Electron microscopy and other ancillary techniques in the diagnosis of small round cell tumors

The current study disusses a new approach to the group of small round cell tumors (SRCTs) independently of their primary anatomical location. We perform this analysis supported mainly by morphological means and particularly with the help of immunohistochemistry and electron microscopy; the last of which continues to play a decisive role in their differential diagnosis. The microscopical similarity of many of these tumors often makes the diagnosis in routine histology extremely difficult, due to the varying degree of heterogeneity present, and may have important therapeutic and prognostic implications. Thus a correct final diagnosis is mandatory for the clinic. Within the group of tumors that express a dominant or occasional small round cell pattern "SRCT" (neoplasms of the Central Nervous System excluded) are included: Ewing's sarcoma and peripheral neuroectodermal tumor (Es/pPNET) comprising its varieties, neuroblastoma, desmoplastic small round cell tumor, rhabdomyosarcoma, alveolar, solid and embryonal, small cell osteosarcoma, chondrosarcoma, myxoid and mesenchymal, round cell and myxoid liposarcoma, synovial sarcoma (monophasic undiffentiated), primitive malignant peripheral nerve sheath tumor (malignant small cell schwannoma), malignant non-Hogdkin lymphoma, Merkel cell tumor of the skin (small cell carcinoma including neuroendocrine carcinoma). This study discusses in each case not only the histology, supported by immunohistochemistry, but also the main ultrastructural characteristics. We are conscious that in some cases further cytogenetic or molecular biology support may be necessary, when considering the limits of morphology today. Thus, short references on molecular genetics, complementing the structural findings, are given.

Differentiating small round cell sarcomas of the soft parts by an innovative immunogold labeling method: an ultrastructural study

A new immunoelectron microscopy procedure was developed by remaking the fixed-frozen tissue specimens into LR White resin blocks suitable for postembedding colloidal gold immunolabeling, and used to examine 16 cases of small round cell soft tissue sarcomas. In rhabdomyosarcoma, ultrastructural double-immunogold staining demonstrated a coexpression of muscle specific actin and desmin in the same tumor cell. In both Ewing's sarcoma and peripheral neuroepithelioma, the heterogeneous expression of MIC2 gene product (p30/32MIC2) in each tumor cell was demonstrated as well. In peripheral neuroepithelioma, the colloidal gold immunolabeling for neurofilament demonstrated the intermediate filaments surrounding microtubules. The procedure for ultrastructural colloidal gold immunolabeling using fixed-frozen tissue is thus considered to be useful not only for tumor diagnosis, but also for investigating various subcellular structures.

Appraisal of the comparative utility of immunohistochemistry and electron microscopy in the diagnosis of childhood round cell tumors

To provide an objective assessment of the comparative utility of fluorescence- and peroxidase-based immunohistochemistry and electron microscopy, an observer blinded study was conducted under realistic study conditions utilizing a large sampling of poorly differentiated pediatric round cell tumors. Working independently, using a single ancillary technique of particular expertise, each of three investigators attempted to render a specific diagnosis with regard to 50 diagnostically challenging tumors. The results were compared against the subsequent "file diagnosis" established by consensus with all relevant information made available. A grading scheme was applied wherein points were awarded based on the accuracy and confidence of diagnosis. A comparative efficiency rating, expressed as a percentage, was formulated by dividing the number of points awarded each technique by the total number of points theoretically available. Electron microscopy proved superior overall, with an efficiency rating of 89%. Immunoperoxidase and immunofluorescence studies yielded efficiency ratings of 71 and 61%, respectively. Used in combination, the techniques achieved an efficiency rating of 95%. Application of these ancillary techniques resulted in a revision of the provisional diagnosis in 11 of 50 cases, and left only two cases without a firm specific diagnosis.

Soft tissue small round cell tumors: morphological parameters

Soft tissue small round cell tumors (SRCTs) comprise a heterogeneous group of neoplasms that predominate in childhood and adolescence and share similar morphological features, consisting of dense cellular proliferation of small round cells with a primitive appearance. Rhabdomyosarcomas, peripheral neuroepitheliomas, Ewing's sarcomas, and lymphomas/leukemias are the prototypic SRCT; other recently described tumors that should be added to the list are the desmoplastic SRCT and the rhabdoid tumor of soft tissues. In addition, several other primary soft tissue neoplasms and metastatic tumors have occasionally been considered in the differential diagnosis of SRCT. The precise identification of a given SRCT is important because of its clinical relevance. However, it may be difficult because the diagnostic criteria are sometimes subtle and several histologic and immunohistochemical features are not specific and/or may be simulated by different tumor types. We discuss the morphological clues that in our opinion are most useful for their diagnosis, the criteria for distinguishing between peripheral neuroepithelioma and Ewing's sarcoma, and the main diagnostic pitfalls.

Histology, immunohistochemistry, and electron microscopy of small round cell tumors of bone

Small round cell tumors (SRCTs) of the bone make up a family of primary bone sarcomas with morphologically, biologically, and clinically specific features. Among them, Ewing's sarcoma (ES) is the most common entity, but several varieties such as atypical ES, large cell ES, and ES with neuroectodermal differentiation (peripheral primitive neuroectodermal tumor of the bone or neuroepithelioma of the bone) have been identified recently. Histology and electron microscopy together with the variable expression of several epitopes (as shown by immunohistochemistry, mainly HBA/71 [Mic2 antigen]) provide the basis for characterizing the group within the context of neuroectodermal-derived neoplasms. A number of other ES-like tumors with small round cells, mimicking those previously described, have been characterized; Askin's tumor of the thoracopulmonary region will be considered as an ES similar to those already described, but within a particularly anatomic location. On the other hand, the presence of an endothelial appearance within a poorly differentiated neoplasm may be present in some ES-like SRCTs (atypical ES with endothelial features). The differential diagnosis with other sarcomas defined by small round to spindle cell contours might prove difficult. Particular attention must be paid to small cell osteosarcoma and mesenchymal chondrosarcoma. Likewise, "primitive sarcoma of bone" is considered in this study because it is a very rare neoplasm differing from the formerly discussed types; its pluripotentiality provides this tumor a blastemic character and a multiphenotypic expression. Malignant non-Hodgkin's lymphoma is an unusual presentation when primary to the bone, previous to any other anatomic location. Several subtypes have been considered within a histology that encompasses that seen in lymph nodes.

Differential diagnosis of small round cell tumors

The differential diagnosis of small round cell tumors includes not only the small round cell tumors of infancy and childhood, but a variety of mesenchymal, cutaneous, and lymphoreticular neoplasms that may occur in adults as well as children. Moreover, lesions considered to be the classical small round cell tumors of childhood may occasionally occur in adults, and lesions characteristically seen in adults may at times occur in children. An awareness of the diverse lesions that may present as small round cell tumors is critical to arrive at the correct diagnosis. Immunohistochemical techniques and ultrastructural studies have now become indispensable in the diagnosis of small round cell tumors. With recent developments in cytogenetic and molecular genetic techniques, the pathologist has acquired an even more powerful and precise armamentarium of diagnostic tools. Judicious use of these methods, with knowledge of their significance and limitations, is the responsibility of the pathologist and laboratory coworkers who play a pivotal role in patient care and treatment. The differential diagnosis of small round cell tumors, including adult-type sarcomas, lymphoreticular malignancies, and neuroectodermal tumors of skin and viscera that simulate classical small round cell tumors, is briefly reviewed with this perspective. Specific cytogenetic and molecular markers are covered in addition to the typical immunohistochemical and ultrastructural features of several diverse lesions.

Desmoplastic small round cell tumor

Desmoplastic small round cell tumor is an undifferentiated tumor associated with serosal surfaces, especially the peritoneum. It is found predominantly in adolescents and young adults and is much more common in males than in females. The tumor has a characteristic histology, with extensive stromal tissue around islands of small and undifferentiated cells revealing the desmoplastic appearance. The coexpression of epithelial and mesenchymal antigens distinguishes this entity from other small round and blue cell tumors of this age group. Cytogenetic studies showed a t(11;22) translocation that differs from the Ewing's tumor translocation and seems to be specific to this entity. The involvement of the WT1 and EWS genes in the translocation makes this tumor an interesting subject for research on histogenesis and differentiation in small round and blue cell tumors.

Small round and spindle cell sarcoma with neuronal differentiation and oncocyte-like features of the thoracic wall: a case report with histological, immunohistochemical and ultrastructural examinations

A case of small round and spindle cell sarcoma with neuronal differentiation and oncocyte-like features is presented. The tumor was encountered in a 32 year old Japanese woman with an initial presentation of palpable tumor in the left lateral region of the thorax. The resected tumor was a partially well encapsulated whitish medullary one and consisted of small round and spindle tumor cells, together with so-called rhabdoid cells in the small round cell area. Although pseudorosettes were often observed, true rosette formation could not be detected anywhere. Ultrastructurally, despite a histologic variety of tumor cells, most tumor cells possessed numerous mitochondria, some of which occasionally contained abnormal filamentous or crystalloid structures. Various amounts of microfilaments were present in most tumor cells and microtubules were present in a few. A minority of small round cells possessed a small number of neurosecretory granules, especially in short cytoplasmic processes. A positive immunoreaction for neuron specific enolase was found by immunohistochemical examination in several small round tumor cells and for neurofilaments in lesser numbers. Despite the lack of S-100 protein, MB2 was detected in both small round and spindle cells. On the basis of these findings, the tumor of the present case corresponds to malignant peripheral nerve sheath tumor with neuronal differentiation and oncocytic features.