Malignant solitary fibrous tumor in the retroperitoneum is rare. We report a 67-year-old male with retroperitoneal malignant solitary fibrous tumor for more than 10 years, which recurred the second time accompanied by lymphatic invasion and lymph node metastasis. Computed tomography scanned multiple retroperitoneal masses and the tumor was widely implanted in the abdominal cavity. Histopathological examination showed that the tumor was mainly composed of short spindle cells, lining up sparse and dense areas. Mitotic figures were noted generally 6–8 per 10 high power fields (HPF) with local necrosis. The tumor invaded the circumferential liver, intestines, lymphatic vessels and lymph nodes. Combined with the immunohistochemical results, it was diagnosed as malignant solitary fibrous tumor, growing up again just two months after the latest surgery. Retroperitoneal malignant solitary fibrous tumors with repeated relapses, infiltrative growth and lymphatic metastasis suggest the need for careful and long follow-up.
Solitary fibrous tumor (SFT) is a mesenchymal neoplasm, most common in pleura. Malignant solitary fibrous tumors in the retroperitoneal space are rare. We report a case of retroperitoneal malignant solitary fibrous tumor of second recurrence with lymphatic metastasis.
A 67-year-old male was firstly checkd out malignant solitary fibrous tumor in retroperitoneum about 12 years ago. After surgical resection and adjuvant chemotherapy, the tumor recurred the second time with hemafecia. Computed tomography revealed multiple retroperitoneal masses and intestinal wall involvement (Fig. 1). Recently, tumor resection was performed in our hospital, and part of liver, intestine, mesentery and omentum were removed at the same time. Yet, many small planted lesions could not be cleared away. The total size of the resected masses was about 18×18×8cm, partially encapsulated with smooth surface. The cut section of the tumor showed lobulated white-brown areas (Fig. 2). Under the microscope, the short spindle-shaped tumor cells were arranged alternatively with hypocellular and hypercellular pattern separated by thick collagen fibers and blood vessels in the interstitium (Fig. 3). Those lesions of high cellular intensity presented obviously cytological atypia, increased mitoses counted about 6–8 per 10 HPF, and focal necrosis. The tumor encroached the surrounding liver, the whole layer of the intestinal wall and lymphatic vessels (Fig. 4). Two lymph nodes (2/18) were observed the same histological finding as that in the hypercellular area (Fig. 5). Also, multiple nodules were seen in the mesentery and omentum.
By immunohistochemical staining, tumor cells were diffusely positive for STAT6 (Fig. 6), CD34, CD99, and Bcl-2, focally and limitedly reactive for p16 and CDK4, and negative for S-100, MDM2, desmin, smooth muscle actin (SMA), myogenin, CD117, and DOG-1. P53 was wild-type expressed and Ki-67 index was about 20% positive. Thus, it was suggested retroperitoneal malignant solitary fibrous tumor.
Although most of the SFTs usually show clinical inertia, malignant solitary fibrous tumors can be primary or transformed/dedifferentiated at a high level in the process of recurrence or metastasis. This case was initially diagnosed as malignant solitary fibrous tumor.
The morphologic features of malignant solitary fibrous tumor may contain dense arrangement, evident atypia, increased mitotic figures, hemorrhage and necrosis, peripheral infiltration, recurrence or metastasis. Because the prognosis of SFTs was not well predicted by histological grading, Demicco et al. [1.2] based on the study on the metastasis and survival rate of non central nervous system SFTs to evaluate the risk of metastasis and death according to the age of onset, tumor size, mitotic count and necrosis, which greatly enhanced the prediction for prognosis. According to the method for risk stratification, Yuan et al.  explored 31 cases of retroperitoneal SFTs and revealed that patients in the high- or intermediate-risk group were prone to metastasis, requiring close follow-up and the Ki67 index ≥ 10% might be used as an important reference to judge the prognosis. In addition, considering the location of the tumor, there was a high risk of recurrence in the retroperitoneum , where metastasis could move into lung, liver or bone[3.5]. Ito et al  was the first time to report a case of primary retroperitoneal malignant solitary fibrous tumor with the paraaortic lymph node metastasis, although it belonged to non high-risk group. Only surgery was performed and the patient had remained free of recurrence for 2.5 years. Comparatively, our case was high-risk group, and the morphology of secondary recurrence became more dense and atypia than that of the previous. Furthermore, infiltrating liver and intestines, tumor thrombi in lymphatic vessels and multiple lymph node metastases might be tips of poor prognosis. Only two months after the latest operation, computed tomography examined out recurrence again during a shorter interval.
The diagnosis of SFT should unite morphology and immunophenotypic markers, as well as differentiating from especially other mesenchymal tumors with spindle-shape cells. Immunohistochemically, SFTs generally express CD34, Bcl-2 and CD99, rarely for S-100, MDM2, desmin, SMA, myogenin, CD117 and DOG-1. Also, Gria2 and ALDH1 could be used as new markers of SFTs. In particular, the results of STAT6 test are of high value for SFT diagnosis. Because liposarcomas can partially express STAT6, the MDM2/CDK4 status must be evaluated by immunohistochemistry and/or molecular detection to exclude liposarcomas. Therefore, in our case the combined use of these antibodies is helpful to distinguish SFT from myogenic and neurogenic tumors, gastrointestinal stromal tumors, synovial sarcomas and liposarcomas .
Nab2‑STAT6 gene is fused into the driving mutation gene of SFT, so molecular detection of Nab2‑STAT6 fusion gene has high sensitivity and specificity for the diagnosis of SFTs . Nonaka et al.  demonstrated for the first time that the down-regulation of Nab2‑STAT6 fusion gene at the transcriptional level was associated with malignant solitary fibrous tumor, which indicated that alert should be required for cases of loss of STAT6, whereas our case was STAT6 protein positive diffusely.
Besides the potential molecular mechanism of p53 mutation promoting SFT to malignancy [1.9.10], Ito et al.  also found Bcl-2 positive only in the hypocellular area, so Bcl-2 might be related to malignant transformation. However, of our patient p53 was wild-type and there was no significant regional difference in Bcl-2 expression.
The first choice for the treatment of retroperitoneal malignant solitary fibrous tumor is still surgery, but it is difficult to complete resection resulting in a very high recurrence rate. Up to now, there is no standardized treatment guidelines, no systematic and effective adjuvant radiotherapy, chemotherapy or targeted treatment. This case exhibit the course of disease can be as long as more than ten years, requiring careful follow-up, and multiple masses, infiltrative growth and lymphatic metastasis might indicate poor prognosis [11.12].
Lei Liu is responsible for the conception and design of the work, and all co-authors contributed to the acquisition and interpretation of the data for the work. Lei Liu drafted the work, and other coauthors revised it. All authors approved the final manuscript to be published and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
Availability of data and materials
All data generated or analyzed during this study are included in this published article.
Ethics approval and consent to participate
Consent to participate was obtained from the patient’s family on Apirl 1, 2022.
Consent for publication
Written informed consent for publication was obtained from the participant.
The authors declare that they have no competing interests.
1Department of Pathology, Peking University International Hospital, Beijing 102206, China