case_images: PMC10716272_01
case_images: T2w axial (A, D) and T1w axial (E) and coronal (C, F) MRI images show a vascularized solid-cystic edematous lesion in the right cerebellar hemisphere, in continuity with the right transverse sinus [(C), arrow]. Diffusion restriction is present due to the high cellularity of the neoplastic tissue (B). Sinus thrombosis is also present [(E), arrow].
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case_images: Computed tomography (CT) and magnetic resonance imaging (MRI) revealed a mass on the right cerebellum that involved the transverse venous sinus ( Figure 1 ).
case_images: T2 (A, B) and Gd T1 TSE (C, D) coronal images show disease recurrence with a solid-cystic, edematous lesion in the right temporo-occipital and right cerebellar regions with involvement of the right transverse sinus [(A, C), arrows]; gliotic-malacic findings coexist in the right hemicerebellar region [(B, D), arrows].
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case_images: After 18 months from the suspension of treatment, a cerebral MRI revealed a local relapse at the surgical site ( Figure 2 ).
case_images: Axial lung CT scans reveal: (A) a nodular lesion in the upper lingular segment, indicated by the arrow. Image (B) shows a reduction in size at the 14-month follow-up after initiation of immunotherapy. Image (C) shows a significant increase in size at the 6-month follow-up after discontinuation of immunotherapy, characterized by spiculated margins.
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case_images: During the diagnostic workup, two metastatic lung lesions (the larger in the upper lingula segment) were discovered with suspicion of a lesion in the right iliac bone ( Figures 3 ,  4 ). After six months of discontinuing therapy, CT and a positron emission tomography (PET) scan revealed a relapse in the lung lesion and in the right iliac bone lesion ( Figures 3A ,  4 ).
case_images: Iliac bone CT scans highlight: (A) a lesion with lytic features located in the right iliac wing and sacral wing (indicated by the arrow) at the onset of bone metastasis. In images (B, C), there is a progressive increase in the extension observed in the right paravertebral soft tissue (indicated by the arrows), as seen at the time of discontinuation of nivolumab. Image (D) depicts the nearly complete resolution observed at the last follow-up, which occurred 8 years after disease onset.
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case_images: During the diagnostic workup, two metastatic lung lesions (the larger in the upper lingula segment) were discovered with suspicion of a lesion in the right iliac bone ( Figures 3 ,  4 ). The patient achieved complete remission in all sites ( Figure 4D ), which was confirmed by a biopsy of the lesion in the right iliac bone, which showed only inflammatory tissue without evidence of neoplastic infiltration. After six months of discontinuing therapy, CT and a positron emission tomography (PET) scan revealed a relapse in the lung lesion and in the right iliac bone lesion ( Figures 3A ,  4 ).
case_images: Both primary (cerebral) and metastatic (bone and lung) tumors consisted of solid sheets of epithelioid cells with clear to eosinophilic cytoplasm. (A, B) show the metastatic site, iliac bone); an abundant lymphocytic infiltrate was present mainly at the periphery of the tumors. (B) Mitoses were brisk (arrow). (C, D) The primary tumor (cerebral) showed a focal expression of synaptophysin (C) and GFAP (D). (E) in the bone metastatic neoplasm, a membranous staining for PD-L1 was seen in 30% of the neoplastic cells. (F) PD-1 was expressed in 40% of the lymphocytes.
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case_images: The iliac bone biopsy confirmed the diagnosis of HGG, with 40% PD-1 expression on lymphocytes and 30% PD-L1 expression on neoplastic cells ( Figure 5 ). Morphological features were similar in the primary and metastatic lesions: the tumor consisted of sheets of epithelioid cells with abundant eosinophilic to clear cytoplasm, large nuclei, and prominent nucleoli and showed a marked lymphoplasmacytic infiltrate at the periphery ( Figures 5A, B ). Although myxoid stroma and blood-filled cystic spaces were not prominent features, the expression of CD99 (membranous), GLUT-1 with a prominent paranuclear/Golgi pattern, focal EMA, and focal GFAP and synaptophysin raised a suspicion of an unusual mesenchymal tumor ( Figure 5 ).
case_images: Analysis of the Archer  FusionPlex Custom Panel-anchored multiplex PCR result showing an EWSR1 exon 13 and CREM exon 5 gene fusion with reads (#/%) of 1493/22.48.
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case_images: A next-generation sequencing (NGS) panel (Archer Custom Fusion Plex Kit, Integrated DNA Technologies, IA) identified the presence of the EWSR1::CREM fusion transcript on both the primary tumor and the metastasis, confirming the diagnosis of metastatic intracranial mesenchymal tumor FET::CREB fusion-positive. ( Figure 6 ).
case_images: Complete timeline, including pathways to reach the diagnosis (surgery) and treatment (red), different histologies (blue), and response to therapy (green).
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