Supplements are?
This appears to be a reasonable write up, with linked studies:
How to Decrease Breast Density to Reduce Breast Cancer Risk
http://mammalivefoundation.org/decrease-breast-density-reduce-breast-cancer-risk/
There is also a possible benefit from iodine:
Changes in Dietary Iodine Explains Increasing Incidence of Breast Cancer with Distant Involvement in Young Women
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5327366/
If taking iodine in larger amounts, do make sure you get selenium to take along with it.