Discussion of case

Solitary Fibrous Tumor (SFT) is a soft tissue neoplasm of fibroblastic origin and uncertain clinical behavior. Originally described as a pleural tumor, it has by now been shown to occur at any site of the body, most commonly intrathoracic and intraabdominal, including retroperitoneum and pelvis ¹. Rarely SFT even arises in the prostate ². SFT may occur at any age of adulthood but typically, patients are in their 5-6th decade ¹. Tumors mostly present as well demarcated lesions and show a characteristic so called “patternless pattern”. Herein, hypo- and hypercellular areas are found. Often tumor cells are separated by thick bundles of collagen. Typically, staghorn like branching vessels can be found and serve as a diagnostic clue. The fat forming type of SFT, which is uncommon, includes a variably prominent component of single to large numbers of mature adipocytes. Tumor cells show a spindled to ovoid morphology with ovoid nuclei ³. On the molecular level, SFT is characterized by a NAB2::STAT6 fusion, which leads to overexpression of STAT6 protein, that can be detected by immunohistochemistry⁴. Features that have been associated with potential aggressive behavior which occurs in approximately 10% of cases are large tumor diameter of more than 100 mm and increased number of mitotic figures (> 4/10 HPF) ³. In addition, higher patient age, high cellularity and nuclear pleomorphism have been associated with a higher risk of relapse. SFT is treated by excision with adjuvant radiotherapy in cases of a malignant SFT ⁵.

Key differential diagnosis

• Well-/De-differentiated liposarcoma is a differential diagnosis especially for the fat forming type of SFT and location in retroperitoneal space; nuclear atypia is more prominent; MDM2 gene amplification is typically found; due to close proximity of STAT6 and MDM2 on chromosome 12q well- and dedifferentiated liposarcoma may show expression of Stat6 upon immunohistochemistry, which is a potential pitfall
• Synovial sarcoma is a spindle cell neoplasm with variable epithelial differentiation which arises most commonly in the extremities but may be found elsewhere. Tumors consist of bland spindle and occasional epithelioid cells sometimes with admixed ropy and wiry or hyalinised collagen, a staghorn like vasculature is occasionally present; molecularly, tumors harbour a characteristic SS18::SSX gene fusion and CD34 and STAT6 are negative
• Extramammary myofibroblastoma, a tumor more common in men, may be found in the inguinal region, shows prominent hyalinised collagen bundles and positivity for CD34, however, STAT6 is negative
• Myopericytoma typically arises in subcutaneous tissue but rarely occurs in deep soft tissue; the variably cellular neoplasm shows a multi-layered perivascular growth sometimes presenting with staghorn like vasculature, focal positivity for CD34 but not STAT6 is seen.

References

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2. Guner, G. et al. The utility of STAT6 and ALDH1 expression in the differential diagnosis of solitary fibrous tumor versus prostate-specific stromal neoplasms. Human pathology 54, 184–188; 10.1016/j.humpath.2016.03.011 (2016).
3. Fletcher, C. D. M. (ed.). WHO classification of tumours of soft tissue and bone. Reflects the views of a working group that convened for a consensus and editorial meeting at the University of Zurich, Switzerland, 18-20 april 2012. 4th ed. (Internat. Agency for Research on Cancer, Lyon, 2013).
4. Doyle, L. A., Vivero, M., Fletcher, C. D., Mertens, F. & Hornick, J. L. Nuclear expression of STAT6 distinguishes solitary fibrous tumor from histologic mimics. Modern pathology : an official journal of the United States and Canadian Academy of Pathology, Inc 27, 390–395; 10.1038/modpathol.2013.164 (2014).
5. Bishop, A. J. et al. Soft Tissue Solitary Fibrous Tumor: Combined Surgery and Radiation Therapy Results in Excellent Local Control. American journal of clinical oncology 41, 81–85; 10.1097/COC.0000000000000218 (2018).