1. A radiation therapy planning system, comprising: an isodose line unit configured to receive or construct isodose lines from a source dose grid planned for a volume of a subject; a region of interest unit configured to define at least one isodose region of interest based on the received or constructed isodose lines; and an optimization unit configured to generate an optimized deliverable radiation therapy plan based on the at least one defined isodose region of interest and at least one dose objective for the defined isodose region of interest.