Fig. 3

Histopathological findings after initial and second surgical debulkings. a-f Histopathological findings after initial surgical debulking. Scale bar found in (a) is applicable to all panels unless otherwise indicated. a H&E, high-power, lymphohistiocytic inflammatory infiltrate with abundant plasma cells. b H&E, low-power, significant storiform fibrosis. c Inflammatory infiltrate is composed predominantly of plasma cells highlighted by CD138. d No ALK-1 immunopositivity was observed. e IgG and (f) IgG4 immunostains revealed > 50 IgG4-positive plasma cells per high-power field and an IgG4/IgG ratio of > 0.5. g-l Histopathological findings after second surgical debulking: (g) H&E, low-power, diffuse inflammatory infiltrate composed of small lymphocytes, histiocytes, and plasma cells with entrapped fat and adjacent skeletal muscle. h H&E, medium-power, highlighting vasculature without evidence of obliterative phlebitis (also not seen with elastic stain, not shown). i H&E, high-power, inflammatory infiltrate with collagen bands; absence of sclerosis. Histiocytes demonstrate S100 immunopositivity at (j) medium-power (scale bar in (i) applies) and (k) high-power (scale bar in (a) applies), and emperipolesis is noted within the larger histiocytes. l The plasma cell component is rich in IgG4, up to 50–60 per high-power field; the IgG/IgG4 ratio is approximately 0.3 (IgG immunostain not shown)