title: Integration of dynamic table translations into dynamic trajectory radiotherapy and mixed photon-electron beam radiotherapy
Creators: name: Guyer, Gian Mauro Carlo
Publisher: name: Universität Bern
Year: 2024
Subjects: subject: 610 Medicine &amp; health

description: Radiotherapy aims at delivering a lethal dose of radiation to tumor cells while sparing the surrounding healthy tissue and organs. Highly specialized devices, such as C-arm linear accelerators (linacs), have been developed for external beam radiotherapy, which deliver high-energy photon and electron beams. Over the last decades, several improvements in photon beam radiotherapy, such as introducing the photon multileaf collimator (pMLC), enabled intensity-modulated radiotherapy (IMRT), resulting in improved target conformality compared to 3D conformal techniques. Volumetric modulated arc therapy (VMAT) improves the delivery efficiency while maintaining the dosimetric plan quality of IMRT by using dynamic gantry rotation during beam on.

Next to the dynamic gantry rotation, also the table and the collimator can rotate dynamically during beam on. This is used in a technique called dynamic trajectory radiotherapy (DTRT). Furthermore, the table can also translate dynamically in three directions, enabling non-isocentric DTRT. However, the potential of dynamic table translations for radiotherapy on a C-arm linac is unexplored. Thus, in this thesis, treatment techniques including dynamic table translations are developed, and potential use cases are shown. 

A treatment planning process (TPP) for non-isocentric DTRT is developed to create treatment plans with photon beams including dynamic gantry, collimator, and table rotation and dynamic table translation. The intensity modulation optimization of the  TPP is based on a hybrid column generation and simulated annealing direct aperture optimization algorithm. The TPP is used to create non-isocentric DTRT plans and several potential use cases for non-isocentric DTRT are demonstrated: While maintaining treatment plan quality, the delivery efficiency is improved by using non-isocentric DTRT instead of multi-isocentric IMRT for craniospinal irradiation. Extending the source-to-target distance in DTRT plans reduces the risk of collision between the gantry and the patient or table and enables additional beam directions, which could be exploited to improve the dosimetric treatment plan quality compared to isocentric DTRT. 

Contrary to photon beam radiotherapy, electron treatments are still applied using patient-specific cut-outs placed in an applicator. By using the pMLC for electron beam collimation instead of the cut-outs, efficient electron beam treatments are possible. Further, the use of the pMLC facilitates mixed photon-electron beam radiotherapy (MBRT). An MBRT technique using pMLC-collimated electron arcs instead of electron beams with a static gantry angle is developed, resulting in improved delivery efficiency while maintaining the dosimetric plan quality of MBRT plans using electron beams with a static gantry angle.

One of the challenges of DTRT on C-arm linacs is accurately predicting potential collisions between the gantry, the patient, and the table during treatment planning. Thus, a collision prediction tool is developed, which is able to predict possible collision interlocks. The tool was successfully validated against measurements.

The created treatment plans for non-isocentric DTRT and MBRT were shown to be accurately deliverable on a C-arm linac. For several treatment plans, the dosimetric accuracy was successfully validated using film measurements. 

In conclusion, this thesis demonstrates the benefits of dynamic table translations in photon and electron beam radiotherapy. With the demonstrated benefits of improved dosimetric treatment plan quality, delivery efficiency, and collision risk, dynamic table translations further facilitate the use of MBRT and DTRT treatment techniques in clinics in the future.