Geometric uncertainties and mitigation strategies in radiotherapy of head & neck cancer

During a radiotherapy course for head and neck cancer, that typically lasts 6-7 weeks, patients show many changes in posture and anatomy. These so-called anatomical deformations are currently accounted for with safety margins. As a result, healthy tissue around the tumor receives high doses, leading to toxicities including mucositis, xerostomia, and the need for a feeding tube. This thesis investigates the magnitude and frequency of anatomical deformations. Strategies to mitigate the effects of these deformations and reduce the safety margins were explored.
We applied Cone Beam CT (CBCT), which is acquired in clinical routine just prior to treatment as part of image guidance radiotherapy. First, deformations of the bony anatomy were studied and a method was evaluated and introduced clinically to take these deformations into account in the image guidance process. Next, tumor shrinkage was investigated by evaluating the day to day motion of gold markers that were surgically implanted close to the tumor. We found that microscopic tumor extensions may not follow shrinkage of the bulk tumor edge and that care should be taken when adapting the treatment to account for shrinkage. With MRI, the tumor is better visible, but the same limitation applies. Then, deformable image registration methods were developed and validated for clinical use in adaptive procedures. Finally, a novel adaptive technique was proposed using the average deformation between the planning and treatment situation. It is shown that this technique, using CBCT scans, effectively reduces uncertainties and allows smaller safety margins to be applied.