Chapter 34: Lymphoid, Leukemic, and Other Cellular Infiltrates

CUTANEOUS T-CELL LYMPHOID HYPERPLASIA AND CUTANEOUS T-CELL LYMPHOMAS

Werner Kempf • Guenter Burg

Lymphoproliferative disease of the skin can be classified into benign reactive disorders—so-called pseudolymphoma (PSLs) or lymphoid hyperplasia—and malignant lymphomas. Primary cutaneous lymphomas (CLs) belong to the group of extranodal non-Hodgkin lymphomas (NHL) and can be defined as lymphoproliferative neoplasms that at the time of diagnosis are confined to the skin (ie, without detectable extracutaneous manifestation at staging investigations). The overall frequency of CLs is about 1 per 100,000 inhabitants per year, similar to that of Hodgkin lymphoma.^1,2 Cutaneous T-cell lymphomas (CTCLs) represent the largest proportion (65%) of all primary CLs. CTCL consists of several nosologic entities with distinct clinicopathologic, phenotypic, genotypic, and biologic features. The biology and prognosis of CL differ significantly from those of nodal lymphomas with the same cytomorphologic or phenotypic profile. This has an important impact on classification and therapy of CL, which is respected in the current lymphoma classification (revised World Health Organization [WHO] classification for hematolymphoid tumors 2018).^3-5

The histologic and phenotypic evaluation is an essential step in the diagnostic workup of CL. Correlation of certain phenotypes with prognosis has been demonstrated for some forms of CTCL. However, a great number of CTCLs exhibit overlapping histologic and phenotypic findings. Thus the final diagnosis of CL must be based on a synthesis of clinical, histologic, cytologic, immunophenotypic, and molecular features as well as the results of staging examinations.²

CLASSIFICATION OF CUTANEOUS LYMPHOMAS

Many attempts have been made over the past decades to classify nodal and CLs. Due to a lack of knowledge about the etiology of most lymphomas, it has not been possible to develop an etiology-based classification.

To emphasize the unique biologic aspects of CL, the EORTC classification for primary CLs was introduced in 1997.⁶ Conceptual differences in the EORTC classification versus the WHO classification⁷ were eliminated during consensus meetings in 2003 and 2004 and resulted in the WHO-EORTC classification for CLs that is now widely accepted.^3,4 The entities listed in this classification were incorporated into the 2008 and 2018 WHO classifications for nodal and extranodal lymphomas. These lymphoma classifications define nosologic entities with distinct clinical and pathologic features. Table 34-1 lists the nosologic entities of CL as recognized by the updated WHO-EORTC classification 2018 and the updated WHO classification for hematolymphoid tumors published in 2018.