Described here are brachytherapy applicators that comprise a modular support matrix. The modular support matrix may define the arrangement of radiation source lumens within the brachytherapy applicator. The modular support matrix can be adjusted to position the radiation sources to obtain a certain dose and dose profile. A series of modular support matrices may be operably connected according to patient anatomy or dosing requirements. Systems and methods for using such modular brachytherapy applicators are also described.