endometrial stromal sarcoma ( ess ) is a rare neoplasm comprising only 0.2% of all uterine malignancies and usually occurs as a primary tumor of the uterine corpus ( 1 ) . ess of extrauterine origin is a very rare type of neoplasm and arises in sites , such as the ovary , the pelvic cavity , mesentery , omentum , and serosal or intramural portion of the large intestine ( 2 - 4 ) . the tumor is usually classified into two subtypes , low - grade esss and undifferentiated sarcomas with the latter subtype carrying a very poor prognosis ( 5 , 6 ) . we describe a 60-yr - old woman with low - grade ess presented as prevesical mass mimicking urachal mass . her history was remarkable in that she had a total hysterectomy by lower median abdominal incision for endometriosis 11 yr ago . routine blood tests , blood chemistry and urine test revealed no abnormal findings . in the previous ultrasonographic examination , heterogeneous hyperechoic pelvic mass was incidentally detected , and she was transferred to our hospital on the impression of urachal tumor ( fig . magnetic resonance imaging of the abdomen and pelvis showed a mass located anterior to the bladder in the prevesical space of retzius and helped to establish the relation of the tumor to the bladder in the sagittal plane ( fig . complete excision with bladder sparing was successful by lower median abdominal incision . at surgery , two tumor masses were founded in the prevesical space of retzius and had not infiltrated the wall of the bladder and urachus . grossly , well - circumscribed yellowish tumors were 4.52.52 cm , 1.511 cm in size , respectively . the tumor consisted of relatively large cells resembling those of the endometrial stroma , which encircled small vessels ( fig . tumor cell nuclei was round to ovoid and a small amount of cytoplasm was present . mitotic activity was less than 10 mitotic figures per 10 high power field ( hpf ) ( fig . most of the cells were also negative for estrogen receptor , and positive for progesterone receptor ( data not shown ) . she was free of symptoms at the last follow - up , 15 months after the abdominal surgery , with no evidence of recurrence . ess usually occurs as a primary tumor of the uterine corpus and is rare neoplasm . however , there have been some reports in which ess is considered to be associated with endometriosis ( 2 - 4 ) . it arises in area of pre - existing endometriosis and also arises in cases that are not associated with endometriosis . histologically , endometrial adenocarcinoma is the most ( about 70% ) , and sarcoma is the second common malignancy ( 12% ) ( 7 ) . ( 7 ) reported that malignant tumor arising in endometriosis involved ovary ( 78.7% ) and extragonadal site ( 21.3% ) . majority of extragonadal sites were rectovaginal septum ( 4.3% ) , colon ( 4.3% ) , vagina ( 2% ) , and pelvic peritoneum ( 5.7% ) . the other theory is a de novo tumor , potentially derived from submesothelial pluripotential mullerian cells . the pluripotential mullerian epithelium is considered to be widely distributed in the abdominal and pelvic cavities ( 4 ) . in the present case , the patient had undergone total hysterectomy for endometriosis 11 yr ago , and the tumor was localized in the prevesical space . so , we assumed that it arose from prevesical endometriosis because the patient had a history of endometriosis but there is no direct evidence . previously , the tumor is usually classified into two categories , low and high - grade , based on differences in mitotic activity , with the latter subtype carrying a very poor prognosis ( 5 ) . recently , the world health organization classification for tumors of the uterus classifies the tumors based primarily on tumor margin status and cytologic features ( 6 ) . low - grade esss are clinically indolent malignancies with minimal cytologic atypia and proliferative activity . undifferentiated sarcomas ( previously categorized as high - grade endometrial stromal sarcomas ) are highly aggressive malignancies that show substantial cytologic atypia and high mitotic activity . young et al . ( 3 ) reported that the mitotic rate of the extrauterine ess correlated with the prognosis but chang et al . ( 2 ) reported that neither mitotic index nor cytologic atypia in primary extrauterine ess were predictive of tumor recurrence or death from tumor . in their series , 77% of patients whose tumors had a mitotic index of less than 10 per hpf had one or more recurrences and 30% died from their neoplasm . the behavior of the primary extrauterine ess was more reminiscent of high - grade primary uterine ess than low - grade primary uterine ess ( 2 ) . therefore , close observation and extended follow - up will be required in extrauterine ess , regardless of mitotic activity . low - grade ess has slow growth and indolent course with a tendency for late recurrence ; approximately 50% of cases of low - grade ess develop recurrent disease ( 6 ) . a relatively long - term survival , despite tumor dissemination , has been reported in low - grade ess cases . both recurrent and metastatic lowgrade sarcomas can be successfully treated with surgical excision , radiation , progestin therapy , or a combination therapy ( 6 ) . considering highly recurrent nature of extrauterine ess or low - grade ess , tumor localized in the prevesical space is very rare but urachal tumor has been most commonly presented in prevesical space . however , when patient with endometriosis presented with prevesical mass in retzius space , endometriosis and its possible malignant changes should be taken into account in 
 differential diagnosis .
endometrial stromal sarcoma ( ess ) is a mesenchymal neoplasm that usually occurs as a primary tumor of the uterine corpus , but rarely arises in other sites , such as the ovary , pelvic cavity , mesentery , omentum and intestine . 
 herein , we present a rare case of low - grade ess presented as prevesical mass . 
 a 60-yr - old woman who had undergone total hysterectomy for endometriosis eleven years ago was presented with incidentally detected prevesical pelvic mass . since malignant transformation of urachal remnants was possible , the mass was suspected to be a urachal tumor . 
 extraction of the mass was performed , and the histopathologic diagnosis was low - grade ess . in summary , 
 prevesical tumor is rare but in patients with endometriosis , we suggest endometriosis and its possible malignant changes should be taken into account in the differential diagnosis of prevesical mass .