Single-Isocenter Multiple-Target SRS Planning of Five to Ten Brain Metastases Using 5 mm Multileaf Collimator: Relationship between Prescription Dose, Number and Volume of Targets

OBJECTIVE: To propose an expression relating the number and volume of targets with the prescription dose in determining normal brain volume receiving 12 Gy dose (V12) for five to ten brain metastases treated in linear accelerator-based stereotactic radiosurgery (SRS) planning. To determine the volume of tumor that can be treated within the brain tolerance dose, for different SRS prescription doses. METHODS: Single-isocenter multiple-target (SIMT) SRS plans were devised for spherical targets that are modeled to simulate 47 tumor scenarios with varying tumor sizes and locations within the brain. Volumetric modulated arc therapy (VMAT) plans were devised using a 5-mm-leaf-width multi-leaf collimator (MLC) with high conformity and dose gradient in the Eclipse treatment planning system for the 21 Gy prescription dose with a 6FFF photon beam. The prescription dose was rescaled to 20 Gy, 18 Gy, 15 Gy and 12Gy to determine the brain V12 volume for a total of 235 SRS plans. RESULTS: Linear correlation was observed between the number, volume and prescription dose of the tumor. The expression relating these parameters was constructed to predict the normal brain V12 volume. The maximum tumor volume that can be treated using SIMT SRS with a 5-mm MLC for 5 to 10 number of targets and for a prescription dose of 21 Gy, 20 Gy, 18 Gy and 15 Gy is determined. CONCLUSION: Using the expression obtained, V12 volume can be calculated using the number of tumors and the total volume of tumors from the pre-planning MRI data. The prescription dose and the SRS fractionation size can be determined before radiotherapy treatment planning.