Concurrent Development of B‐Cell Lymphoblastic Lymphoma and Histiocytic Sarcoma in a C57BL/6 Laboratory Mouse: A Case Report

We report a rare case of a spontaneous composite tumour with the histopathological characteristics of B-cell lymphoblastic lymphoma and histiocytic sarcoma in a 104-week-old male C57BL/6 mouse. It appeared to be a large, well-circumscribed intra-abdominal mass, presumably originating from a mesenteric lymph node. Histological examination revealed two distinct neoplastic cell populations, including small- to intermediate-sized CD20- and Pax5-positive lymphoid cells and large pleomorphic histiocytes expressing Iba1. Both cell types were strongly stained by proliferating cell nuclear antigen, which is indicative of high proliferative activity. No evidence of metastases was observed in other organs. While single cases of similar composite neoplasms have been reported in humans, this is the first documented case in a laboratory mouse. These findings expand the known spectrum of spontaneous hematopoietic tumours in aged mice and highlight the importance of comprehensive histopathological and immunophenotypic evaluation in diagnosing atypical lesions.