Multiseptated Cystic Mass in the Pelvic Cavity: A Case Report of a Leiomyoma with Cystic Degeneration Mimicking a Peritoneal Inclusion Cyst

BACKGROUND: Uterine leiomyomas are common benign neoplasms, but atypical presentations such as extensive cystic degeneration are rare and can pose significant diagnostic challenges. When pedunculated and predominantly cystic, these tumors can closely mimic other pelvic pathologies, particularly Peritoneal Inclusion Cysts (PIC). CASE PRESENTATION: A 23-year-old nulliparous woman presented with lower abdominal pain. Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) revealed a large multiseptated cystic mass in the pelvic cavity, located posterolaterally to the uterus. An adjacent smaller cystic mass demonstrated T1 hyperintensity and T2 shading on MRI, suggestive of an endometrioma. Due to the multiseptated appearance and the presence of underlying endometriosis as a predisposing factor, the initial radiologic diagnosis was a PIC with an associated endometrioma. The patient subsequently underwent laparoscopic surgery, which revealed a large pedunculated subserosal leiomyoma arising from the uterine fundus with extensive cystic degeneration. Coronal MRI demonstrated a connecting stalk observed intraoperatively, and histopathology confirmed the diagnosis of leiomyoma with cystic degeneration and associated endometriosis. CONCLUSION: Uterine leiomyoma with cystic degeneration should be considered in the differential diagnosis of multiseptated cystic pelvic masses, especially when ovaries are identified separately. Careful assessment using multiplanar reconstruction to identify a connecting stalk or the bridging vessel sign is essential to avoid diagnostic pitfalls and ensure appropriate surgical planning.