Large Retroperitoneal GIST in a Patient With Neurofibromatosis Diagnosed With EUS

Neurofibromatosis type I (NF-I) is a rare autosomal dominant genetic disorder that predisposes patients to various neoplasms, including gastrointestinal stromal tumors (GIST) which are commonly found in the small intestine in the setting of NF-I. We report a rare case of a patient with NF type I who was found to have a large retroperitoneal GIST diagnosed with Endoscopic Ultrasond Guided Fine Needle Aspiration (EUS-FNA). A 56 year-old male patient with NF-I who is otherwise healthy presented to the emergency department complaining of sharp epigastric abdominal pain of one week duration. Computed tomography (CT) of the abdomen and pelvis was remarkable for a 15 cm cystic mass inferior to the pancreas and hence the patient was referred to the Gastroenterology clinic for further evaluation. Physical examination was unremarkable and repeat laboratory test results were within normal limits. The patient underwent EUS-FNA which revealed a 15 cm heterogeneous retroperitoneal mass and an unremarkable pancreas. Microscopically the tumor consisted of spindle cells with focal areas of necrosis and abscess formation. Mitoses were absent, and the overall tumor morphology was consistent with a low-grade neoplasm. The tumor cells were positive with CD117 and CD34 immunohistochemical stains, diagnostic of GIST (Image 1). The patient underwent an exploratory laparotomy with retroperitoneal mass excision and 6 cm small bowel resection with side-to-side functional end-to-end anastomosis as a 0.9 cm subserosal small bowel nodule was noted during gross examination which was later confirmed as a second primary GIST. Due to the tumor size, location and mitotic index, the tumor was classified as having a high-risk of progression and the patient was referred to Oncology for initiation of adjuvant therapy on discharge. This case highlights a rare co-existence between two already rare pathologies and the importance of close monitoring of patients with NF-I. GISTs should be considered in the differential diagnosis of a mass in a patient with NF-I, even outside of the GI tract. Obtaining tissue biopsy may prove to be challenging, however, EUS-FNA provides an extremely helpful diagnostic tool especially in those patients where prompt diagnosis and early intervention provide better outcomes.1869 Figure 1. H&E stain at 20x magnification showing tumor infiltrating the muscularis propria (asterisk) without involvement of the overlying small bowel mucosa (A). At 400x magnification, the retroperitoneal mass is composed of a spindle cell proliferation with low-grade morphology and absent mitotic figures, identical to the histologic findings of the small bowel lesion (B). Strong, diffuse positivity is seen within the tumor with CD117 (C) and CD34 (D) immunohistochemical stains, diagnostic of gastrointestinal stromal tumor.