TABLE 71-1 Salient Features of Histiocytic and Dendritic Cell Sarcoma
Histiocytic and Dendritic Cell Sarcomas
Histiocytic and Dendritic Cell Sarcomas
Louis P. Dehner
The subject of histiocytic lesions of the skin is a topic usually dominated by Langerhans cell histiocytosis and juvenile xanthogranuloma, both regarded as clonal proliferations and benign in the usual clinical sense of treatment response and limited disease progression, if any. However, in the setting of multifocal and generally organ-based lesions, both may behave in a clinically aggressive fashion despite their morphologically innocuous appearance.
Another histiocytic lesion of the skin is the common dermatofibroma and its several histiocytic variants which are thought to be derived from the dermal dendrocyte. This category of dermal-based neoplasms may have a considerable degree of nuclear pleomorphism and an unsettling degree of mitotic activity, but in the absence of anaplasia and atypical mitotic figures. There is some degree of overlapping histologic features between dermatofibroma and juvenile xanthogranuloma, which is not surprising given that the dermal dendrocyte is regarded as the common progenitor.
Unequivocal malignant neoplasms of monocyte/macrophage lineage are in general very uncommon to rare regardless of their primary site. The skin is just one of the several reported sites, and in some cases, multifocal tumors are apparent on clinical presentation.
The past “malignant histiocytosis” was a diagnosis that today is recognized as anaplastic large cell lymphoma (ALCL) and lymphomatoid papulosis, which are CD30 T-cell proliferations, and with the presence of ALK-1 translocation in subsets of ALCL. Also regarded as malignant histiocytosis in the past was histiocytic medullary reticulosis of Scott and Robb-Smith, which is recognized today as primary or secondary hemophagocytic lymphohistiocytosis or macrophage activation syndrome.
Before a diagnosis of a malignant histiocytic lesion is made in the skin or elsewhere, it is right to keep in mind that reactive histiocytes can acquire a marked degree of atypia with nuclear enlargement, hyperchromatism, and mitotic activity.
Through the application of a series of well-characterized membrane and cytoplasmic markers, various groups or clusters of antigens are exposed by antigen processing and antigen presenting mononuclear bone marrow–derived cells which are ubiquitously distributed in virtually organs but have a specific localization in some organs such as the liver, spleen, and lymph nodes or a more random or diffuse dispersion in other sites such as the skin and supporting soft tissues.
Malignant cutaneous neoplasms of the monocyte/macrophage lineage can be broadly classified into histiocytic and dendritic cell neoplasms, as tumors of antigen processing or presenting function, based on their morphology, immunophenotype, and ultrastructural characteristics.1,2 There are four distinct diagnostic categories: (i) histiocytic sarcoma (HS); (ii) follicular dendritic cell sarcoma (FDCS); (iii) interdigitating dendritic cell sarcoma (IDCS); and (iv) indeterminate dendritic cell sarcoma (INDCS) (Table 71-1). The exact cell of origin of some of these neoplasms remains the subject of continued study and discussion without a consensus or resolution in all cases. Some authors also classify Langerhans cell proliferations under this diagnostic umbrella, which have been discussed in depth in Chapter 67. Hematolymphoid malignancies of lymphocytic (T and B) lineage are almost always considered in the differential diagnosis and laboratory workup, before arriving at a final diagnosis of these rare neoplasms.
HS is defined by the World Health Organization (WHO) as a malignancy with morphologic and immunophenotypic features that resemble those of mature tissue histiocytes.3 HS is an aggressive neoplasm that presents across a wide range of ages, as a primary neoplasm or subsequent to a diagnosis of another hematologic malignancy. Two peaks have been reported, the first between 0 and 29 years and the second between 50 and 69 years.4 The first peak is smaller and corresponds to the cases associated with pre–B-cell and T-cell leukemia/lymphoma; the second peak corresponds to those cases associated with B-cell lymphoma.4,5 HS can present as solitary or multiple lesions involving lymph nodes or extranodal sites.2,4 The skin is the fourth most common site of extranodal involvement that can manifest as solitary or multiple lesions or as a rash. Excisional biopsy of localized lesion or a punch biopsy of a cutaneous rash usually provides adequate tissue for diagnosis with ancillary testing.
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