Floor van Leeuwen has been working as an epidemiologist at the Netherlands Cancer Institute (NKI) since 1981, where she led the Psychosocial Research and Epidemiology Department between 2010 and 2022. Her research focuses on risk factors of breast cancer and ovarian cancer as well as the long-term effects of cancer treatment. Her research group at the Netherlands Cancer Institute investigates the risk of secondary tumors, cardiovascular disease, and infertility after radiation treatment and chemotherapy for lymphoma, testicular cancer, and breast cancer.
Van Leeuwen was one of the first in the world to prove that radiotherapy increases the risk of secondary cancer in the irradiated regions. This has led to a more precise follow-up of patients who have received radiotherapy, and increased precision in the irradiated regions. In her more recent work, Van Leeuwen showed that especially patients with a hereditary risk of breast cancer are extra sensitive to damage caused by radiation.
During her career, Van Leeuwen also delved into the role of hormonal and genetic risk factors in the development of breast cancer and ovarian cancer. She was especially interested in the long-term effects of fertility treatment and the development of cancer in women with an abnormality in the BRCA1 or BRCA2 gene. Her research showed that there is no increased risk of developing breast cancer in women who have undergone IVF treatment, nor in children who were born after IVF or ICSI treatment.
In the 1990s, Van Leeuwen and her colleagues founded the Hereditary Breast and Ovarian cancer study Netherlands (HEBON), which exists to this day. The study proves that the effect of birth control pills on the development of breast cancer or ovarian cancer in women with a BRCA1- of BRCA2 mutation is no different from that in women in the general population.