200 Section - A Case Report of a Tumor-like Lesion Following COVID-19 Vaccination: A Literature Review and Ultrasonographic Findings

Delayed hypersensitivity reactions (DHRs) presenting as a tumor-like soft tissue mass following COVID-19 vaccination (Moderna mRNA-1273).

Case Description: We report a 66-year-old man who developed a firm, painless, enlarging mass in the left deltoid three months after a Moderna mRNA-1273 booster. Ultrasound showed a well-defined, heterogeneous hypoechoic intramuscular lesion without vascularity or bony involvement. MRI revealed a 5.4 cm intramuscular mass with heterogeneous signal and a hypointense capsule. Histopathologic examination demonstrated necrotic tissue with capillary proliferation.

Discussions:

Delayed hypersensitivity reactions following COVID-19 vaccination are well recognized. Localized cutaneous reactions typically present as erythematous, tender, and swollen plaques near the injection site and are generally benign and self-limiting. However, presentation as a tumor-like intramuscular mass is rare. The mechanism likely involves a T-cell–mediated immune response to vaccine components, causing localized inflammation, necrosis, and reactive vascular proliferation that can mimic neoplastic growth. Ultrasound in post-vaccine inflammatory lesions usually shows superficial, well-defined, and minimally vascular features in the subcutaneous layer. This case demonstrates a rare large intramuscular inflammatory mass after vaccination detected by ultrasound, which should be differentiated from soft-tissue malignancy. Sonographic features suggesting malignancy include deep location, irregular margins, heterogeneous echotexture, increased vascularity, and rapid growth. When findings are indeterminate or suspicious, such as lesions larger than 5 cm or with atypical vascularity, further MRI or core needle biopsy is recommended.

Conclusions:

This case highlights a rare DHRs following the Moderna COVID-19 vaccination, presenting as a tumor-like intramuscular mass. Clinicians should recognize that vaccine-related inflammation can mimic soft-tissue sarcoma and perform MRI or biopsy when imaging findings are indeterminate to avoid misdiagnosis and unnecessary surgery.