Are tumefactive lesions classified as sclerosing mesenteritis a subset of IgG4-related sclerosing disorders?

Background: The relationship between tumefactive lesions classified as sclerosing mesenteritis and IgG4-related sclerosing disorders (eg, lymphoplasmacytic sclerosing pancreatitis/autoimmune pancreatitis) remains uncertain. Aims: To review lesions coded as "sclerosing mesenteritis" for findings in keeping with lgG4-related sclerosing disorders. Methods: Inclusion in the study required available paraffin blocks for lgG4 staining and documentation of a mass lesion. Results: A total of nine mesenteric lesions (3-14 cm) were identified in 6 male and 3 female patients. On H&E-stained sections, all were characterised as loosely marginated fibroinflammatory processes with variable amounts of fat necrosis. Lymphocytic venulitis/phlebitis was identified in 8 of 9 cases. IgG and lgG4 expression in lesional plasma cells was assessed by immunohistochemistry. lgG4-positive plasma cells were counted in the areas of greatest density in ≥3 high power fields (HPFs). The highest number per HPF was recorded and a score assigned based on the following scale: <5/HPF, none/minimal; 5-10/HPF, mild; 11-30/HPF, moderate; >30/HPF, marked. The relative proportion of lgG4-reactive plasma cells to total IgG-positive plasma cells was assessed. lgG4-reactive plasma cells ranged from 0 to >100 in the most dense zones (3 cases, none/minimal; 4 cases, moderate; 2 cases, marked). Conclusions: Although this study is limited by small numbers, findings suggest that some tumefactive lesions regarded as sclerosing mesenteritis may be a subset of lgG4-related sclerosing disorders.