| Objective: To Study the clinicopathologic, immunohistochemical, ultrastructural features and the diagnostic significance of the expression of EWS-FLI1/ERG fusion transcripts resulting from t(11;22) for Extraskeletal Ewing sarcoma/peripheral primitive neuroectodermal tumors (ES/pPNETs).Methods:The Clinicopathologic , immunohistochemical and ultrastructural features of 55 cases of ES/pPNETs were analyzed by light, electron microscopy and immunohistochemistry assay. One-step reverse transcriptase-polymerase chain reaction(RT-PCR) was employed to detect the characteristic EWS-FLI1/ERG or other specific fusion transcripts in 30 cases of ES/pPNETs and 70 cases of other malignant tumors including 50 cases of small round-cell tumors (SRCTs) using formalin fixed, paraffin embedded tissues. The t(11;22) translocation was detected by Chromogen in situ hybridyzation(CISH) in 15 cases of ES/pPNETs and 4 cases of control tumors.Results: Clinically, 55 cases of ES/pPNETs aged from 17 months to 59 years (mean 21.9 years) with male 28 cases and female 22 cases were situated in limbs (52.7%, 29/55), thoracic region (14.5%, 8/55) and trunk (9.1%, 5/55), head-neck region (7.3%,4/55), 2 cases in retroperitonium, 2 cases in pelvic cavity, 1 case in groin and 1 testis, respectively. Grossly, tumors were nodular or lobular, of 25 cases, the tumors ranged in size from 2.0cm to 20.0cm in largest diameter(mean 7.0cm). Microscopically, undifferentiation small round cell arranged in variable pattern of diffuse growth(65.5%),lobular or nested(47.3%), alveolar(32.7%), organoid (10.9%)and papillary(16.4%). Over two patterns were presented simultaneously in about 80% cases. The tumor composed of relatively uniform small cell with Project supported by National Natural Science Foundation of China (NO:39860073 ) dense clumping of chromatin or fine chromatin network and with scanty cytoplasm. The cell with clear cytoplasm were seen in 21 cases(38.2%); PAS staining was positive in 22 cases(40.0%). Homer-Wright rostte were seen in 19 cases of pPNETs,Flevner-wintersteiner rostte in 9 cases (16.4%)of 55 ES/pPNETs, the necrosis presented in 38 cases(69.1%) of ES/pPNETs. The histologic distinction among the pPNET,Ewing Sarcoma and Askin tumor is not possible. Immunohistochemically, tumor cells were positive for CD99(85.5%), NSE(80.0%),Vim(63.6%),S-100(47.3%) and Syn(21.3%), respectively, diffuse or focal expression. Over two kinds of neural marker were expressed in 30 of 55(56.4%) ES/pPNETs. There was a focal and weak reactivity for CK in 3 (16.7%) cases of ES/pPNETs. The FLI1 protein expression was seen in 28 of 36 ( 77.8%) ES/pPNETs. Of the 69 non-ES/pPNETs cases FLI1 protein were expessed in 23 cases(33.3%), including 10 of 20 rabdomyosarcomas, 2 of 5 neuroblastomas, 4 of 20 synovial sarcomas, 1 of 5 malignant lymphomas, 1 myxiod liposarcoma, 1 granurlocytic sarcoma, 1 mesenchymal chondrosarcoma, 1 undifferentiated sarcoma and 1 leiomyosarcoma. Ultrastructurally, tumor cell demonstrated paucity of organelles, variable glycogen, occasional neurosecretory granules and cell junction-like structure. The total RNA were successfully extracted from 25 of 30 cases of ES/pPNETs using paraffin embedded tumor tissue. The expression of EWS-FLI1/ERG mRNA were detected in 20 cases of ES/pPNETs(80.0%), No other fusion transcripts were detected.Among the 70 cases of control cases, PAX3/PAX7-FKHR fusion transcripts were detected in 4 of 6 aveolar rabdomyosarcomas, the expression of SSX-SYT mRNA were detected in 14 of 16 (87.5%) synovial sarcomas, COL1A1-PDGFB mRNA were detected in 5 of 8(62.5%) Dermatofibrosarcoma protuberans and ASPL-TFE3 mRNA were detected in all 4 cases of alveolar soft part sarcomas. The t(11;22) were detected in 10 of 15 (66.7%) ES/pPNETs by CISH technique that consistent with the results of EWS-FLI1/ERG fusion transcripts detection. Conclusion: 1. ES/pPNETs occur primarily in children and young adults, most commonly located in limbs, trunk and thoracic region. Histologically, tumor are composed of relatively uniform small...
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