Superficial mesenchymal tumours expressing epithelial markers on immunohistochemistry: Diagnostic clues and pitfalls

The diagnosis of mesenchymal neoplasms arising in the superficial soft tissue can be challenging as some entities are rare and show overlapping features. Moreover, the spectrum of mesenchymal tumours has expanded recently to include potential new entities, some of which have been described after the 5th edition of the World Health Organisation (WHO) classification of soft tissue and bone tumours published in 2020. In the skin and superficial soft tissue, tumours of epidermal, melanocytic and appendageal origin are more commonly encountered than mesenchymal neoplasms. However, specific entities from the latter category can occasionally express epithelial markers on immunohistochemistry, some of them in a strong and diffuse manner. It is therefore crucial to be aware of diagnostic pitfalls when encountering cytokeratin positivity in superficial soft tissue neoplasms. This article provides an overview on the differential diagnosis of these mesenchymal tumours that can sporadically occur also in the skin, including myoepithelial neoplasms, epithelioid sarcoma, keratin positive giant cell tumour of soft tissue / xanthogranulomatous epithelial tumour, superficial CD34-positive fibroblastic tumour / PRDM10-rearranged soft tissue tumour, and perineurioma.     

Primary clear cell sarcoma of the ileum

Clear cell sarcoma or malignant melanoma of soft tissue is a high-grade sarcoma with melanocytic differentiation, essentially involving tendons and aponeuroses of distal extremities in young adults. Visceral locations are exceptional and only 8 cases involving the gastrointestinal tract are reported in the literature. We present a case of an ileal clear cell sarcoma discovered in a 26-year-old woman. Moreover, we emphasize on the diagnosis difficulties of this rare type of sarcoma, characterized by the specific translocation t(12;22)(q13;q12).     

Melan-A/Mart-1 expression in various melanocytic lesions and in non-melanocytic soft tissue tumours

AIMS: The purpose of this study was to test different malignant non-melanocytic tumours with the commercially available antibody Melan-A to examine its diagnostic specificity and to compare the S100, Melan-A and HMB-45 reactivity in various melanocytic lesions. METHODS AND RESULTS: Seventy-three benign and malignant melanocytic lesions and 31 cases of non-melanocytic tumours, sarcomas, carcinomas and carcinoids, were selected. Immunohistochemical staining of paraffin sections, following a high temperature antigen unmasking technique, was performed. Melan-A stains junctional and dermal melanocytes in all benign melanocytic lesions with the exception of neuro-naevoid areas. The epithelioid and the spindle cells in malignant melanomas did not show considerable difference in their Melan-A reactivity. The predominantly spindle cell type mucosal melanomas contained more Melan-A-positive cells than HMB-45-positive cells and similar results were observed in metastatic malignant melanomas. In desmoplastic melanomas the positivity of Melan-A was not consistent. None of the sarcomas, carcinomas and carcinoids expressed Melan-A. Almost all soft tissue tumours, except for two malignant gastrointestinal stromal tumours, were unreactive for HMB-45. These two cases did not react with Melan-A antibody. CONCLUSIONS: Melan-A is a useful additional marker to differentiate non-melanocytic tumours from primary or metastatic melanoma. In melanocytic lesions the Melan-A staining pattern is similar to S100, but seems to be more specific. In desmoplastic melanomas, however, the variable Melan-A staining further necessitated detailed histological examination and the use of the S100 reaction.     

Melanotic Neuroectodermal Tumor of Infancy Occurring in the Left Thigh of a 6-Month-Old Female Infant

We report an exceptional case of melanotic neuro-ectodermal tumor of infancy (MNTI) occurring in the soft tissue of the left thigh of a 6-month-old female infant. The tumor consisted mainly of small round cells (neuroblasts) arranged in cords and nests that were separated by broad fibrovascular areas. In addition, there were a few medium-sized tumor cells containing melanin pigment (melano-cytic cells) that in electron microscopy contained melanosomes as well as tonofilaments. Both tumor cell types immunostained for neuron-specific enolase (NSE) and vimentin, and the melanocytic cells reacted additionally with the antikeratin antibody KL1. Within the tumor stroma, neurofilament- and S-100-protein-positive neural cells and vimentin- and desmin-positive myofibroblasts were seen. Although densecore granules were demonstrated ultrastructurally in some neuroblasts, no immunostaining for chromogranin A, Leu-7, serotonin, or regulatory peptides was found. MNTI located in an extremity can be confused with malignant small round and blue cell tumors of childhood. The distinction between MNTI and these tumors is of clinical significance because MNTI, in most cases, is a benign tumor that, in contrast to the latter, can be cured by complete excision. The presence of a biphasic cell population with neuroblasts and melanocytic cells must be considered the main diagnostic feature of MNTI.     

Epithelioid angiosarcoma involving the lungs

Diffuse lung involvement by metastatic tumor from an unknown primary site often constitutes a diagnostic dilemma. Although cytologic features and pattern of metastatic spread can guide in narrowing the list of possible primary neoplasms, immunohistochemistry remains pivotal in determining the phenotype of metastatic disease. We report a case with extensive involvement of lung parenchyma by a metastatic epithelioid neoplasm exhibiting a variety of distinctive patterns with a predominance of intra-arterial and lymphangitic spread. Immunohistochemical studies showed no evidence of epithelial, melanocytic, or lymphoid differentiation. The neoplastic cells were strongly positive for vimentin and CD31 but negative for CD34 and factor VIIIR:Ag. Electron microscopy of formalin-fixed tissue revealed multiple Weibel-Palade bodies and pinocytosis, supporting the diagnosis of epithelioid angiosarcoma. Doppler studies performed after pathologic diagnosis was rendered demonstrated 2 discrete hypoechoic masses within the medial aspect of the left proximal calf musculature, suggestive of solid soft tissue neoplasm-a possible source of pulmonary metastatic disease.     

Malignant melanoma simulants arising in congenital melanocytic nevi do not show experimental evidence for a malignant phenotype.

Proliferative neoplasms that resemble malignant melanoma may develop in large congenital melanocytic nevi, prenatally or in the neonatal period, although these lesions rarely show the progressive growth or behavioral characteristics of melanoma. This report describes the genetic, biologic, and immunologic characteristics of six tissue culture cell lines derived from two neoplasms present in congenital melanocytic nevi in two newborn infants. Both neoplasms had clinical and histologic features of malignant melanoma. Despite these features, cells from all lines were phenotypically benign, as evidenced by a normal karyotype, their expression pattern of pigment cell-associated antigens, absence of melanoma-associated ganglioside GD2, their mitogenic response to the tumor-promoting phorbol ester 12-0-tetradecanoyl phorbol-13-myristate, their inability to grow anchorage independently in soft agar, and prolonged but finite life span. The cells did not produce tumors in nude mice, but they remained viable at the injection site for over 7 months.     

Reduced expression of p16 and p27 is correlated with tumour progression in cutaneous melanoma

AIMS: To determine if the cyclin dependent kinase inhibitors (CDKIs) p16 and p27 show reduced expression in the progression from benign to malignant melanocytic tumours, and to correlate these findings with patient prognosis. METHODS: Ninety-two melanocytic tumours were assessed for immunohistochemical expression of p16 and p27. These specimens included nine compound naevi, 10 dysplastic naevi, 17 thin (<1 mm) melanomas, 22 thick (>1 mm) melanomas, nine in-transit metastases, 13 lymph node metastases, and 12 soft tissue metastases. Clinicopathological information on the 39 patients with melanoma primaries was obtained from the Sydney Melanoma Unit database. The median follow up period was 43.3 months. RESULTS: A significant loss of expression of p16 and p27 was found with tumour progression. Positive expression of p27 was found in all compound and dysplastic naevi but only 43.6% of melanoma primaries. Expression of p27 was greater in lymph node and in-transit metastases (63.6%), but lower in soft tissue metastases (36.4%). Positive expression of nuclear p16 was evident in 73.7% of benign naevi, 28.2% of primary melanomas and 14.7% of metastatic melanomas. Neither p16 nor p27 expression was significantly correlated with overall survival, disease free survival or other clinicopathological markers. CONCLUSIONS: The CDKIs p16 and p27 are associated with tumour progression in melanoma, but do not reliably predict recurrence or survival.     

PEComa: what do we know so far?

PEComas (tumours showing perivascular epithelioid cell differentiation) are a family of related mesenchymal neoplasms that include angiomyolipoma, lymphangiomyomatosis, clear cell "sugar" tumour of the lung, and a group of rare, morphologically and immunophenotypically similar lesions arising at a variety of visceral and soft tissue sites. These tumours all share a distinctive cell type, the perivascular epithelioid cell or "PEC' (which has no known normal tissue counterpart). PEComas show a marked female predominance and are composed of nests and sheets of usually epithelioid but occasionally spindled cells with clear to granular eosinophilic cytoplasm and a focal association with blood vessel walls. PEComas appear to arise most commonly at visceral (especially gastrointestinal and uterine), retroperitoneal, and abdominopelvic sites, with a subset occurring in somatic soft tissue and skin. Nearly all PEComas show immunoreactivity for both melanocytic (HMB-45 and/or melan-A) and smooth muscle (actin and/or desmin) markers. A subset of PEComas behave in a malignant fashion. This review examines the members of the PEComa family, with an emphasis on lesions arising outside of the kidney, lung and liver, and discusses preliminary evidence for pathological features that might predict malignant behaviour.     

Non-melanocytic mimics of melanocytic neoplasms

On histopathologic examination, many non-neoplastic conditions mimic benign or malignant neoplasms. Alternatively, some benign and malignant neoplasms can also mimic non-neoplastic lesions. This is true of all organ systemsskin is no exception. Examples of these mimickers can be found in skin lesions of almost all tissue types, including those that are melanocytic, lymphoid, epithelial, neural, vascular, neuroendocrine, and fibrohistiocytic. Melanocytic neoplasms are particularly important as it's challenging to differentiate them as being benign or malignant, and it's difficult to tell them apart from non-melanocytic neoplasms. This review illustrates examples of non-melanocytic lesions that disguise themselves as melanocytic.     

Histiocytic sarcoma - a case with evenly distributed multinucleated giant cells

Histiocytic sarcoma is an uncommon neoplasm of mature histiocytes with a poor clinical outcome. We report a case of a true histiocytic sarcoma with prominent and evenly distributed multinucleated giant cells that mimics a giant cell tumor of soft tissue. The tumor was located between the appendix, right ovary, and the terminal ileum with severe adhesion. The liver and spleen were not enlarged. Grossly, the tumor appeared grayish white, solid, and soft. Microscopically, polygonal mononuclear tumor cells aggregated to form somewhat epithelioid nests, which occasionally showed coagulative necrosis. Prominent and evenly scattered giant cells were present in all sections. In addition, tumor cell infiltration was noted in regional lymph nodes. The tumor cells were positive for lysozyme, CD68, CD163, and negative for T- and B-cell lineage markers, follicular dendritic cell, megakaryocytic, epithelial, muscular, and melanocytic markers, CD1a and CD30. This case posed great difficulty in clinical and pathological diagnoses. Gross pictures, microscopic findings, and extensive immunostains are important for the differential diagnosis.     

Mismatch repair protein expression and microsatellite instability: a comparison of clear cell sarcoma of soft parts and metastatic melanoma.

Clear cell sarcoma of soft parts is a rare soft tissue malignancy that shows phenotypic overlap with cutaneous melanoma but can be distinguished by the presence of a t(12;22) translocation. Microsatellite instability (MSI), a variation in the lengths of short repeat DNA segments in the genome, has been implicated in melanoma tumorigenesis, but is rare or absent in clear cell sarcoma. Defects in the mismatch repair (MMR) enzyme complex correlate with MSI in some tumor types, allowing the use of immunohistochemistry for the MMR proteins hMLH1 and hMSH2 to predict the presence of MSI. To determine if the association between MMR defects and MSI extends to clear cell sarcoma, we compared a group of nine clear cell sarcomas to 11 metastatic melanomas on the basis of MSI and the expression of MMR proteins. MSI was studied using fluorescence-based multiplexed PCR of five loci. Immunohistochemistry was evaluated on formalin-fixed paraffin-embedded tissue for hMLH1, hMHS2 and hMSH6. MSI was present in only 1/9 (11%) clear cell sarcoma case and in 8/11 (73%) melanoma cases. Immunostaining for hMLH1 and hMSH2 was preserved in all the clear cell sarcomas but loss of immunostaining for one or both proteins was seen in 6/11 melanomas (55%). hMSH6 was detected in 7/9 (78%) clear cell sarcomas and 10/11 (91%) of melanomas. Clear cell sarcoma and metastatic melanoma differed significantly with respect to the presence of MSI (P=0.010) and staining for hMLH1 and/or hMSH2 (P=0.014) but not hMSH6 (P=0.57). Mismatch repair, and consequently genomic instability may contribute to tumorigenesis in melanoma but not clear cell sarcoma. Immunostaining for hMLH1 and hMSH2 and MSI analysis may be helpful in the differential diagnosis of large soft tissue or visceral malignancies with melanocytic differentiation.     

Proliferative activity of primary cutaneous melanocytic tumours

The expression of proliferating cell nuclear antigen (PCNA) was examined in formalin-fixed paraffin-embedded tissue sections from 41 lesions (27 melanocytic nevi, 3 atypical nevi and 11 malignant melanomas) to determine the proliferative activity of primary cutaneous melanocytic tumours. Most of the malignant melanomas had more than 7% PCNA-positive cells (9.2±0.5%), while the melanocytic nevi manifested less than 1% PCNA-positive cells (0.4±0.1%). Atypical nevi exhibited an intermediate, but still significantly lower, labelling ratio when compared with malignant melanomas (0.8±0.2%). The proliferative activity of the lesions was compared between portions at different depths in the skin (epidermal, upper dermal and lower dermal location). In cases of malignant melanoma, the proliferative activity was higher in the deeper portion of dermis whereas PCNA-positive cells in melanocytic nevi were located in the upper dermis predominantly. Thus the PCNA labelling ratio of malignant melanoma and/or melanocytic nevus cells located in the epidermodermal junction was not necessarily higher than that of malignant melanoma and/or melanocytic nevus cells in the dermis. These results indicate that staining with PCNA would be very useful in the differentiation of malignant melanoma from melanocytic nevi manifesting cellular and/or structural atypia by virtue of a significant difference in the proportion of PCNA-positive cells. Although malignant melanomas have higher proliferative activity than melanocytic nevi in the deeper dermis, junctional activity in melanocytic tumours does not indicate cell proliferation.     

Incidental perivascular epithelioid cell tumor in an inguinal hernia sac

Perivascular epithelioid tumors (PEComa) are uncommon mesenchymal neoplasms demonstrating positivity for muscular and melanocytic immuno-markers. Included in this category are angiomyolipoma, lymphangioleiomyomatosis, and clear cell sugar tumors. Lesions which do not fit into these categories are classified as “not otherwise specified”. We present a case of an incidentally discovered PEComa within inguinal hernia sac contents in a 70-year-old woman. It consisted of spindled and epithelioid cells with bland oval nuclei, small nucleoli and clear to light eosinophilic cytoplasm. There was no atypia or mitoses. The lesion was strongly positive for HMB45 and smooth muscle actin. Pelvic soft tissue and peritoneal PEComas are rarely reported in literature and very little is known about their prognosis. We discuss the immunohistochemistry, differential diagnosis, and pathogenesis of PEComas.     

Paraganglioma-like dermal melanocytic tumor: a case report with particular features

Childhood dermal tumors with melanocytic features is an unusual tumor that can create diagnostic confusion. Among them, paraganglioma-like melanocytic tumors- previous included in melanocytic tumors of uncertain malignant potential- has some particular histopathologic and immunohistochemical features. We describe a case of 13 years old girl with a paraganglioma-like dermal melanocytic tumor of the left leg.     

CD34<Superscript>+</Superscript> fibrocytes in melanocytic nevi and malignant melanomas of the skin

CD34(+) fibrocytes are constitutive elements of the human connective tissue. The stroma associated with invasive carcinomas is characterized by a stereotypic loss of CD34(+) fibrocytes and a phenotype change towards CD34(-) alpha-Smooth muscle actin (SMA)(+) myofibroblasts. Secreted protein acidic and rich in cysteine (SPARC) is an important mediator of tumor-associated stromal remodeling. Melanocytic lesions of the skin have not been investigated as to this aspect up to now. Thus, we investigated a total of 20 malignant melanomas and 29 melanocytic nevi. The normal dermis and benign melanocytic nevi showed numerous CD34(+) fibrocytes, whereas malignant melanomas were devoid of this cell type. alpha-SMA-positive myofibroblasts were absent from the normal dermis, melanocytic nevi, and malignant melanomas. SPARC was positive in malignant melanoma cells and negative in their associated stroma, while all melanocytic nevi were completely negative. The stromal phenotype of malignant melanomas (CD34(-) alpha-SMA(-)) differs from that of invasive carcinomas (CD34(-) alpha-SMA(+)) suggesting different pathogenic mechanisms involved in tumor-associated stromal remodeling. SPARC expression appears to be closely related to malignancy in melanocytic lesions.     

Molecular pathology of melanocytic tumors

Genetic and genomic analyses of melanocytic tumors have yielded new opportunities for improvements in diagnostic accuracy for the distinction of nevus from melanoma and better selection of patients affected by melanoma for targeted treatment. Since chromosomal copy number changes are commonly found in malignant melanoma, but rare in melanocytic nevi, cytogenetic assays have emerged as a promising ancillary study for the workup of melanocytic tumors with ambiguous light microscopic features. Comparative genomic hybridization (CGH) permits assessment of the full set of chromosomes, but requires a significant amount of lesional tissue, and may fail to detect aberrations in a minor subpopulation of tumor cells. Fluorescence in situ hybridization (FISH) is the cytogenetic assay of choice for limited amounts of tissue. FISH targets only specific chromosomes, with inherent limitations in test sensitivity and specificity. FISH analysis is also heavily dependent on individual experience. Molecular studies have identified distinct sets of mutations in melanoma and/or nevi. These mutations have become clinically relevant for targeted therapy of patients with advanced disease, especially for the treatment of patients with metastatic melanoma carrying the BRAF^V600 or KIT mutations. However, mutation analysis can on occasion also be used for diagnostic purposes.