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Müllerian adenosarcoma (MAS) is a rare uterine neoplasm, typically arising from the endometrial surface. MAS originating from adenomyosis is exceptionally uncommon, with fewer than 10 cases reported, and poses diagnostic, staging, and therapeutic challenges. We report a 35-year-old nulliparous woman presenting with 6 months of abnormal uterine bleeding and iron-deficiency anemia. Initial imaging suggested benign endometrial polyps and fibroids, and two hysteroscopic curettages yielded benign findings. Persistent symptoms prompted further MRI and CT evaluation, revealing a large, heterogeneously enhancing intramural mass with junctional zone disruption and deep myometrial infiltration. Uterus-preserving excision and second-opinion pathology review ultimately confirmed low-grade MAS likely arising from adenomyosis. Given persistent radiologic concerns, definitive management with total laparoscopic hysterectomy and salpingo-oophorectomy was recommended. MAS arising from adenomyosis frequently leads to false-negative superficial biopsies and presents with nonspecific imaging characteristics that may mimic benign lesions or endometrial carcinoma. Accurate diagnosis requires deep tissue sampling and comprehensive histopathologic review. Current FIGO staging does not account for intramural tumors, complicating risk assessment and management. This case emphasizes the importance of considering MAS in patients with discordant clinical, imaging, and biopsy findings. Thorough histopathologic evaluation, multidisciplinary collaboration, and reporting of such rare cases are essential to improve recognition, diagnosis, and management strategies for MAS.