Extraskeletal osteosarcoma (ESOS), also known as soft tissue osteosarcoma, is a rare malignant neoplasm that produces osteoid, bone or chondroid material with no involvement of bone or periosteum. Although the first report described in 1941[1], few cases have been reported in the literature so far. [2] ESOS are most frequently found in the lower extremity, particularly in the deep soft tissue of the thigh (42%-77%), followed by the upper extremity (12%). It has also been reported that have been found in the retroperitoneum (12%). Other relatively rare sites such as the larynx, kidney, esophagus, small intestine, liver, heart, urinary bladder, parotid and breast have been reported before.[2, 4] In contrast to skeletal osteosarcoma, which always occurs in patients in the first 3 decades of life, the majority of ESOS occurs in the fifth and seventh decades of life and at a mean reported age of 60 years.[6, 8] Males are slightly more than females, with a ratio of 1.9:1.[7, 9] Controversially, there were also statistics showed the male predominance seen in primary osteosarcoma was not found among patients with ESOS. [2]Through the etiology of the ESOS is difficult to pin down precisely, some reports revealed it could be related to radiation, such as previous exposure to X-rays and radioactive thorium dioxide,or at least 4 years after receiving high-dose radiation therapy.[10] In addition, some reports revealed 12–30% of patients have been experienced trauma, and some cases could occur after ossifying myositis.[11, 12] However in our case, the patient had no history of trauma and radiation. Symptoms mainly included a painful or painless mass that grew slowly and progressively of ESOS in the abdominal cavity. The mass was large generally when the patient sought treatment. If the mass invades the bowel, changes in defecation characteristics may occur, including constipation and blood-tinged stool. It is visible on ultrasonic, CT, and MRI as a large soft-tissue mass with a lack of osseous involvement.[3, 7, 11, 13, 14] The ESOS is not specific on imaging, the radiological features described in some cases is a calcified mass on CT, but is a solid-cystic mass in our case.[3, 7] On MRI, the lesion shows slightly hyperintense to muscle also nonspecifically on T1-weighted imaging and high signal intensity on T2-weighted imaging, which is not same with the results in our case.[15] The diagnosis of ESOS should be determined in combination with clinical manifestations, radiographical and pathological findings and only after excluding the possibility of primary bone tumor or bone tumor metastatic to soft tissue.[4] For atypical clinical and radiographic manifestations such as the patient in our case, pathology may be the final diagnostic criterion particularly for ESOS in the abdominal cavity. Pathological subtypes of ESOS can be divided into six types. One of the most common is the osteoblastic variant such as in our case with abundant osteoid, outside of that, followed by chondroblasts, fibroblasts/pleiomorphic malignant fibrous histiocytoma like, telangiectasis, small cell and mixed types.[4, 8] Surgery is the main treatment for ESOS. Depending on differences in location, range and development of the tumor, a simple resection, wide resection or radical resection could be selected. Besides, preoperative radiotherapy and adjuvant chemotherapy are also available as treatment options for ESOS. According to statistical reports that expanding the scope of surgery can reduce the local recurrence rate, but it has no special significance for prolonging the survival time.[9] According to the current situation, the regimens and effects of chemotherapy remain controversial for ESOS. Ahmad et al.[16] reported that 27 patients were treated with the doxorubicin-based chemotherapy with an effective rate of 19% in 60 ESOS patients. Wang et al.[8] reported that the majority of cases received chemotherapy regimens based on methotrexate, adriamycin, and cisplatin. A minority received adriamycin-based and ifosfamide-based therapy. However, survival benefits have not been demonstrated between different chemotherapy regimens or between the people who received chemotherapy and those who did not. Besides when patients are unable to accept the surgical treatment or tolerate high dose chemotherapy or advanced disease, radiotherapy could be chosen as palliative treatment. There have been reported that preoperative or postoperative radiotherapy was thought to be beneficial in reducing the volume of tumors and local recurrence, but no specific improvement in overall survival or progression-free survival, also no difference in death due to disease or event-free survival.[2, 8] For patients who cannot achieve negative surgical margin, radiotherapy is significant to improve overall survival.[8] Regardless of the origin and location of the tumor, the prognosis is poor of ESOS. EOSO has a high risk of local recurrence as well as distant metastasis. Combined with the results of multiple reports, the local recurrence rate was approximately 18%-19% and distant metastasis was 37%-38%.[2, 16] According to the reports, approximately 39% patients died of the disease at 3 years,[2] and approximately 75% die within 5 years of diagnosis.[13] Tumor size is an important factor affecting the prognosis, as the patients with the tumor larger than 5cm have worse clinical outcomes. Bane et al.[4] reported that the mortality rate associated with the disease for patients was about 14.3% (1 of 7 patients) with the tumor smaller than 5cm, but was 87.5% (14 of 16 patients) with the tumor larger than 5cm. Besides, positive margin after operating is also an important factor that affects overall survival and local recurrence. Tumors with positive margins have the higher risk of local recurrence and lower survival rate after 5 years. As for patients presenting with non-metastatic disease, the local control rate was about 89% at 5 years, which was no significant difference between positive and negative margins. At 10 years, local control rate with negative margins was no significant change, but reduced significantly with positive margins.[8] In the presented case, the patient was an elderly women with a large tumor(>5cm in size), negative margin and without adjuvant treatment. No systemic metastases were observed at 2 months postoperatively. It was also difficult for us to predict the prognosis of the patient.
Otherwise, we summarize the reports of ESOS of mesentery so far in English literature (Table 1). It includes the basic characteristics of the patient, the condition of the tumor, treatment and prognosis during initial diagnosis.[3, 7, 13, 14, 17–21] The average age of the 10 patients (5 males) was 57 years (range, 39 to 75 years). The tumor was larger than 10cm of 7 patients. All of them underwent surgery, only 3 accepted chemotherapy. By comparison, there was no significant improvement in prognosis. It remains a challenge to determine the optimal treatment for ESOS of the mesentery.