Plexiform Angiomyxoid Myofibroblastic Tumor (PAMT) of the Stomach: an Extremely Rare Mesenchymal Tumor Masquerading as Gastrointestinal Stromal Tumor or Leiomyoma

Abstract

A 76-year-old Caucasian male with a past medical history of hypertension, hyperlipidemia, peptic ulcer disease, right bundle branch block, and stroke presented with a submucosal gastric lesion. It was found on a screening endoscopy due to his history of peptic ulcer disease, which revealed a subepithelial bulge within a thickened fold on the lesser curvature in the distal antrum. The initial clinical impression was a leiomyoma or gastrointestinal stromal tumor (GIST). An endoscopic ultrasound (EUS) was performed which showed a 35 × 29 mm hypoechoic, heterogeneous, and lobulated lesion originating from the muscularis propria (Fig. 1). Fine needle aspiration of the submucosal mass was performed which revealed spindle cells consistent with leiomyoma on immunohistochemistry (positive for desmin and smooth muscle actin; negative for CD-117). Given the benign diagnosis and lack of symptoms, the patient was followed by gastroenterology. Two years later, a CT scan showed interval enlargement of the lesion to 47 × 33 × 30 mm with enlargement of the mesenteric lymph nodes (Fig. 2). A PET scan was performed which showed a 49-mm gastric antrum mural mass without hypermetabolic lymphadenopathy or evidence of distant metastasis. He was initially referred to our advanced therapeutic endoscopist for an endoscopic resection, but after evaluation, he was referred for a surgical resection. Due to the size, location, and entirely endoluminal component, a wedge resection with appropriate margins was not feasible. He subsequently underwent a robotic distal gastrectomy with Billroth II gastrojejunostomy. The patient’s postoperative course was uneventful. He was discharged from the hospital on post-operative day three on a regular diet and has recovered very well. The final pathology confirmed a 5.4-cm plexiform multinodular tumor involving the submucosa, muscularis propria, and focally bulged through the serosa (Fig. 3a). The tumor was composed of bland spindle cells with small and thin-walled blood vessels and myxoid stroma (Fig. 3b). Immunohistochemical stains revealed the tumor cells were positive for CD10, caldesmon, desmin, SMA, vimentin, and ALK and negative for CD-117, DOG-1, S100, neurofilament, and EMA. A prominent blood vessel component was highlighted by CD34 and ERG and myxoid stroma was confirmed on Alcian blue s ta in . A diagnos is of p lex i form angiomyxoid myofibroblastic tumor (PAMT) was rendered based on the growth pattern and immunophenotype. There was no necrosis and the proliferation was low on Ki-67 immunostain. All margins were negative for tumor.