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Objectif de l’étude En cas de radiothérapie prostatique, la région antéro-inférieure du rectum a été identifiée comme étant hautement prédictive de saignement rectal [1] . L’objectif de cette étude dosimétrique était d’évaluer la faisabilité de diminution de la dose dans cette sous-région rectale, tout en conservant une bonne couverture du volume cible prévisionnel. Deux stratégies originales de planification inverse ont été testées : en préservant la sous-région rectale et/ou en utilisant un modèle dosimétrique permettant d’identifier la dose moyenne minimale atteignable par la structure rectale [2] . Matériel et méthode À partir de la scanographie de 60 patients irradiés à la dose de 78 Gy pour un cancer de prostate, quatre plans d’arcthérapie volumétrique modulée (VMAT) ont été générés pour chacun des patients : un plan standard correspondant aux pratiques standard, un plan utilisant des objectifs spécifiques sur la sous-région rectale, un plan utilisant le modèle de Moore appliqué à la paroi rectale seule (« modèle ») et un plan en utilisant le modèle appliqué à la fois à la paroi rectale et à la sous-région rectale. Résultats Le plan utilisant le modèle appliqué à la fois à la paroi rectale et à la sous-région rectale permettait l’épargne la plus importante de la sous-région rectale avec une diminution de la dose moyenne de 4,7 Gy, 5,3 Gy et 7,7 Gy par rapport respectivement au plan utilisant le modèle de Moore appliqué à la paroi rectale seule, plan utilisant des objectifs spécifiques sur la sous-région rectale et au plan standard. Les valeurs moyennes des Normal Tissue Probability of Complication (NTCP ; modèle Lyman–Kutcher–Burman, prédiction de saignement rectale de grade 1 ou plus, suivi de 3 ans) étaient de 22 % avec le plan standard, 19 % avec le plan utilisant des objectifs spécifiques sur la sous-région rectale, 18 % avec le plan utilisant le modèle de Moore appliqué à la paroi rectale seule et 17 % plan utilisant le modèle appliqué à la fois à la paroi rectale et à la sous-région rectale. La dose délivrée dans le volume cible prévisionnel dans les plans utilisant le modèle appliqué à la fois à la paroi rectale et à la sous-région rectale était légèrement moins homogène. La conformation de la dose était similaire pour tous les plans. Conclusion En comparaison aux plans de VMAT standard, l’application de contraintes de dose spécifiques à la sous-région rectale et à la paroi rectale en utilisant une application de la méthode de Moore permet une diminution moyenne de 8 Gy dans la sous-région rectale et une diminution de 23 % du risque de saignement rectal prédit. Une étude prospective va débuter pour confirmer le bénéfice de cette approche pour diminuer la toxicité rectale. [ABSTRACT FROM AUTHOR]

In external beam radiotherapy, the dose planning is currently based on computed tomography (CT) images. A relation between Hounsfield numbers and electron densities (or mass densities) is necessary for dose calculation taking heterogeneities into account. In image-guided radiotherapy process, the cone beam CT is classically used for tissue visualization and registration. Cone beam CT for dose calculation is also attractive in dose reporting/monitoring perspectives and particularly in a context of dose-guided adaptive radiotherapy. The accuracy of cone beam CT-based dose calculation is limited by image characteristics such as quality, Hounsfield numbers consistency and restrictive sizes of volume acquisition. The analysis of the literature identifies three kinds of strategies for cone beam CT-based dose calculation: establishment of Hounsfield numbers versus densities curves, density override to regions of interest, and deformable registration between CT and cone beam CT images. Literature results show that discrepancies between the reference CT-based dose calculation and the cone beam CT-based dose calculation are often lower than 3%, regardless of the method. However, they can also reach 10% with unsuitable method. Even if the accuracy of the cone beam CT-based dose calculation is independent of the method, some strategies are promising but need improvements in the automating process for a routine implementation., (Copyright © 2017 Société française de radiothérapie oncologique (SFRO). Published by Elsevier SAS. All rights reserved.)

MRI-based radiotherapy planning is a topical subject due to the introduction of a new generation of treatment machines combining a linear accelerator and a MRI. One of the issues for introducing MRI in this task is the lack of information to provide tissue density information required for dose calculation. To cope with this issue, two strategies may be distinguished from the literature. Either a synthetic CT scan is generated from the MRI to plan the dose, or a dose is generated from the MRI based on physical underpinnings. Within the first group, three approaches appear: bulk density mapping assign a homogeneous density to different volumes of interest manually defined on a patient MRI; machine learning-based approaches model local relationship between CT and MRI image intensities from multiple data, then applying the model to a new MRI; atlas-based approaches use a co-registered training data set (CT-MRI) which are registered to a new MRI to create a pseudo CT from spatial correspondences in a final fusion step. Within the second group, physics-based approaches aim at computing the dose directly from the hydrogen contained within the tissues, quantified by MRI. Excepting the physics approach, all these methods generate a synthetic CT called "pseudo CT", on which radiotherapy planning will be finally realized. This literature review shows that atlas- and machine learning-based approaches appear more accurate dosimetrically. Bulk density approaches are not appropriate for bone localization. The fastest methods are machine learning and the slowest are atlas-based approaches. The less automatized are bulk density assignation methods. The physical approaches appear very promising methods. Finally, the validation of these methods is crucial for a clinical practice, in particular in the perspective of adaptive radiotherapy delivered by a linear accelerator combined with an MRI scanner., (Copyright © 2017. Published by Elsevier SAS.)

The development of both image-guided and intensity-modulated radiotherapy has underlined the question of treatment adaptation to anatomical and/or biological changes occurring during radiotherapy course and modifying delivered dose to the patient. Adaptive radiotherapy has been introduced when several plans are used to treat a patient during radiotherapy. Adaptation may be performed online, offline or in a hybrid way. New images of the patient are needed for adaptive radiotherapy to perform many processes: image registration, segmentation and evaluation of cumulative dose. Deformable image registration methods are generally used to image registration and contours propagation. Fraction and cumulative dose evaluations use deformable image registration methods or more complex methods based on Monte-Carlo calculation. These methods have uncertainties and have to be evaluated. However, evaluation and validation tools are still being developed. The physicist's mission is to ensure that every new technology, such as adaptive radiotherapy, is deployed with highest safety, by technical validation and by implementing a specific quality assurance program. Adaptive radiotherapy implementation still raises many questions, so its potential clinical application requires great caution and should be carefully explored in prospective clinical trials., (Copyright © 2015 Société française de radiothérapie oncologique (SFRO). Published by Elsevier SAS. All rights reserved.)

Understanding the mechanical properties of human liver is one of the most critical aspects of its numerical modeling for medical applications or impact biomechanics. Generally, model constitutive laws come from in vitro data. However, the elastic properties of liver may change significantly after death and with time. Furthermore, in vitro liver elastic properties reported in the literature have often not been compared quantitatively with in vivo liver mechanical properties on the same organ. In this study, both steps are investigated on porcine liver. The elastic property of the porcine liver, given by the shear modulus G, was measured by both Transient Elastography (TE) and Dynamic Mechanical Analysis (DMA). Shear modulus measurements were realized on in vivo and in vitro liver to compare the TE and DMA methods and to study the influence of testing conditions on the liver viscoelastic properties. In vitro results show that elastic properties obtained by TE and DMA are in agreement. Liver tissue in the frequency range from 0.1 to 4 Hz can be modeled by a two-mode relaxation model. Furthermore, results show that the liver is homogeneous, isotropic and more elastic than viscous. Finally, it is shown in this study that viscoelastic properties obtained by TE and DMA change significantly with post mortem time and with the boundary conditions., (© 2011 – IOS Press and the authors. All rights reserved)