2021 Update on Meningeal Solitary Fibrous Tumor

Meningeal Solitary Fibrous Tumor (SFT) is an uncommon tumor, accounting for less than 1% of CNS tumors. It is a fibroblastic neoplasm with a genomic inversion at the 12q13 locus, leading to NAB2-STAT6 gene fusion and surrogate nuclear STAT6 immunohistochemical expression. SFT generally affects adults in the fifth to seventh decades. It is dural-based and typically supratentorial, but 10% have a spinal location. SFT has a high propensity for recurrence and metstasis, at times occuring decades after initial diagnosis. Imaging often prompts a pre-operative assumption of meningioma as the diagnosis. Histologically, the tumor has a spindle cell appearance and variably cellular with abundant stromal keloid-type collagen. Some examples are very cellular tumors with densely packed round-to ovoid cells and little intervening stroma. Mitoses and necrosis can be present. Can a true SFT be STAT6 negative on immunohsitochemical examination? Yes, but it is just as likely that the pathologist is looking at something else on the differential diagnosis, including other unusual mesechymal tumors. In such cases, molecular testing for STAT6 fusion is required to render a confident diagnosis. Grading of SFT has changes since the last iteration of the WHO Classification in 2016. The 2016 criteria depended on the histologic hemangiopericytoma phenotype as a factor in raising this tumor from a grade I up to a grade II tumor. Now, in the soon-to-be-released 2021 WHO Classification, this phenotype is not considered important -- with emphasis instead on mitotic count as a means of creating grading cut-offs. Specifically, SFTs with fewer than 5 mitoses per ten high-power fields are considered grade 1, while a mitotic count of 5 or greater justifies a grade 2 designation. If, in addition to this elevated mitotic count, necrosis is present, then a grade 3 designation is assigned. There have been reported some rare pattern in SFT, including lipomatous, pappillary, giant cell, and myxoid; but these do not affect prognosis. If STAT6 is negative, the pathologist shoud think about other entities in the differential diagnosis, including fibrous meningioma, a variety of mesenchymal tumors (such as phosphaturic mesenchymal tumor), malignant peripheral nerve sheath tumor, and primary non-pigmented melanocytic tumors of the CNS. To reiterate, three grade of SFT will be recognized in the 2021 WHO classification of CNS tumors: Grade 1 - SFT with less than five mitotic figures per ten high power fields; Grade 2 - SFT with five or greater mitotic figures per ten high power fields; Grade 3 - SFT with five or greater mitotic figures per ten high power fields and necrosis. This information is taken from an excellent lecture on this SFTs delivered by Dr. Caterina Giannini at this year's annual USCAP meeting (see photo of her below delivering her lecture remotely). Thanks Dr. Gianinni for distilling this topic down to a concoction suitable for us blue-collar neuropathologists to imbibe!