Gluteal injection site granulomas are a very common finding on CT and plain radiographs. They occur as a result of subcutaneous (i.e. intra-lipomatous) rather than intramuscular injection of drugs, which cause localized 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.
Fat necrosis, which can liquefy. This cavity is surrounded by fibrous tissue and reactive inflammatory cells (lymphocytes, foamy histiocytes, and giant cells). Capillary growth can be prominent. Dystrophic calcification can eventually occur.
Usually seen as well-defined small nodules that often contain calcification.
Typical signal characterisitcs include:
- T1: hypointense
- appearance depends on the temporal evolution of the granuloma
- may be T2 hyperintense if the reaction is inflammatory
- may be T2 hypointense if the reaction is fibrous
The differential depends on whether or not the granuloma is calcified.
- soft tissue sarcoma
- subcutanous metastases
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