Development of an optimization process for dose treatment planning for multiple arc stereotactic radiosurgery

The goal of stereotactic radiosurgery is to deliver conformal high dose irradiation to a lesion in the brain while simultaneously minimizing the radiation delivered to the surrounding normal tissue. The trial-and-error approach to dose treatment planning in stereotactic radiosurgery is very time-consuming, and there is no assurance that the resulting treatment plan is optimized. Previously used dose calculation methods and optimization algorithms like the downhill simplex and simulated annealing methods are slow.;A new and faster optimization process incorporating a new objective function was developed. The new objective function is consistent with the goal of stereotactic radiosurgery, which is the delivery of a high dose to the target and a low dose to the surrounding normal tissue. Mathematically, the new objective function involves maximizing the minimum difference between doses on the target boundary and doses in the surrounding normal tissue. It also allows the calculation of fewer doses, hence less CPU time, at each iteration than other objective functions that utilized all or pad of the 3-D dose calculation matrix. The new objective function is mathematically differentiable, and was optimized using the relatively fast sequential quadratic programming method.;When compared to existing optimization processes such as the downhill simplex and simulated annealing methods, the new optimization process achieved dose distributions at least equal to and most often better than those achieved using other optimization processes. Furthermore, the new optimization process is faster.;Since collimator diameters and beam weights had been used independently as adjustable parameters in optimized dose design, the two parameters were investigated. The results indicate that beam weight parameters alone are superior to collimator diameter parameters alone, or both beam weight and collimator diameter parameters, whenever emphasis is on low doses in the normal tissue. On the other hand, when the emphasis is on destroying the target as completely as possible, collimator diameter parameters alone produce superior treatment dose plans.;Often times, a lesion lies next to some critical tissue in the brain. The new optimization process can be and has been modified and used to determine optimized dosing of a target while avoiding high dose to neighboring critical tissue. For some irregularly shaped lesions, more than one isocenter is needed for a conformal dose. The new optimization process was also modified to determine a treatment plan for an ellipsoidal target requiring two isocenters. The resulting treatment dose is more conformal than those determined using a single isocenter. Finally, the new optimization process was used to determine an optimized treatment plan for a real tumor.