A 34-year-old man presented with a mediastinal mass that raised concern for a malignancy, but upon biopsy was found to have IgG4-RD. This picture shows obliterative phlebitis, a histopathologic finding that is nearly pathognomonic of IgG4-RD. The vein is clearly outlined, but its lumen is nearly completely replaced with the same inflammatory infiltrate present elsewhere in the lesion. Obliterative arteritis (as opposed to phlebitis) can also occur, particularly in the lung, but is much less common than is venous involvement in IgG4-RD.
