Dose uncertainty in radiotherapy of patients with head and neck cancer measured by in vivo ESR/alanine dosimetry using a mouthpiece.

In order (i) to evaluate the dose uncertainty of the mouthpiece in daily use during intensity-modulated radiotherapy of patients with head and neck cancer, and (ii) to present a system for in vivo dosimetry of the oral mucosa, we equipped the mouthpiece with alanine dosimeter probes for in vivo dosimetry. The aim was to determine the dose uncertainty caused by the daily positioning of the mouthpiece during dynamic treatment techniques. During IMRT radiotherapy of patients with head and neck cancer, the doses accumulated next to the mucosa were measured in five patients and compared to the dose calculated by the treatment planning system. The comparison of the applied and measured dose for each measurement point showed in six of the eight alanine probe positions a good agreement within the given relative combined standard uncertainty of less than 4.5% for a accumulated dose of 30 Gy and less than 4.6% for an accumulated dose of 8 Gy, respectively. In two of the eight alanine probe positions the applied and measured doses differed by 7.7% and 8.2% from each other. The dominant contribution to the overall uncertainty for the in vivo measurements was the positioning of the dosimeter probes in the patient's body and their corresponding localization in the CT data as well as the inaccuracy of the available algorithm for dose distribution calculation at the low-density material/soft tissue interface between the mouthpiece and the mucosa. Regarding our results, we refrain from the use of a mouthpiece during dynamic treatments such as IMRT.