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.