Teaching Case

A 74-year-old male patient with hypertension, an unremarkable past medical history and no abnormal laboratory tests, underwent upper digestive endoscopy for vague complaints including pyrosis and dyspepsia.

Endoscopy (Fig.1) disclosed a sizeable intraluminal bulge at the gastric body along the lesser curvature, lined by normal mucosa. Weeks later, endoscopic ultrasound (EUS, Fig.2) confirmed a demarcated, ovoid-shaped submucosal mass originating from the muscularis propria, with a mildly inohomogeneous hypechoic structure and intralesional vascular signals. EUS-guided biopsy diagnosed fusocellular proliferation without atypias, necrosis and mitoses. Immuno-phenotype (positive CD117/c-kit and CD34, negative Actine and S100, proliferative index Ki67<2%) was consistent with low-grade gastrointestinal stromal tumour (GIST).
Gastroenterologists opted for initially nonsurgical management. Six months later, MRI monitoring was performed with pharmacological hypotonisation and gastric distension using 750 ml water. MRI (Fig.3) depicted the known gastric submucosal mass with stable size (nearly 3x2 cm), predominantly exophytic growth, intermediate T2-weighted signal intensity (higher than the lifted mucosa) without necrotic/cystic changes, non-restricted diffusion and progressive contrast-enhancement. These MRI findings corroborated low-grade GIST amenable to conservative treatment.

Although rare, gastrointestinal stromal tumours (GISTs) are the most frequent mesenchymal tumours of the digestive tract and are defined by the 95%-specific immunohistochemical marker c-kit (CD117). The stomach represents the most frequent (55-60% of cases) site of involvement. Gastric GISTs generally occur in the 6th-7th decade of life and are discovered incidentally in up to 40% of cases at either endoscopy or cross-sectional imaging, and are submucosal masses with exophytic (80% of cases), intraluminal or mixed (dumbbell-shaped) growth [1, 2].
GISTs have greatly variable aggressiveness, which dictates their treatment and prognosis: among them, approximately 20-30% are malignant tumours with metastatic potential. Although different classification systems exist, there is increasing consensus that gastric GISTs below 2 cm and those with low mitotic index on histology have a low or very low malignant potential, and therefore do not warrant surgical resection unless interval growth occurs [1-3].
Furthermore, the most recent literature showed that small-sized GISTs have different imaging features from the well-known appearance of larger lesions. Specifically, the former appear as roundish, demarcated submucosal masses, most usually with exophytic (80% of cases) rather than intraluminal or mixed growth. Small GISTs generally appear homogeneous with solid attenuation and MRI signal intensity, and positive contrast enhancement which persists from the arterial over the later phases. Conversely, features suggesting malignancy include lobulated or irregular margins, mucosal ulceration, and heterogeneity from necrosis or haemorrhage. The differential diagnosis includes other submucosal gastric masses, which are typically mesenchymal in origin [4-9].
Therefore, the use of MRI is appealing to assess size, growth pattern and structural features of gastric GISTs, with high tissue contrast and excellent sensitivity for internal cystic or necrotic changes which would suggest a different therapeutic approach. Moreover, the apparent diffusion coefficient (ADC) calculated from diffusion-weighted MRI sequences shows an inverse linear correlation with the malignancy risk. As this case exemplifies, a dedicated MRI study of the stomach using fluid luminal distension and pharmacological hypotonisation offers the potential for non-invasive monitoring of small GISTs during conservative treatment, thus obviating repeated endoscopic ultrasound [5, 8, 9].