Extraventricular neurocytoma

Axial non-enhanced CT shows mostly isodense lesion with hyperdense elements lesion situated in the left frontobasal region. The bone windows display multiple linear hyperdense lesionsintralesional calcifications, which appear as rings and arcs calcifications-and-arcs calcification. The lesion exhibitscauses marked bone resorption evidencedevident at the orbitaryorbital portion of the frontal bone and the sphenoid bone's greater wing of left sphenoid.

The heterogeneous left frontobasal lesion presentsdisplays both cystic and solid intraaxial segmentsintralesional components with hyperintense elements on T1, T2, and fat-saturated sequences, resulting from subacute blood components or dense proteinaceous fluid. Some blooming artifacts on the GRE sequence probably correspond to calcifications without ruling out the possibility of intralesional hemorrhages. The contrast enhancement is starkvivid and heterogeneous, with some nodular enhancement along the lesion's inner border.

Gross pathology image shows the macroscopic appearance of the lesion (black arrows). 

Hematoxyline-EosinHematoxylin-eosin (H&E) stains show multiple small round blue cells within a neuropil-like matrix, forming rosettesrosette aggregates. Some calcifications can be observed at the superior and inferior right quadrants of the second picture (big irregular violet structures). There are also small disperse vessels. In the third picture, a few hemosiderin deposits can be seen at the superior left quadrant (brown small irregular forms). No chondroid matrix could be appreciated. The fourth picture shows diffuse and intense Synaptophysinsynaptophysin (SYN) positivity.The last picture demonstrates negativity for Glial Fibrillary Acidic Proteinglial fibrillary acidic protein (GFAC) stain. 

InmunohystochemicInmunohystochemical analysis also revealed an absence of IDH mutations or 1p19q co-deletion. 

Differential diagnosis in this case would include the following entities:

• anaplastic oligodendroglioma. The principal differential diagnosis, as this lesion can also manifest with multiple calcifications, bone remodeling, and lack of perilesional edema. On the other hand, this type of neoplasm tends to have coarse and larger calcifications and it is typically encountered in middle-aged adults.
• chondrosarcoma. The presence of rings and arcs calcifications and bone destruction raised the possibility of chondrosarcoma. Also, the T2 hyperintense regions resembling the cartilaginous matrix favored this option. However, chondrosarcoma does not usually appear as an intracranial mass and is rare in young patients. 
• grade III meningioma. Cystic components are less frequent in this type of lesion, and enhancement is usually more significant and homogeneous. 
• gangliocytoma / ganglioglioma. Ordinarily seen in children and young adults, these tumors can have a similar clinical presentation. Nevertheless, they have a more homogeneous radiological appearance and do not show bone resorption.
• anaplastic astrocytoma / GBM. These neoplasms rarely show calcifications and bone resorption, and frequently present with marked peripheral FLAIR hyperintensity.
• metastases. Although this diagnosis should always be considered, there was no oncological history in this patient. 

The first diagnostic possibility was chondrosarcoma, especially due to the T1 and T2 hyperintense components and the rings and arcs calcifications. The patient went to have surgical resection of the lesion, and the pathological examination is shown above. No chondroid matrix was found, therefore chondrosarcoma was discarded. There were also no IDH mutations, 1p19q co-deletion, or glial cells to think aboutconsider an oligodendroglioma. Finally, the histologic appearance together with the synaptophysin positivity was in keeping with an extraventricular neurocytoma. The T1 hyperintense components could be explained either by blood products, the dense proteinaceous fluid, or areas of microcalcificationsmicrocalcification.

Extraventricular neurocytomas are extremely rare WHO grade II primary CNS neoplasms, only described in a few case reports. They can have significant overlap in radiologic presentation with other primary brain tumors; consequently, it is difficult to make an accurate preoperative diagnosis. 

Acknowledgments to Dra. Celia Lizi Oramas Requejo and Hospital de Palamós for providing the radiological images, to Dr. Pablo Benito Peña for sharing the macroscopic picture, to Dra. Laura Ferrazza Martín for contributing with the microscopic images, and to Dr. Nerses Nersesyan for his broad knowledge and incredible support.