B cell lymphoma, unclassifiable, with features intermediate between diffuse large B cell lymphoma and classical Hodgkin lymphoma : diagnosis by fine-needle aspiration cytology

A 58-year-old lady presented with mediastinal lymphadenopathy. A thoracoscopic ultrasound-guided fine-needle aspiration showed large atypical epithelioid cells arranged in cohesive sheets and dispersed as single cells with intact cytoplasm amid a background of lymphocytes and histiocytes. A cytological diagnosis of "a malignant neoplasm" was made, raising a broad list of differential diagnoses. A broad panel of immunocytochemical stains performed on the cell block was indicative of a lymphoproliferative disorder, but the immunophenotype was intermediate between diffuse large B cell lymphoma (DLBCL) and classical Hodgkin lymphoma (cHL). Diffuse and strong reactivity to CD20, CD79a, and PAX-5, and weak reactivity to CD30, was in favor of a DLBCL, or more precisely mediastinal (thymic) large B cell lymphoma (MLBL). However, there were negative staining for LCA, OCT-2, and BOB-1 as well as positive staining for EBV-encoded RNA, which were against a diagnosis of MLBL and raised the possibility of cHL. The absence of RS cells and the typical mileu, the negativity for CD15 and the strong positivity of CD20 and PAX-5 were against a diagnosis of cHL. On this basis, the diagnosis of "B-cell lymphoproliferative disorder with features intermediate between DLBCL and cHL" was rendered. The diagnosis was subsequently confirmed on excisional biopsy. This case report demonstrates broad differential diagnoses raised by this diagnostic entity and the importance of an adequate cell block for accurate designation.