MR-guided proton therapy: a review and a preview

Abstract

Background: The targeting accuracy of proton therapy (PT) for moving soft-tissue tumours is expected to greatly improve by real-time magnetic resonance imaging (MRI) guidance. The integration of MRI and PT at the treatment isocenter would offer the opportunity of combining the unparalleled soft-tissue contrast and real-time imaging capabilities of MRI with the most conformal dose distribution and best dose steering capability provided by modern PT. However, hybrid systems for MR-integrated PT (MRiPT) have not been realized so far due to a number of hitherto open technological challenges. In recent years, various research groups have started addressing these challenges and exploring the technical feasibility and clinical potential of MRiPT. The aim of this contribution is to review the different aspects of MRiPT, to report on the status quo and to identify important future research topics.

Methods: Four aspects currently under study and their future directions are discussed: modelling and experimental investigations of electromagnetic interactions between the MRI and PT systems, integration of MRiPT workflows in clinical facilities, proton dose calculation algorithms in magnetic fields, and MRI-only based proton treatment planning approaches.

Conclusions: Although MRiPT is still in its infancy, significant progress on all four aspects has been made, showing promising results that justify further efforts for research and development to be undertaken. First non-clinical research solutions have recently been realized and are being thoroughly characterized. The prospect that first prototype MRiPT systems for clinical use will likely exist within the next 5 to 10 years seems realistic, but requires significant work to be performed by collaborative efforts of research groups and industrial partners.

Keywords: Image guidance; Magnetic resonance imaging; Proton therapy.

Fig. 1

Model of the magnetic interaction between an MRI and PBS assembly when delivering a scanning pattern down the bore of the MRI. Example of a scanned pencil beam with the MRI scanner turned off (a) and on (b). c Scanning magnet (SM) settings and MFH over the scanning pattern shown in (d). e Changes to the scanning pattern due to the presence of the magnetic field from the MRI scanner

Fig. 2

Proton dose distortions within uniform transverse magnetic field of 0.5 T (a), 1.5 T (b), and 1.5 T with delivery corrections (c), for a prostate plan (i.e. large range representing the worst-case scenario). d Dose volume histogram for the 3 above scenarios with the magnetic field on, compared to the planned dose with no magnetic field. Adapted from [8]

Fig. 3

Artist impressions of (a) a rotating biplanar open in-beam MRI scanner integrated in a (b) compact proton therapy gantry treatment room (Image courtesy: Ion Beam Applications SA, Louvain-la-Neuve, Belgium)

Fig. 4

Calculated two-dimensional relative dose distribution for a 240 MeV proton beam in a homogeneous water phantom within a transverse 3 T magnetic field

Fig. 5

a From left to right: HU and dose profile of a proton spread-out Bragg peak (SOBP) for a beam entering via the frontal sinus. SOBP dose and dose difference distribution in a 2D sagittal plane as planned on the sCT and then delivered on the CT using a prescribed dose of 2 Gy. Adapted from [48]. b Original MRI, CT and pseudoCTs generated with a 2D and a 3D Unet for an exemplary brain case. The SFUD proton dose distribution for a single gantry angle is depicted on the original CT and the two pseudoCTs. The generic target volume is marked in red, the 95% iso-dose line in green. Adapted from [49]