Physique de la radiothérapie (GRT)

Impact of the uncertainties in the management of respiratory motion for modern radiotherapy

In the thorax-abdominal region, parts of the lung, liver, and other organs move appreciably due to breathing influencing the accuracy of imaging, treatment planning, and delivery in radiotherapy. In most cases, the duration of a beam irradiation is longer than several respiratory cycles. As a consequence, the target volume, but also the organs at risk, moves during the irradiation process, making the achievement of treatment’s efficiency difficult and challenging. In this context the use of 4D-CT and the respiratory correlated cone beam CT (4D-CBCT) are powerful tools for the respiratory motion management in radiotherapy.

In 4D-CT, the images are taken over several breathing cycles. The respiratory motion is recorded using a respiratory monitoring device where the respiratory information is detected by a pressure sensor fixed on the patient abdomen. The synchronisation of the device with the CT- scanner allows the acquisition of 4D-CT images which provides substantial information on tumor movement in the lung, allowing for the creation of custom planning margins explicitly including respiratory motion.

4D-CBCT performed on a linear accelerator (Symmetry, 4D image guidance) allows monitoring prior to treatment, the mean position, trajectory, and shape of a moving tumor. Such pre-verification has been reported to reduce respiration-induced geometrical uncertainties, enabling the safe delivery of 4D radiotherapy with small margins.

The aim of this project is to implement at CHUV a workflow for free-breathing-lungs treatments, for modern radiotherapy modalities, as Volumetric Arc Therapy (VMAT) and Tomotherapy by exploiting 4D-CT and 4D-CBCT images. The goal is to reduce the uncertainties in dose delivery particularly for the “interplay effect” between modulated intensity beam and moving organ and to determine the optimal safe margins for hypo-fractionnated treatments.

Decision making in radio-oncology with multi-criteria optimization

Radiation therapy is the treatment of malignant cancer tumors with external radiation beams. The beam configuration and settings superposed on the patient’s CT scan slices results to a patient-specific radiation therapy treatment plan. Given the multiple beam configurations, multiple initial conditions and parameter settings there is a wide range of clinically acceptable treatment plans which express different trade-offs between target coverage and organs at risk (OAR). The standard technique in inverse radiation therapy treatment planning involves the search of the optimal solution through an optimization process based on pre-defined objectives and constraints. These initial conditions are defined by the planner who, based on his experience and training, searches the optimal solution through a trial-and-error loop. This method may lead to suboptimal choices of final treatment plans because the total solution space of plans is not fully explored. Multi-criteria Optimization (MCO) is a new method in radiation therapy that offers the possibility of creating a set of optimal plans representing different trade-offs. In the context of MCO problems there is no unique optimal solution that can satisfy all the objectives at the same time but rather a set of mathematically equivalent compromised solutions, called Pareto optimal solutions. For these solutions no objective can be improved without deteriorating another. In order to ensure real time navigation, the Pareto surface that represents the set of Pareto optimal plans in N dimensions, is approximated by a limited number of pre-calculated Pareto optimal solutions. The majority of the solutions available to the decision maker during navigation are near-Pareto optimal solutions that result as a linear combination of the above mentioned pre-calculated Pareto optimal solutions. 

The general aim of the project is to explore the possibilities offered by a multi-criteria based inverse treatment planning system, to determine the differences with the classical optimization methods and evaluate the impact of the available decision making tools on the quality of the final plan. 

This work is partly founed by Swiss National Science Foundation (SNSF)  project N° 320030_149489/1.

Managing tumor movement with Tomotherapy units

Moving targets is one of the most challenging situations in radiotherapy. There are different techniques to get rid of the movement: gating, Mid-ventilation techniques and tracking. Nowadays, only few treatment devices are able to track the tumor movement during the irradiation. 

Tomotherapy units have two main features that make them original compared to other treatment units. First the build-in detector allows the user to position the patient with accuracy thank MVCT. Moreover, it allows the user to extract the beam data during the irradiation. Second, the MLC conception allows the possibility to change the leaves configuration very fast. These two features are of outmost interest in the development of this project. 

The main objective of the project is to manage the tumor movement with tomotherapy units.   

This work is founed by Accuray Inc. 

Dose calculation for CBCT imaging in radiation therapy

Outcome of radiotherapy is strongly dependent not only on the planning dosimetry but also on the tumor localization at the time of beam delivery. Standard method of localization is based on image guidance by wall or gantry mounted X-ray tubes leading to extra dose delivered to the patient inside and outside the treated volume. Dose distributions of these acquisitions inside patients are complex depending not only on kV energy, gantry arc and filter, but also on patient anatomy especially in heterogeneous regions such as H&N and thorax. Thus they cannot be characterized by only one quantity such as CTDI and should be fully calculated.

Calculation of kV dose distributions is not yet implemented in commercial softwares used for treatment planning systems, but several studies have demonstrated the feasibility of extending them to the kV domain. In the previous part of this project we have commissioned such an extension for the Philips Pinnacle TPS by adding the necessary dose deposition monoenergetic kernels calculated in the 20 kV to 100 kV range.

This second part of the project aims at calculating a set of dose distributions of breast, H&N and SBR lung treatments, at extracting from them the engaged effective dose as well as its variability and at comparing the results to the one’s coming from CT exams. Proposition of new more specific indicators are also be considered.

This work is partly founded by Federal Office of Public Health.

 Dernière mise à jour le 25/06/2019 à 08:52