Breast Tumor Diversity Tied to Metastasis Risk and Tamoxifen Timing

Not all hormone-sensitive breast cancers behave the same, and that difference shows up in who metastasizes and who benefits from treatment. That's the takeaway from a new doctoral thesis at Karolinska Institutet by Oscar Danielsson, at the Department of Oncology-Pathology, supervised by docent Linda Lindström.

Danielsson's work split hormone-sensitive disease into its two common subtypes, luminal A and luminal B. Both gained from tamoxifen over the long haul, but on very different clocks. Luminal B patients saw most of the benefit within five years. Luminal A patients gained less at five years, with the payoff growing as follow-up stretched on.

That gap has a practical consequence for research. Danielsson said a study that stopped at five years would have missed much of tamoxifen's later benefit for luminal A patients, which is why long-term follow-up matters when studying this cancer. The slow burn isn't hypothetical: tumor cells can lie dormant and start growing again 10 to 20 years later.

Variation within a single tumor also carried weight. Patients whose tumors showed more variation in progesterone receptor levels had roughly a 40% higher rate of metastasis during follow-up than those with less variation, yet they still responded to endocrine therapy. As Danielsson put it, differences both within tumors and between patients appear tied to metastasis risk and treatment benefit.

Two of the thesis studies handed tumor tissue images to artificial intelligence, which flagged patterns linked to higher metastasis risk. One model also picked out a group of patients who clearly benefited from goserelin. Those results still need confirmation in other patient groups before the approach reaches clinical practice.

The work was published in the International Journal of Cancer.