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Is ovulation induction with letrozole in breast cancer patients still safe even if it could increase progesterone levels?

Very high progesterone levels (mean 186.6 ± 43.6 ng/mL) during the luteal phase were found in a small study of breast cancers patients undergoing controlled ovarian stimulation (COS) with letrozole plus recombinant FSH. Results highlight the need to further evaluate this in larger series. While waiting, the clinical significance of high progesterone levels can be drawn from epidemiological and experimental data here reviewed in order to give reassurance to the clinician involved in fertility preservation. If the progesterone increase will be confirmed, epidemiological and experimental data do not seem to indicate a detrimental effect or they could even be protective. As this possible rise of levels is a very short event in the very long lasting and multifactorial breast carcinogenesis, it is unlikely that it will significantly influence breast cancer prognosis.