The study had to be terminated due to the new European legislation on drug studies. This legislation requires all studies aimed at evaluating treatments to be registered as such, even in the context of Standard of Care.

This is a multicenter hospital-based prospective cohort study conducted in institutions with known expertise in performing oocytes/embryo freezing for fertility preservation. The study aims at refining the understanding of the efficacy and safety of controlled ovarian stimulation with or without letrozole in young women with newly diagnosed breast cancer who are candidates to receive (neo)adjuvant chemotherapy.

Patients enrolled in this study undergo standard or "random start" ovarian stimulation with Gonadotropins using antagonist protocol before the beginning of chemotherapy. Ovulation is triggered in all patients with a Gonadotropin Releasing Hormone-GnRH agonist.

After retrieval, oocytes are denuded and matured oocytes are subjected to fertilization before embryo freezing or direct vitrification.

Primary objective is to evaluate the efficacy of performing a controlled ovarian stimulation with or without letrozole in young women with newly diagnosed breast cancer who are candidates to receive (neo)adjuvant chemotherapy in terms of mature oocytes collected.

Inclusion Criteria:

• Diagnosis of invasive non-metastatic breast cancer (i.e. stage I to III);
• Breast cancer diagnosis ≥18 and ≤ 40 years;
• No prior history of gonadotoxic treatments;
• Fertility preservation counseling for fertility preservation;
• Written inform consent;
• FSH < 20 UI/L and/or antra-follicular count ≥ 6 (follicles of 2-9 mm) and/or AMH ≥ 6 pmol (only applicable for patients who undergo controlled ovarian stimulation for embryo/oocyte cryopreservation).

Exclusion Criteria:

• Newly diagnosed stage IV breast cancer (i.e. de novo metastatic breast cancer);
• Prior diagnosis of other malignancies before breast cancer.