Title: Online MR-guided radiotherapy – A new era in radiotherapy
Abstract: Technical progress has repeatedly heralded new eras of radiotherapy. Over the last decades, 3D-treatment planning, intensity modulated radiotherapy, 4D-radiotherapy, complex dose calculation algorithms and image guided radiotherapy have been game-changers permitting increasingly precise target coverage and sparing of normal tissues. Stereotactic radiotherapy for lung cancer is just one example where these new technologies have led to improved outcomes and increased use of radiotherapy. Currently we are witnessing the implementation of the latest technological revolution: The MR-Linac. The level of precision that can be achieved in radiotherapy is limited by the imaging available. Until recently we had to rely on cone-beam imaging to detect anatomical inter- and intrafractional changes. MR images with their superior soft-tissue contrast are already in use for many years in radiotherapy treatment planning. The union of a linear accelerator and an MRI in a single device to facilitate online adaptive MR guided radiotherapy was complex and required almost 20 years from an idea to a clinically available system [[1]Raaymakers B.W. Lagendijk J.J. Overweg J. Kok J.G. Raaijmakers A.J. Kerkhof E.M. et al.Integrating a 1.5 T MRI scanner with a 6 MV accelerator: proof of concept.Phys Med Biol. 2009; 54: N229-237https://doi.org/10.1088/0031-9155/54/12/N01Crossref PubMed Scopus (456) Google Scholar]. The reasons for this were manifold: Unlike with cone-beam CT guided radiotherapy; the magnetic field of an MR-linac is permanent and impacts the dose distribution [[2]Malkov V.N. Hackett S.L. Wolthaus J.W.H. Raaymakers B.W. van B. Asselen Monte Carlo simulations of out-of-field surface doses due to the electron streaming effect in orthogonal magnetic fields.Phys Med Biol. 2019; https://doi.org/10.1088/1361-6560/ab0aa0Crossref PubMed Scopus (19) Google Scholar]. In addition, the moving linear accelator influences the magnetic field. Besides superior soft tissue imaging, the MR-linac enables us for the first time to continuously acquire images while the beam is on. Plan adaptations strategies had to be developed to account for day to day changes in anatomy [3Bohoudi O. Bruynzeel A.M.E. Senan S. Cuijpers J.P. Slotman B.J. Lagerwaard F.J. et al.Fast and robust online adaptive planning in stereotactic MR-guided adaptive radiation therapy (SMART) for pancreatic cancer.Radiother Oncol. 2017; 125: 439-444https://doi.org/10.1016/j.radonc.2017.07.028Abstract Full Text Full Text PDF PubMed Scopus (184) Google Scholar, 4Winkel D. Bol G.H. Kiekebosch I.H. Van Asselen B. Kroon P.S. Jurgenliemk-Schulz I.M. et al.Evaluation of online plan adaptation strategies for the 1.5T MR-linac based on "First-In-Man" treatments.Cureus. 2018; 10https://doi.org/10.7759/cureus.2431Crossref Google Scholar]. However, these tasks have been mastered and first centers are now treating patients using the new technology [5Acharya S. Fischer-Valuck B.W. Kashani R. Parikh P. Yang D. Zhao T. et al.Online magnetic resonance image guided adaptive radiation therapy: first clinical applications.Int J Radiat Oncol Biol Phys. 2016; 94: 394-403https://doi.org/10.1016/j.ijrobp.2015.10.015Abstract Full Text Full Text PDF PubMed Scopus (199) Google Scholar, 6Raaymakers B.W. Jurgenliemk-Schulz I.M. Bol G.H. Glitzner M. Kotte A. van Asselen B. et al.First patients treated with a 1.5 T MRI-Linac: clinical proof of concept of a high-precision, high-field MRI guided radiotherapy treatment.Phys Med Biol. 2017; 62: L41-L50https://doi.org/10.1088/1361-6560/aa9517Crossref PubMed Scopus (311) Google Scholar]. In the current special issue of ctRO world-renowned experts in the field will give insights in opportunities and challenges of online MR guided radiotherapy. Uulke van der Heide, Benjamin Knowles and Joakim Jonsson will discuss what a radiation oncologist needs to know about MRI physics and MR based treatment planning, while Dennis Winkel and Sebastian Klüter will provide an overview of the MRI Linac by Elekta and Viewray, respectively [7van der Heide U.A. Frantzen-Steneker M. Astreinidou M. Nowee M.E. van Houdt P.J. MRI basics for radiation oncologists.Clin Transl Radiat Oncol. 2019; 18: 74-79Abstract Full Text Full Text PDF PubMed Scopus (14) Google Scholar, 8Knowles B. Friedrich F. Fischer C. Paech D. Ladd M. Beyond T2 and 3T: new MRI techniques for clinicians.Clin Transl Radiat Oncol. 2019; 18: 87-97Abstract Full Text Full Text PDF Scopus (8) Google Scholar, 9Jonsson J. Nyholm T. Söderkvist K. The rationale for MR-only treatment planning for external radiotherapy.Clin Transl Radiat Oncol. 2019; 18: 60-65Abstract Full Text Full Text PDF PubMed Scopus (41) Google Scholar, 10Winkel D. Bol G.H. Kroon P.S. van Asselen B. Hackett S. Werensteijn-Honingh A.M. et al.Adaptive radiotherapy: the Elekta Unity MR-linac concept.Clin Transl Radiat Oncol. 2019; 18: 54-59Abstract Full Text Full Text PDF PubMed Scopus (251) Google Scholar, 11Klüter S. Technical design and concept of a 0.35 T MR-Linac.Clin Transl Radiat Oncol. 2019; 18: 98-101Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar]. The RTT perspective on the online adaptive workflow will be discussed by Robin Botman [[12]Botman R. Tetar S.U. Palacios M.A. Slotman B.J. Lagerwaard F.J. Bruynzeel A.M.E. The clinical introduction of MR-guided radiation therapy from a RTT perspective.Clin Transl Radiat Oncol. 2019; 18: 140-145Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar]. Which tumor sites might benefit most from online adaptive MR guided radiotherapy? Sonja Stieb shares her view on the potential repetitive imaging and changes to organs at risk in head and neck cancers [[13]Stieb S. Elgohari B. Fuller C.D. Repetitive MRI of organs at risk in head and neck cancer patients undergoing radiotherapy.Clin Transl Radiat Oncol. 2019; 18: 131-139Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar]. Can higher doses be applied in pancreatic cancer with MR guidance? Anna Bruynzeel [[14]Bruynzeel A. Lagerwaard F. The role of biological dose-escalation for pancreatic cancer.Clin Transl Radiat Oncol. 2019; 18: 128-130Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar] will provide her initial experience from this field. Advantages and disadvantages of MRgRT in prostate cancer will be balanced against by Julia Murray [[15]Murray J. Tree A. Prostate cancer – advantages and disadvantages of MR-guided RT.Clin Transl Radiat Oncol. 2019; 18: 68-73Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar]. Cihan Gani and Giuditta Chiloiro will discuss new opportunities of online MR guided treatment in rectal cancer and report the first experience of online MRgRT for this tumor site [16Gani C. Boldrini L. Valentini V. Online MR guided radiotherapy for rectal cancer. New opportunities.Clin Transl Radiat Oncol. 2019; 18: 66-67Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar, 17Chiloiro G. Boldrini L. Meldolesi E. Re A. Cellini F. Davide C. et al.MR-guided radiotherapy in rectal cancer: first clinical experience of an innovative technology.Clin Transl Radiat Oncol. 2019; 18: 80-86Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar]. Oligometastatic disease is now widely considered an indication for local treatment - Dennis Winkel will shed a light on how to optimally tackle individual lymph nodes [[18]Winkel D. Werensteijn-Honingh A.M. Kroon P.S. Eppinga W.S.C. Bol G.H. Intven M.P.W. et al.Individual lymph nodes: "See it and Zap it".Clin Transl Radiat Oncol. 2019; 18: 46-53Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar]. And finally, Matthias Guckenberger [[19]Rühle A. Andratschke N. Siva S. Guckenberger M. Is there a role for stereotactic radiotherapy in the treatment of renal cell carcinoma?.Clin Transl Radiat Oncol. 2019; 18: 104-112Abstract Full Text Full Text PDF Scopus (21) Google Scholar] will introduce online MRgRT as alternative to surgery in renal cell carcinoma. The opportunities of online MR guided radiotherapy appear plenty and we are excited to discuss these in this issue and in the future. BS: VUMC receives a research from Viewray, Inc. BS has received speaker honoraria from Viewray, Inc. CG: The department of Radiation Oncology Tübingen receives financial and technical support from Elekta AB (Stockholm, Sweden) under a research agreement. CG has received travel grants from Elekta AB (Stockholm, Sweden).