Photomedicine and stem cells : the Janus face of photodynamic therapy (PDT) to kill cancer stem cells, and photobiomodulation (PBM) to stimulate normal stem cells / Heidi Abrahamse (DST/NRF SARChI Chair: Laser Applications in Health, Laser Research Centre, Faculty of Health Sciences, University of Johannesburg, South Africa), Michael R Hamblin (Wellman Center for Photomedicine, Massachusetts General Hospital, Boston, MA, USA, Department of Dermatology, Harvard Medical School, Boston, MA, USA, Harvard-MIT Division of Health Sciences and Technology, Cambridge, MA, USA)