Testing for immune cell "hotspots" around tumours could help identify women at high risk of relapsing breast cancer, say scientists.
Women with large numbers of immune cells clustered in and around tumours were more likely to see their treatment fail within 10 years, research showed.
Scientists analysed tissue samples from 1,178 women with the most common form of oestrogen-sensitive breast cancer.
Patients with clustered immune cells were 25% more likely to relapse within 10 years of starting treatment than those whose immune cells were evenly dispersed.
The likelihood of cancer returning within five years was 23% higher in women with the hotspots.
Lead scientist Dr Yinyin Yuan, from The Institute of Cancer Research in London, said: "We have developed a new, automated computer tool that makes an assessment of the risk of relapse based on how cells are organised spatially, and whether or not immune cells are clustered together in the tumour.
"In the most common form of breast cancer, oestrogen receptor positive, the presence of hotspots of immune cells clustered together in the tumour was strongly linked to an increased risk of relapse after hormone treatment.
"Larger studies are needed before an immune hotspot test could come to the clinic, but in future such a test could pick out patients at the highest risk of their cancer returning. It might also be possible to predict which patients would respond to immunotherapy."
The findings appear in the Journal of the National Cancer Institute.
Professor Paul Workman, chief executive of The Institute of Cancer Research, said: "What this study tells us is that the immune system probably has a key role to play in how breast cancer responds to hormone treatment. Measuring the immune response to cancer could be important in the future to help identify patients who could benefit from immunotherapy."
Katherine Woods, from the charity Breast Cancer Now, which co-funded the study, said: "That this exciting immune tool could be added to existing prognostic tests to more accurately identify women at high risk of their breast cancer coming back is very promising.
"Automatically analysing the distribution of immune cells in a tumour is a big achievement, and if this approach is validated it could help doctors guide chemotherapy treatment."
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