When size misleads: CT attenuation and enhancement characteristics predict benignity in a giant ovarian mucinous cystadenoma

Giant ovarian cysts are uncommon due to widespread early imaging detection; however, when encountered—particularly in perimenopausal women—they raise significant concern for malignancy. We report a 53-year-old woman presenting with progressive abdominal distension and 4 months of amenorrhea. Ultrasound demonstrated a 27 cm cystic adnexal mass with internal debris, categorized as O-RADS US 3. Contrast-enhanced CT showed a well-circumscribed cyst with homogeneous near-water attenuation (0-4 Hounsfield units), no enhancement, and no solid components, ascites, or lymphadenopathy—features demonstrating benign imaging characteristics analogous to O-RADS 2. Serum tumor markers (CA-125 15.9 U/mL, CEA 0.999 ng/mL, AFP 1.21 ng/mL, β-hCG < 0.2 mIU/mL, and LDH 215 U/L) were overall within or near normal limits. The patient underwent staging laparotomy, and histopathology confirmed benign mucinous cystadenoma with reactive changes in lymph nodes, omentum, and peritoneal biopsies. This case highlights that near-water CT attenuation and absence of enhancement are strong predictors of benignity even in extremely large adnexal masses, and that lesion size alone should not be considered a surrogate marker for malignancy.