Relapsing ANCA-associated vasculitis presenting as a mediastinal mass.

Anti-neutrophil cytoplasmic antibody-associated vasculitides (AAV) represent a heterogeneous multisystem group of disorders typified by necrotising inflammation of smaller blood vessels, classically yielding a pauci-immune, crescentic glomerulonephritis. Without prompt treatment, there is a significant risk of irreversible damage and ensuing renal impairment.Diagnosis is often challenging, exacerbated by the disorder's often vague and insidious presentation. Even with known AAV, relapse can mimic other similarly serious conditions including infection or malignancy, with significant ramifications on respective downstream management options.Here, we present the case of a young man with pre-existing myeloperoxidase-positive vasculitis, describing a subacute cluster of symptoms and subsequently found to have an anterior mediastinal mass. Urgent tissue sampling excluded other aetiologies and ultimately delineated this as a surprising complication of vasculitis, allowing for prompt immunosuppression escalation. This case highlights the diagnostic challenge these patients represent, and the utility timely access to multimodality imaging holds alongside a multidisciplinary approach to expedite correct management decisions.