Breast tumor microenvironment cells may predict tamoxifen treatment effectiveness, study finds

A study from Karolinska Institutet published in August 2026 suggests that the composition of cells in a breast tumor's microenvironment may predict the long-term effectiveness of tamoxifen treatment. Analyzing samples from 513 postmenopausal women in the STO-3 trial followed for up to 25 years, researchers found that patients with low immune cell abundance experienced the greatest benefit from tamoxifen. However, because the cohort was treated under older standards of care, these findings require confirmation in contemporary populations before clinical application.

Tumor microenvironment composition

• ▪The tumor microenvironment of a breast tumor consists of immune cells, connective tissue cells, and other cells found within and around the tumor.
• ▪A study by Karolinska Institutet researchers published in August 2026 mapped the types of cells present within and around breast tumors to investigate their link to cancer recurrence.

Tamoxifen treatment response

• ▪Tamoxifen is a drug used to treat hormone-sensitive breast cancer by blocking the effects of the hormone estrogen.
• ▪Different levels of blood vessel-related cells and connective tissue cells within and around breast tumors are associated with how much benefit patients gain from tamoxifen treatment.

STO-3 trial cohort

• ▪Researchers analyzed tumor samples from 513 postmenopausal women with estrogen receptor-positive, HER2-negative breast cancer who participated in the randomized Stockholm Tamoxifen (STO-3) trial.
• ▪Participants in the Stockholm Tamoxifen (STO-3) trial were assigned to receive either tamoxifen or no endocrine treatment and were followed for up to 25 years.

Immune cell abundance

• ▪Breast cancer patients with a low abundance of immune cells in the tumor microenvironment experienced the greatest long-term benefit from tamoxifen compared to patients who did not receive the treatment.
• ▪A study published in BMC Medicine in August 2026 suggests that immune cell abundance in the tumor microenvironment is linked to the risk of breast cancer recurrence.

Treatment benefit prediction

• ▪Karolinska Institutet researcher Nick Tobin stated in August 2026 that cells surrounding a breast tumor may provide important information about how patients respond to treatment, whereas tumors are currently assessed mainly on cancer cell characteristics.
• ▪A study from Karolinska Institutet published in August 2026 suggests that the types of cells in the microenvironment of a breast tumor may predict the long-term effectiveness of tamoxifen treatment.

Future clinical validation

• ▪Karolinska Institutet researchers emphasized in August 2026 that because their study is based on patients treated according to older standards of care, the findings must be confirmed in contemporary patient populations before clinical application.
• ▪Karolinska Institutet researcher Paula Camargo Romera stated in August 2026 that understanding tumor microenvironment variations may eventually help researchers tailor breast cancer treatments more effectively to the biological characteristics of individual tumors.

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