Correct!
3. Inflammatory myofibroblastic tumor

The surgical biopsy material, evaluated with immunohistochemistry and light microscopy, showed no evidence of malignancy, so both lymphoma and non-small cell carcinoma are excluded diagnoses. Immunoglobulin G4 (IgG4)-related sclerosing disease is a consideration, but is unlikely given the observation of the low number of IgG4 positive plasma cells in the biopsy material. Similarly, fibrosing mediastinitis is a prominent diagnostic consideration as well, but usually histopathologic material in patients with fibrosing mediastinitis shows less cellular and more fibrotic findings, which is not observed in this case. The most likely diagnosis is inflammatory myofibroblastic tumor.

Further clinical course: The patient was treated with corticosteroid therapy, with marked improvement in her systems. A repeat 18FDG-PET scan showed diminished metabolism within the right paramediastinal mass, and the mass decreased slightly in size as well.

Diagnosis: Inflammatory myofibroblastic tumor

References

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3. Coffin CM, Humphrey PA, Dehner LP. Extrapulmonary inflammatory myofibroblastic tumor: a clinical and pathological survey. Semin Diagn Pathol. 1998; 15:85-101. [PubMed]
   
4. Cerfolio RJ, Allen MS, Nascimento AG, Deschamps C, Trastek VF, Miller DL, Pairolero PC. Inflammatory pseudotumors of the lung. Ann Thorac Surg. 1999; 67:933-936. [CrossRef] [PubMed]
   
5. Kim TS, Han J, Kim GY, Lee KS, Kim H, Kim J. Pulmonary inflammatory pseudotumor (inflammatory myofibroblastic tumor): CT features with pathologic correlation. J Comput Assist Tomogr. 2005; 29:633-639. [CrossRef] [PubMed]
   
6. Chen CH, Lin RL, Liu HC, Chen CH, Hung TT, Huang WC. Inflammatory myofibroblastic tumor mimicking anterior mediastinal malignancy. Ann Thorac Surg. 2008; 86:1362-1364. [CrossRef] [PubMed]
   
7. Schaeffer CJ, Minai OA, Sharma N, Kanne JP, Mohammed TL. Inflammatory myofibroblastic tumor of the lung: recurrence after steroid treatment. J Thorac Imaging. 2008; 23:191-193. [CrossRef] [PubMed]

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