Léo Adenis

Séminaires Jeunes docteurs du GDR du 7 juin 2021

Title: Modelling radiotherapy effect on low-grade gliomas

Abstract:

Part of my thesis work was focused on the design and validation of a patient-scale tumor growth model and the effect of radiotherapy on this tumor. This model is based on partial differential equations, describing the processes of proliferation, diffusion, radiation damage and cell death. I have developed a genetic automatic adjustment algorithm, and used the CMA-ES stochastic automatic adjustment method, in order to reproduce the evolution of the radius of diffuse low-grade gliomas as a function of time, before and after radiotherapy of a sample of patients.

Titre: Modélisation de l'effet de la radiothérapie sur les gliomes de bas-grade

Résumé: 

Une partie de mes travaux de thèse a porté́ sur la conception et la validation d’un modèle de croissance tumeur à l’échelle d’un patient et de l’effet de la radiothérapie sur cette tumeur. Ce modèle est basé sur des équations aux dérivées partielles, décrivant les processus de prolifération, diffusion, endommagement par la radiothérapie et mort cellulaire. J'ai développé́ un algorithme génétique d’ajustement automatique, et utilisé la méthode stochastique d’ajustement automatique CMA-ES, afin de reproduire l’évolution du rayon de gliomes de bas-grade diffus en fonction du temps, avant et après radiothérapie sur un échantillon de patients.

Tim Schneider

Séminaires Jeunes docteurs du GDR du 7 juin 2021

Title: Improving proton therapy with magnetically focussed minibeams

Abstract: Despite major advances over the last decades, the dose tolerance of normal tissue continues to be a central problem in radiation therapy, limiting for example the effective treatment of hypoxic tumours and high-grade gliomas. Proton minibeam radiation therapy (pMBRT) is a novel therapeutic strategy, combining the improved ballistics of protons with the enhanced tissue sparing potential of submillimetric, spatially fractionated beams (minibeams), that has already demonstrated its ability to significantly improve the therapeutic index for brain cancers in rats. In contrast to conventional proton therapy which uses comparatively large beam diameters of five millimetres to several centimetres, minibeams re- quire beam sizes of less than 1 mm which are challenging to create in a clinical context. So far, every implementation of pMBRT at clinically relevant beam energies could only be achieved with the help of mechanical collimators (metal blocks with thin slits or holes). However, this method is inefficient, inflexible and creates high levels of unwanted secondary particles. The optimal approach may therefore be the generation of minibeams through magnetic focussing.

In my thesis, I investigated how magnetically focussed proton minibeams can be realised in a clinical context. Starting from the computer model of a modern pencil beam scanning nozzle (the term "nozzle" describes the final elements of a clinical beamline), it could be shown that current nozzles will not be suitable for this task, since their large dimensions and the presence of too much air in the beam path make it impossible to focus the beam down to the required sizes. Instead, I developed an optimised nozzle design and evaluated it with different clinical beam models. It could be demonstrated that this design allows the generation of proton minibeams through magnetic focussing and that the new nozzle can be used with already existing technology. Moreover, a Monte Carlo study was performed to compare and quantify the differences between magnetically focussed minibeams and mechanically collimated minibeams.

Titre: Améliorer la protonthérapie avec des mini-faisceaux générés par focalisation magnétique

Résumé: Malgré d’importants progrès, la tolérance des tissus sains aux rayonnements demeure un facteur central en radiothérapie, limitant par exemple l’efficacité du traitement des gliomes de haute grade. La proton thérapie avec mini-faisceaux (proton minibeam radiation therapy, pMBRT) est une nouvelle stratégie thérapeutique qui a pour objectif d’améliorer la préservation des tissus sains en combinant les avantages balistiques des protons et le fractionnement spatial de la dose obtenu avec des faisceaux submillimétriques. Dans ce contexte, la pMBRT a déjà démontré sa capacité à augmenter l’index thérapeutique dans le traitement des tumeurs cérébrales de rats. Un défi important est la génération des mini- faisceaux dans un cadre clinique : contrairement à la radiothérapie conventionnelle qui utilise des faisceaux larges (diamètre d’environ 5 mm à plusieurs centimètres), les mini-faisceaux se caractérisent par un diamètre de moins d’un millimètre. Actuellement, la génération des mini-faisceaux de protons est réalisée à l’aide de collimateurs mécaniques (blocs en métal avec plusieurs fentes ou trous) ce qui comporte plusieurs inconvénients (notamment une très faible flexibilité, une réduction importante du débit de dose ainsi que la génération de particules secondaires indésirables). Une solution optimale pourrait être la génération des mini-faisceaux par focalisation magnétique.

Le point de départ de ma thèse était donc la question : Comment la génération des mini-faisceaux de protons par focalisation magnétique peut-elle être réalisée dans un cadre clinique ? En utilisant le modèle numérique d’un pencil beam scanning nozzle (le "nozzle" est la dernière partie d’une ligne de faisceau clinique), il a été démontré que les nozzles actuels ne sont pas adéquats pour focaliser les faisceaux de protons à la taille requise, les principales raisons étant une distance focale trop grande et une présence d’air excessive. En partant de ces conclusions, un nouveau design de nozzle optimisé a été développé. Ce nouveau modèle est capable de générer des mini-faisceaux de protons par focalisation magnétique dans des conditions réalisables avec les technologies existantes. Une étude Monte Carlo a également été menée afin de comparer et de quantifier les différences entre la génération de mini-faisceaux par collimation mécanique et par focalisation magnétique.

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Feriel Khellaf

Séminaires Jeunes docteurs du GDR du 12 avril 2021

Title: List-mode proton CT reconstruction

Abstract:

Proton therapy is used for cancer treatment to achieve better dose conformity by exploiting the energy-loss properties of protons. Proton treatment planning systems require knowledge of the stopping-power map of the patient’s anatomy to compute the absorbed dose. In clinical practice, this map is generated through a conversion from X-ray computed tomography (CT) Hounsfield units to proton stopping power relative to water (RSP). This calibration generates uncertainties as photon and proton physics are different, which leads to the use of safety margins and the reduction of dose conformity. In order to reduce uncertainties, proton CT (pCT) was proposed as a planning imaging modality since the reconstructed quantity is directly the RSP. In addition to energy loss, protons also undergo multiple Coulomb scattering (MCS) inducing non-linear paths, thus making the pCT reconstruction problem different from that of X-ray CT and degrading spatial resolution. The use of a most likely path (MLP) formalism for protons to account for the effects of MCS has improved the spatial resolution in pCT, although this formalism assumes a homogeneous medium.
The objective of this work was to improve image quality of pCT list-mode reconstruction.  First, we study the accuracy of the MLP formalism in heteregeneous media by comparing the theoretical MLP against Monte Carlo generated proton paths. Results in terms of spatial, angular, and energy distributions were analyzed to determine the maximum systematic error on the MLP and assess the impact on reconstruction.
The MLP formalism provides an additional information to the MLP estimate, which is the uncertainty envelope around the MLP. This information, included in a reconstruction algorithm, could help improve spatial resolution. In addition to MCS, the resolution of the trackers used to measure the protons' position and angle has also an impact on spatial resolution. We propose a deconvolution method using the uncertainty on the MLP estimate and the tracker resolution to improve the spatial resolution of pCT images. Results on simulated data show an improved spatial resolution in simple phantoms as well as anthropomorhpic phantoms.

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Floriane Poignant

Séminaire Jeunes Docteurs du GDR MI2B  du 12 avril 2021

Title: Physical, chemical and biological modelling for gold nanoparticle-enhanced radiation therapy: towards a better understanding and optimization of the radiosensitizing effect

Abstract:

In radiation therapy, high-Z nanoparticles such as gold nanoparticles (GNPs) have shown particularly promising radiosensitizing properties. At an early stage, an increase in dose deposition and free radical production throughout the tumor (photoelectric effect) and at sub-cellular scale (Auger cascade) might be responsible for part of the effect for low-energy X-rays. In this work, these early mechanisms are investigated with simulation tools to better quantify them and understand their impact on cell survival.

This work was based on Monte Carlo (MC) models developed to track electrons down to low energy both in water (meV) and gold (eV). In particular, the accuracy of electron transport in gold was assessed by comparing the MC predictions with experimental data in the literature.

Once validated, the MC simulation was used to quantify the energy deposited in nanotargets located near the GNP, which correlates with the probability to generate damages. These nanodosimetry results showed a significant increase of the probability of having an energy deposition in the nanotarget larger than a threshold, within 200 nm around the GNP. This suggests that GNPs may be particularly efficient at destroying biological nanotargets in its vicinity.

The MC simulation was then used to quantify chemical effects. At the macroscale, the increase of free radical production for a concentration of GNPs was calculated. Such increase correlated well with the increase of dose deposition at the macro-scale.

Finally, MC results were used together with the biophysical model NanOx to predict cell survival in presence of GNPs. NanOx was originally developed to calculate the biological dose in hadrontherapy. The Local Effect Model (LEM), currently the main biophysical model implemented for GNP-enhanced radiation therapy, was also used to calculate cell survival and to compare NanOx and LEM predictions. For a simple system where GNPs were homogeneously distributed in the cell, the increase of cell death with the biophysical model NanOx was purely due to the increase of the macroscopic dose. No increased biological effectiveness due to Auger electrons was obtained, which comes in contradiction with the LEM predictions.

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