Histiocytic Sarcoma Associated With Follicular Lymphoma: Evidence for Dramatic Response With Rituximab and Bendamustine Alone and a Review of the Literature.

Histiocytic sarcoma (HS) is a rare aggressive malignancy with a dismal prognosis and no agreed-upon standard treatment. Classically, the diagnosis of HS has been difficult to confirm and has relied on inaccurate, crude techniques. Therapy often involves intensive chemotherapeutic regimens, surgery, and/or radiotherapy, which are poorly tolerated with variable response rates. Patients often die of diffusely metastatic disease. Modern diagnostic techniques are helping to slowly uncover more uniquely customized therapeutic approaches in this enigmatic disease. We present a review of the current literature regarding HS diagnosis, treatment, and outcomes. Additionally, we describe the first reported case of HS transdifferentiated from follicular lymphoma that had a dramatic and durable response to rituximab/bendamustine alone as initial treatment. Unlike traditional chemotherapy regimens, this treatment was well tolerated and had a good toxicity profile. The combination of rituximab and bendamustine warrants further investigation in the treatment of HS, especially those originating from prior follicular lymphoma. Modern immunohistochemical and molecular profiling techniques are beginning to reveal heterogeneity among HS tumors and potentially therapeutic targets.