Comparison of total MU and segment areas in VMAT and step-and-shoot IMRT plans.

We compared treatment plans for volumetric intensity-modulated arc therapy (VMAT) and step-and-shoot intensity-modulated radiation therapy (IMRT) in terms of their monitor unit (MU) and segment area at each control point to investigate the difference between the two methods. We investigated three sites: prostate (three cases), head and neck (three cases), and pleura (two cases). We used the total MU and the MU weighted average of segment area (MWSA) in each plan to compare VMAT and IMRT plans. VMAT plans tended to have a larger MWSA and a lower total MU than did IMRT plans in all sites, although there was little difference between dose indices in either irradiation technique. We conclude that VMAT is a better treatment technique due to its higher MU efficiency caused by the larger segment area.