Cancers arising in dense breasts are more genomically unstable and have increased immune cells

A new study led by InforMD member A/Prof Kara Britt has found that breast cancers arising in dense breast tissue, which are known to be more aggressive than those from low density, have greater genome instability and immune activity. This might explain their aggressive nature but also, importantly, indicate that they may be responsive to immunotherapy.

The findings, published in the journal npj Breast Cancer, come after screening guidelines were recently updated to ensure Australian women with dense breasts are informed of this when receiving their mammogram result. This is important because both dense tissue and early-stage cancers appear as white on a mammogram, and high breast density is a risk factor for breast cancer.

“This study really highlights that breast density affects the biology of how cancer arises, and that maybe we can use this information to tailor better treatments for women with dense breasts”, said A/Prof Britt, from the Peter McCallum Cancer Centre in Melbourne.

The research identified elevated numbers of immune cells such as different types of T cells and B cells had infiltrated the tissue at the edge of tumours in dense breasts. While these cells are key drivers of the body’s immune response, they can also be co-opted by the cancer in ways that support rather than prevent tumour growth.

Higher density breasts were also seen to have an increased genomic instability – or an increased tendency for DNA mutations and genetic alterations during cell division, a known trigger for cancer.

The greater genomic instability and immune activity could influence response to treatment, and particularly immune-based or targeted approaches.

In Australia, around 40 per cent of women aged 50 –74 who attend BreastScreen meet criteria for having dense breasts. The findings help to explain the elevated breast cancer risk seen in these women, noting their cancers are typically harder to detect.

The results of this study are consistent with other reports showing that the tumours arising is dense breasts are more aggressive. Another important reminder for women with dense breasts to consider talking with their GP about their breast density, how to screen for cancer, and also how to best manage their other breast cancer risk factors.

Reference: Cheasley et al., Breast cancers arising in high mammographic density tissue harbor tumor-promoting immune profiles, 2026, NPJ Breast Cancer