ALK-1 positive orbital inflammatory myofibroblastic tumour (IMT) associated with prominent numbers of IgG4 plasma cells - a case report.

A 14-year-old boy presented with a 2-year history of a supraorbital mass, associated with loss of vision and phthisis. A lack of response to systemic steroids led to an incisional biopsy. This showed a spindle cell lesion with a prominent inflammatory component, with numerous plasma cells. The spindle cells were positive for anaplastic lymphoma kinase (ALK-1). Over 90% of the plasma cells were surprisingly positive for IgG4. The presence of ALK-1 positivity within the spindle cells, coupled with the prominent inflammation indicated a diagnosis of ALK-1 positive inflammatory myofibroblastic tumour (IMT-the neoplastic member of the so called inflammatory pseudotumours). However, the level of IgG4 positivity within the plasma cell population would have otherwise lead to a diagnosis of IgG4 related disease, if the ALK-1 positive spindle cells population was not represented. Recent literature from systemic IMT has alluded to the presence of IgG4 plasma cell positivity in IMT and argues that in the absence of other supporting histological features of IgG4 disease (phlebitis and lymphoid aggregates), as in this case, the presence of IgG4 plasma cells, even in high numbers should not lead to a kneejerk diagnosis of co-existing IgG4 disease. This case report is the first to make this association in the orbit and argues that in the presence of IMT, the IgG4 plasma cells are not necessarily pathogenic.