The immunophenotype of Ewing's sarcoma. An immunohistochemical analysis

Clinicopathologic Features of the Non-CNS Primary Ewing Sarcoma Family of Tumors in the Head and Neck Region

Ewing sarcoma family of tumor (ESFT) is a group of malignant neoplasms that affect children and young adults. Primary ESFT does not commonly arise from the head and neck region. This study aimed to elucidate the clinicopathologic characteristics of ESFT of the head and neck region except for central nervous system primitive neuroectodermal tumors. Among the 207 cases of ESFT of the bone and soft tissue, diagnosed at Asan Medical Center during a 20-year period, 25 (12.1%) involved the head and neck region. Of those, 21 were available for histologic, immunohistochemical, and molecular studies. EWSR1 rearrangement was detected in 19 cases by reverse transcriptase-polymerase chain reaction and/or fluorescence in situ hybridization. Primary sites included the cranial area (6 cases, 31.6%), sinonasal tract (6 cases, 31.6%), paraspinal space (4 cases, 21.0%), and other spaces (3 cases, 15.8%). The 5-year overall survival and disease-free survival rates for all cases were 69.7% and 33.6%, respectively. A large tumor size (>5 cm) correlated significantly with overall survival (P=0.009), but not with disease-free survival (P=0.210). Microscopically, 8 cases (42.1%) showed nested growth pattern. Clear and/or eosinophilic cytoplasm was observed in 68.4% cases. Immunopositivity for CD99, Friend leukaemia integration-1 (FLI-1), CD57, and caveloin-1 were detected in 100%, 88.9%, 83.3%, and 50% cases, respectively. ESFT in the head and neck region had a favorable prognosis and frequent atypical and epithelioid features. An awareness of these histologic and immunophenotypic characteristics will improve the diagnostic accuracy for head and neck round cell malignancies.

Primary paraesophageal Ewing's sarcoma: an uncommon case report and literature review

Ewing’s sarcoma is a rare and highly aggressive cancer most frequently arising in people under 20 years of age. We report an uncommon case of primary paraesophageal Ewing’s sarcoma in a 25-year-old male harboring the infrequent EWSR1/ERG fusion transcript with multiple splice variants coexisting in the same tumor. The patient was totally refractory to chemotherapy and died 17 months after diagnosis. We underscore the need for better understanding of the molecular pathogenesis of the disease and improved systemic therapy options.

Primary spinal extradural Ewing's sarcoma (primitive neuroectodermal tumor): Report of a case and meta-analysis of the reported cases in the literature

Background:

Primary spinal primitive neuroectodermal tumors (PNET) and/or spinal extraskeletal Ewing's sarcoma family tumors (ESET) are rare lesions appearing in the spinal extradural space. One hundred forty-one primary spinal PNETs, including 29 intramedullary lesions, have been reported in the literature. Encountering a case of primary epidural EES/peripheral PNET (pPNET) in sacral level, which is the fifth one occurring at this level in the literature, we have tried to conduct a meta-analysis of the reported cases.

Case Description:

A 44-year-old lady with epidural EES/pPNET is reported here. She was once operated for L5/S1 herniated disc, which did not ameliorate her symptoms. The clinical, imaging, surgical, and histopathologic characteristics of our case are presented and wide search of the literature is also done. All the reports were level 3 or less evidences and most of the series had missing parts. 106 cases of primary intraspinal (extradural/extramedullary-intradural) EES/pPNET and 29 cases of primary intramedullary PNET (CNS-PNET) have been reported in the literature. The most common clinical presentation in both entities was muscle weakness proportionate to the tumor location. Distant metastasis occurred in 38 of 99 (38%) cases of primary intraspinal EES/pPNET, while the rate of metastasis was 48% in patients with PNETs occurring in the intramedullary region (P > 0.05). One-year survival rate of the patients who underwent chemo-radiation after total or subtotal resection was better than those who did not receive chemotherapy or radiotherapy, or did not have total or subtotal resection. However, this difference was not repeated in 2-year survival rate in any of the tumor groups.

Conclusion:

It seems that total or subtotal removal of the tumor and adjuvant chemo- and radiation therapy can improve the outcome in these patients.

Ewing sarcoma: clinical state-of-the-art

Ewing sarcoma, a rare malignancy of childhood and adolescence, has become a model of advances in diagnosis, treatment, and outcome through long-standing research efforts in multinational clinical trials. With modern multimodal regimens consisting of local surgery and/or radiotherapy plus intensive systemic chemotherapy, survival can be achieved for ≈ 70% of patients with localized disease. However, in the last decade, improvement in survival curves has slowed down. Also, a relapse rate of ≈ 30% remains unacceptable, since salvage strategies for Ewing sarcoma recurrence are discouraging and prognosis is unfavorable in most cases. Metastatic disease at diagnosis poses a similar challenge, since even if remission is achieved, relapse frequently occurs despite the most intensive treatment. Urgently needed, novel biology-driven treatment options are now beginning to emerge on the horizon, but have not yet reached the standard of care. An overview of the current clinical state-of-the-art is provided in this article.

Primitive neuroectodermal tumors of the spine: a comprehensive review with illustrative clinical cases

Primary spinal primitive neuroectodermal tumors (PNETs) are uncommon malignancies that are increasingly reported in the literature. Spinal PNETs, like their cranial counterparts, are aggressive tumors and patients with these tumors typically have short survival times despite maximal surgery, chemotherapy, and radiation. Because no standard management guidelines exist for treating these tumors, a multitude of therapeutic strategies have been employed with varying success. In this study the authors perform a comprehensive review of the literature on primary spinal PNETs and provide 2 new cases that highlight the salient features of their clinical management.

Histological heterogeneity of Ewing's sarcoma/PNET: an immunohistochemical analysis of 415 genetically confirmed cases with clinical support

Ewing's sarcoma (ES)/peripheral neuroectodermal tumor (PNET) are malignant neoplasms affecting children and young adults. We performed a study to typify the histological diversity and evaluate antibodies that may offer diagnostic/prognostic support. In total, 415 cases of genetically confirmed paraffin-embedded ES/PNET were analyzed on whole sections and in tissue microarrays. This study confirms the structural heterogeneity of ES/PNET, distinguishing three major subtypes: conventional ES (280 cases); PNET (53 cases); and atypical ES/PNET (80), including large cells, vascular-like patterns, spindle pattern, and adamantinoma-like configuration. All cases presented positivity for at least three of the four tested antibodies (CD99, FLI1, HNK1, and CAV1). CAV1 appeared as a diagnostic immunomarker of ES/PNET being positive in CD99-negative cases. Hence, the immunohistochemical analysis confirmed the diagnostic value of all four antibodies, which together cover more than 99% of the tumors, independently of the histological variety. The univariate analysis for survival revealed atypical ES as the only histological parameter apparently associated with less favorable clinical outcome, particularly in the subgroup of patients treated with surgery. In conclusion, the diagnosis of atypical ES is a challenge for the pathologist and needs support from molecular techniques to perform an optimal differential diagnosis with other small round cell tumors.

Ewing's sarcoma family of tumors: current management

Ewing's sarcoma is the second most frequent primary bone cancer, with approximately 225 new cases diagnosed each year in patients less than 20 years of age in North America. It is one of the pediatric small round blue cell tumors, characterized by strong membrane expression of CD99 in a chain-mail pattern and negativity for lymphoid (CD45), rhabdomyosarcoma (myogenin, desmin, actin) and neuroblastoma (neurofilament protein) markers. Pathognomonic translocations involving the ews gene on chromosome 22 and an ets-type gene, most commonly the fli1 gene on chromosome 11, are implicated in the great majority of cases. Clinical presentation is usually dominated by local bone pain and a mass. Imaging reveals a technetium pyrophosphate avid lesion that, on plain radiograph, is destructive, diaphyseal and classically causes layered periosteal calcification. Magnetic resonance best defines the extent of the lesion. Biopsy should be undertaken by an experienced orthopedic oncologist. Approximately three quarters of patients have initially localized disease. About two thirds survive disease-free. Management, preferably at a specialist center with a multi-disciplinary team, includes both local control-either surgery, radiation or a combination-and systemic chemotherapy. Chemotherapy includes cyclic combinations, incorporating vincristine, doxorubicin, cyclophosphamide, etoposide, ifosfamide and occasionally actinomycin D. Topotecan in combination with cyclophosphamide has shown preliminary activity. Patients with initially metastatic disease fare less well, with about one quarter surviving. Studies incorporating intensive therapy followed by stem cell infusion show no clear benefit. New approaches include anti-angiogenic therapy, particularly since vascular endothelial growth factor is an apparent downstream target of the ews-fli1 oncogene.

Cluster analysis of immunohistochemical profiles in synovial sarcoma, malignant peripheral nerve sheath tumor, and Ewing sarcoma

As a result of overlapping morphologic and immunohistochemical features, it can be difficult to distinguish synovial sarcoma, malignant peripheral nerve sheath tumor, and Ewing sarcoma/primitive neuroectodermal tumor in core biopsies. To analyze and compare immunohistochemical profiles, we stained tissue microarrays of 23 synovial sarcomas, 23 malignant peripheral nerve sheath tumors, and 27 Ewing sarcomas with 22 antibodies potentially useful in the differential diagnosis, and analyzed the data with cluster analysis. Stain intensity was scored as none, weak, or strong. For CD99, tumors with membranous accentuation were independently categorized. Cluster analysis sorted five groups, with like tumors clustering together. Synovial sarcoma clustered into two groups: one cytokeratin and EMA positive (n = 11), the other mostly cytokeratin negative, EMA positive, bcl-2 positive and mostly CD56 positive (n = 9). Malignant peripheral nerve sheath tumor clustered into two groups: one S100 positive, with nestin and NGFR positivity in most (n = 10), the other mostly S100 negative, and variably but mostly weakly positive for nestin and NGFR (n = 11). Ewing sarcomas clustered into a single group driven by membranous CD99 staining. Thirteen cases failed to cluster (outliers), while three Ewing sarcomas clustered into groups of other tumor types. Paired antibodies for each tumor type determined by visual assessment of cluster analysis data and statistical calculations of specificity, sensitivity, and predictive values showed that EMA/CK7 for synovial sarcoma, nestin/S100 for malignant peripheral nerve sheath tumor, and membranous CD99/Fli-1 for Ewing sarcoma yielded high specificity and positive predictive values. Cluster analysis also highlighted aberrant staining reactions and diagnostic pitfalls in these tumors. Hierarchical cluster analysis is an effective method for analyzing high-volume immunohistochemical data.

A systematic review of molecular and biological markers in tumours of the Ewing's sarcoma family

The aims of this study were to perform the first systematic review of molecular and biological tumour markers in tumours of the Ewing's sarcoma family (ESFT), and evaluate the current evidence for their clinical use. A well-defined, reproducible search strategy was used to identify the relevant literature from 1966 to February 2000. Papers were independently assessed for tumour markers used in the screening, diagnosis, prognosis or monitoring of patients with ESFT. Eighty-four papers studying the use of 70 different tumour markers in ESFT's were identified. Low-quality, inconsistent reporting limited meta-analysis to that of prognostic data for 28 markers. Patients with tumours lacking S-100 protein expression have a better overall survival (OS) (hazard ratio (HR)=0.41, 95% confidence interval (CI) 0.19, 0.89) than those with expression; patients with high levels of serum LDH had a worse OS and disease-free survival (DFS) (OS: HR=2.92, CI 2.16, 3.94, DFS: HR=3.38, 95% CI 2.28, 4.99); patients with localised disease and tumours expressing type 1 EWS-FLI1 fusion transcripts had an improved DFS compared with those with other fusion transcript types (HR=0.17, 95% CI 0.079, 0.37). The knowledge base formed should facilitate more informative future research. Improved statistical reporting and large, multicentre prospective studies are advocated.

Intracranial Ewing sarcoma/'peripheral' primitive neuroectodermal tumor of dural origin with molecular genetic confirmation

Ewing sarcoma/'peripheral' primitive neuroectodermal tumor (ES/pPNET) is the designation given to a family of small cell neoplasms that typically arise in bone or soft tissue and are unified by their common expression of the MIC2 antigen and specific translocations involving a gene on chromosome 22q12 [the most common being t(11;22)(q24;q12)]. ES/pPNET of intracranial origin is extraordinary. We report the case of a 6-year-old boy with a large left frontal region mass that adhered to dura and was extracerebral at surgery. Histologic study revealed a high-grade, undifferentiated-appearing neoplasm of small cell type that was negative on immunostudy for glial fibrillary acidic protein, synaptophysin, desmin, leukocyte common antigen, smooth muscle actin and epithelial membrane antigen, but positive for vimentin and neuron-specific enolase and diffusely labeled by antibody O13 (which recognizes the MIC2 gene product). RNA-based polymerase chain reaction assay confirmed the diagnosis of ES/pPNET by demonstrating fusion transcripts indicative of t(11;22) translocation. Bone scan, computerized tomography of the chest and bone marrow examination revealed no systemic tumor. The limited observations published to date suggest that primary intracranial ES/pPNET is most likely to present in childhood as a circumscribed, contrast-enhancing and dural-based extracerebral mass. It must be distinguished from a variety of small cell neoplasms, particularly PNETs of central neuroepithelial origin.

Hu Immunolabeling as a Marker of Neural and Neuroendocrine Differentiation in Normal and Neoplastic Human Tissues: Assessment Using a Recombinant Anti-Hu Fab Fragment

The recombinant antibody fragment Fab GLN 495 recognizes an epitope shared by members of the neuron-associated Hu protein family (including HuC, HuD, and HelNl). This novel reagent labels the nuclei of neurons throughout the peripheral and central neuraxes and has been shown to recognize pulmonary small cell carcinomas and central nervous system (CNS) tumors of mature neuronal phenotype or neuronogenic differentiating capacity. Using this Fab fragment, we have undertaken a systematic survey of normal human tissues and an assessment of 554 non-CNS tumor samples for immunohistochemical evidence of Hu expression. Adrenomedullary cells, pancreatic islet cells, paraganglial chief cells, isolated adenohypophyseal cells, and spermatogonia were the only nonneuronal normal tissue elements to bind Fab GLN 495. In addition to labeling all 10 small cell carcinomas studied (six of which were extrapulmonary in origin), this recombinant anti-Hu Fab proved immunoreactive with neuroblastomas (four/four), esthesioneuroblastomas (one/one), typical (three/four) and atypical (one/four) pulmonary carcinoids, pancreatic islet cell tumors (two/six), large-cell neuroendocrine carcinoma of lung (one/four), Merkel cell tumors (two/three), medullary carcinomas of the thyroid (four/six), pheochromocytomas (two/four) and paragangliomas (four/four). Nonneural/neuroendocrine tumor labeling was restricted to the neuronal and immature neuroepithelial components of teratomas, to extraskeletal myxoid chondrosarcomas (three/four) and to small subsets of cells within examples of renal rhabdoid tumor (one/four), desmoplastic small cell tumor (one/four), alveolar rhabdomyosarcoma (two/four), Ewing sarcoma/PNET (two/nine), and Wilms tumor (one/four). Immunoreactivity was principally nuclear, with variable cytoplasmic labeling. Our findings support the largely restricted expression of Hu by neural/neuroendocrine neoplasms, suggest a potential role for Fab GLN 495 in the identification of small cell carcinomas irrespective of primary site, and support a recent proposal that at least some extraskeletal myxoid "chondrosarcomas" actually represent neuroendocrine tumors of soft parts. Int J Surg Pathol 8(2):109-117, 2000

Cytokeratin immunoreactivity in Ewing's sarcoma: prevalence in 50 cases confirmed by molecular diagnostic studies

Ewing's sarcoma (ES) and primitive neuroectodermal tumor (PNET) are characterized by the presence of the specific t(11;22)(q24;q12) or variants thereof, producing diagnostic EWS fusion transcripts. Cytokeratin has been reported sporadically to be expressed in some cases of ES/PNET. However, its prevalence has not been assessed systematically in a series of cases with confirmatory molecular or cytogenetic evidence of a diagnostic translocation. We present in detail three index patients in whom strong cytokeratin immunoreactivity was a confounding factor in the diagnosis. To establish further the prevalence of cytokeratin immunoreactivity in a series of well-characterized ES/PNET, we then performed immunohistochemical studies with antibodies CAM5.2 and AE1/AE3 on 50 cases of ES/PNET diagnosed at Memorial Sloan-Kettering Cancer Center in which molecular evidence of a specific ES/PNET-associated translocation were available. Immunoreactivity to cytokeratin was present in 10 cases (20%), in five diffusely and five focally. There was no significant association between cytokeratin expression and the following parameters: patient age, sex, skeletal and extraskeletal primary site, and the type of EWS fusion transcript. Cytokeratin expression, a manifestation of epithelial differentiation, is present in as many as 20% of ES/PNET in either a diffuse or focal pattern.

Epithelial Phenotype in Ewing's Sarcoma/Primitive Neuroectodermal Tumor

Neural differentiation is an integral component of Ewing's sarcoma/primitive neuroectodermal tumor (PNET), which exhibits a continuous spectrum from minimal to prominent neural phenotype. Differentiation of Ewing's sarcomas/PNETs along other lineages or the expression of an epithelial phenotype is less common and-if present-may cause diagnostic difficulties. In this study we evaluated the frequency of epithelial differentiation in formalin-fixed and paraffin-embedded tissues of 33 (22 primary and 11 metastatic) Ewing's sarcomas/PNETs by using an immunohistochemical assay with several antikeratin antibodies. Focal positivity for low- or high-molecular-weight keratins was documented in 18% of the cases, and diffuse coexpression of low- and high-molecular-weight keratins was observed in two cases. Expression of the MIC-2 gene product was documented in 94% of the tumors. The primitive neural phenotype as revealed by expression of either neuron-specific enolase or synaptophysin was observed in 30% of the cases, but coexpression of both neural markers was present in only 15% of the tumors. This study documents that, in addition to primitive neural differentiation, Ewing's sarcomas/PNETs frequently exhibit focal positivity for keratins, with rare strong diffuse coexpression of both low- and high-molecular-weight keratins. The findings indicate that the expression of an epithelial phenotype, at least in a focal fashion, is a relatively frequent finding in otherwise typical Ewing's sarcomas/PNETs. Int J Surg Pathol 8(1):59-65, 2000

Relation of neurological marker expression and EWS gene fusion types in MIC2/CD99-positive tumors of the Ewing family

The Ewing family of tumors (EFT) is characterized by high MIC2/CD99 expression and specific EWS/ETS gene rearrangements, resulting in different chimeric transcripts. Further division into peripheral primitive neuroectodermal tumors and Ewing's sarcoma is still debated and, in the absence of distinct morphological parameters, has been based on the reactivity with neuroglial markers (NgM). We investigated 44 EFT in terms of a possible correlation between the type of EWS chimeric transcripts and reactivity with the following NgM: polyclonal and monoclonal neuron-specific enolase (NSE), S-100, chromogranin A, synaptophysin, Leu-7, glial fibrillary acid protein, and neurofilament. EWS/Fli1 fusion type 1 was detected in 30 of 44 and type 2 in 11 of 44 tumors. Three tumors, presenting with an uncommon morphology, carried rare chimeric transcripts. Our results indicate an association of lack of NgM staining with type 1 EWS/Fli1 translocations, found in 16 of 18 tumors with no NgM expression as detectable by any of the antibodies we applied. Using the monoclonal NSE antibody, 21 of 26 tumors without NgM staining expressed type 1 EWS/FLI1chimeric RNA, whereas in the groups with 1 or more and 2 or more NgM, only 9 of 17 and 1 of 5 tumors, respectively, carried type 1 EWS/Fli1 fusion transcripts. Despite this association of increased NgM expression with a non-type 1 EWS/Fli1 gene fusion, a strict correlation between the extent of NgM expression and certain EWS fusion types was not evident. This fortifies the concept to consider EFT as a spectrum of tumors and suggests the type of EWS fusion transcripts as one, but not the only parameter influencing the extent of differentiation.

Poorly differentiated synovial sarcoma: immunohistochemical distinction from primitive neuroectodermal tumors and high-grade malignant peripheral nerve sheath tumors

Synovial sarcoma is a relatively common sarcoma in adults, which in its classic bimorphic form infrequently poses a diagnostic problem. Monomorphic spindled variants, as well as the less common poorly differentiated variants, may be confused with other soft-tissue sarcomas; the poorly differentiated variant (PDSS), in particular, may be histologically indistinguishable from other small, blue, round cell tumors, including primitive neuroectodermal tumors (PNETs). Detection of the synovial sarcoma-associated t(X;18) by either cytogenetic or molecular genetic approaches may be necessary to confirm the diagnosis of synovial sarcoma in difficult cases. We evaluated 10 cases of PDSS from eight patients using a panel of antibodies (including those to intermediate filament proteins, nerve-sheath associated markers, and neuronal and neuroectodermal associated markers) in order to better establish the immunophenotype of this tumor and to help distinguish it from the tumors with which it may be confused, particularly PNETs and high-grade malignant peripheral nerve sheath tumors (MPNSTs). Our results showed PDSS to have significant immunophenotypic overlap with both PNETs and MPNSTs. In most instances these three entities may be differentiated by a panel of antibodies that should include those to both low and high molecular weight cytokeratins, epithelial membrane antigen, type IV collagen, CD99, CD56, and S-100 protein. Our results also suggest that synovial sarcoma may be a tumor showing combined neuroectodermal and nerve sheath differentiation--perhaps because of translocation-associated expression of specific proteins--rather than a carcinosarcoma of soft tissues or a tumor of specialized arthrogenous mesenchyme.

Ewing's sarcoma of bone: oncologic and functional results

This paper describes 29 patients with Ewing's sarcoma of bone treated between 1975 and 1990 at the University of Nijmegen Hospital, Nijmegen, The Netherlands. Osteomyelitis was the primary diagnosis in 24%. Treatment consisted of chemotherapy in combination with surgery and/or radiotherapy. Nine patients received radiotherapy only; five of them died of disease. Five patients underwent an intralesional excision; four of them died of disease. Twelve patients underwent a wide excision; there is no evidence of disease in any of them. Three patients underwent a radical disarticulation; all died of disease. The disease-free survival at 1.5 years was 66%. This figure at 5 years was 55%. After wide excision and reconstruction in tumors of expendable, femoral or radial bones good functional results were obtained in all cases.

Small cell osteosarcoma of bone: an immunohistochemical study with differential diagnostic considerations

Seventy-nine cases of small round cell tumors involving bone were studied in an attempt to learn whether the immunohistochemical features of the lesions might allow distinction of small cell osteosarcoma from other potential differential diagnostic considerations, including Ewing's sarcoma, atypical Ewing's sarcoma, primitive neuroectodermal tumor, mesenchymal chondrosarcoma, lymphoma, and the Askin tumor. The tissues studied were all formalin-fixed, decalcified, paraffin sections from patients between the ages of 16 and 48 years. With one exception (a small cell osteosarcoma), none of the lesions was cytokeratin positive. Moreover, none of the lesions was epithelial membrane antigen, desmin, factor VIII-related antigen, synaptophysin, or Leu-M1 positive. Accordingly, strong positivity for these antibodies in a majority of tumor cells should prompt inclusion of tumor types other than those listed above in the differential diagnosis. Vimentin positivity was seen in a majority of the tumors studied irrespective of histologic type. Scattered tumor cells (< 25%) showed positivity with antibodies to muscle-specific actin and smooth muscle actin in several of the different tumor types studied. No lesions other than lymphoma were leukocyte-common antigen (LCA) positive; all but two lymphomas were LCA positive, while all but one lymphoma were L26 positive. One (lymphoblastic) lymphoma was LCA and L26 negative. S-100, neuron-specific enolase, and Leu-7 did not prove to be specific for "neural-associated" tumors, but rather appeared in some small cell osteosarcomas, Ewing's sarcomas, atypical Ewing's sarcomas, primitive neuroectodermal tumors, mesenchymal chondrosarcomas, lymphomas, and Askin tumors. Antibody to cell surface antigen HBA71 was positive in three Ewing's sarcomas (two typical and one atypical) and negative in small cell osteosarcoma (three cases), mesenchymal chondrosarcoma (two cases), and lymphoma (one case). While some guidance may be derived from analysis of immunohistochemical staining patterns in a given lesion, the results reported in the present study do not suggest that routine immunohistochemistry alone will permit distinction of these small cell tumors of bone from one another. The value of immunohistochemical studies appears to lie particularly in the use of antibodies to LCA and S-100 protein to distinguish lymphoma and mesenchymal chondrosarcoma, and perhaps antibody to HBA71 to distinguish neural family lesions (such as Ewing's sarcoma), from other small cell tumors, such as small cell osteosarcoma.

The prognostic significance of the DNA content in Ewing's sarcoma: a retrospective cytophotometric and flow cytometric study

The DNA content of the cell nuclei of Ewing's sarcoma was analysed by means of cytophotometry in situ with image analysis in Feulgen-stained sections in 37 patients, and by retrospective flow cytometry according to the method of Hedley in 26 patients. Different histogram patterns were obtained: normal unimodal or bimodal DNA distributions and abnormal DNA distributions with one or two stem lines, or an abnormal DNA distribution with no stem lines. Both methods enabled us to make a distinction between two groups of Ewing's sarcomas with a different prognosis. All patients with aneuploid tumours died within 5 years after the initial diagnosis. Eleven of 19 (58%) patients with a normal DNA distribution in their tumour, as determined by cytophotometry, are still alive and in good health with a mean survival period of 7.5 years, ranging from 2 to 19 years. Of the group of patients in which flow cytometry revealed a normal DNA pattern, eight of 15 (53%) are still alive and in good health, with a mean survival period of 8 years. These results indicate that both techniques are reliable methods for obtaining prognostic information in Ewing's sarcomas. However, cytophotometry in situ yielded a better discrimination for the overall survival (P < 0.01) than did flow cytometry (P < 0.05). In 19% of the cases there was a discrepancy between the DNA histograms obtained with the two techniques. In five of 26 cases the DNA distributions were classified as normal by one method and aneuploid by the other. Tumour cell representation or selective loss of cells during enzymatic treatment may be responsible for this discrepancy.