title: Cutaneous B-cell lymphoma with loss of CD20 immunoreactivity after rituximab therapy.
abstract: BACKGROUND
  Antibodies to the B-cell-specific antigen CD20 are widely used for immunohistochemical identification of B-cell lymphomas, approximately 95% of which are strongly CD20-positive.
  
  
  METHODS
  We report a 51-year-old male with a CD20-positive systemic B-cell lymphoma who developed a CD20-negative relapse with secondary cutaneous involvement after therapy with the anti-CD20 monoclonal antibody rituximab (Rituxan).
  
  
  RESULTS
  Biopsy of a skin nodule demonstrated an atypical lymphocytic infiltrate that was negative for CD20, CD3, lysozyme, and myeloperoxidase, but strongly positive for CD45rb (LCA) and the B-cell marker CD79a.
  
  
  CONCLUSIONS
  We conclude that loss of CD20 expression in cutaneous B-cell lymphoma (both primary and secondary) is important to recognize, that immunohistochemistry for CD79a, another widely expressed B-cell marker, is useful in the identification of CD20-negative B cells in such cases, and that loss of CD20 expression may become more common, as use of rituximab is expected to increase.
authors.affiliation: Department of Pathology, The Pennsylvania State University College of Medicine/Hershey Medical Center, C7633 Hershey Medical Center (H083/Box 850), Hershey, PA 17033, USA. lclarke816@aol.com
authors.forename: Loren E
authors.initials: LE
authors.lastname: Clarke
authors.forename: Michael G
authors.initials: MG
authors.lastname: Bayerl
authors.forename: W Christopher
authors.initials: WC
authors.lastname: Ehmann
authors.forename: Klaus F
authors.initials: KF
authors.lastname: Helm
mesh_terms: D000911:Antibodies, Monoclonal / Q000627:therapeutic use*; D058846:Antibodies, Monoclonal, Murine-Derived; D015703:Antigens, CD / Q000378:metabolism; D018951:Antigens, CD20 / Q000378:metabolism*; D000970:Antineoplastic Agents / Q000627:therapeutic use*; D014408:Biomarkers, Tumor / Q000378:metabolism; D051925:CD79 Antigens; D006801:Humans; D007150:Immunohistochemistry; D017493:Leukocyte Common Antigens / Q000378:metabolism; D016393:Lymphoma, B-Cell / Q000188:drug therapy / Q000378:metabolism* / Q000473:pathology; D008297:Male; D008875:Middle Aged; D009364:Neoplasm Recurrence, Local; D011947:Receptors, Antigen, B-Cell / Q000378:metabolism; D012008:Recurrence; D000069283:Rituximab; D012878:Skin Neoplasms / Q000188:drug therapy / Q000378:metabolism* / Q000473:pathology
chemical_list: D000911:Antibodies, Monoclonal; D058846:Antibodies, Monoclonal, Murine-Derived; D015703:Antigens, CD; D018951:Antigens, CD20; D000970:Antineoplastic Agents; D014408:Biomarkers, Tumor; D051925:CD79 Antigens; C497881:CD79A protein, human; D011947:Receptors, Antigen, B-Cell; D000069283:Rituximab; D017493:Leukocyte Common Antigens