# IgG4‐Related Sclerosing Mediastinitis Mimicking Thymoma: A Case Report

## TL;DR

A case of IgG4-related disease in the chest was mistaken for a tumor, showing the difficulty in diagnosing this rare condition.

## Contribution

Highlights the diagnostic challenge of IgG4-related disease mimicking thymoma despite normal serum IgG4 levels.

## Key findings

- A mediastinal mass was initially suspected as thymoma but was diagnosed as IgG4-related disease post-surgery.
- Histopathology confirmed dense lymphoplasmacytic infiltration and IgG4-positive plasma cells.
- The patient remained disease-free for 3 years without corticosteroid therapy.

## Abstract

IgG4‐related disease (IgG4‐RD) rarely presents as an anterior mediastinal mass and may radiologically resemble thymic neoplasms. We report a 70‐year‐old man with an incidentally detected anterior mediastinal tumour on chest CT. The lesion measured 33 mm, was well‐circumscribed, homogeneous and partially calcified. Tumour markers and serum IgG4 were within the normal range. The patient underwent thoracoscopic resection under the diagnosis of thymoma. Histopathological examination revealed dense lymphoplasmacytic infiltration and abundant IgG4‐positive plasma cells, confirming IgG4‐related sclerosing mediastinitis. No corticosteroid therapy was administered, and the patient remained disease‐free over 3 years of follow‐up. This case highlights the diagnostic challenge of mediastinal IgG4‐RD and the importance of considering it in the differential diagnosis of thymic tumours, even when serum IgG4 is normal.

This case highlights the diagnostic challenge of mediastinal IgG4‐RD and the importance of considering it in the differential diagnosis of thymic tumours, even when serum IgG4 is normal.