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Bone marrow involvement occurs in about 5% of cases of classical Hodgkin lymphoma (3% of cases of the nodular sclerosis subtype; 10% in mixed cellularity; and a higher proportion of cases [perhaps over 50%] of lymphocyte-depleted classical Hodgkin lymphoma, which tends to present at higher stage), most frequently in the form of focal nodular fibrotic lesions.  Immunostains are useful for highlighting the neoplastic cells in the mixed inflammatory cell infiltrate recruited by the soup of cytokines that are elaborated, and are essential when examining follow-up specimens.