MR-only guided proton therapy: advances, future perspectives and challenges

Introduction
At the end of January 2018, proton therapy was
introduced at the University Medical Center Groningen (UMCG). The Groningen Proton Therapy Center (GPTC) is one of approximately 60 proton therapy centers in operation worldwide. Over the last decade, the number
of proton therapy centers has been constantly increasing. Proton therapy allows for radiation of tumor tissue with high precision, while minimizing normal tissue damage. This is due to the intrinsic physical properties of protons
that allow for decrease of radiation dose issued to tissue surrounding the target volumes compared to conventional photon therapy [1, 2]. However, to fully utilize the benefit
of protons, very accurate identification of tumor location is required. The advantage of highly conformal dose distributions in proton therapy may be compromised by spatial distortions, as they increase the range uncertainty of the proton beam. Due to the energy deposition of
protons with steep dose gradients, accurate positioning of these gradients is critical to successful treatment planning and treatment delivery [1]. Geometric errors and uncertainties in Computed Tomography (CT) and Magnetic
Resonance (MR) images can have a significant dosimetric impact, especially when the radiation is targeted to a small volume or a volume close to organs at risk [3]. In other words, small uncertainties (e.g. a few millimeters)
can lead to underdosage in the target volume and overdosage to healthy surrounding tissue [1, 4]