Faculty of Veterinary Medicine, Urmia University Veterinary Research Forum 2008-8140 17 7 2026 07 01 Spontaneous primary B cell lymphoblastic lymphoma of the spleen of a 24-month-old male C57BL/6 mouse 519 522 737487 10.30466/vrf.2025.2065919.4839 EN Soo-Eun Sung Preclinical Research Center, Daegu-Gyeongbuk Medical Innovation Foundation, Daegu, South Korea Minkyoung Sung Preclinical Research Center, Daegu-Gyeongbuk Medical Innovation Foundation, Daegu, South Korea Kyung-Ku Kang Preclinical Research Center, Daegu-Gyeongbuk Medical Innovation Foundation, Daegu, South Korea Jae Woo Park Preclinical Research Center, Daegu-Gyeongbuk Medical Innovation Foundation, Daegu, South Korea Ye-jin Kim Preclinical Research Center, Daegu-Gyeongbuk Medical Innovation Foundation, Daegu, South Korea Sun Hee Park Biotoxtech Co. Ltd., Cheongju, South Korea Duyeol Kim Biotoxtech Co. Ltd., Cheongju, South Korea Sijoon Lee Preclinical Research Center, Daegu-Gyeongbuk Medical Innovation Foundation, Daegu, South Korea Journal Article 2025 07 20 Spontaneous lymphoid neoplasms are common among aged C57BL/6 mice, predominantly manifesting as T cell lymphomas. However, primary B cell lymphoblastic lymphomas are rare and poorly characterized, especially in laboratory animals. This case report describes a primary B cell lymphoblastic lymphoma in the spleen of a 24-month-old male C57BL/6 mouse. Gross examination revealed an enlarged spleen with a homogenous, white, firm mass replacing the normal parenchyma. Histopathological analysis revealed that the splenic architecture was diffusely replaced by neoplastic lymphoid cells with high mitotic activity. Immunohistochemical staining confirmed the B cell origin of the neoplasm, demonstrating strong CD20 positivity and CD3 negativity. This case underscores the importance of detailed pathological evaluation in aged laboratory mice, especially to distinguish spontaneous lesions from treatment-induced effects in experimental studies. These findings contribute to the understanding of lymphoid neoplasms in laboratory animals, as well as provide valuable reference data for pre-clinical research in toxicology and oncology. https://vrf.iranjournals.ir/article_737487_3a0cc47e57ab00dc9ba87aa222753611.pdf