The Backward Monte-Carlo (BMC) paper has been accepted for publication in Computer Physics Communications. It is now published online with the following DOI: 10.1016/j.cpc.2018.04.001, or from arXiv.
The paper contains detailed explanations and proofs of the BMC procedure used in PUMAS. The implementation is also discussed in details, in section 3. Some validation figures can be found in section 4.