Monitor Unit-Efficient Planning For Stereotactic Body Radiation Therapy And Stereotactic Radiosurgery

Studies have shown that greater modulation results in less accurate plan delivery due to dose calculation algorithm uncertainties and interplay effects between tumor and multileaf collimator motion. Greater modulation also leads to an increased number of monitor units (MUs) per unit absorbed dose (intensity modulated radiation therapy [IMRT] factor) which lengthens the beam on time and treatment duration. The aim of this study was to develop and analyze an MU-efficient treatment planning method that minimized the number of MUs while maintaining the dosimetric quality of the treatment plan. We compared 2 volumetric modulated arc therapy (VMAT) planning methods, conventional VMAT planning, and MU-efficient VMAT planning on 14 SBRT/SRS plans. Treatment sites included brain, liver, spine, and lung. Both methods utilized a commercially available treatment planning system. The MU-efficient planning method used the MU objective with a strength setting of 100, a lower objective of 0, and an upper objective of 1.5 times the prescription dose per fraction. The plan normalization and the optimization objectives for organs at risk (OARs) and targets were identical between the conventional VMAT plan and the MU-efficient plan. Dosimetric quality metrics such as the conformity index (CI), gradient measure, and planning target volume dose coverage (minimum, maximum, and mean dose) were compared between the initial clinical plan and the MU-efficient plan. MU efficiency metrics including the total MUs, IMRT factor, and delivery time were also compared between the conventional and MU-efficient plans. The CI (P=0.863) and gradient measure (P=0.192) were statistically equivalent for both sets of plans. The average IMRT factor was 3.1 MU/cGy for the clinical plans, and 2.3 MU/cGy for the MU-efficient plans. Improvement in MU-efficient was less for targets with concavities or asymmetry (spine or multiple metastases treated with a single iso). The average reduction in IMRT factor for concave/asymmetric targets (0.4 MU/cGy) was less than other sites (0.9 MU/cGy). Assuming a dose rate of 600 MU/minute, the MU-efficient plans resulted in reduction in treatment delivery time of 5.3 minutes (0.6-14.9 minutes) per course of treatment. MU-efficient treatment planning facilitated by the MU objective is a simple way to reduce the IMRT factor for SBRT/SRS plans and reduce table time without sacrificing target coverage, dose gradient, conformity, or any other plan quality metric studied.