Constant Dose Rate Vmat in Raystation

When optimizing variable dose rate VMAT (VDR) beams, the leaf positions and the MU of each control point are optimized simultaneously (Figure 1). Since the MU is not allowed to be individually optimized for each control point for a constant dose rate VMAT (CDR) beam (Figure 2), it is likely that the plan quality is reduced compared to a VDR beam with the same angle settings. To obtain a good quality dose distribution using CDR, the delivery time is usually increased compared to the VDR beam. The reason is that the gantry is not allowed to move faster from the directions where little dose is desired. Instead the optimizer closes the leaves faster from these directions to create smaller MLC openings.