Surgical exploration revealed a lobulated fleshy mass with solid and cystic areas and few regions of osseous consistency. The mass appeared to be arising from the psoas muscle tendon sheath. Debulking of the tumor was attempted and chemotherapy was initiated. Following gross and histopathologic examination, a diagnosis of synovial sarcoma arising from the tendon of the psoas muscle was made.
A CT scan performed at the 6-month follow-up revealed recurrent soft tissue masses in the postoperative tumor bed and in other sites within the abdomen (the left subphrenic region, the lesser sac, the left renal fossa, and the perisplenic region) (Figure 3). Contiguous invasion of adjacent organs (the left kidney, the pancreatic tail, and the spleen) was noted. Compared with previous scans, the tumor masses showed more heterogenous enhancement and confluent necrotic nonenhancing areas. No calcific foci were identified within the recurrent tumor deposits. Multiple subpleural cavitating metastatic nodules were also seen in posterobasal segments of the lungs bilaterally (Figure 4). The patient died within 3 months of the follow-up scans.
Computed tomography (CT) showed a large, heterogeneously enhancing, lobulated soft tissue mass. Lumps and specks of calcification and few central, poorly enhancing regions suggestive of necrosis were seen within the mass (Figure 2A). The tumor was inseparable from the iliopsoas muscle complex and was seen predominantly in the left iliac fossa and lumbar region (Figure 2B). Inferiorly and medially the rectosigmoid and descending colon were displaced medially by the tumor and superiorly the mass extended up to the lower pole of the left kidney. With these findings, the possibility of an extraperitoneal tumor likely originating from the iliopsoas muscle was proposed. No focal lesions were noted in the liver. There was no evidence of associated lymphadenopathy or extension in to the intervertebral foramen.
The differential diagnosis included primary calcifying extraperitoneal neoplasms like malignant fibrous histiocytoma, malignant mesenchymoma leiomyosarcoma, liposarcoma, osteosarcoma, and, rarely, hemangiopericytoma.
Historically, synovial sarcomas are thought to originate from primitive pluripotent mesenchyme that is capable of synovial differentiation. This theory is consistent with the origin of the malignancy from sites devoid of normal synovium, such as the retroperitoneum, pelvis, head and neck, mediastinum, and pleural cavity. However, these sites are infrequently reported. The tumor occurs most often near tendons or tendon sheaths and next to joint capsules, most commonly affecting the extremities in the vicinity of large joints. These malignancies typically occur in adolescents and young adults between 15 and 40 years of age. The reported 5-year survival rates of synovial sarcoma range from 25% to 51%.2
Synovial sarcomas in the abdominal wall tend to occur with a much greater frequency in females, which such tumors in the extremities or the neck tend to occur with a much greater frequency in males.3 Primary intra-abdominal synovial sarcoma is rare; there are only approximately 33 previously reported cases. Most of these cases are retroperitoneal and few have pelvic origin. Retroperitoneal synovial sarcomas represent approximately 1% of retroperitoneal tumors and remain confined to the abdomen. Synovial sarcomas elsewhere do not metastasize remotely. Conversely, tumors arising from the pelvis often develop remote metastasis to the lungs. This is supported by the findings in our patient, in whom the tumor originated in the pelvic sidewall and soon developed pulmonary secondary sites. This likely reflects the close relationship of pelvic tumors to skeletal structures.4
The most important factor in the recognition of synovial sarcoma on histopathologic examination is its biphasic pattern-ie, the presence of both epithelial and spindle cell types; hence, it is considered a carcinosarcoma. The monophasic synovial sarcomas consist of spindle cells only.
CT is valuable in detecting aggressive tumors, predicting resectability, detecting distant metastases, and evaluating response to treatment. Tumor masses appear large, irregular, and necrotic with heterogeneous contrast enhancement. MR imaging of synovial sarcomas reveals a mosaic of mixed low, intermediate, and high signal intensity on T2-weighted images as a result of the admixture of solid components, cystic degeneration, hemorrhage, myxoid stroma, and fibrous tissue (known as the "bowl-of-fruit" sign). This feature is also seen in other retroperitoneal masses, such as Ewing's sarcoma, and malignant fibrous histiocytoma, and is, therefore, nonspecific.5
Primary intra-abdominal synovial sarcoma is a rare aggressive tumor with high fatality rates. Synovial sarcoma most commonly affects the extremities, especially the lower thigh and knee region; other primary sites have been reported infrequently. We report a rare case of malignant intra-abdominal synovial sarcoma arising from the pelvic side-wall and review reported cases of intra-abdominal synovial sarcoma. These tumors should be included in the differential diagnosis of extraperitoneal masses in young adults and middle-aged patients.
Primary intra-abdominal synovial sarcoma. Appl Radiol.