Lymphomatoid keratosis (LK) is a rare epidermotropic type of cutaneous lymphoid hyperplasia. It can be differentiated from mycosis fungoides and benign lichenoid keratosis by comparing clinocopathological, immunohistochemical and molecular biological findings and should be classed as a pseudolymphoma.

Figure 1. Keratosic and papulous lesion, badly limited, not pigmented, measuring 1.5 cm in its larger diameter.

A 51-year-old patient exhibits a lesion evolving for 1 month, without any associated symptoms, located in the left lumbar paravertebral region. The patient is healthy. He admits exposing himself regularly to natural and artificial UV. Clinically, the lesion is keratosic and papulous, badly limited, not pigmented, measuring 1.5 cm in its larger diameter (Figure 1). This clinical aspect suggests a squamous-cell carcinoma, an achromic melanoma or a cutaneous lymphoma.

Figure 2. Histological features of epidermal hyperplasia and epidermotropism in the epidermis and formation of lymphoid follicle and lichenoid infitration of lymphocytes in the reticular dermis. H&E stain, ×40 and ×200.

A skin biopsy is performed. Histological examination shows a benign keratosic lesion, characterized by considerable inflammatory reorganization, located in the dermo-epidermal junction (Figure 2). The epiderm is acanthosic, hyperkeratosic and focally parakeratosic. Spongiosis, intercellular edema and Civatte bodies can be observed, in limited numbers. In fact, these histological features suggest an interface dermatosis of lichen planus type, but the extensive inflammatory infiltrate, particularly rich in lymphocytes, implies complementary immunohistochemistry in order to exclude a lymphoma. Immunohistochemistry dyes reveal T lymphocytes expressing CD3, CD4, CD5 and CD7, together with rare CD20 B lymphocytes. The latter are sometimes found in the epidermis. T lymphocytes expressing CD8 are sometimes observed within the infiltrate. Some rare cells are CD30 positive. Finally, numerous Langerhans cells expressing CD1a are noted. At first, PCR analysis of extracted DNA reveals rearrangement of gene coding for the heavy chain of immunoglobulins. After review, this result is considered as a PCR artefact, due to a small proportion of B lymphocytes within the infiltrate. This same PCR technique did not show rearrangement of gene coding for gamma sub-unit of T receptor (TCR).

Routine blood test (complete blood count, urea, creatinine, protein electrophoresis, C Reactive Protein, liver enzymes, beta 2 microglobulin) does not show any abnormality.

Considering the ambiguous feature of the lesion, it is totally surgically removed. Clinical examination does not show any recurrence, one year after surgical excision.

In 1997, Kossard was the first author to suggest the term "lymphomatoid keratosis" (LK), further to the observation of a lichenoid keratosis showing histological lymphomatoid characteristics simulating mycosis fongoides. Clinically, it was a solitary lesion, measuring 1x2 cm, with recent apparent changes and associated pruritus. Based on these observations, Kossard suggested that the case of unilateral mycosis fongoide previously described by Evans et al corresponded most probably to a lymphomatoid keratosis [1, 2]. To date, to our knowledge, there have been 24 case reports of LK in the English literature.

In 2002, Al Hoqail et al reported 15 cases of benign lichenoid keratosis with epidermotropic characteristics reminiscent of histological LK according to the definition of Kossard [3]. Patients were aged 28 to 83 years, the sex ratio was 3 men for 2 women and the lesions were mostly located on the trunk with an 0.6 cm average diameter. LK clinically presents as an asymptomatic, scaly, erythematous plaque on the face or upper trunk of a middle-aged patient. Differential diagnosis included actinic keratosis, seborrhoeic keratosis, squamous-cell and basal-cell carcinomas.

In 2007, Arai et al reported six cases of LK, aged 36 to 78 years, with an identical sex ratio [4]. Mostly located on the face, the lesions formed squamous patches measuring 0.85 cm on average, suggesting a seborrhoeic keratosis or an actinic keratosis. A delay ranging from 3 months to several years was necessary to reach the diagnosis. Histology systematically showed hyperkeratosis, parakeratosis, acanthosis with a bandlike lymphocyte infiltration within the epidermis. There was no keratinocyte nor lymphocyte atypia. The immunohistochemical study showed CD20 and CD79a cells within the epidermis. The CD4/CD8 cellular ratio was normal. Rearrangements of the TCR gene were demonstrated in 3 cases. Further to these observations, the authors suggested to consider the LK as a particular variant of pseudolymphoma. Follow-up was carried out only in 2 cases. The lesions did not recur after 13 and 34 months, respectively.

In 2010, Choi et al described a case report of a 60-year-old woman presented with a 10-year history of a pruritic, solitary, well-demarcated, scaly, brown to black plaque on the abdomen, measuring 3.8×1.2 cm in size, which had started as a tiny papule. Histopathological and immunophenotypical study reveal the diagnosis of LK [5].

In conclusion, according to the literature, our study definitely illustrates a case of LK. This unusual lesion is associated with the characteristics of lichenoid keratosis and an abundant lymphocyte infiltrate, of reactional nature, though resembling mycosis fongoides. It is a benign lesion considered as a pseudolymphoma, in which a lymphocyte population with clonal appearance is sometimes observed. In our patient, the presence of gene rearrangement coding for the heavy chain of immunoglobulin was considered as PCR artifact, due to a small proportion of B lymphocytes within the infiltrate (pseudoclonality) [6].

All authors (M.Caucanas, M. Lejeune, J.-L. Dargent, O.Vanhooteghem) certify that they have no conflicts of interest, including specific financial interests and relationships and affiliations relevant to the subject of this manuscript.