Two Giant Cystic Uterine Adenomyomas in a Premenopausal Woman: The Largest Case to Date With Immunohistochemical Findings.

Background and Aims: Cystic adenomyoma is a rare focal cystic variant of adenomyosis, and giant lesions are particularly uncommon. Malignant transformation has been reported in endometriosis-related disease, but the molecular features of cystic adenomyoma, especially in adults, remain unclear. We are aimed at describing a premenopausal patient with two giant cystic adenomyomas, including the largest lesion reported to date, and to explore a possible pathogenic mechanism using immunohistochemistry. Methods: A 47-year-old nulliparous premenopausal woman presented with progressive abdominal distension and urinary symptoms. Imaging showed two large hemorrhagic cystic masses adjacent to a mildly enlarged fibroid uterus, and ovarian endometriotic cysts were suspected preoperatively. The patient underwent hysterectomy with bilateral salpingo-oophorectomy. Gross, histologic, and immunohistochemical examinations were performed on the uterus and cystic lesions. Clinical follow-up was obtained for 10 months. Results: Surgery revealed two cystic adenomyomas measuring 30 and 10 cm, contiguous with the uterus but separate from both ovaries. The thick cyst walls were composed of smooth muscle bundles, and the inner surfaces were lined by a single layer of endometrial-type epithelium; multiple foci of conventional adenomyosis were also present. In the cystic adenomyomas, glands were HNF-1β+, pAKT+, estrogen receptor+, PTEN-, PIK3CA-, and ARID1A- with a p53 wild-type pattern. Eutopic endometrial glands were HNF-1β+, PIK3CA+, pAKT+, ARID1A+, and p53+, with mixed PTEN-positive and -negative glands. The postoperative course was uneventful, and no recurrence was observed at 10 months. Conclusion: This case represents the largest cystic adenomyoma reported to date and the first adult case characterized in detail by immunohistochemistry. Differential PTEN and ARID1A expression between eutopic endometrium and cystic adenomyomas supports a model in which PTEN-deficient endometrial clones invade the myometrium to form adenomyosis, with additional ARID1A loss and pAKT activation driving cystic enlargement without malignant transformation.