Embodiments of the present disclosure provide improved techniques for dosimetry in radiotherapy treatment. In a fractionated radiotherapy treatment, the expected dose buildup is computed in advance of a fraction delivery based on the instructions in the treatment plan. The progress of the fraction is then monitored using an electronic portal image detector (EPID), and fluence data from the EPID used to calculate an estimate of the actual dose build-up in real time. This estimate can be compared to the expected dose buildup in order to validate the fraction.