Toward MR-integrated proton therapy: modeling the potential benefits for liver tumors

Abstract

Magnetic resonance imaging (MRI)-integrated proton therapy (MRiPT) is envisioned to improve treatment quality for many cancer patients. However, given the availability of alternative image-guided strategies, its clinical need is yet to be justified. This study aims to compare the expected clinical outcomes of MRiPT with standard of practice cone-beam CT (CBCT)-guided PT, and other MR-guided methods, i.e. offline MR-guided PT and MR-linac, for treatment of liver tumors. Clinical outcomes were assessed by quantifying the dosimetric and biological impact of target margin reduction enabled by each image-guided approach. Planning target volume (PTV) margins were calculated using random and systematic setup, delineation and motion uncertainties, which were quantified by analyzing longitudinal MRI data for 10 patients with liver tumors. Proton treatment plans were created using appropriate PTV margins for each image-guided PT method. Photon plans with margins equivalent to MRiPT were generated to represent MR-linac. Normal tissue complication probabilities (NTCP) of the uninvolved liver were compared. We found that PTV margin can be reduced by 20% and 40% for offline MR-guided PT and MRiPT, respectively, compared with CBCT-guided PT. Furthermore, clinical target volume expansion could be largely alleviated when delineating on MRI rather than CT. Dosimetric implications included decreased equivalent mean dose of the uninvolved liver, i.e. up to 24.4 Gy and 27.3 Gy for offline MR-guided PT and MRiPT compared to CBCT-guided PT, respectively. Considering Child–Pugh score increase as endpoint, NTCP of the uninvolved liver was significantly decreased for MRiPT compared to CBCT-guided PT (up to 48.4%, p < 0.01), offline MR-guided PT (up to 12.9%, p < 0.01) and MR-linac (up to 30.8%, p < 0.05). Target underdose was possible in the absence of MRI-guidance (D90 reduction up to 4.2 Gy in 20% of cases). In conclusion, MRiPT has the potential to significantly reduce healthy liver toxicities in patients with liver tumors. It is superior to other image-guided techniques currently available.

Magnetic resonance imaging (MRI) guided proton therapy: A review of the clinical challenges, potential benefits and pathway to implementation

“…A modelling study in liver tumours found that MRI-guided proton therapy can reduce the PTV volume by 40% compared with CBCT-guided proton therapy. MRI-guided proton therapy reduced the normal tissue complication probability by 48% compared with CBCT-guided proton therapy, and 31% compared with MRI-Linac [58].…”

Management of Motion and Anatomical Variations in Charged Particle Therapy: Past, Present, and Into the Future

“…While it has already been implemented for photon radiotherapy via the MR-linac, the application to charged particles is currently challenged by the need to mitigate the deflection of the treatment beam by the beamline magnetic field. Modelling on MR-integrated proton therapy for liver cancer has shown clear dosimetric benefits as well as significant reduction in normal tissue complication probability in comparison with other imaging modalities, including offline MR-guided proton therapy and MR-linac ( 137 ). Recent studies have made progress in quantifying the impact of the magnetic field on detectors for proton dosimetry and demonstrating the technical feasibility of low-field MR guidance on phantoms in a static research beam line ( 138 , 139 ).…”

Technical note: Experimental dosimetric characterization of proton pencil beam distortion in a perpendicular magnetic field of an in‐beam MR scanner

“…In particular, PT is highly susceptible to positioning errors, anatomical differences, and organ motion 4 . The combination of magnetic resonance (MR) imaging and PT has the potential to significantly improve the targeting accuracy and precision for moving tumors through real‐time imaging at unprecedented soft‐tissue contrast in the absence of ionizing imaging dose, offering an approach to solve the problem of limited soft tissue contrast associated with x‐ray guided PT 5 …”

“…4 The combination of magnetic resonance (MR) imaging and PT has the potential to significantly improve the targeting accuracy and precision for moving tumors through real-time imaging at unprecedented soft-tissue contrast in the absence of ionizing imaging dose, offering an approach to solve the problem of limited soft tissue contrast associated with x-ray guided PT. 5 The technical integration of MR and PT into a hybrid in-beam MR-PT system is challenging for several reasons. First, the influence of the magnetic fringe fields of the beam delivery system originating from the accelerator, beam transport, and dose delivery system on the magnetic field (MF) of the MR scanner potentially affects the imaging quality during dose application and must be taken into account.…”