We herein present an overview of the upcoming 5^thedition of the World Health Organization Classification of Haematolymphoid Tumours focussing on lymphoid neoplasms. Myeloid and histiocytic neoplasms will be presented in a separate accompanying article. Besides listing the entities of the classification, we highlight and explain changes from the revised 4^thedition. These include reorganization of entities by a hierarchical system as is adopted throughout the 5^thedition of the WHO classification of tumours of all organ systems, modification of nomenclature for some entities, revision of diagnostic criteria or subtypes, deletion of certain entities, and introduction of new entities, as well as inclusion of tumour-like lesions, mesenchymal lesions specific to lymph node and spleen, and germline predisposition syndromes associated with the lymphoid neoplasms.

Cutaneous pseudolymphomas (PSLs) belong to a group of lymphocytic infiltrates that histopathologically and/or clinically simulate lymphomas. Different causative agents (e.g., Borrelia sp., injected substances, tattoo, arthropod bite) have been described, but in many cases no cause can be identified, hence the term idiopathic PSL. Clinicopathological correlation is important to make the diagnosis. Four main groups of cutaneous PSL can be distinguished based on histopathologic and/or clinical presentation: (a) nodular PSL; (b) pseudo‐mycosis fungoides (pseudo‐MF) and simulators of other CTCLs; (c) other PSL (representing distinct clinical entities); and (d) intravascular PSL. This article gives an overview of the histopathologic and clinical characteristics of cutaneous PSLs and proposes a new classification.

Primary cutaneous lymphomas (CL) are the second most common form of extranodal lymphomas. Cutaneous T‐cell lymphomas represent the majority. They are classified according to the WHO classification 2017 and the updated WHO‐EORTC 2018 published in the fourth edition of the WHO classification for Skin Tumors monograph. Primary cutaneous acral CD8+ T‐cell lymphoma and EBV‐positive mucocutaneous ulcer have been listed as new provisional entities. Moreover, the histological and genetic spectrum of lymphomatoid papulosis has been expanded. Recently, prognostic subtypes were delineated for some entities and subtypes of CL such as folliculotropic mycosis fungoides and marginal zone lymphoma. Since CL show overlapping histological features, clinico‐pathological correlation is of outmost importance for the diagnosis. Recent studies revealed new biomarkers and genetic alterations underlying the pathogenesis of CL. Moreover, targeted therapies have widened the treatment options particularly for aggressive lymphomas.

Cutaneous T‐cell lymphomas (CTCL) represent the majority of primary cutaneous lymphomas (CL). Mycosis fungoides (MF) and cutaneous CD30+ lymphoproliferative disorders account for 80% of all CTCL. CTCL show overlapping histological features. Thus clinical‐pathological correlation is of importance to achieve final diagnosis. MF shows a characteristic evolution with patches, plaques, and in a subset of patients (10%–20%) with tumors. Therapy is stage‐adapted with skin‐directed therapies such as UV‐light therapies and corticosteroids in early disease stage (i.e., patch and limited plaque stage) and systemic therapies (retinoids, interferon, mono chemotherapy, targeted therapy) and/or radiation therapy (local or total skin beam electron) in advanced stages. Novel therapies include targeted therapy such as mogamulizumab (anti‐CCR4) or brentuximab vedotin (anti‐CD30) and histone deacetylase inhibitors. Considering the impact of targeted therapies, biomarkers such as CD30 are not only crucial for the diagnosis and correct classification of an individual lymphoma case, but also for therapy as they may represent therapeutic targets. In the recently revised WHO classification 2017 and the updated WHO‐EORTC classification for CL 2018, primary cutaneous CD8+ acral T‐cell lymphoma has been introduced as a new still provisional entity. It displays characteristic clinical, histological, and phenotypic features and exhibits an excellent prognosis. Rare, but aggressive CTCL include cutaneous primary cutaneous aggressive epidermotropic CD8‐positive T‐cell lymphoma and cutaneous gamma/delta T‐cell lymphoma, which present with rapid onset of necrotic or ulcerated plaques and tumors. As they have a poor prognosis, treatment includes multiagent chemotherapy and hematopoietic stem cell transplantation.