Optimizing target volume definition and treatment accuracy in esophageal cancer

The rise of the multimodality treatment of esophageal cancer has resulted in a considerable improvement of long-term oncologic outcomes. The accuracy of esophageal cancer radiotherapy did contrariwise not improve over the last decennium. Visualization of the esophageal tumor extent using a planning computed tomography scan for delineation purposes, or cone-beam CT scan for verification purposes, can be notoriously challenging. The research presented in this thesis aimed to increase the accuracy of the gross target volume and the clinical target volume definition in esophageal cancer radiotherapy with the aid of fiducial markers placed on (echo)endoscopically determined tumor borders. We also investigated an alternative strategy to improve the bony-anatomy based setup verification, and the best strategy to take respiration-induced target motion into account; geometrical uncertainties typically compensated by the clinical target volume-to-planning target volume margin.
We demonstrated that fiducial markers are capable of improving accuracy of esophageal cancer radiotherapy, and made it possible to understand and potentially mitigate different kinds of uncertainties. We endorse the use of fiducial makers placed on (echo)endoscopically determined tumor borders, an anisotropic and region-specific margin to construct the planning target volume margin, and a mid-position strategy for esophageal cancer radiotherapy.