Factors Influencing Normal Brain Dose for Single-Isocenter Multi-Target Radiosurgery
Y Yuan*, E Thomas, G Clark, J FIveash, R Popple, Univ Alabama Birmingham, Birmingham, ALSU-E-T-673 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: To evaluate the effects of number of arcs, collimator angle, variable secondary collimator size, and optimization objectives on normal brain dose.
Methods: Ten patients treated for multiple cranial metastases were retrospectively planned for single-isocenter VMAT. All plans were optimized using a standardized objective function and were normalized such that 99% of the total target volume received 18 Gy. Multiple variables were utilized in planning, including numbers of arcs (2 versus 4), collimator angles (45° versus selected per beam to minimize area of normal brain exposed in the beams-eye-view), jaw tracking (fixed versus trailing MLC leaf), and a low dose constraint for healthy brain (mean dose 2 Gy versus no constraint). Plan quality was evaluated by conformity index, gradient index, mean absolute volume of normal brain receiving 1.8 Gy (V10), 4.5 Gy (V25), 12 Gy (V12Gy), and normal brain mean dose.
Results: Conformity index, gradient index, and V12Gy were not significantly dependent on the factors considered. The low dose constraint reduced the brain mean dose by 56 cGy (13.7% reduction relative to no low dose constraint). Other factors did not have a significant effect on mean dose. The low dose constraint decreased V25 and V10 by 136 cc (28.8% reduction) and 128 cc (12.7% reduction), respectively. Collimator angle and jaw tracking each reduced V25 and V10 by up to 32 cc and 40 cc, respectively. Compared to 2 arcs, 4 arcs decreased V25 by 20 cc and increased V10 by 38 cc.
Conclusion: Limiting the normal brain mean dose in the optimization objective function significantly reduces the low dose spill into the normal brain without changing target coverage. Jaw tracking and appropriate selection of collimator also reduces the low dose volume, but to a lesser extent.