Chemosensitivity During Phases of the Menstrual Cycle in Breast Cancer Patients

Inclusion Criteria: * Women with a new diagnosis of triple negative breast cancer who have not yet started systemic treatment. * Patients with stage I-III disease, patient with locoregional recurrence who have not been treated with chemotherapy before. , * Aged \< 60 years * Women having a (regular) physiological menstrual cycle * Patients who are assigned to receive neoadjuvant chemotherapy with or without immunotherapy, targeted therapy and endocrine therapy * Patients must be systemic treatment naïve for current malignancy (e.g. no chemotherapy, hormonal therapy or targeted therapy) * Signed written informed consent Exclusion Criteria: * current use of hormonal contraception or in the six weeks prior to start of neoadjuvant systemic treatment for breast cancer, such as: * Oral contraception (OAC) * Hormonal intra-uterine device (IUD, Mirena) * No ovarian function suppression to preserve fertility * Other forms of hormonal contraception, including but not limited to: nuva-ring, Implanon, prikpil * Currently pregnant and / or breast feeding. In case of use of hormonal contraception or breast feeding in de last year: patients should have had at least 2 menstrual cycles since stopping hormonal contraception. \- active other malignancy * IVF-trajectory for egg cell preservation prior to start of neoadjuvant systemic treatment