INTRODUCTION: Multicentric Castleman's disease is a rare lymphoproliferative 
disorder whose hallmark is atypical lymph node hyperplasia. Symptoms can include 
fever, splenomegaly, and abnormal blood cell counts. High levels of interleukin 
6 (IL-6) are observed frequently in this disorder and are believed to drive the 
disease. Recently, therapies that target interleukin-6 or its receptor have been 
shown to be effective in Castleman's disease.
CASE PRESENTATION: We report the case of a 76-year-old Caucasian man with 
aggressive biopsy-proven Castleman's disease who experienced pulmonary and lymph 
node involvement, as well as fever and weight loss. He was treated with 
siltuximab, a chimeric anti-interleukin-6 antibody. After 5 months, 
fluorodeoxyglucose positron emission tomography computed tomography scans showed 
marked improvement in his lungs, but worsening mediastinal disease, consistent 
with a mixed response. Biopsy of the mediastinal disease revealed 
lymphoplasmacytic infiltrate with non-caseating, ill-defined granulomas and 
scarring consistent with sarcoidosis. Prednisone 50mg by mouth daily was 
started, which was tapered to 2.0-5.0mg daily. Siltuximab was continued. A 
subsequent fluorodeoxyglucose positron emission tomography computed tomography 
scan showed near-complete resolution of lung and mediastinal disease, now 
ongoing for 3.5+ years without serious adverse events.
CONCLUSIONS: Lymphomas have previously been reported to coexist with 
sarcoidosis, albeit rarely, but there has been only a single previous case of 
this type with Castleman's disease. Of importance, early recognition of the 
presence of sarcoidosis in our patient prevented discontinuation of siltuximab 
therapy due to "progression". Our experience may also have broader implications 
in that it suggests that etiology of "mixed responses" should be confirmed by 
performing biopsies on the progressive tumor.