PF is a rare benign gastric mesenchymal tumor, accounting for 0.2% of all gastrointestinal mesenchymal tumors, with its incidence 150 times lower than that of gastric GIST(1). Although PF is a global disease, East Asia is the most prevalent region. The tumor is widely distributed in various age groups, ranging from 5 to 81, with a median age of 46, and has no gender difference(3). The typical clinical presentations of PF are gastrointestinal symptoms, which are primarily non-specific, such as abdominal pain, abdominal distension, and abdominal discomfort. Almost all PF occurs within the stomach, of which the antrum is the most commonly involved site, followed by the body and fundus, and the size of the mass is between 0.8cm and 17cm with a median maximum diameter of 4cm(3, 4). Extragastric PFs have also been reported, such as the mediastinum, duodenum, jejunum and gallbladder, but are relatively uncommon(3, 5). For gastrointestinal PFs, gastroscopy reveals that the lesions are frequently round, elevated and submucosal tumor-like appearance with a clear boundary as shown in the case we presented above. The CT imaging often suggests a solid or cystic-solid mass with mild enhancement in the arterial phase and reinforced enhancement throughout the venous phase(6, 7). However, compared with other gastrointestinal mesenchymal tumors, those imaging features are non-specific, in which case the final diagnosis is relied on histopathological examination.
Microscopically, PF is characterized by oval to spindle-shaped bland tumor cells arranged in multinodular plexiform growth pattern and separated by abundant intercellular myxoid stroma rich in small thin-walled, arborizing blood vessels(8). Cellular atypia and mitosis are absent in the majority of cases. The tumors are usually centered in the submucosa or muscularis propria, and often intrude into the mucosa and/or the serosa with a pushing or infiltrative border(1). Vascular or lymphatic involvement can be seen in a few cases, but distant metastasis has not been reported yet. By immunohistochemistry, almost all tumor cells of PF are positive for SMA according to available data, which implies that they possess a myofibroblastic nature. Variable staining results demonstrated for desmin, caldesmon, PR, CD10 and CD34 reported by others due to use of different antibodies, technical errors or misinterpretation(3, 4). Genetic abnormalities including glioma-associated oncogene homologue 1 (GLI1) amplification, metastasis-associated lung adenocarcinoma transcript 1 (MALAT1)-GLI1 oncogenic fusion(9), and Patched 1 (PTCH1) deletion(10) have been identified in a subset of PF cases.
In our case, the most essential histologic differential diagnosis is myxoid GIST. It should be noted that the negative immunohistochemical staining of CD117 and DOG-1, and the lack of c-KIT and PDGFRα gene mutations can distinctively distinguish PF from GIST. Other differential diagnoses should include plexiform schwannoma, plexiform neurofibroma, and inflammatory fibroid polyp (IFP). Both schwannoma and neurofibroma originate from the peripheral nerve sheath and can show plexiform growth pattern(1, 11). However, in contrast to the immunohistochemical findings in PF, these two neural tumors are both positive for S100 and SOX10. IFP consists of CD34-positive bland spindle cells, which are concentrated around small blood vessels forming an “onion skin”-like appearance, accompanied by a myxoedematous inflammatory stroma often dominated by eosinophils(12). Therefore, PF can be distinguished from IFP by both histological features and immunophenotype.
Because PF often involves the muscularis propria, and it is challenging to exclude GIST clinically, the mainstream treatment for PF reported in the literature is surgical resection(3). However, PF shows a benign biological behavior with neither local recurrence nor distant metastasis have been reported so far. Conservative management, such as endoscopic resection, has been tried to treat PF. Together with our case, 12 patients underwent gastroscopic resection(3, 4, 13, 14). The common features for the twelve cases are that the tumors are located in the superficial layer (mucosa or submucosa) of the gastric wall and are small enough (with a median size of 1.65cm, ranged from 0.8cm to 5cm) to be resected under gastroscopy. Hence, it is important to diagnose and treat PF at an early stage. Follow-up results were obtained in 10/12 patients. Over a median follow-up period of 6 months (range 1-32 months), all the patients were uneventful after endoscopic resection. However, due to the limited follow-up time, the long-term outcome of endoscopic treatment for PF needs to be further studied.
In conclusion, gastric PF is a rare benign mesenchymal tumor with a unique histological appearance and immunophenotype. Endoscopic resection can be used as an alternative treatment for small PF involving only the superficial layer of the gastric wall. The pathogenesis, genetic abnormalities, and long-term clinical follow-up studies remain to be established for the deep comprehension of PF.