The rounded mass may initially be concerning for renal cell carcinoma. However, notice that it distorts the contour of the kidney rather than arising from it (no "claw sign"), and thus other diagnoses should be considered. Also there is another smaller mass adjacent to it along the superior/lateral aspect (best seen on coronal images). The history of splenectomy due to gunshot wound (and overlying soft tissue metallic fragments) indicate that the retroperitoneum was likely violated at the time of injury and thus could be seeded with splenic tissue. This benign diagnosis was confirmed with sulfur colloid SPECT.