Cutaneous T-cell lymphoma (mycosis fungoides and Sezary syndrome)

What changed in 2022. Four of those entities, the gamma/delta lymphoma, the CD8-positive aggressive epidermotropic lymphoma, the acral CD8-positive disorder and the CD4-positive small or medium disorder, had been grouped in the previous classification under a single heading, cutaneous peripheral T-cell lymphoma, rare subtypes. WHO-HAEM5 separated them because their clinical behaviour, their appearance and their genetics differ, and the behaviour differs enormously: two of the four are indolent and two are aggressive, so a single heading hid the only fact a patient needed.

Why the dermatologist is part of the diagnosis. These conditions overlap under the microscope, and WHO-HAEM5 says so directly: because the appearances and the surface markers overlap across the primary cutaneous T-cell lymphomas, correlation with the clinical history, the signs and the symptoms is a key element of the work-up, and dermatological examination and clinical photographic documentation are indispensable. A biopsy of lymphomatoid papulosis read without the history is reported as an aggressive lymphoma; a biopsy of early mycosis fungoides read without the history is reported as eczema. Dated photographs and a record of how lesions have behaved over months are therefore part of the diagnostic material, not a courtesy.

What is not in this family. Sezary syndrome, although it is a disease of the skin and the blood and is managed by the same teams, is classified by WHO-HAEM5 among the mature T-cell and NK-cell leukaemias rather than among the primary cutaneous lymphomas, because it is leukaemic from the start; it has its own page here and is kept beside mycosis fungoides because that is how it is treated. The primary cutaneous B-cell lymphomas, primary cutaneous marginal zone lymphoma, primary cutaneous follicle centre lymphoma and primary cutaneous diffuse large B-cell lymphoma of the leg type, arise in the skin from B cells and belong to the B-cell side of the classification; the first two are indolent and the third is not. Primary cutaneous anaplastic large cell lymphoma is in this family, while the systemic anaplastic large cell lymphomas are not, which is the distinction that most often goes wrong because the cells look the same.

How common they are, and how they behave. The skin lymphomas are rare. In the United Kingdom population series that reports lymphoma by subtype, mycosis fungoides accounted for 39 of 5,796 lymphomas and the CD30-positive lymphoproliferative disorders for 37, European age-standardised rates of 0.12 and 0.13 per 100,000 a year, with five-year relative survival of 86.6 and 88.3 per cent. Those two figures carry the character of the family: most of these conditions are long-term skin diseases that are managed for decades rather than cancers that are cured or not cured, and the usual harm is over-treatment rather than under-treatment. The aggressive members, the gamma/delta lymphoma and the CD8-positive aggressive epidermotropic lymphoma, are the exceptions and are treated as systemic disease from the start.

How they are staged. Mycosis fungoides and Sezary syndrome are staged by the ISCL and EORTC system revised in 2007, which classifies the skin, the lymph nodes, the viscera and the blood separately; that system is on the mycosis fungoides page. The other cutaneous lymphomas use a separate ISCL and EORTC system that records the number, size and distribution of skin lesions and whether lymph nodes or other organs are involved. Neither is the Lugano classification used for nodal lymphoma, because counting lymph node regions does not describe a disease that lives in the skin.