Pseudolymphomas of the skin are benign lymphocytic proliferations that simulate cutaneous malignant lymphomas clinically and/or histopathologically [1-5]. The term pseu-dolymphoma is not specific but is merely descriptive as it encompasses reactive skin conditions with different aetiologies, pathogeneses, clinicopathological presentations and behaviours. Cutaneous pseudolymphomas are traditionally divided into T- and B-cell pseudolymphomas according to the histopathological and immunophenotypical features , although in many conditions this distinction is artificial. For example, pseudolymphomas induced by drugs may present with either a T- or a B-cell pattern and the same drug may induce different patterns in different patients. Thus, in what follows we classify cutaneous pseudolymphomas according to specific clinicopathological entities (Table 20.1).
In recent years, many reactive skin diseases have been added to the list of cutaneous pseudolymphomas, mainly because of the presence of histopathological features similar to those observed in malignant lymphomas of the skin. In contrast, several entities classified in the past as cutaneous pseudolymphomas have been reclassified as low-grade malignant lymphomas, based on clinicopathological and genetic features as well as on follow-up data. Nevertheless, most of the diseases reported as 'pseudolymphoma' in the past are benign reactive skin disorders and need to be clearly separated from cutaneous malignant lymphomas. In this context, the introduction of the concept of 'clonal dermatoses', that is, reactive skin conditions with monoclonal populations of T or B lymphocytes showing a possible evolution into clear-cut cutaneous malignant lymphoma, has brought confusion to an already confused field [7-9]. True 'evolution' from a clear-cut cutaneous pseudolymphoma into a malignant lymphoma of the skin is exceptional, if it occurs at all.
There are no exact data concerning the incidence, prevalence and geographical distribution of cutaneous pseu-dolymphomas. Cutaneous pseudolymphomas associated with infectious organisms (Borrelia burgdorferi lymphocy-toma) commonly arise in regions with endemic B. burgdor-feri infection. There has also been a rise in the number of cases of Borrelia lymphocytoma in countries where Borrelia species are absent, in patients returning from travels in endemic regions.
The clinical manifestations of cutaneous pseudolym-phomas are protean. The lesions are often solitary although they may be regionally clustered or generalized in distribution. Cutaneous pseudolymphomas may also show the features of generalized erythroderma. The course of pseu-dolymphomas varies considerably. The lesions may persist for weeks, months or even years; they may resolve spontaneously and they may recur unpredictably.
Histological criteria for the diagnosis of cutaneous pseudolymphomas include two main features: (i) the architectural pattern of the infiltrates; and (ii) the cellular composition of those infiltrates, which frequently show a mixed character. These histological features then need to be compared carefully with the immunophenotypical data obtained on routinely fixed, paraffin-embedded sections [10 -12]. The recent introduction of polymerase chain reaction (PCR) analysis of the rearrangement of the T-cell receptor (TCR) and immunoglobulin heavy-chain (JH) genes allows the clonality of cutaneous T- and B-cell infiltrates to be established [13-16]. Although, as a rule, malignant lymphomas reveal a monoclonal population of lymphocytes whereas pseudolymphomas show a polyclonal infiltrate, it must be underlined that demonstration of monoclonality may be lacking in true malignant lymphomas and that a distinct proportion of cutaneous pseudolymphomas harbour a monoclonal T- or B-cell population. In this context, it must be clearly stated that differentiation of benign from malignant lymphoid infiltrates of the skin is possible only after a careful synthesis and integration of the clinical, histopatho-logical, immunophenotypical and molecular features. In some cases, only careful follow-up will reveal the true diagnosis.
Table 20.1 Classification of cutaneous pseudolymphomas.
Simulated malignant lymphoma
Lichenoid pigmented purpuric dermatitis (including lichen aureus)
Lichen sclerosus et atrophicus
CD8+ cutaneous infiltrates in HIV patients
Pseudolymphomas in tattoos, T-cell type
Pseudolymphomas at sites of vaccination, T-cell type
Lymphomatoid drug reaction, B-cell type Pseudolymphoma after vaccination, B-cell type Pseudolymphoma in tattoos, B-cell type
Morphoea, inflammatory stage Syphilis (secondary)
'Acral pseudolymphomatous angiokeratoma' (small papular pseudolymphoma) Lymphocytic infiltration of the skin (Jessner-Kanof) Inflammatory pseudotumour
Mycosis fungoides/Sézary syndrome
Lymphomatoid papulosis/anaplastic large cell lymphoma—CD30+
Subcutaneous T-cell lymphoma
Follicle centre cell lymphoma Marginal zone B-cell lymphoma Large B-cell lymphoma
Follicle centre cell lymphoma Marginal zone B-cell lymphoma
Marginal zone B-cell lymphoma
Chronic lymphocytic leukaemia, B-cell type Plasmacytoma
Marginal zone B-cell lymphoma
Intravascular large B-cell lymphoma
SPECIFIC CLINICOPATHOLOGICAL ENTITIES
The concept of chronic actinic dermatitis encompasses four chronic photodermatoses: persistent light reactivity, photosensitivity dermatitis, photosensitive eczema and actinic reticuloid [17-20]. Actinic reticuloid is a severe persistent photodermatitis that usually affects older men. The disease is characterized by extreme photosensitivity to a broad spectrum of UV radiation . Clinically and histologically, it has many of the features of mycosis fungoides and Sezary syndrome. The patients present in the early stages with erythemas on the face and neck and on the back of the hands (Fig. 20.1). Ectropion may be present. As the eruption pro gresses, it becomes lichenified as a consequence of chronic scratching and scaly plaques may develop. In some areas, the lesions may consist of lichenoid papules. Recurrent erythroderma is common in these patients . A 'leonine' face with deep furrowing of markedly thickened skin as well as diffuse alopecia can also be seen. Pruritus is generally severe and intractable and may lead to attempts at suicide. The disease is chronic and shows no tendency to spontaneous remission . Although 'progression' into T-cell lymphoma has been reported, it seems more likely that these cases represented examples of mycosis fungoides from the onset, and that actinic reticuloid is not a potential precursor of cutaneous T-cell lymphoma [24,25].
Histological examination reveals dense, superficial or deep perivascular mixed-cell infiltrates of lymphocytes, histio-cytes, plasma cells and eosinophils as well as some atypical mononuclear cells with hyperchromatic lobulated nuclei
(Fig. 20.2). In the upper part of the dermis, the infiltrate is band-like or patchy. The papillary dermis is usually thickened. Stellate and multinucleated fibroblasts are present. Exocytosis of lymphocytes within the hyperplastic epidermis can be found. When present, the features of lichen simplex chronicus superimposed upon an inflammatory process are helpful in distinguishing actinic reticuloid from mycosis fungoides and Sezary syndrome. Immunohistology is characterized by the predominance of CD8+ T cells [26,27].
The clinical differentiation of actinic reticuloid from cutaneous T-cell lymphomas (mycosis fungoides and Sezary syndrome) can be difficult because circulating Sezary cells may be found in the peripheral blood of patients with actinic reticuloid . A low helper : suppressor ratio in the peripheral blood has been found in patients with erythrodermic actinic reticuloid, as opposed to the high ratio commonly observed in patients with Sezary syndrome . Unlike patients with mycosis fungoides and Sezary syndrome, on phototesting, patients with chronic actinic dermatitis are sensitive to UVB, UVA and, in most instances, to visible light. Fluorescent light may lead to an exacerbation of the disease. In patients with actinic reticuloid, the minimal erythema dose is lower than normal.
Treatment of chronic actinic dermatitis is difficult and numerous therapeutic approaches have been proposed . Photoprotection is crucial. Any relevant associated contact or photocontact allergens have to be identified and avoided. Some patients have been reported to respond to corticoster-oids, photochemotherapy with psoralen and UVA (PUVA), interferon-a or to a combination treatment with azathio-prine, hydroxychloroquine and prednisone. Ciclosporin (sometimes combined with bath PUVA) or topical tacrolimus ointment (especially for facial lesions) also appears to be effective [31,32].
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