Real-time dose reconstruction and dose coverage forecasting using the magnetic resonance linear accelerator

Intrafraction patient motion during radiotherapy treatment affects the delivered dose, thereby, deviating from the planned dose. To monitor the deviation, we propose a workflow capable of performing and displaying real-time dose calculations during treatment with a magnetic resonance linear accelerator (MR-linac). Additionally, the workflow can forecast the accumulated dose and enables evaluation of dose guidance in real time for triggering automatic plan adaptation or intrafraction drift corrections (IDCs).

Information on the treated anatomy, rigid translations or three-dimensional (3D) volumes, and the linac state were collected to perform clinical grade dose calculations. Real-time dose reconstructions and the accumulated dose forecasts were performed for two dynamic scenarios. The first, rigid translations derived from two-dimensional orthogonal MR images where the calculated dose was compared to diode measurements. The second, deformable dose accumulation using deformable image registration, based on 3D MR imaging and compared to a dose film measurement.

The real-time dose reconstruction had a maximum standard deviation error of 2.3 % with the measured dose, compared to 13.1 % for the planned dose. The workflow maintained real-time performance using a single graphics processing unit up to 92 % of the time that the beam was on. Finally, dose coverage forecasts were updated every 6.5 s at the start of treatment, with the update frequency increasing towards the end of treatment.

We demonstrated the feasibility of displaying real-time delivered dose distributions and forecasted dose coverage using an MR-Linac. This enables triggering automatic plan adaptations or IDCs.