Vanishing mediastinal mass associated with mRNA SARS-CoV-2 vaccination: a rare case report

Background: Thymic hyperplasia has occasionally been reported as an immune response to vaccination for severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2). We herein report a rare case of a vanishing mediastinal mass associated with mRNA SARS-CoV-2 vaccination. Case Description: A woman in her forties presented with continuous fever after receiving her first mRNA-1273 SARS-CoV-2 vaccination. Chest computed tomography (CT) examination revealed an anterior mediastinal mass of 66 -mm in diameter. She was referred to our hospital for evaluation of the anterior mediastinal mass. 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) revealed markedly increased uptake in the mediastinal mass. The differential diagnosis included thymoma, thymic cancer, and lymphoma. Although a CT scan performed 25 days after vaccination showed that the mediastinal mass had shrunk to 32 mm, the lesion was still present and a needle biopsy was performed. Unfortunately, accurate pathological examination could not be achieved because of small specimen. A surgical biopsy by the thoracoscopic approach was next planned to rule out malignancy. However, a CT scan prior to surgical biopsy performed 2 months after vaccination revealed that the mediastinal mass had completely disappeared. The surgical biopsy was cancelled. Because there was no residual lesion at 6 months, we concluded that the mediastinal mass was an adverse effect of the SARS-CoV-2 vaccination, presenting as a transient hyperplasia of the thymus. Conclusions: It is important to distinguish an immune response from a malignancy to avoid unnecessary surgery or treatment. Our case warns us that we should take into account the timing of SARS-CoV2 vaccination when we encounter a patient with mediastinal mass.