Killing cancer with light, not scalpels | with Stephen Bown
What if you could kill a cancer cell without touching the healthy tissue around it? That is the central promise of photodynamic therapy, and Stephen Bown has spent forty years making good on it.Filmed at the Royal Institution on 29 May 2026.
PDT works by combining two ingredients that are harmless on their own: a drug called a photosensitiser that makes tissue sensitive to light, and light itself. Together they trigger a reaction that destroys living cells while leaving the structural scaffolding of the surrounding tissue intact. That selectivity makes it possible to treat cancers in places where a scalpel cannot safely go, and to achieve cosmetic results that surgery rarely matches.
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In this Discourse, Professor Bown draws on decades of clinical and research experience to explain how PDT works, why getting light deep inside the body is a formidable physics problem, and what the most promising applications look like today. He covers treatments for cancers of the skin, bladder, lungs, oesophagus, mouth and brain, describes an early-stage proposal to treat type 2 diabetes using the same principles, and explains how PDT may prime the immune system to fight back against cancers long after treatment has ended.
Stephen Bown is Emeritus Professor of Laser Medicine and Surgery at University College London and founder of the National Medical Laser Centre. His connection with the Royal Institution dates to 1981, when he gave his first Discourse here.
Chapters:
0:00 Introduction
0:16 A history of light in medicine
2:27 The accidental birth of photodynamic therapy
5:22 From laboratory to clinic
8:06 Treating skin cancers
11:00 Getting light inside the body
19:41 Bladder, lungs and mouth
27:36 PDT in rural India
33:14 The oesophagus and photosensitiser selection
39:04 Could PDT treat type 2 diabetes?
44:04 Brain tumours and bioluminescence
55:47 Immunotherapy, infection and macular degeneration
1:00:18 Summary
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