A 68-year-old woman with no history of immunosuppression presented with posterior cervical lymphadenopathy and a normal complete blood count. Lymph node excision demonstrated reactive follicular hyperplasia and sheets of monocytoid B cells (panel A [hematoxylin and eosin (H&E) stain, original magnification ×20; inset, original magnification ×400]) with rare mononuclear cells with prominent nuclear and cytoplasmic inclusions, which were confirmed to harbor cytomegalovirus (CMV) (panel B [H&E stain, original magnification ×1000; inset, CMV immunostain, original magnification ×1000]). Immunostains also revealed a subtle population of mantle zone B cells (panel C [CD20 immunostain, original magnification ×20]) with dim aberrant expression of CD5 (panel D [CD5 immunostain, original magnification ×400]) and cyclin D1 (panels E-F [cyclin D1 immunostain, original magnifications ×40 and ×400, respectively]), which replaced the entire mantle zone of some follicles (panel F). These findings were diagnostic of mantle cell lymphoma with a mantle zone growth pattern (MCL-MZGP), which upon subsequent staging was found to involve the bone marrow.

MCL-MZGP is distinguished from in situ mantle cell neoplasia by the presence of full- thickness mantle zone involvement. As demonstrated here, MCL-MZGP has an insidious morphologic presentation that can be both difficult to detect by morphology alone and easily overlooked when a more overt cause of lymphadenopathy, such as viral lymphadenitis, is also present.

A 68-year-old woman with no history of immunosuppression presented with posterior cervical lymphadenopathy and a normal complete blood count. Lymph node excision demonstrated reactive follicular hyperplasia and sheets of monocytoid B cells (panel A [hematoxylin and eosin (H&E) stain, original magnification ×20; inset, original magnification ×400]) with rare mononuclear cells with prominent nuclear and cytoplasmic inclusions, which were confirmed to harbor cytomegalovirus (CMV) (panel B [H&E stain, original magnification ×1000; inset, CMV immunostain, original magnification ×1000]). Immunostains also revealed a subtle population of mantle zone B cells (panel C [CD20 immunostain, original magnification ×20]) with dim aberrant expression of CD5 (panel D [CD5 immunostain, original magnification ×400]) and cyclin D1 (panels E-F [cyclin D1 immunostain, original magnifications ×40 and ×400, respectively]), which replaced the entire mantle zone of some follicles (panel F). These findings were diagnostic of mantle cell lymphoma with a mantle zone growth pattern (MCL-MZGP), which upon subsequent staging was found to involve the bone marrow.

MCL-MZGP is distinguished from in situ mantle cell neoplasia by the presence of full- thickness mantle zone involvement. As demonstrated here, MCL-MZGP has an insidious morphologic presentation that can be both difficult to detect by morphology alone and easily overlooked when a more overt cause of lymphadenopathy, such as viral lymphadenitis, is also present.

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