The main consideration here is for a well-differentiated pancreatic neuroendocrine tumor, with differential mode with an intrapancreatic accessory spleen. Further single-photon emission CT with technetium 99–labeled red blood cells (not shown) confirmed that it was an accessory spleen. 

Intrapancreatic accessory spleen it is not that uncommon, with an autopsy study with 3000 subjects finding accessory spleens in 364 and, in 17% of those, it was located in the tail of the pancreas ¹.

Endocrine tumors of the pancreas comprise tumors which arise from the pancreatic islet cells and include some distinct tumors which match the cell type of origin. Each has different clinical presentation, on account of them elaborating various hormones and different biological behaviors.