The Geant4 Monte Carlo radiation transport toolkit provides the basic services and infrastructure required for the development of flexible simulation frameworks and applications, which have found generalized use in high energy physics, nuclear physics, astrophysics, and medical physics research. Object-oriented design provides the possibility to implement or modify any physics process in Geant4 without changing other parts of the code. This feature makes Geant4 open to extension of its physics modeling capabilities and to the implementation of alternative physics models. In this paper, the development of a simulation platform for performance studies and detector optimization of the Clear-PEM scanner, a high-performance positron emission mammography prototype, and the implementation of precise low energy bremsstrahlung angular generators for the Geant4 LowE category are described.