Development of localized cul-de-sac endometrioid carcinoma associated with deep infiltrating endometriosis during remission of early endometrial cancer

•A cul-de-sac endometrioid carcinoma adjacent to extraovarian endometriosis was identified during remission of endometrial cancer.•The origin of the cul-de-sac tumor was malignant transformation of deep infiltrating endometriosis.•Endometriosis-related cancer was identified in a woman with endometrial cancer during remission.•Hyperestrogenism due to infertility treatment may contribute to malignant transformation of deep infiltrating endometriosis.