Mycosis fungoides (MF) is the most common cutaneous T-cell lymphoma. Clinical features are crucial for diagnosing this entity, especially on its early stages. The initial presentation is that of small scaly patches, usually on sun-protected areas such as the buttocks, lower abdomen, and thighs. In a minority of patients the patches disseminate and indurated raised plaques appear. The third stage is called tumor stage, which results in nodules that frequently ulcerate and may involve sun-exposed skin. Extracutaneous dissemination may be seen as the disease progresses. The histopathological findings on early, patch-stage mycosis fungoides are subtle and can be easily overlooked. The lymphocytic infiltrate is usually composed of small lymphocytes with minimal or no atypia, arranged around vessels within the papillary dermis and superficial plexus. Epidermotropism by lymphocytes, considered a strong sign for suspecting MF (especially in the absence of important spongiosis), may be very focal. As the lesion progresses, the lymphocytes start to display more obvious cytologic atypia. Plaques of MF usually exhibit cells with “cerebriform” nuclei and often show Pautrier’s microabscesses. Dense infiltrates occupying the reticular dermis characterize tumor lesions. The lymphocytes in tumor stage MF may display an anaplastic morphology making the distinction between MF and ALK negative anaplastic large cell lymphoma (ALCL) impossible without clinical information. Many clinical and histopathological variants of MF have been described. Although scoring systems using clinical, histopathological, immunophenotypic, and molecular characteristics are in use [38], the diagnosis of early MF can still be cumbersome.

Staging of MF, mainly based on clinical and histopathological findings, correlates with prognosis [39]. Immunophenotyping is used as an adjunct for diagnosis. Most of the cases display a CD3+, CD4+, CD8− phenotype. Demonstration of loss of CD2, CD3, CD5, or CD7 may help in the diagnosis [11]. However, some controversy still exists about the exact role of immunohistochemistry in the diagnosis of early MF, since overlapping findings may be seen in inflammatory conditions [40] and the prognosis of early MF does not seem to be influenced by phenotype [41].

Molecular studies can be helpful in the diagnosis of MF, if taken in conjunction with clinical and histopathological findings. T-cell receptor (TCR) gene rearrangement studies are used to demonstrate T-cell clonality in MF [42]. Although Southern blot analysis is still considered the gold standard for determination of T-cell clonality, [43] PCR-based methods are routinely used for this purpose. Investigation of TCR-γ chain gene rearrangements is the most commonly used test in clinical laboratories, due to its high sensitivity [44, 45]. Evaluation of TCR-β gene rearrangements is being increasingly performed in clinical settings. It is expected that the assessment of both TCR-γ and TCR-β gene rearrangements will increase the sensitivity of T-cell clonality detection in MF [8].

When interpreting TCR gene rearrangement studies the dermatopathologist should be aware of a few caveats. Inflammatory conditions in which the lymphocytic infiltrates are scant not uncommonly yield oligoclonal or even monoclonal bands [3]. Duplicate analyses are needed to rule out the possibility of pseudoclonality in these cases [14]. On the other hand, the sensitivity of the PCR-based methods used for TCR gene rearrangement determination widely varies: it is not uncommon that patch MF lesions show oligoclonality [14]. Therefore, a negative or oligoclonal TCR gene rearrangement pattern does not rule out MF.

Sézary syndrome (SS) is characterized by erythroderma, lymphadenopathy, and the presence of the so-called Sézary cells in peripheral blood. Controversy still exists about the distinction between erythrodermic MF (a condition that may evolve through patch/plaque disease and has variable levels of blood involvement) and SS [46]. Some evidence suggests that these are two different processes and that they constitute malignancies of thymic memory T cells (SS) and skin resident effector memory T cells (MF) [47]. The prevailing view of SS as the leukemic counterpart of MF seems therefore not completely accurate. On clinical grounds, however, there is frequent overlapping between these two entities and a diagnosis of SS should be made under strict criteria, such as the demonstration of a T-cell clone in peripheral blood, peripheral blood lymphocytosis with a CD4+/CD8+ ratio greater than 10, and circulating Sézary cells greater than 1 × 10⁹/L.

Histologically, the changes seen in SS may be indistinguishable from those seen in MF. However, lack of epidermotropism and low grade cytologic atypia are common in SS [48]. Most of the cases exhibit a CD3+, CD4+, CD8− phenotype.

Clonal TCR gene rearrangements are commonly demonstrated in peripheral blood of patients with SS [1]. These monoclonal TCR gene rearrangements can be identical to those detected in skin [49] and this finding has been used as evidence of multiple site involvement by the same malignant process. However, different clonal TCR gene rearrangements in peripheral blood and skin have been detected in a number of patients, suggesting clonal heterogeneity in some cases [14, 19]. Moreover, monoclonal T-cell populations in peripheral blood have been associated with autoimmune disorders [17], advanced age [50], and can be identified in association with other non cutaneous lymphomas and inflammatory skin conditions [1]. Patients with SS may be negative for T-cell clones in peripheral blood [51].

Primary cutaneous CD30-positive T-cell lymphoproliferative disorders (CD30+ TLPD) constitute the second most common cutaneous T-cell lymphoma/lymphoid proliferations. The WHO classification recognizes three types: primary cutaneous anaplastic large cell lymphoma (C-ALCL), lymphomatoid papulosis (LyP), and borderline lesions. It is recognized that these disorders represent a clinical and histological spectrum of entities going from LyP, a recurrent, benign diffuse eruption of papulonodular lesions that regress spontaneously, to C-ALCL, usually presenting as one or multiple localized ulcerated nodules they may in some cases regress [52]. Distinction should be made from systemic ALCL with secondary involvement of the skin and from other lymphomas showing CD30 expression, such as mycosis fungoides with large cell transformation [53].

LyP has been classified classically in three histological types [54]. Type A, that resembles Hodgkin lymphoma, i. e., shows aggregates of large Reed-Sternberg-like CD30-positive cells in a background of inflammatory cells; type B, or papular mycosis fungoides-like; and type C, or ALCL-like in which the large CD30-positive cells are arranged in sheets with minimal inflammatory background. Recently, a “type D” denomination has been proposed for CD8-positive lesions [55]. Cutaneous anaplastic large cell lymphoma (C-ALCL) reveals a dermal infiltrate composed of large anaplastic, pleomorphic cells, most of them expressing CD30.

The demonstration of clonal rearrangements of TCR in CD30+ TLPD has been variably reported [56, 57]. It is important to take in consideration that identical clones can be detected in different CD30+ TLPD lesions [58] as well as in LyP and concomitant lesions of mycosis fungoides [57]. When dealing with ALCL lesions, it is important to distinguish C-ALCL from involvement of skin by systemic ALCL. Cutaneous anaplastic large cell lymphoma (C-ALCL) cases usually lack t(2;5) and ALK expression by immunohistochemistry [59]. Translocations involving IRF4 detected in C-ALCL but not in systemic ALCL, have been reported as useful in this differential diagnosis [35].

This group encompasses rare lymphomas in which a diagnosis of MF has been excluded, clinically and histopathologically .

Primary cutaneous pleomorphic CD4-positive small/medium sized T-cell lymphoma is a provisional category in the 2008 WHO classification. There is controversy about whether this entity should be considered a true lymphoma or an atypical lymphoid proliferation of undetermined significance [60]. It presents as a single or multiple localized lesions on the upper trunk, neck, or face. The course is usually indolent although some aggressive cases have been reported [61]. Histologically, a dermal infiltrate composed of small to medium-sized T cells expressing CD4 admixed with numerous B cells, variable number of CD8-positive T cells, plasma cells, eosinophils, and histiocytes, is present. Expression of BCL6, PD-1, and CXCL13 has been reported in these cases and linked this lesion to a possible follicular T helper origin [62]. In a large series of cases, 60 % of cases were found to have a monoclonal rearrangement of the TCR gamma [60].

Primary cutaneous CD8-positive aggressive epidermotropic cytotoxic T-cell lymphoma (Berti’s lymphoma) on the other hand, is characterized by multiple skin nodules that rapidly disseminate [63]. The prognosis is usually poor. On histological examination, the tumor cells—which can range from small to large—show marked epidermotropism and cytotoxic phenotype. Monoclonal TCR gene rearrangement has been reported in most of the few reported cases [64, 65].

Subcutaneous panniculitis-like T-cell lymphomas (SPTCL) include only cases with the alpha-beta phenotype [66]. Patients usually present with nodules or plaques on lower extremities with or without systemic symptoms. A common association with hemophagocytic syndrome was originally reported [67]. The skin biopsies show a lobular panniculitis pattern. Epidermal or marked dermal involvement is uncommon. The infiltrating cells are an admixture of small, medium, or large cells with variable numbers of histiocytes. Rimming of adipocyte spaces by lymphoma cells, erythrophagocytosis, fat necrosis, and karyorrhexis is variably seen. The atypical cells express CD3 and CD8 and are usually negative for CD4. Distinction of this entity from inflammatory conditions such as lupus erythematosus profundus, Weber-Christian disease, histiocytic cytophagic panniculitis, and reactive panniculitis to drugs or injections can be very challenging. Molecular analysis of TCR gene rearrangement helps in the differential diagnosis, since most of the cases of SPTCL are monoclonal [68].

Primary cutaneous gamma-delta T-cell lymphoma is an aggressive tumor that occurs in young adults and presents with multiple lesions, commonly on proximal lower extremities, and shows frequent mucosal involvement [69]. Three histologic patterns have been described: epidermotropic, dermal, and subcutaneous [70, 71]. These patterns may be observed within a single biopsy. Epidermotropism may be present. Rimming of adipocytes, similar to that seen in SPTCL, may be seen [70]. The tumor cells have a cytotoxic phenotype, with strong expression of granzyme B, TIA-1, and perforin. The T-cell phenotype of the tumor cells is confirmed by expression of CD3. The cells are usually double negative for CD4 and CD8, [72] although CD8 expression has been observed in some cases. CD56 is commonly positive. The tumor cells are negative for βF1 and positive for TCRγ and TCRδ by immunohistochemistry (the latter requires frozen tissue.) Most of the cases show monoclonal TCR gene rearrangement [71].

Cutaneous adult T-cell leukemia/lymphoma (ATLL) refers to skin-limited lesions without lymph node or peripheral blood involvement [73, 74]. ATLL is endemic in Japan, the Caribbean islands, South America, and regions of Central Africa. It is caused by the human T-cell leukemia virus type I (HTLV-1) [73]. Secondary cutaneous involvement is frequently seen in the systemic form. On histologic examination, ATLL cells in skin display a predominantly perivascular distribution and varying degrees of epidermotropism in a pattern sometimes indistinguishable from that seen in mycosis fungoides. The tumor cells are positive for CD3, CD4, and CD25 [75]. Monoclonal integration of HTLV-1 proviral DNA can be detected by Southern blot analysis in most cases of ATLL [76]. Interestingly, extraordinary integration patterns of HTLV-1 proviral DNA are associated with distinct clinical and pathological subtypes and prognosis [77].

Primary cutaneous extranodal NK/T-cell lymphoma, nasal type includes NK-cell lymphomas along with some cytotoxic T-cell lymphomas and lymphomas with indeterminate lineage (NK/T) [78]. These tumors, although rare in Western populations, are not uncommonly seen in Asian and Latin American countries [79–81]. Associated EBV infection is detected in almost all the cases [82, 83]. Patients are present with multiple ulcerated nodules with necrotic centers. The prognosis is poor [84]. Histologically, the lesions show a dense dermal infiltrate of variably sized cells with angiocentric distribution and necrosis [85]. Subcutaneous infiltration by the lymphoma cells, similar to that seen in SPTCL, can be seen. The tumor cells are usually positive for CD56, cytoplasmic CD3, and cytotoxic markers. It is important to demonstrate the presence of EBV, commonly by EBER ISH testing. T-cell receptor (TCR) gene rearrangement is not detected in the true NK-cell neoplasms; however, a subset of cases shows monoclonal TCR gene rearrangements [86, 87]. Assessment of the NK-cell killer immunoglobulin-like receptor (KIR) repertoire through reverse transcriptase PCR has been used to demonstrate monoclonality in these tumors [88].

Another EBV-associated lymphoid proliferation with distinct clinical presentation, regarded in the WHO/EORTC classification as a variant of the previous category, is the hydroa vacciniforme-like cutaneous T-cell lymphoma (HVLL) [89]. It usually affects children and young adults and occurs in regions of Latin America and Asia [90, 91]. Patients present with vesicles and papules on sun-exposed areas that leave scars clinically resemble hydroa vacciniforme. Hypersensitivity to mosquito bites is common. It seems that this disease is a part of the spectrum of EBV-related T-cell lymphoid proliferations of the skin that may include typical hydroa vacciniforme and aggressive lymphomas [92]. The proliferating cells in HVLL usually express CD3, CD2, CD8, and cytotoxic markers. T-cell receptor (TCR) gene rearrangement is detected in a variable proportion of cases [93, 94]. Detection of EBER-1 and BamHI A rightward transcripts (BARTs) by real time PCR from skin crusts and scales of patients with EBV-associated cutaneous lesions has been reported as highly sensitive and specific [95].

A 38-year-old male presented with multiple papules on chest, neck, and upper extremities. Some of the lesions were crusted with pustule formation. The lesions failed to improve on multiple antibiotic and topical treatments. A biopsy demonstrated dermal lymphocytic infiltrate with minimal epidermotropism. Acute inflammation was present in superficial dermis and epidermis. A dense infiltrate was identified around eccrine coils. This infiltrate was demonstrated to be CD3-positive with some CD4-positive cells involving adnexal structures. Numerous CD8-positive cells were also present. Molecular studies for TCR gamma chain gene rearrangement revealed a clonal peak. A diagnosis of syringotropic mycosis fungoides was made (Fig. 2.6) .

Comment: Syringotropic mycosis fungoides may be challenging to diagnose in both clinical and histopathological grounds. The lesions seen in this variant of mycosis fungoides are usually multiple papules or nodules, sometimes associated with anhidrosis [96]. Patches or plaques of typical mycosis fungoides are not usually seen. On histology, careful examination is needed to identify the syringotropic cells. In this case, the associated reactive infiltrate composed of neutrophils and reactive CD8-positive T lymphocytes makes the interpretation of the histopathological findings even more difficult. Demonstration of clonality by TCR gene rearrangement studies, along with an adequate interpretation of the pathological and clinical findings, is useful to support a diagnosis of mycosis fungoides in this case. The patient was treated with total skin electron beam therapy with good clinical response.

Primary cutaneous marginal zone B-cell lymphoma (PCMZL) is an indolent tumor with almost 100 % 5-year survival [66]. It is composed of small B-cells including marginal zone (centrocyte-like) cells, lymphoplasmacytoid cells, and plasma cells. It is considered a part of the broader mucosa-associated lymphoid tissue (MALT) type lymphomas. It affects adults over 40 years and presents as red to violaceous papules, plaques, or nodules, predominantly on the upper extremities and less often on head and trunk.

Histologically, PCMZL is characterized by nodular to diffuse cellular infiltrate with residual reactive lymphoid follicles. The infiltrating cells comprise predominantly centrocyte-like cells or monocytoid cells of small to medium size with slightly irregular nuclei, moderately dispersed chromatin, inconspicuous nucleoli, and moderately abundant pale cytoplasm. These cells are admixed with variable numbers of lymphoplasmacytoid cells and plasma cells, small numbers of centroblasts and numerous reactive T-cells. Intranuclear (Dutcher bodies) or intracytoplasmic periodic acid-schiff (PAS)-positive inclusions may be present, particularly in cases with predominance of lymphoplasmacytoid cells. Diffuse infiltration involving epithelium and sweat glands and the presence of very immature plasma cells suggests secondary involvement by a systemic lymphoma [98].

Immunophenotypically, the neoplastic lymphocytes express B-cell markers CD19, CD20, CD22, and CD79a, and are negative for CD5, CD10, and CD23. In addition, the neoplastic lymphocytes are BCL2 positive and BCL6 negative. The dendritic cell marker CD21 highlights residual or distorted follicular dendritic cell meshworks, where only rare or absent CD10+ or BCL6+ residual germinal center cells may be found; however, most cells in the infiltrate are CD10− and BCL6−. The lymphoplasmacytoid cells and the plasma cells show monotypic cytoplasmic immunoglobulin light chain expression [99]. If evidence of clonality by light chain restriction is demonstrated by immunohistochemistry, there is no compelling need of performing molecular analysis to demonstrate clonality.

Immunoglobulin heavy chain (IGH) gene rearrangement studies show monoclonal rearrangements in most cases, in the range of 60 % using one set of FR primers, and almost 100 % using three sets of FR primers [23, 100]. Sequence analyses demonstrate somatic hypermutation in the clonally rearranged IGH variable region. Although most cases do not have recurrent chromosomal translocations, a minority of cases show t(14;18)(q32;q21) involving the IGH and MALT1 genes [31] and t(3;14)(p14.1;q32) involving IGH and FOXP1 genes [101]. The t(11;18)(q21;q21) involving the API2 and MALT1 genes that frequently occur in non-cutaneous marginal zone lymphoma of MALT does not occur in PCMZL [102]. Gene rearrangement involving BCL10 has not been reported. Gains of chromosome 3 and 18 may be present [31]. FAS gene mutations are detected in a minority of cases, similar to MZL of other extranodal sites. Mutations of other genes occur rarely and these include the oncogenes PIM1, PAX5, RhoH/TTF, and MYC. Inactivation of the tumor suppressor genes CDKN2A and DAPK by hypermethylation has been documented. These findings suggest different pathogenetic pathways between PCMZL and extracutaneous marginal zone lymphoma of MALT and thus support their distinct classification. It is of interest that cases of PCMZL in Europe, but not in the USA have been associated with Borrelia burgdorferi [97].

Primary cutaneous follicle centre lymphoma (PCFCL) is an indolent lymphoma of follicle center B-cells, with a > 95 % 5-year survival [66]. As its indolent behavior, independent of histologic appearance, grading is not recommended for PCFCL. It predominantly affects middle-aged adults, and the sites most frequently affected are trunk, and head and neck regions [103]. It presents as solitary or grouped firm erythematous plaques or nodules. In contrast to PCMZL, multifocal skin lesions are rare.

Histologically, PCFCL shows a spectrum of growth patterns including a follicular, a follicular and diffuse, and a diffuse pattern, with consistent sparing of the epidermis. Follicles are ill-defined with lack of tingible body macrophages and attenuated or absent mantle zones. The abnormal follicles are composed of a mixture of centrocytes, relatively few centroblasts, and many reactive T-cells, enmeshed in a network of follicular dendritic cells. In advanced lesions, the follicular structures are no longer discernible, and the infiltrating cells are generally large centrocytes with variable admixture of centroblasts and immunoblasts .

Immunophenotypically, the neoplastic lymphocytes express B-cell markers, with co-expression of the germinal center marker BCL6. CD10 is detected in cases with follicular growth pattern but is uncommon in cases with diffuse growth pattern. Focal positivity for MUM-1 is possible, although the tumor cells are usually negative [104]. The neoplastic cells are negative for CD5 and CD43. Unlike nodal and secondary follicular lymphomas, PCFCL cells generally do not express or are faintly positive for BCL2. A strong expression of BCL2 suggests secondary skin infiltration. Monotypic immunoglobulins can be detected by flow cytometry or frozen sections but rarely by routine paraffin immunohistochemistry.

Molecular testing reveals monoclonal IgH gene rearrangements in 67 % of the cases. Somatic hypermutations of the variable IgH gene are common, thus the chance of false negatives can be decreased with the use of multiple primers sets [23, 100]. In contrast to systemic follicular lymphoma, PCFCL commonly lack the t(14;18)(q32;q21) or BCL2 gene rearrangement [105, 106]. Mutations of oncogenes BCL6, MYC, RhoH/TTF, and PAX5 have been reported. Inactivation of the CDKN2A tumor suppressor gene by deletion or promoter hypermethylation is rare [107]. Array-based comparative genomic hybridization (aCGH) and FISH studies may rarely reveal amplification of chromosome region 2p16 (containing the c-REL and BCL11A genes) [108] and loss of chromosome 14q32 (containing the IGH gene) [107].

Primary cutaneous diffuse large B-cell lymphoma, leg type (PCDLBCL, LT) is a subtype of diffuse large B-cell lymphoma with an aggressive behavior, and histologically is composed of large transformed cells with a diffuse growth pattern. Primary cutaneous diffuse large B-cell lymphoma, leg type (PCDLBCL, LT) characteristically involves the legs, [109] but sometimes can occur in trunk and rarely in the head. Primary cutaneous diffuse large B-cell lymphoma, leg type (PCDLBCL, LT) predominantly affects elderly females and presents as multiple rapidly growing nodules [110]. In contrast to indolent cutaneous B-cell lymphomas, they more often disseminate to extracutaneous sites and have a poorer prognosis.

Histologically, PCDLBCL, LT, shows a diffuse infiltrate of large cells in the dermis with a destructive growth pattern obliterating adnexal structures and extending into the subcutaneous tissue, with sparing of the epidermis. The infiltrating tumor cells monotonous population or confluent sheets of centroblasts and immunoblasts [111]. Mitotic figures are frequently observed.

Immunophenotypically, the neoplastic lymphocytes express B-cell markers, and light chain restriction can be demonstrated by flow cytometry immunophenotype or frozen section, but no paraffin section immunohistochemistry. The neoplastic cells commonly express BCL2 and IRF4/MUM1 [112, 113]. They variably express BCL6 and generally do not express CD10 .

Molecular testing reveals monoclonal IgH gene rearrangements in almost 100 % of cases [23]. The t(14;18) involving the BCL2 gene is usually not present [114], however, cases with gene amplification of BCL2 have been reported [115]. Gene expression profiling studies showed gene expression profile similar to that of activated B-cell type of nodal or systemic DLBCL [116]. Array-based comparative genomic hybridization (aCGH) and FISH studies showed frequent deletion of chromosome 9p21 (containing the CDKN2A and CDKN2B genes) and amplification of 18q21 (containing the BCL2 and MALT1 genes). Other chromosomal aberrations include gains in chromosomes 1, 2, 3, 7, 12, and 18q, and losses in 6q, 13, and 17 [108]. Inactivation of CDKN2A by promoter hypermethylation is also detected. The inactivation of CDKN2A, either by deletion or promoter hypermethylation is a poor prognostic marker [107].

A 54-year-old male presented with nodules in the scalp. The patient did not have a significant medical history. On physical examination, scalp nodules, less than 1 cm in diameter were palpated, and a punch biopsy was obtained. The histologic sections showed a dense dermal infiltrate without epidermal involvement. There were scattered lymphoid follicles with germinal center formation. Immunohistochemical studies showed the lymphoid follicles to react with B-cell markers CD20 and PAX-5. BCL2 was negative in the germinal centers (Fig. 2.7 a and b).

Comment: The differential diagnosis is between follicular lymphoid hyperplasia and primary cutaneous follicular lymphoma. Molecular testing for IGH gene rearrangements yielded a polyclonal pattern, thus it failed to detected monoclonality. However, because of the possibility of follicular lymphoma, testing for t(14:18)(q32;q21) IGH/BCL2 by PCR was performed and demonstrated the presence of the rearrangement (Fig. 2.7c). This rearrangement is commonly associated with nodal follicular lymphoma, but it can also be present in some primary cutaneous follicular lymphomas, as illustrated in this case. On follow up, clinical staging was negative, and the disease was confined to the scalp. Local radiation, but no systemic therapy was administered, and the patient was asymptomatic on last follow up 5 years after diagnosis .