Opening statement

There is no doubt that the ideal situation for radiotherapy would be a treatment under more or less static conditions in an ideal dosimetric situation with permanent online image‐based control of the position of tumor, OARs, and patient surface. Online MR‐guidance is therefore an appealing concept and it has already been applied to brachytherapy.9 However, to provide clinical results beyond what current image guidance strategies in external beam radiotherapy can achieve, several requirements must be fulfilled on the way, and the allocation of a large amount of resources has to be justified.

We have already come very close to the objective of treating a quasi‐static geometric situation if advanced image guidance strategies already available at moderate cost are fully used. Several such strategies are now available but are underutilized, typically for lack of funding or perceived complexity. Recent developments, such as flattening‐filter‐free (FFF)‐delivery and fast collimators have, however, dramatically shortened treatment time and thus rendered advanced imaging strategies more feasible. Considerable expertise is needed, as it is also for MR‐guidance. Continuous 2D‐tracking based on fiducials placed by minimally invasive procedures has entered the clinical routine for the ablation of small lesions without complex interference of OARs and achieved precision is near‐perfect.10

3D‐imaging with CBCT, particularly in conjunction with breath‐hold strategies,11 still has considerable potential. Accuracies in the range of 3 mm can be consistently achieved across treatment targets using deep inspiration breath‐hold, resulting in favorable dose distributions and straightforward dose accumulation. 4D‐approaches are available, and ultrafast “snapshot” volume imaging is ready to be deployed clinically.12 Ultrasound, where applicable, allows not only for positioning but also for tracking in 2D and 3D.13 Surface scanning as a complementary positioning and gating tool not using ionizing radiation may simultaneously provide patient surveillance and gating signals during a therapy session, further improving overall precision of a treatment.14

The integration of functional MR‐data into the treatment process is desirable, but the possibilities at the currently available field strengths in integrated machines are limited. Another aspect is that non‐coplanar treatment strategies have recently gained renewed interest outside the cranial area15 and high‐LET radiation, too, may have further potential to improve clinical results independent of imaging strategy. Both strategies are currently not feasible in conjunction with in‐room MR‐guidance. Finally, local control of small, mobile lesions is already excellent. For larger lesions, overcoming integral dose limits using particle strategies may be more important than minimally further improving geometric precision.

In conclusion, if online MR‐guidance is necessary, then there is a general necessity for broad use of advanced image guidance strategies, particularly as successful screening programs such as those for lung cancer and, potentially, even pancreatic cancer, are established, as this potentially leads to more localized disease being treated. These opportunities should be exploited immediately with available technology while, in parallel, online MR‐guidance is scientifically developed to provide added value in applications such as intratumoral dose painting, conformal treatment of individual lymph nodes identified as positive by novel markers, or other situations not yet identified that go beyond just providing geographic precision.