Scientists have created detailed maps of breast tumors, revealing hidden pockets of cancer cells that remain inactive, or "dormant," and may explain why cancer can return after treatment. These cells do not divide actively, allowing them to avoid detection and destruction by chemotherapy. They can remain in this inactive state for a long time before reactivating and causing tumor regrowth. This research was conducted by teams from the MRC Laboratory of Medical Sciences (LMS), Imperial College London, and UCL Genetics Institute, and was published in the journal Genome Medicine. The study used advanced techniques like single-cell RNA sequencing and spatial transcriptomics to examine the structure and genetic activity of breast cancer tumors. These methods allowed researchers to identify distinct clusters of dormant cancer cells surrounded by other cells, such as immune cells and connective tissue cells. These surrounding cells may act as a protective shield, helping the dormant cancer cells avoid the effects of cancer treatments. One key finding was that dormant cancer cells are often located near specific types of cells, such as CXCL10-positive macrophages and myofibroblastic cancer-associated fibroblasts. These neighboring cells may create a physical or biological barrier that prevents cancer-killing immune cells or treatments from reaching the dormant cells effectively. This suggests that different parts of a tumor may respond differently to treatment, and that therapies targeting both active and dormant cancer cells, as well as the surrounding environment, could be more effective. The study also found that the complement pathway—a part of the immune system—may be more active in the areas where dormant cells reside. This discovery suggests that treatments targeting this pathway could make those areas more vulnerable to attack. The research was primarily funded by UKRI Future Leaders Fellowship, the Medical Research Council, and the Biotechnology and Biological Sciences Research Council, highlighting the importance of understanding the complex interactions within tumors to improve cancer treatment strategies.