Sonke et al. 39 have shown that although the shape of the tumor trajectory was found to be stable interfractionally, baseline shifts might invalidate the 4DCT planning **dataset**. Although MLC tracking is expected to cope with these shifts (depending on the motion acquisition method), this might result in a mismatch with the dose-influence data. Therefore, to apply the proposed online 4D dose reconstruction method, the validity of the dose-influence **data** should be assessed prior to each fraction. To allow for dose reconstruction on anatomy not captured on 4DCT, patient models could be utilized. 40 Innovations in image-guided radiation therapy technology like the MR-linac 41 may ultimately bring 4D online-imaging and have the potential to continuously update a patient model, robust to baseline shifts and other anatomy changes. Extending this work to volumetric arc therapy (VMAT) is hindered by the substantial increase in dose-influence data. We are currently investigating the trade-off between the number of beam angles and the dosimetric accuracy for online dose reconstruction of VMAT deliveries. Generating dose-influence **data** on the fly is currently too slow and we therefore envision future online 4D dose calculation directly, based on the patient model geometry and a real-time Monte Carlo dose calculation.