The Continuum of 3D to 4D Treatment Delivery

Treatment planning based on respiration-correlated (“4D”) CT has become more common in recent years. Still, treatment delivery is typically 3D, i.e., a “one plan fits all” (motion phases) approach. 4D delivery in principle offers greater flexibility but also greater challenges, both computational and practical. We develop a computational framework to open up some of the 4D flexibility without going all the way to 4D delivery. We apply our method to lung cases. The key result is that even for very complicated cases it is not necessary to do full unconstrained 4D delivery. A quasi-static delivery with 5% variability of intensity maps seems to suffice.