mesuré et le matériel du capuchon d'accumulation. Ceci fut accompli en décomposant le signal provenant de la cavité de la chambre d'ionisation en dose primaire et diffusée. Un facteur liant le PSF et les mesures fut calculé à l'aide d'études Monte Carlo.