A Rare Case of Trans‐differentiation of Follicular Lymphoma into Histiocytic Sarcoma

A 55-year-old previously healthy gentleman presented with a few months’ history of generalised lymphadenopathy and drenching night sweats. PET CT scan showed enlarged lymph nodes above and below the diaphragm with an SUV max of 17.4 at the left external iliac lymph node. The lymph node biopsy showed focal effacement by a solid appearing diffuse infiltrate of large neoplastic cells with abundant cytoplasm and pleomorphic nuclei (upper panel, left H&E ×20) with high mitotic rate, reflected by high Ki-67 index (Figure 1). These cells were diffusely positive for CD68 (upper panel, middle), CD163 (upper panel, right), CD4 (lower panel, left), S-100 and lysozyme, with variable OCT2 and cyclin D1 positivity. These features were strongly suggestive of high-grade histiocytic sarcoma (HS). A small focal lymphoid infiltrate of low-grade follicular lymphoma (FL) was noted in areas unaffected by the HS that was CD10+, CD20+ and strongly BCL2+ (lower panel, middle) with low Ki-67 index. Molecular analysis revealed that the HS and FL infiltrates shared t(14;18), suggesting trans-differentiation of FL into HS. The bone marrow (BM) morphology showed heavy infiltration by the HS (lower panel, right ×100). A small kappa+ CD10+ FL infiltrate was detected by flow cytometry analysis of the BM. The patient was treated with CHOP regimen but, unfortunately, passed away due to severe cytokine release syndrome, tumour lysis and multi-organ failure. HS is a very rare and extremely aggressive disorder with somewhat better prognosis if diagnosed at an early stage [1]. It can arise de novo or via trans-differentiation from B-cell malignancies [2], as was demonstrated by this case. Dina Osman and Reem Ahmed wrote the manuscript. Dina Osman took the photographs. The authors declare no conflicts of interest. The authors have nothing to report Verbal consent for publication was obtained from the Late's next of kin. The authors have confirmed clinical trial registration is not needed for this submission The slides can be found at the Queen Elizabeth Hospital Birmingham and Midlands Integrated Reporting for Haemato-oncology (MIRHO).