synovial sarcoma is a clinically rare neoplasm that accounts for ~10% of all malignant soft tissue tumors . synovial sarcoma most commonly arises in the soft tissues of the extremities in young and middle - aged adults ( 1 ) . this tumor may also arise in other parts of the body , but primary omental synovial sarcoma is extremely rare . to the best of our knowledge , only 3 cases of primary omental synovial sarcoma ( 2 cases in women and 1 case in a man ) have been documented in the literature to date , and they were all associated with an unfavorable clinical course ( 24 ) . surgical resection and adjuvant chemotherapy involving doxorubicin - ifosfamide are the preferred first - line treatments for synovial sarcoma , whereas pazopanib ( a tyrosine kinase inhibitor ) and trabectedin ( an anticancer alkaloid agent ) ( 5,6 ) have recently been recommended as treatments for patients with advanced disease , although the optimal therapeutic strategy for primary omental synovial sarcoma has not been clearly determined due to the rarity and severity of the disease . large complex abdominal masses that are incidentally discovered in postmenopausal women are often initially suspected to be pelvic malignancies . the lack of characteristic symptoms and specific imaging characteristics may lead to the preoperative misdiagnosis of omental synovial sarcoma as a gynecological malignancy , particularly ovarian cancer . a specific chromosomal aberration , t(x;18)(p11.2;q11.2 ) , and its product , the syt - ssx fusion protein , are identified in > 90% of cases of synovial sarcoma ( 7 ) , but the fusion gene may only be detected postoperatively based on detailed pathological and immunohistochemical examinations of the tumor . we herein report a rare case of aggressive primary omental synovial sarcoma mimicking ovarian cancer that was treated with multiple surgical resection procedures and adjuvant chemotherapy involving doxorubicin - ifosfamide , pazopanib and trabectedin . a 51-year - old multigravida woman who experienced abdominal distension for 4 months was referred to the department of obstetrics and gynecology of the hashimoto municipal hospital ( hashimoto , japan ) in april 2014 . the patient had no history of lower abdominal or pelvic discomfort , pelvic surgery , or other relevant medical conditions . transvaginal and transabdominal ultrasound examinations revealed a large solid abdominal mass , which exhibited iso - echogenicity . magnetic resonance imaging ( mri ) revealed a large heterogeneous mass with an irregular component occupying the lower abdominal cavity , with an intact uterus . based on these radiological findings , the mass was suspected to be an ovarian malignancy ( fig . 1 ) . the results of a laboratory analysis of the patient 's peripheral blood revealed normal tumor marker levels [ cancer antigen ( ca)125 , ca19 - 9 and carcinoembryonic antigen ] and an elevated lactate dehydrogenase level ( 291 the resected mass was 12.510.07.2 cm in size and weighed 502 g. the consistency of the mass was elastic - hard and partially soft , and its cut surface was whitish - yellow and contained a cystic and necrotic component measuring 2 cm in diameter . microscopically , the tumor included a well - circumscribed region composed of hyaline - fibrous and mucinous - myxomatous components . the hyaline - fibrous region exhibited diffuse proliferation of spindle - shaped and small round tumor cells , with capillaries arranged in a spider 's web - like pattern ( fig . the mucinous - myxomatous region was composed of highly cellular tissue , which displayed a high nuclear : cytoplasmic ratio and mitotic figures . immunohistochemistry revealed positive staining of the tumor cells for b - cell lymphoma ( bcl)-2 , and partially positive staining for cd99 and epithelial membrane antigen ( ema ) . the tumor cells were negative for s100 , -smooth muscle actin ( sma ) , desmin , cd34 , cd31 , cytokeratin ( ck)7 and ck ae1/3 . the ki-67 labeling index was 4060% . immunohistochemistry helped to exclude leiomyosarcoma , malignant schwannoma , gastrointestinal stromal tumor and solitary fibrous tumor . although a qualitative analysis of mrna expression based on a reverse transcription polymerase chain reaction ( rt - pcr ) assay was performed using an rna sample extracted from formalin - fixed paraffin - embedded tumor tissue , the syt - ssx fusion gene transcript was not detected . the diagnosis of primary omental poorly differentiated synovial sarcoma was confirmed based on the pathological findings . the patient 's postoperative course was uneventful , and she was discharged from the hospital on postoperative day 7 . the patient was treated with a combination of doxorubicin and ifosfamide as adjuvant chemotherapy . despite normal findings on physical examination , 7 months after the operation a metastatic tumor was detected in the liver by positron emission tomography - computed tomography ( pet - ct ) , and laparoscopic partial liver resection was performed , followed by adjuvant pazopanib . twelve months after the operation , another metastatic tumor was detected in the liver by pet - ct , and the patient was again treated with laparoscopic partial liver resection followed by adjuvant pazopanib . at 14 months after the first operation , pet - ct revealed multiple recurrent metastatic tumors in the liver , lungs and pleura . although the patient was treated with partial lung resection , aggressive metastatic tumors were detected in the liver , lungs and abdominal cavity at 2 months after the first operation . due to the difficulty of surgical resection , the patient was treated with trabectedin , and she remained alive with stable disease at 24 months after the first operation . the patient provided written informed consent regarding the publication of the case details and associated images . synovial sarcoma does not necessarily arise from synovial or soft tissue and has been reported to develop at other sites , such as the kidneys , lungs and pleura . synovial sarcoma most commonly develops in the extremities , particularly in the knee , but primary intra - abdominal synovial sarcoma has rarely been reported , with only 8 cases published in the literature to date ( 2 ) . only 3 cases of primary omental synovial sarcoma have been reported in the english literature since 1945 , according to a search through the pubmed and medline databases ( 24 ) : 2 of the cases involved women , aged 16 and 37 years ( 2,3 ) , and the remaining case involved a man , aged 66 years ( 4 ) . systemic symptoms , such as abdominal pain , fullness , tenesmus and vomiting , occurred in these cases , and the tumors ranged in size from 9.5 to 20 cm . none of these cases were diagnosed as omental synovial sarcoma preoperatively based on radiological examinations ; instead , they were diagnosed postoperatively based on detailed pathological and immunohistochemical examinations of the tumor . the diagnosis of primary omental synovial sarcoma is difficult in female patients , as surgery is required for a definitive diagnosis . in a previous study , the results of imaging analysis were non - specific ( 8) : on mri , t1-weighted imaging ( wi ) revealed areas of isointensity or hyperintensity relative to muscle in the tumor masses , and t2wi frequently showed heterogeneous signal intensity , including areas of triple signal intensity involving regions of high signal intensity that appeared fluid , isointense , or hyperintense relative to fat , and hypointense relative to fibrous tissue . in a case involving a postmenopausal female , a malignant tumor ( ovarian cancer ) was initially suspected due to the detection of a large abdominal mass and signs of necrosis . histologically , poorly differentiated synovial sarcoma is formed from sheets of undifferentiated round cells with hyperchromatic nuclei , which exhibit frequent mitoses , whereas biphasic synovial sarcoma is composed of a combination of epithelial and spindle cell components , and monophasic synovial sarcoma is diagnosed when only an epithelial or a spindle cell component is observed ( 1 ) . poorly differentiated synovial sarcoma is usually positive for bcl-2 and cd99 , and focally positive for ema , but it is negative for s-100 , -sma , desmin , cd34 and cd31 . poorly differentiated synovial sarcoma differs from biphasic or monophasic synovial sarcoma in that it is also negative for ck7 and ck ae1/3 , which emphasizes its poorly differentiated status and the non - epithelial nature of the tumor cells . the syt - ssx fusion gene , which is produced by the chromosomal translocation t(x;18)(p11.2;q11.2 ) , is found in ~90% of synovial sarcomas during fluorescence in situ hybridization or rt - pcr analysis of the gene transcript ( 7 ) . although previous studies have detected the syt - ssx fusion gene in omental synovial sarcoma ( 2,3 ) , its true frequency in omental poorly differentiated synovial sarcoma remains unknown . even if molecular testing for the syt - ssx fusion gene produces a negative result , the diagnosis of synovial sarcoma may be confirmed based on immunohistochemical findings . while the product of the syt - ssx fusion gene was not detected in the present case , immunohistochemistry detected positivity for bcl-2 , focal positivity for cd99 and ema , and negativity for s100 , -sma , desmin , cd34 , cd31 , ck7 and ck ae1/3 . the most appropriate clinical management strategy for primary omental synovial sarcoma may not be clear , as our experience with such cases is limited . according to the 3 previous reports on this tumor ( 24 ) , omental synovial sarcoma appears to have a poor prognosis , although synovial sarcomas characteristically progress slowly . the primary treatment for this disease is surgery , but in all 3 reported cases of omental synovial sarcoma , multiple aggressive metastases developed following complete resection . despite receiving combined treatment with surgery and chemotherapy , the patients succumbed to the disease within 2 months , 13 months and 3 years , respectively . patients who present with local recurrence , distant metastasis , incomplete resection , positive margins , lymph node involvement , or large bulky tumors , should undergo adjuvant chemotherapy involving doxorubicin - ifosfamide ( 5,6 ) . some molecular - targeted drugs , including pazopanib , a selective multi - targeted receptor tyrosine kinase inhibitor , and trabectedin , an anticancer alkaloid agent , have been recommended as second - line chemotherapies in cases of poorly differentiated synovial sarcoma in which combined treatment with doxorubicin and ifosfamide fails to control metastasis , as they block tumor growth and inhibit angiogenesis ( 5,6 ) . in the present case , the tumor progressed rapidly , with multiple metastases and locally recurrent lesions developing during the short follow - up period , which were treated with aggressive surgical resection and adjuvant chemotherapy . the rapid tumor progression observed in this case was consistent with the previous 3 reports . although treatment with pazopanib and trabectedin was well - tolerated , it was ultimately unsuccessful . to improve the treatment of omental synovial sarcoma , , we encountered a rare case of primary omental synovial sarcoma , in which aggressive metastasis developed and proved difficult to control with a combination of surgical and chemotherapeutic interventions , including molecular - targeted drugs . gynecologists should be aware of the aggressive metastatic potential of this rare tumor and the importance of an intensive diagnostic approach . furthermore , a new therapeutic strategy for primary omental synovial sarcoma is required to effectively control this disease .
synovial sarcoma is clinically rare , and cases of synovial sarcoma arising in the omentum are particularly rare . 
 only 3 cases have been reported in the literature to dtae , and they were associated with a poor prognosis . 
 we herein report a rare case of aggressive primary omental synovial sarcoma presenting as an ovarian malignancy . 
 a 53-year - old multigravida woman was referred to our hospital due to progressive abdominal distension . 
 magnetic resonance imaging revealed a large heterogeneous mass with an irregular component occupying the lower abdominal cavity , with an intact uterus . 
 intraoperative examination revealed a solid mass arising from the lower omentum . 
 the diagnosis of omental synovial sarcoma was established based on postoperative pathological and immunohistochemical examination of the tumor . 
 the patient underwent multiple surgical resection procedures due to the development of metastases in the liver , lungs and abdominal cavity and received adjuvant chemotherapy including doxorubicin - ifosfamide , pazopanib and trabectedin . 
 such rare neoplasms may be difficult to diagnose preoperatively based on radiological examinations alone . 
 thus , gynecologists should be aware of the possibility of omental synovial sarcoma , and combined surgical and chemotherapeutic interventions are required to control such aggressive tumors .