Gluteal injection site granulomas are a very common finding on CT and plain abdominal xrays. They occur from subcutaneous (”intralipomatous”) rather than intramuscular injection of drugs, resulting in fat necrosis, scar formation and dystrophic calcification.
Once familiar with the entity they rarely pose any diagnostic confusion, although they have been reported to be metabolically active on FDG-PET.
For more on this topic please visit Radiopaedia.org here.
1. H Prosch et al “Gluteal injection site granulomas: false positive finding on FDG-PET in patients with non-small cell lung cancer” British Journal of Radiology ,78 (2005),758 – 761
2. L Michaels and RW Poole “Injection granuloma of the buttock” Can Med Assoc J. 1970 March 28; 102(6): 626–628.
3. University of Washington: Musculoskeletal radiology. 
Credit: Dr Frank Gaillard