Simultaneous reduction of number of spots and energy layers in intensity modulated proton therapy for rapid spot scanning delivery.

Abstract

Reducing proton treatment time improves patient comfort and decreases the risk of error from intrafractional motion, but must be balanced against clinical goals and treatment planquality. To improve the delivery efficiency of spot scanning proton therapy by simultaneously reducing the number of spots and energy layers using the reweighted regularizationmethod. We formulated the proton treatment planning problem as a convex optimization problem with a cost function consisting of a dosimetric plan quality term plus a weighted regularization term. We iteratively solved this problem and adaptively updated the regularization weights to promote the sparsity of both the spots and energy layers. The proposed algorithm was tested on four head-and-neck cancer patients, and its performance, in terms of reducing the number of spots and energy layers, was compared with existing standard and group regularization methods. We also compared the effectiveness of the three methods ( , group , and reweighted ) at improving plan delivery efficiency without compromising dosimetric plan quality by constructing each of their Pareto surfaces charting the trade-off between plan delivery and planquality. The reweighted regularization method reduced the number of spots and energy layers by an average over all patients of and , respectively, with an insignificant cost to dosimetric plan quality. From the Pareto surfaces, it is clear that reweighted provided a better trade-off between plan delivery efficiency and dosimetric plan quality than standard or group regularization, requiring the lowest cost to quality to achieve any given level of deliveryefficiency. Reweighted regularization is a powerful method for simultaneously promoting the sparsity of spots and energy layers at a small cost to dosimetric plan quality. This sparsity reduces the time required for spot scanning and energy layer switching, thereby improving the delivery efficiency of protonplans.