Typical imaging features of a gastric GIST, which, on CT was not entirely convincing to not have pancreatic involvement. Further workup with MRI showed the lesion to be confined to the gastric wall. Biopsies were performed via gastroscopy and the tumor was further resected:  

Macroscopy: Labeled "GIST tumor'.  A segment of gastric wall 40 x 18 x 14 mm, with a lobulated tumor arising from the wall 52 x 43 x 28 mm, with a dull, violaceous surface bearing patchy areas of ulceration, and moderately firm consistency.  The cut surface is tan, firm and lobulated with heterogeneous friable, brown areas, focal apparent surface fat and admixed areas of hemorrhage.  The lesion abuts the mucosal resection margin. 

Microscopy: The sections show a tumor within the gastric wall comprising intersecting fascicles and nests of spindled cells with elongate nuclei with vesicular chromatin, small nucleoli and abundant eosinophilic cytplasm. The mitotic rate is 1/ 5 mm^2. The tumor is present within the submucosa, muscularis propria and subserosa. No definite serosal invasion is seen. No definite vascular invasion is seen but an immunohistochemical stain for CD31 is being performed to better assess this. No perineural invasion is identified. The tumor is clear of excision margins.

Using immunohistochemical stains, the tumor cells are strongly and diffusely positive for CD117, show patchy positive staining for CD34 and are negative for CD31, S100 and mixed cytokeratins (AE1/AE3 and CAM5.2).

Gastric body-type mucosa included in the specimen. There is no significant inflammation. No Helicobacter pylori are identified and there is no intestinal metaplasia or glandular dysplasia.

Conclusion: Gastric resection: Gastrointestinal stromal tumor
 - Size 52mm
 - Mitotic rate 1/5 mm^2 (WHO prognostic group 3a/ AJCC 8th ed. stage IB);
 - Invasion into subserosa  
 - Negative resection margins.

Note: No vascular invasion is identified with an immunohistochemical stain for CD31.