A Comprehensive Evaluation of Single Isocenter Multiple Target SRS Plans and the Analytical Relationship between Plan Quality Indices with the Number and Volume of Targets

OBJECTIVE: To study the relationship of plan quality indices with the number and volume of target for 5 to 10 brain metastases treated with LINAC-based Single Isocenter Multiple Target (SIMT) Stereotactic radiosurgery (SRS) planning and to determine the maximum volume of spherical targets treated without exceeding the normal tissue tolerances. METHODS: Spherical targets of 5 to 10 numbers per plan, with individual target volumes ranging from 0.025 cc to 11.5 cc, were simulated with randomly drawn planning target volumes (PTVs) within the brain. SIMT SRS plans were generated for the 21 Gy prescription dose with a 6 MV Flattening Filter Free photon beam. Target coverage, organ at risk sparing, plan quality indices, R(50%), and gradient measure were studied. Mean brain dose, V(12) for Brain minus PTV (BmP), V(10), V(12), V(15), V(18), V(20), and V(24) for brain volume were evaluated. Equations relating the gradient index, mean brain dose, and V(12) (BmP) to the given number and volume of the targets were constructed. RESULTS: PTV coverage D(98) was 98.77 ± 1.37 %. The mean CI(RTOG), Q(RTOG), HI(RTOG), CI(P), GI, and R(50%) of the individual targets were 1.02 ± 0.08, 0.94 ± 0.02, 1.49 ± 0.11, 0.91 ± 0.06, 4.74 ± 2.3, and 4.95 ± 2.67, respectively. The gradient measure achieved was in the range of 0.49 to 1.35 cm. The mean brain dose was in the range of 1.62 to 6.69 Gy. The mean V(12) (BmP) per target obtained was 3.85 ± 2.83 cc. CONCLUSION: Equations relating the number and volume of targets to the gradient measure, mean brain dose, V(12) (BmP), and V(10-24) can serve as a baseline for multiple brain metastases SIMT planning. The target volume for 5, 6, 7, 8, 9, and 10 targets that can be treated without exceeding V(12) (BmP) is 6, 5, 4.7, 4, 3.7, and 3.4 cc, respectively, for the 21 Gy prescription dose.