S6468 When Stomach Meets Ovary: A Rare Collision Tumor in the Gastric Region

Case Description/Methods: Sixty-one-year-old woman with no significant past medical history presented to emergency department with 3-week history of abdominal distension and epigastric pain. A computed tomography (CT) scan of the abdomen and pelvis revealed extensive abdominopelvic ascites associated with serosal and peritoneal implants, concerning for malignant ascites. She subsequently underwent interventional radiology-guided paracentesis. Cytological analysis of the ascitic fluid was positive for metastatic adenocarcinoma consistent with Mullerian primary. Follow up magnetic resonance imaging (MRI) abdomen demonstrated a 5 cm lesion in the gastric antrum extending into the pylorus, suspicious for primary gastric malignancy. Subsequent upper endoscopy revealed a large, fungating, ulcerated circumferential mass at the pylorus. Endoscopic biopsies demonstrated poorly differentiated invasive adenocarcinoma with signet ring cells consistent with gastric primary with discrete fragments of metastatic mullerian type adenocarcinoma. Based on these findings, she was diagnosed with 2 high-grade histologically distinct primary malignancies. Initial systemic therapy was directed towards the gastric carcinoma with FLOT: 5-fluorouracil, leucovorin, oxaliplatin, and docetaxel. She subsequently underwent total abdominal hysterectomy with bilateral salpingo-oophorectomy followed by multiple cycles of carboplatin and paclitaxel. As of 9 months post-diagnosis, she has experienced no further hospitalizations. Discussion: The incidence of synchronous primary cancers is increasing, defined as the diagnosis of 2 or more distinct primary malignant tumors occurring simultaneously or within a short interval of one another, typically within a 6-month timeframe (1). A rare subset of these are collision tumors, which involves 2 histologically distinct neoplasms arising independently within the same anatomical location. These tumors may exist adjacent to one another or intermingled, without a transitional zone between them (2). Although collision tumors are uncommon, the majority of reported cases have been documented in dermatologic pathology (3). However, there are isolated reports describing collision tumors involving distinct primaries in other organ systems – for example, concurrent thyroid and lung carcinomas identified within a single cervical lymph node (4). To our knowledge, this case represents the first reported instance of a collision tumor involving both gastric and ovarian primary malignancies, identified within the same gastric tissue biopsy.