Because of variable clinical presentations and histologic appearances, the diagnosis of SFTs can be a challenge, as we observed in our case, when the result of an extemporaneous examination is not confirmed by the results obtained in paraffin-embedded tissues and immunohistochemistry. SFTs can be misdiagnosed as other mesenchymal lesions (haemangiopericytomas, giant cell angiofibromas, monophasic synovial sarcomas or fibrous histiocytomas), smooth muscle tumours (leiomyomas and leiomyosarcomas), neural tumours (gliomas, fibrous meningiomas, schwannomas, or neurofibromas), etc. In the past, some SFTs have probably been misdiagnosed as other spindle cell neoplasms. An extensive histopathological analysis of 41 fibroblastic lesions of the orbit that had different diagnoses, including haemangiopericytomas, fibrous histiocytoma, mixed tumours and giant cell angiofibroma, led to their reclassification as SFTs [
11].
Histopathological and immunohistochemical staining features and, more recently, molecular analyses are the basic tools used to diagnose SFTs and also have prognostic value. Additionally, fine needle aspiration has been mentioned as useful for diagnosing SFT [
12]. Similar to other locations, SFTs occurring in the lacrimal fossa can histologically mimic haemangiopericytoma [
13]. The histological differentiation of these two types of lesions is difficult because cases of haemangiopericytoma can show features of SFT and vice versa [
14‐
16]. Many pathologists consider haemangiopericytoma and solitary fibrous tumour to represent a continuum of tumours [
17]. Solitary fibrous tumours and haemangiopericytomas overlap, even if they are not identical entities. For this reason, the WHO has created a combined term, SFT/haemangiopericytoma, to describe these lesions. According to the 2016 CNS WHO classification, which uses molecular parameters in addition to histology, the SFT/hemangiopericytoma classification sets three tumour grades: grade I (often diagnosed as SFT), grade II (corresponding typically to haemangiopericytoma) and grade III (previously termed anaplastic haemangiopericytoma) [
18]. The differential diagnosis of haemangiopericytoma is mandatory due to its malignant course and its rate of recurrence and metastasis, which contrast with the benign evolution of SFT. In our case, spindle cells with long processes (fibroblast-like cells) and minimal cytoplasm, small elongated nuclei and small visible nucleoli were observed to have a haphazard distribution. The stroma was collagenized, with thick bands of collagen interspersed between the tumour cells. Branching staghorn vasculature, similar to that seen in haemangiopericytoma, was found. Mature adipocytes and giant multinucleated stromal cells, which have been reported in other cases, were absent [
19]. The histological criteria for malignancy include tumours greater than 5 cm, cellular pleomorphism, infiltrative growth, necrosis, haemorrhage, nuclear atypia and high mitotic activity (> 4 mitoses/10 hpf) [
18]. In our case, no signs of malignancy were identified, there was no nuclear atypia, and mitotic figures were rare.