A lung tumour can leave traces of itself in the bloodstream long before it is easy to study with a scan or biopsy. These traces are tiny fragments of genetic material, shed by cancer cells as they grow, die, and interact with the body. For lung cancer, where early diagnosis and timely treatment choices can change the course of care, that signal matters. Circulating tumour DNA, often shortened to ctDNA, is part of the wider field of “liquid biopsy”. Instead of taking tissue fr