eP164 - Collision Tumor Between a Low Grade Appendiceal Mucinous Neoplasm and a Neuroendocrine Tumor

Purpose/Background: Tumors of the appendix occur in 1% of appendectomies. They may be benign or malignant, and majority are mucinous and neuroendocrine neoplasms. In the unlikely event that the tumor components of two adjacent yet separate neoplasms arise from different proliferative cellular lines, this is called a collision tumor.Methods/Interventions: We present a 44 year-old female with abdominal enlargement and right lower quadrant pain. Physical exam showed a distended, but soft and nontender abdomen. Abdominal CT revealed an abdominopelvic mass (10.5 x 17.6 x 21.1 cm), considered as a gynecologic malignancy. CEA and CA 125 levels were elevated. Noted on surgery was a 26 x 23 cm right abdominopelvic mass (Fig 1A), with an appendiceal tumor (1.7 x 1.5cm) (Fig 1B). For purposes of complete oncologic resection, she underwent a right salpingo-oophorectomy, total hysterectomy with left salpingo-oophorectomy, and right hemicolectomy.Results/Outcomes: The patient was discharged on the 5th postoperative day. On biopsy, the appendiceal tumor was a well-differentiated grade 1 neuroendocrine tumor (NET), while the ascending colon, uterus, bilateral adnexa, and right ovary were that of a well-differentiated low grade appendiceal mucinous neoplasm (LAMN).Conclusion/

Discussion: Mucinous neoplasms may develop aggressive growth pattern due to its high luminal mucin content leading to appendiceal rupture and spillage into the peritoneal cavity. NETs are based on a combination of histomorphology and features that lead to expression of the tumor markers synaptophysin and CD56. LAMN (73%) and appendiceal NETs (30-80%) are the most frequent benign and malignant lesions of the appendix, respectively. A rare entity is that of a collision tumor, where two independent tumors are within the same site with absence of transitional change between two proliferative cellular lines. They are rare, especially for appendiceal lesions. Prior to this case, only 17 cases were reported.

On literature review, collision tumors are seen in females (60%) with an age range of 23-60, and a median age of 49. Of the 15 cases of LAMN and appendiceal NETs, 12 patients underwent a right hemicolectomy, 6 patients received adjuvant chemotherapy, with a recurrence rate of 16.7% (3/18). Clinical presentation varies with appendicitis symptoms, pelvic mass, diarrhea, and weight loss. Diagnosis is based on intraoperative and histologic findings, and emphasis on recognizing these tumors during surgery is crucial.

Surgical approach is still the gold standard, where appendectomy is sufficient for LAMN confined to the appendix with negative margins, or NETs smaller than 1 cm. For advanced NETs larger than 2cm with base involvement, high grade, lymphovascular invasion, and lymph node involvement, resection with right hemicolectomy and lymph node dissection is recommended. Resection is considered for NETs between 1-2 cm with features of serosal involvement, Ki-67 proliferation index of >2%, appendiceal base location, and angioinvasion. LAMN with pseudomyxoma peritonei may necessitate CRS with HIPEC. Adjuvant chemotherapy is recommended for high grade LAMN and NETs that have LVSI and nodal involvement.

At present, no studies or trials have been reported on appendiceal collision tumors. Managing them has become a dilemma due to different staging for both tumor lines. We recommend to tailor treatment on the more aggressive histologic pattern and stage. A multidisciplinary approach is also recommended.Collision tumor of the appendixAppendix (A) Gross specimen. The yellow arrow is a gray gelatinous mass corresponding to the LAMN occupying the entire appendix. The wall of the appendix is outlined (blue arrow). The red arrow is a distinctly separate, light yellow, ovoid mass corresponding to the NET at the junction of the cecum and appendix. (B-D) Microscopic sections of the appendix. (B). Hematoxylin and eosin, 20x. Low grade appendiceal neoplasm (arrowheads) and NET(arrow). PGH, 2023.