Incidence and General Characteristics

Following the gastrointestinal tract, the skin is the second most common site of extranodal non-Hodgkin lymphomas (NHLs). Overall, the incidence of primary cutaneous NHLs is 1: 100,000. Mycosis fungodies, the most common type of primary cutaneous NHL, accounts for approximately 0.5% of hematopoietic neoplasms. While T-cell lymphomas represent the majority of primary cutaneous lymphomas, the diagnosis is often difficult, requiring multiple biopsies and complex immunophenotypic and molecular diagnostic tests. Further complicating the diagnosis is the heterogeneity of entities that fall into the CTCL category. While mycosis fungoides is the most common type, anaplastic large cell lymphoma and CD30-negative peripheral T-cell lymphomas are also included in this category. The microscopic features of CTCLs are varied. Mycosis fungoides consists of small epidermal collections of lymphocytes with folded, "cleaved" nuclei, and Pautrier microabscesses. Anaplastic large T-cell lymphoma presents with large pleomorphic cells with effacement of the dermal and epidermal architecture. When the histology is classic, a diagnosis of CTCL is easier.

However, clonal T-cell proliferations can be difficult to detect when features overlap with benign entities, including reactive lymphoid hyperplasia, parapsorasis, lichenoid dermatitis, and spongotic dermatitis. Poor inter- and intraobserver agreement among pathologists further complicates diagnostic accuracy. Immunophenotyping often provides no clearcut answer regarding the benign or malignant nature of a lymphoid infiltrate. While clonal T-cell disorders may lose one of the common surface T-cell markers (CD2, CD3, CD4, CD5, CD7, and CD8), this is neither a sensitive nor a specific finding.

The clonal nature of primary CTCLs can be confirmed by molecular testing methods. When malignant clones arise, the V, D, and J elements that encode the peptide chains of the T-cell receptor (TCR) undergo rearrangement at the DNA level. T Cells have two types of antigen receptors: ab and gS. Beause of its limited number of V and J elements (15 and 5, respectively) and lack of a D element, the TCR-g family produces the fewest possible recombinations (Fig. 54.3). Thus, TCR-g has become a target for examination in cases of suspected T-cell lymphomas. T-Cell clon-ality can be identified in about 79-100% of true cutaneous T-cell lymphoma cases by primers designed to amplify the DNA of the rearranged TCR-g loci. However, studies have shown that a small percentage of benign lymphoid infiltrates are clonal as well. Thus, while molecular studies are of great diagnostic value, they must be combined with skin biopsy histology and immunophenotype findings and the patient's clinical features and history to achieve an accurate final diagnosis.

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