Towards real-time plan adaptation for MRI-guided radiotherapy

The introduction of hybrid MRI and linear accelerator (MRI-linac) machines enables the online volumetric imaging during radiation delivery with the superior soft tissue contrast of the diagnostic quality MRI. In this context, conventional radiotherapy workflow will gradually transfer from an offline to an online setting, where in every treatment fraction radiation will be adapted on-the-fly to the changing anatomy of the patient. A new generation of planning systems is therefore needed for online plan adaptation based on the MRI-derived position and motion data. This work formulates the requirements for such a system and proposes a new sequencing methodology to facilitate fast online replanning along with indicative inter- and intrafraction adaptive applications. The newly developed Adaptive Sequencer (ASEQ), is able to incorporate anatomical changes into the IMRT planning process, essentially allowing the successful optimization and delivery of a static clinical prescription on a dynamic patient anatomy. ASEQ is an iterative process which gradually converges to an input prescribed dose. Each iteration produces unique segments which target the latest patient anatomy. By coupling ASEQ to a Segment Weight Optimization (SWO) in a conventional static environment, we show that valid clinical plans can be generated for multiple treatment sites. Furthermore we demonstrate that by omitting SWO, and instead transferring any missing/excess dose in a voxel-by-voxel basis to the next fraction's prescription (Inter-Fraction Scheme (IFS)), the intended dose can be successfully delivered enabling the non-deterministic plan adaptation during treatment. We then demonstrate that ASEQ can be utilized for intrafraction plan adaptation based on 3D anatomical deformations in kidney cases with artificially induced baseline shifts. In both single and multifraction treatments ASEQ converges to the prescribed dose and spares the surrounding structures outside of the target region, by generating segments that target the different instances of the moving patient anatomy. By reimplementing ASEQ to facilitate very fast applications we showcase inter- and intrafraction adaptive treatments for the current and future MRI-guided clinic. We present a daily replanning application for rotational correction in prostate radiotherapy based on implanted fiducial markers. The inclusion of the daily recorded prostate rigid transformations into fast online replanning leads to consistent target coverage and OAR high dose exposure. This in turn enables the reduction of the planning margins and thus further healthy tissue sparing. Finally, we present a single fraction SBRT treatment based on 3D deformations calculated from online MR data for renal tumours. The MRI pipeline includes a pre-beam 4DMRI and multiple sets of orthogonal 2D-cine MR images acquired during the beam-on phase which are processed by a statistical motion model to produce high frequency 3D deformation vector fields along with their corresponding volumes. We simulate fast inter-beam replanning utilizing a novel mid-position update scheme while including the previously delivered dose to the patient, accurately calculated on the online anatomies. We demonstrate that this method is able to account for baseline variations/drifts that might occur during treatment and leads to higher target coverage and dose escalation while greatly decreasing the dose delivered to the surrounding tissue

On-line MR imaging for dose validation of abdominal radiotherapy

For quality assurance and adaptive radiotherapy, validation of the actual delivered dose is crucial.Intrafractional anatomy changes cannot be captured satisfactorily during treatment with hitherto available imaging modalitites. Consequently, dosecalculations are based on the assumption of static anatomy throughout the treatment. However, intra- and interfraction anatomy is dynamic and changes can be significant. In particular, hypofractionated and escalated radiotherapy thus demand for reliable dose reconstruction based on periodic imaging.In this paper, we investigate the use of an MR -linac as a dose tracking modality for the validation of treatments in abdominal targets where both respiratory and long-term peristaltic and drift motion occurs.The on-line MR imaging capabilities of the modality provides the means to perform respiratory gating of both delivery and acquisition yielding a model-free respiratory motion management under free breathing conditions.In parallel to the treatment, the volumetric patient anatomy was captured and used to calculate the applied dose. Subsequently, the individual doses were warped back to the planing grid to obtain the actual dose accumulated over the entire treatment duration. Eventually, the planned dose was validated by comparison with the accumulated dose.Representatively for a site subject to breathing modulation, two kidney cases (25Gy target dose) demonstrated the working principle on volunteer data and simulated delivery. The proposed workflow successfully showed its ability to track local dosimetric changes. Integration of the on-line anatomy information could reveal local dose variations −2.3 to 1.5Gy in the target volume of a volunteer dataset. In the adjacent organs at risk, high local dose errors ranging from −2.5 to 1.9Gy could be traced back

Feasibility of stereotactic radiotherapy using a 1.5 T MR-linac : Multi-fraction treatment of pelvic lymph node oligometastases

Online adaptive radiotherapy using the 1.5 Tesla MR-linac is feasible for SBRT (5 × 7 Gy) of pelvic lymph node oligometastases. The workflow allows full online planning based on daily anatomy. Session duration is less than 60 min. Quality assurance tests, including independent 3D dose calculations and film measurements were passed