Primary Peritoneal Low-Grade Serous Carcinoma in a 16-Year-Old Female: A Case Report.

Background: Primary peritoneal carcinoma (PPC) is an uncommon malignancy typically diagnosed in postmenopausal women, accounting for less than 1% of all gynecologic cancers. Its occurrence in adolescents is extremely rare. We present a case of a 16-year-old female with low-grade serous carcinoma (LGSC) arising from the anterior rectal peritoneum, highlighting diagnostic challenges and therapeutic considerations. Case Presentation: A 16-year-old girl presented with a 7-day history of lower abdominal pain. Ultrasound revealed an 8 cm mixed cystic-solid pelvic mass anterior to the rectum. Laboratory tests showed elevated CA-125 (106 U/mL). Exploratory laparotomy demonstrated an 8 cm solid mass attached to the anterior rectal wall, extending into the right mesorectum with peritoneal nodules at the bladder reflection. The uterus and adnexa appeared grossly normal. Frozen section analysis revealed adenocarcinoma with psammoma body formation. Histopathology and immunohistochemistry confirmed low-grade serous carcinoma: PAX8(+), WT1(+), CK7(+), ER(60%), PR(40%), CK20(-), and P53 wild-type. Peritoneal washings contained rare malignant cells. Postoperatively, the patient underwent total abdominal hysterectomy, bilateral salpingo-oophorectomy, and omentectomy. Final pathology confirmed low-grade serous carcinoma involving the anterior rectal wall, bilateral adnexal surfaces, and peritoneum. She completed six cycles of adjuvant chemotherapy (paclitaxel + carboplatin, TC regimen). No recurrence was observed during follow-up. Conclusions: This case underscores the importance of considering PPC in the differential diagnosis of pelvic masses in young females, even when the ovaries appear normal.