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 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 () was optimized without using the MU Objective tool, and it was re-optimized with different values of the Maximum MU () parameter of the MU Objective tool. MU differences were analyzed with a paired samples -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  = 400 (-4.2%,  < 0.001). For prostate plans, the lowest [Formula: see text] was obtained (-3.7%,  < 0.001). For head and neck plans [Formula: see text] was -7.3% ( < 0.001) and for gynecological plans [Formula: see text] was 7.0% ( = 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  = 400 led to the optimum MU reduction, maintaining the original dose distribution and dosimetric parameters practically unaltered.