A molecular test can identify which patients will have a very low risk of death from breast cancer up to 20 years after diagnosis, according to a new clinical study by researchers at Karolinska Institutet and University of California, San Francisco (UCSF).

The results were published in JAMA Oncology journal.

Since the introduction of screening, more breast cancer tumours with indolent, low–risk behaviour are detected. Better tools to identify such tumours are therefore needed to avoid overtreatment.

In the new study, researchers sought to determine whether a 70–gene test with an ultralow risk threshold could accurately and reliably identify tumours with indolent behaviour to assess the risk of fatal breast cancer up to 20 years after diagnosis. The study was run in collaboration with the Stockholm breast cancer study group and the Stockholm tamoxifen (STO–3) trial. The new analysis included postmenopausal breast cancer patients enrolled in the STO–3 trial from 1976 until 1990, randomised to receive tamoxifen treatment versus no systemic therapy.

“We concluded that the ultralow risk threshold identifies patients at very low long–term risk of dying from breast cancer,” says senior author Linda Lindström, researcher at the Department of Biosciences and Nutrition at Karolinska Institutet. “Since breast cancer is such a diverse disease, it is important to be able to identify patients at very low long–term risk.”

The test classified 15 per cent of the patients as very low risk patients, showing that such tumours are an inherent part of the spectrum of breast cancers. Women with ultralow risk tumours had an excellent prognosis, whether or not they received tamoxifen.

“This is an important step forward for personalising care for women with breast cancer,” says Laura Esserman, breast cancer specialist and professor at UCSF. “We can now test small node–negative breast cancers, and if they are in the ultralow risk category, we can tell women that they are highly unlikely to die of their cancers and do not need aggressive treatment.”

The study was financed by California Breast Cancer Research Program BCRP, the Swedish Research Council, the Swedish Research Council for Health, Working life and Welfare (FORTE) and the Gösta Milton Donation Fund.

The article is titled, “Use of Molecular Tools to Identify Patients With Indolent Breast Cancers With Ultralow Risk Over 2 Decades.”