Dermatopathology, Cutaneous Lymphomas

Primary cutaneous lymphomas (CLs) are a heterogeneous group of skin-based non-Hodgkin lymphomas lacking extracutaneous manifestations at the time of diagnosis and are the second most common form of extranodal lymphomas.  In 2018, the World Health Organization–European Organization for Research and Treatment of Cancer (WHO-EORTC) published an updated classification of CLs in the fourth edition of the WHO Classification of Skin Tumors Blue Book, which serves as the gold standard in the diagnosis. CLs more frequently originate from a T-cell than a B-cell lineage. In the Western world, cutaneous T-cell lymphomas (CTCLs) make up approximately 75% of all CL, with the majority classified as Mycosis fungoides (MF). Around 6.4 per million persons worldwide are affected by CTCL, and the likelihood of the disease increases significantly with age. The median age of diagnosis is approximately 54 years old, although rarely, it may occur in children and adolescents.  Patients with CTCL have increased risks of secondary cancers such as other non-Hodgkin lymphomas, melanoma, lung cancer, and bladder cancer. When neoplastic MF cells are found in the blood, the condition is known as Sézary syndrome. CLs are often complex and require clinical evaluation, light microscopic examination, immunohistochemistry analysis, and molecular work-up of skin biopsies. Cutaneous B-cell lymphomas (CBCLs) are often distinguished clinically by their indolent behavior. Prominent dermal B cell infiltrates are often present, which is remarkable in that B cell collections are not typically found in normal skin and are rarely observed in reactive conditions or other types of CL. See  Dermal B-Cell Infiltrate (CBCL). A complete list of both CTCL and CBCL diagnoses according to the 2018 WHO-EORTC is listed below with respective abbreviations, which will be used throughout the rest of this chapter. Provisional diagnoses are included:  : Mycosis fungoides (MF). MF variants: Folliculotropic MF (FMF). Pagetoid reticulosis (PR). Granulomatous slack skin (GSS)/Granulomatous mycosis fungoides. Sézary syndrome (SS). 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). 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).