Purpose: Identify an MU-efficient treatment planning methodology that maintains the dosimetric quality of the treatment plan.
Methods: Seventeen hypofractionated (7-18Gy/fx) and twenty-five conventional fractionated (1.8-3Gy/fx) volumetric modulated arc therapy (VMAT) treatment plans were selected utilizing the Varian Eclipse™ version AcurosXB_15.6. The treatment sites included H&N, lung, liver, and prostate. A base plan was created without utilizing the MU objective tool. Then, an MU-efficient plan was created by utilizing the MU objective tool with a strength setting of 70, a lower objective of zero, and an upper objective (MaxMU) of different values. Dosimetric quality and MU-efficient metrics were used to compare the base plan and the MU-efficient plan. The MaxMU parameter was identified for each treatment site that best reduced the total MU, but at the same time gave a similar to or better plan than the base plan by as determined by fulfilment of the dose constraint metrics.
Results: For the hypofractionated lung and liver plans, a MaxMU of 1.25 times the prescription dose per fraction (1.25x) resulted in the total MU reduction by 24% and 25% respectively. For the hypo-fractionated prostate, a MaxMU of 2x resulted in the total MU reduction by 13%. For the conventional fractionated bilateral H&N and prostate with lymph nodes plans, a MaxMU of 600MU resulted in the total MU reduction by 19% and 15% respectively. For the conventional fractionated unilateral H&N and prostate minus lymph nodes plans, a MaxMU of 400MU resulted in the total MU reduction by 13% and 18% respectively. MU-efficient plans with the above-mentioned MaxMUs resulted in similar plans compared to the base plans, and in some cases, gave better dosimetric results for OARs.
Conclusion: The MaxMU for four studied sites were identified that led to the optimum MU reduction without sacrificing target coverage or any other studied plan quality metrics.