Ki-67 index increased from 15% to 70% in a recurrent intracranial solitary fibrous tumorRare Brain Tumor Turns Aggressive After Surgery

BackgroundIntracranial solitary fibrous tumor (SFT) is a rare mesenchymal neoplasm. Malignant progression of low-grade SFT typically occurs over an extended time course and in the context of TERT promoter mutations. Rapid progression with marked proliferative escalation following a previously reported TERT-wild-type primary tumor is exceptionally rare.Case presentationA 70-year-old male had undergone resection of an anterior skull base lesion at an outside hospital 12 months prior. The outside pathology report diagnosed CNS WHO grade 2 intracranial SFT, describing 2–3 mitotic figures/10 HPF with small foci of necrosis, STAT6 nuclear positivity, a Ki-67 index of approximately 15%, and wild-type TERT promoter status. At our institution, the patient presented with left-sided limb weakness and facial deviation. Preoperative imaging was interpreted as recurrent meningioma. Gross total resection (Simpson grade I) was performed. Histopathology of the recurrent tumor revealed a hypercellular spindle cell neoplasm with patternless architecture, staghorn vasculature, diffuse nuclear atypia, up to 10 mitotic figures/10 HPF, geographic necrosis, and a Ki-67 index of 70%. Diffuse nuclear STAT6 expression supported the diagnosis of recurrent SFT in the appropriate clinical and anatomical context. Notably, the original slides from the first surgery were not available for direct histopathological review at our institution; the primary tumor grading relies exclusively on the outside pathology report. Preoperative chest CT showed no evidence of thoracic metastasis; however, abdominopelvic staging was not performed.ConclusionThis case illustrates the potential for rapid clinical and proliferative deterioration of intracranial SFT following a previously reported TERT-wild-type primary tumor. The Ki-67 index serves as a core quantitative indicator of proliferative activity. Given the unavailability of original slides for independent review, the precise WHO grade of the primary tumor cannot be independently verified, which limits definitive conclusions regarding cross-grade progression. Dura-based SFT shares substantial imaging overlap with meningioma, necessitating STAT6 immunohistochemistry for accurate differential diagnosis. Individualized postoperative surveillance should be informed by validated risk stratification models.