Proton therapy of head and neck cancer: evaluation of PTV-based and robust optimized IMPT versus VMAT

Abstract

Radiation therapy for head and neck cancers has had an essential role for curative treatment for many years. Due to the location of the cancer, several critical organs will get irradiated and this may lead to serious side-effects, such as xerostomia, dysphagia and secondary malignancies. In this thesis the possible benefits of proton therapy is investigated by comparing Volumetric modulated arc therapy (VMAT) and Intensity modulated proton therapy (IMPT) for patients with oropharynx cancer. Two different techniques to achieve adequate target coverage were tried out: planning target volume (PTV)-based IMPT and robust optimized IMPT.

The IMPT plans were kept equal (or better) in terms of target dose coverage as VMAT. The mean parotid dose was then reduced as much as possible without losing target coverage. The mean dose to both parotid glands were reduced for 8 of 12 patients with a corresponding reduction in the normal tissue complication probability (NTCP). It was shown that robust optimized IMPT achieved the lowest mean parotid dose and also the lowest NTCP.

A repeat CT was taken halfway through the treatment and was used to check IMPTs robustness for patient anatomy changes. The robust optimized IMPT was deemed the most robust, where 11 of 12 patients still had adequate target coverage. For medulla the maximum dose (\SI{50}{\gray}) was exceeded for 1 patients for PTV-based IMPT and for none of the patients for robust optimized IMPT. For 2 patients perturbed dose distributions were made and the robust optimized IMPT plan was again deemed the most robust against set up uncertainties.

This study showed that some patients might benefit from IMPT, but for other patients VMAT may yield the same treatment effects. Robust optimized IMPT should be preferred over PTV-based IMPT.