Assessment of the Monitor Unit Objective tool for VMAT in the Eclipse treatment planning system.

Aim: This work aims to achieve the highest possible monitor units (MU) reduction using the MU Objective tool included in the Eclipse treatment planning system, while preserving the plan quality.

Background: The treatment planning system Eclipse (Varian Medical Systems, Palo Alto, CA) includes a control mechanism for the number of monitor units of volumetric modulated arc therapy (VMAT) plans, named the MU Objective tool.

Material and methods: Forty prostate plans, 20 gynecological plans and 20 head and neck plans designed with VMAT were retrospectively studied. Each plan ( base plan ) was optimized without using the MU Objective tool, and it was re-optimized with different values of the Maximum MU ( MaxMU ) parameter of the MU Objective tool. MU differences were analyzed with a paired samples t -test and changes in plan quality were assessed with a set of parameters for OARs and PTVs.

Results: The average relative MU difference [Formula: see text] considering all treatment sites, was the highest when MaxMU  = 400 (-4.2%, p  < 0.001). For prostate plans, the lowest [Formula: see text] was obtained (-3.7%, p  < 0.001). For head and neck plans [Formula: see text] was -7.3% ( p  < 0.001) and for gynecological plans [Formula: see text] was 7.0% ( p  = 0.002). Although similar MU reductions were observed for both sites, for some gynecological plans maximum differences were greater than 10%. All the assessed parameters for PTVs and OARs sparing showed average differences below 2%.

Conclusion: For the three studied clinical sites, establishing MaxMU  = 400 led to the optimum MU reduction, maintaining the original dose distribution and dosimetric parameters practically unaltered.