Online Radiotherapy: The Paradigm Shift to Real-Time Adaptive Radiotherapy.

Adaptive radiotherapy (ART) involves the continuous adaptation of the radiation plan according to patient- and tumor-specific feedback. In online ART, the plan is optimized in real time during the treatment; in offline ART, the plan is recalculated between treatment sessions. Hybrid linear accelerators with integrated CT, MRI, or PET are required to perform online ART.