Subepithelial lesions (SELs) of the stomach are reported to be found in less than 3% of endoscopic examinations. After initial discovery, a diagnostic workup is performed to differentiate between benign and malignant lesions, followed by either surgical treatment or appropriate surveillance. Symptoms are typically absent or include mild abdominal pain or discomfort, but bleeding or strictures can also occur. In cases with symptoms such as bleeding, surgical treatment is often necessary. If the patient is asymptomatic, the treatment plan is usually determined through a combination of endoscopy, endoscopic ultrasound (EUS), and abdominal computed tomography (CT). It is important to approach and evaluate these lesions very carefully, as submucosal gastric adenocarcinoma can also present as an SEL.

Plexiform fibromyxoma (PF) of the stomach is a very rare SEL, known as a highly vascular tumor that can cause ulceration of the gastric mucosa, making it a potential cause of gastrointestinal bleeding [1-4].

We report the case of a 68-year-old female patient who was diagnosed with an SEL accompanied by an ulcer. After initially declining surgical treatment and opting for observation, she presented with bleeding during follow-up and underwent surgical intervention. The lesion was subsequently diagnosed as PF.

A 68-year-old female patient visited our outpatient clinic for further evaluation after being diagnosed with an SEL at an outside hospital. At the time of her visit, the patient had no specific symptoms or discomfort, and no other abnormal findings were observed on physical examination. No specific abnormalities were noted on hematological tests. Her body temperature was 36.3°C, blood pressure was 145/84 mm Hg, heart rate was 65 beats/min, and respiratory rate was 22 breaths/min. Laboratory findings revealed a hemoglobin of 10.7 g/dL. The patient had no past medical history of conditions or medication treatments that could cause anemia at the time of their hospital visit.

Endoscopic findings showed an approximately 1 cm SEL in the gastric antrum (AW side), with a central ulceration observed within the lesion (Fig. 1). A biopsy of the ulcer was diagnosed as chronic gastritis. EUS revealed a 12.4 mm SEL in the submucosa, with a slightly heterogeneous echogenicity (Fig. 2A). An abdominal CT scan performed at the same time showed an approximately 1 cm contrast-enhancing lesion in the gastric antrum (Fig. 2B). Due to the risk of bleeding, endoscopic resection or surgical treatment was recommended to the patient. However, she opted for follow-up observation without treatment, and a plan for regular monitoring was established.

Five months later, the patient presented to the emergency room with hematemesis. Her body temperature was 37.0°C, blood pressure was 155/87 mm Hg, heart rate was 90 beats/min, and respiratory rate was 22 breaths/min. Laboratory findings revealed a hemoglobin of 8.7 g/dL. Endoscopy showed the previously observed SEL with ulceration (Fig. 3A), but no active bleeding was present, so surgical treatment was performed. Pathology examination after surgery revealed a 1.5 cm mass with ulceration resected specimen consists of ring-shaped stomach tissue (4.0×2.7 cm). Cavity, measuring 2.4×0.7 cm is found in the center. On sections, 1.5×1.0 cm sized solid mass with surface ulceration is present (Fig. 3B). Pathological examination and immunohistochemical (IHC) staining of the lesion confirmed the diagnosis of lesion. The tumor cells showed an ovoid to spindle shape and characteristically displayed a plexiform growth pattern (Fig. 4A-C). The pathological features included a mitotic count of less than 1 per 10 HPFs, absence of necrosis, and no evidence of lymphovascular or perineural invasion. Tumor cells stained positive for smooth muscle actin (SMA) but negative for CD117, DOG-1, desmin, ALK (Fig. 4D). Pathological examination and IHC staining of the lesion confirmed the diagnosis of PF.

A report indicates that PF occurs slightly more frequently in women. It is typically reported to occur in people in their 40s and 50s [2]. It is most commonly reported to occur in the gastric antrum, followed by the duodenum, jejunum, etc. Symptoms are generally non-specific, and in many cases, there are no symptoms, so it is often discovered incidentally. It usually appears with symptoms such as abdominal pain and discomfort [2]. Some patients present to the hospital with bleeding due to a central ulcer in the lesion. It generally exhibits benign behavior [2].

It has been reported that the size of PF on endoscopy is typically less than 20 mm (6–17 mm), and a central ulceration of the lesion is a common finding. Bleeding from the ulcer is not uncommonly the patient’s main symptom. The tumor is often reported to be found in the gastric antrum [2,3].

Imaging studies have shown that differentiation from gastrointestinal stromal tumor (GIST) is often difficult when it presents alongside other SELs. While small tumors may not be visible on abdominal CT scans, their increased vascularity often causes them to appear as contrast-enhanced lesions. They are typically reported as solid, cystic, or solid/cystic masses with well-defined borders [1].

Histologically, PF shows a multinodular, plexiform growth pattern with a proliferation of ovoid to spindle cells within myxoid stroma, and an increased vascularity. The spindle cells are bland, without significant atypia or mitotic activity [2,5]. The primary differential diagnosis between GISTs, leiomyomas, and PF relies heavily on specific IHC markers. Although they can all present as spindle cell lesions within the gastrointestinal tract, their distinct immunophenotypes guide the final diagnosis and subsequent clinical management. GISTs are typically strongly positive for CD117 (c-KIT) and DOG-1, while negative for SMA and desmin. Leiomyomas are negative for CD117 (c-KIT) and DOG-1, but positive for both SMA and desmin. In the case of PF, the tumor cells are positive only for SMA.

Malignancy is reported to be determined by tumor size, location, and mitotic activity, but precise data is still lacking. PF typically appears ovoid to spindle-shaped and is positive for SMA, but is characteristically negative for C-KIT (-), DOG-1 (-), S-100 (-), desmin (-), and CD34 (-) [2].

The treatment is reported to be most effective with surgical resection [2].