Solitary fibrous tumor: WHO classification, histopathological correlation contribution to the Radiology-Pathology Conference, FDG PET-CT imaging, and biological behaviors

According to the 2002 World Health Organization (WHO) classification scheme for soft tissue tumors [1], the definition of solitary fibrous tumor (SFT) is a ubiquitous mesenchymal tumor of probable fibroblastic type that shows a prominent haemangiopericytoma-like branching vascular pattern. SFTs can be pleural or extrapleural and have been detected in different locations such as in the scalp, orbit, thoracic wall, mediastinum, pericardium, retroperitoneum, abdominal cavity, pelvis, meninges, spinal cord, periosteum, as well as organs such as the lungs, thyroid, salivary glands, liver, gastrointestinal tract, adrenals, prostate, spermatic cord, testes, urinary bladder, and others [2–4].Morphologically, pleural SFTs resemble extrapleural SFTs. Histopathologically, SFTs typically show a patternless architecture characterized by a combination of alternating hypocellular and hypercellular areas separated from each other by thick bands of hyalinized, somewhat keloidal, collagen and branching haemangiopericytoma-like vessels [1]. Mitoses are generally scarce, rarelyexceeding threemitosesper10high-powerfields inSFTs [1]. According to the definition by the 2002WHO classification,malignant SFT are usually hypercellular lesions, showing at least focallymoderate to marked cytological atypia, tumor necrosis, numerous mitoses (≥4 mitoses per 10 high-power fields) and/or infiltrative margins [1,4–6]. We would like to thank F Lococofor et al. for their interests and comments on our Clinical Imaging manuscript [7], and we appreciate the Editors of Clinical Imaging for giving us the opportunity to further discuss SFTs. In their comments, Lococofor et al. presented a case of malignant SFT with 0–2 mitotic figures per 10 high-power fields [8]. Similar as in the case we presented, the mitotic rate and immunocytochemical profile of the tumors [7,8] were similar. As the fluorodeoxyglucose (FDG) imaging appearances were also similar in each case, it suggested that the biological potential of the tumors were similar, and bothwould be expected to exhibit benign to low-grade biological behavior in spite of the difference in pathological diagnosis interpretation, which may vary in local circumstances and institutional experiences. The WHO tumor grading systems are created to identify risks for malignant tumor behavior, and adherence to these descriptions is routinely used to establish tumor risk. Tumormitotic rate is a very significant feature of these systems for biological aggressiveness prediction. In these two SFTs cases, the low FDG uptake, low mitotic rate, and the presence of tissue differentiation from the positive immunohistochemical tumor tissue markers all suggested benign to low-grade tumor behavior that would be benefitted with complete tumor resection. The teaching point to be gained from both imaging cases under discussion is that the FDG positron emission tomography-computed tomography imaging derived standard uptake value [9] indicates likely biological behavior of SFTs of the pleura. Thank you very much again for your interests, attentions, comments, and the opportunity for further discussion on this interesting topic.