A rare case of follicular lymphoma with transformation to diffuse large B-cell lymphoma and transdifferentiation to histiocytic sarcoma

This is an interesting case of a 53-year-old Caucasian male with a history of recurrent follicular lymphoma (FL) treated with chemotherapy. After 12 years, he developed lymphadenopathy and had an axillary lymph node biopsy which showed transformation to Diffuse Large B-cell lymphoma (DLBCL) with background FL. The patient was treated with chemotherapy and his course was complicated by cellulitis/abscess formation of the right thigh. Computerized Tomography scan showed a large necrotic nodal conglomerate within the right inguinal region measuring 7.8 cm × 5.1 cm and an excisional biopsy was performed. The lymph node architecture was effaced by sheets of large, atypical, pleomorphic cells with open/vesicular chromatin and scattered prominent nucleoli. Focal areas showed residual/recurrent FL, grade I-II. Immunohistochemical stains showed that the large neoplastic cells express CD4, CD68, CD163, BCL-6, lysozyme, CD45 and S-100. The morphologic and immunophenotypic findings are consistent with histiocytic sarcoma (HS). Fluorescence in-situ hybridization studies of the HS showed translocation t(14;18)(q32;q21), a gene rearrangement involving 3q27(BCL6), and gain of 14q32(IgH). The patient’s prior FL and DLBCL were shown to have the same translocation t(14;18)(q32;q21). The molecular findings show a common cell lineage between all three diagnoses confirming that FL can transform into DLBCL and transdifferentiate into HS.