Dr Brennan is a specialist in Paediatric Oncology and Clinical Trials. 

At the National Cancer Research Initiative (NCRI), Bernadette is a member of the Sarcoma Clinical Studies Group and chairs the Young Onset Soft-tissue Sarcoma (YOSS) subgroup. She is a member of the Children's Cancer and Leukaemia Group (CCLG) Rare Tumour Group and EXPeRT European Rare Tumour Group.

She is clinical lead on the European paediatric Soft tissue sarcoma Study Group (EpSSG) non-rhabdyomyosarcoma soft tissue sarcomas (NRSTS) study for the UK and International/UK Euro Ewing 2012 Study and a member of the Euro Ewing Consortium.