Premalignant alterations in breast and endometrium associated with a PTEN mutation in a woman with Cowden syndrome: implications for preventive care

•This case report documents the in vivo process of malignant transformation in a patient with Cowden syndrome.•PTEN-driven oncogenesis may proceed through a stage of pre-invasive intra-epithelial neoplasm.•Our findings have important implications for preventive care and for pathologic sampling at the time of prophylactic surgery.