Injection granuloma mimicking soft tissue sarcoma following seasonal influenza vaccine administration: A case report.

Rationale: Soft tissue masses are common within the general population with a minority diagnosed as soft tissue neoplasms. Differing between benign and malignant soft tissue processes can be a challenge given the overlapping clinical and imaging characteristics. We present the case of a 69-year-old female referred to the Orthopaedic Oncology Service for evaluation of a suspected soft tissue sarcoma in the upper arm.

Patient Concerns: She reported a mass localized over the deltoid with associated tenderness 1 month after influenza vaccination.

Diagnosis: After thorough consideration of the patient's clinical course, history, advanced imaging, and physical examination, the diagnosis of injection granuloma associated with recent influenza vaccination was considered.

Interventions: Biopsy was deferred and close interval follow-up with clinical and imaging evaluation revealed a resolving process.

Outcomes: The patient was followed until complete resolution of all symptoms, which occurred 5 months after initial presentation.

Lessons: It was hypothesized that due the patient's body habitus, the injection contents intended for intramuscular administration remained in the subcutaneous tissues and elicited a granulomatous reaction. This case highlights several important factors for physicians to consider in the work up of suspicious masses for which injection granuloma is on the differential diagnosis.