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Primary cutaneous lymphomas are a heterogeneous group of T- and B-cell neoplastic entities impacting millions of individuals worldwide annually. In 2018, the World Health Organization updated the diagnostic criteria for the various subtypes of this group of tumors to assist dermatologists and pathologists in classifying them. Cutaneous B-cell lymphomas (CBCLs) are often distinguished clinically by their indolent behavior. Prominent dermal B cell infiltrates are often present, which is remarkable because B cell collections are not typically found in normal skin and are rarely observed in reactive conditions or other types of CL. The following activity outlines the evaluation of these tumors and highlights many pathologic underpinnings when managing patients with primary cutaneous lymphomas. Objectives: Identify all subtypes of cutaneous T- and B-cell lymphomas according to updated World Health Organization guidelines. Summarize general histopathologic features of cutaneous T- and B-cell lymphomas. Outline immunophenotypic and molecular findings of cutaneous lymphomas. Review pertinent clinicopathologic findings associated with different cutaneous T- and B-cell lymphomas. Access free multiple choice questions on this topic.

Primary cutaneous lymphomas are a heterogeneous group of skin-based non-Hodgkin lymphomas that lack extracutaneous manifestations at diagnosis and are the second most common type of extranodal lymphoma.[1] In 2018, the World Health Organization–European Organization for Research and Treatment of Cancer (WHO-EORTC) published an updated classification of cutaneous lymphomas in the 4th edition of the WHO Classification of Skin Tumors Blue Book, which serves as the gold standard in the diagnosis.[2] Primary cutaneous lymphomas more frequently originate from a T-cell than a B-cell lineage.[3] In the Western world, cutaneous T-cell lymphomas (CTCLs) make up approximately 75% of all cutaneous lymphomas, with the majority classified as Mycosis fungoides.[4] Around 6.4 per million persons worldwide are affected by CTCL, and the likelihood of the disease increases significantly with age.[5] The median age at diagnosis is approximately 54 years, although it may rarely occur in children and adolescents.[6][7] Patients with CTCL have increased risks of secondary cancers such as other non-Hodgkin lymphomas, melanoma, lung cancer, and bladder cancer.[8] When neoplastic Mycosis fungoides cells are found in the blood, the condition is known as Sézary syndrome.[9] Primary cutaneous lymphomas are often complex and require clinical evaluation, light microscopy, immunohistochemistry, and molecular analysis of skin biopsies.[10] Cutaneous B-cell lymphomas (CBCLs) are often distinguished clinically by their indolent behavior. Prominent dermal B cell infiltrates are often present, which is notable because B cell collections are not typically found in normal skin and are rarely observed in reactive conditions or other types of CL.[11] See Image. Dermal B-Cell Infiltrate (CBCL). A complete list of CTCL and CBCL diagnoses according to the 2018 WHO-EORTC is provided below, along with their respective abbreviations, which are used throughout the remainder of this chapter. Provisional diagnoses are included:[2] Cutaneous T-cell Lymphoma Mycosis fungoides Mycosis fungoides variants Folliculotropic Mycosis fungoides (FMF) Pagetoid reticulosis Granulomatous slack skin (GSS)/Granulomatous mycosis fungoides Sézary syndrome Adult T-cell leukemia/lymphoma (ATL) Primary cutaneous CD30+ lymphoproliferative disorders Primary cutaneous anaplastic large cell lymphoma (C-ALCL) Lymphomatoid papulosis (LyP)

Mycosis fungoides variants Folliculotropic Mycosis fungoides (FMF) Pagetoid reticulosis Granulomatous slack skin (GSS)/Granulomatous mycosis fungoides Sézary syndrome Adult T-cell leukemia/lymphoma (ATL) Primary cutaneous CD30+ lymphoproliferative disorders Primary cutaneous anaplastic large cell lymphoma (C-ALCL) Lymphomatoid papulosis (LyP) Large cell transformation of mycosis fungoides Subcutaneous panniculitis-like T-cell lymphoma (SPTCL) Extranodal natural killer (NK)-cell/T-cell lymphoma, nasal type Primary cutaneous peripheral T-cell lymphoma, rare subtypes Primary cutaneous gamma/delta T-cell lymphoma (PCGDTL) Primary cutaneous aggressive epidermotropic CD8+ cytotoxic T-cell lymphoma (CD8+ AECTCL, provisional) Primary cutaneous CD4+ small/medium T-cell lymphoproliferative disorder (CD4+SMT-LPD, provisional) Primary cutaneous acral CD8+ T-cell lymphoma (provisional) Primary cutaneous peripheral T-cell lymphoma, not otherwise specified (NOS) Cutaneous B-cell Lymphoma Primary cutaneous marginal zone lymphoma (PCMZL) Primary cutaneous follicle center cell lymphoma (PCFCL) Primary cutaneous diffuse large B-cell lymphoma, leg type (PCDLBLC, LT) EBV+ mucocutaneous ulcer (EBVMCU, provisional) Intravascular large B-cell lymphoma (IVBCL)