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Abstract. We introduce, in this paper, a variational framework for the construction of improved treatment plans in volumetric modulated arc therapy for cancer treatment. This framework consists of a shape optimization component, from the molding of beam shapes, as well as strong constraints, from the equipment involved, on both beam shapes and intensities. We apply the binary level-set method, to handle complex shape topologies, and the fast sweeping method, to handle beam intensity constraints. The result is a fast, flow-based algorithm, in a simplified setting, that guarantees energy decrease. We include numerical simulations of clinical cases to demonstrate the efficacy of the approach.

The interplay between geometry and electrostatics contributes significantly to hydrophobic interactions of biomolecules in an aqueous solution. With an implicit solvent, such a system can be described macroscopically by the dielectric boundary that separates the high-dielectric solvent from low-dielectric solutes. This work concerns the motion of a model cylindrical dielectric boundary as the steepest descent of a free-energy functional that consists of both the surface and electrostatic energies. The effective dielectric boundary force is defined and an explicit formula of the force is obtained. It is found that such a force always points from the solvent region to solute region. In the case that the interior of a cylinder is of a lower dielectric, the motion of the dielectric boundary is initially driven dominantly by the surface force but is then driven inward quickly to the cylindrical axis by both the surface and electrostatic forces. In the case that the interior of a cylinder is of a higher dielectric, the competition between the geometrical and electrostatic contributions leads to the existence of equilibrium boundaries that are circular cylinders. Linear stability analysis is presented to show that such an equilibrium is only stable for a perturbation with a wavenumber larger than a critical value. Numerical simulations are reported for both of the cases, confirming the analysis on the role of each component of the driving force. Implications of the mathematical findings to the understanding of charged molecular systems are discussed.

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