Calcified Pseudoneoplasm of the Neuraxis

DISCUSSION

CAPNONs, also known as fibro-osseous lesions of the CNS, are rare non-neoplastic lesions, first described by Rhodes and Davis,⁶ in 1978. They are found essentially everywhere in the central nervous system; lesions can be intra- or extra-axial and occur in both the brain and spine.^7,8 However, CAPNONs may have a slight predilection for intracranial involvement, specifically occurring along the dura of the skull base.⁹ The lesions have no predilection for age or sex. Cases have been reported in patients as young as 12 and as old as 90 years of age, with about half of patients presenting between 40 and 60 years of age.^9,10 Because symptoms are related to local mass effect, clinical presentations are extremely variable. Headaches, epileptic seizures, cranial nerve dysfunction, and neck/back pain have all been reported; some CAPNONs are discovered incidentally.^11⇓-13

The diagnosis of a CAPNON can be strongly suggested by its imaging appearance, especially when intracranial. True to their name, CAPNONs are highly calcific, a feature that is readily apparent on CT. Although lesions are classically described as hypointense on both T1WI and T2WI, T1 hypointensity is not a reliable characteristic, likely due to interactions between calcification and free water molecules, as was seen in the case presented here.¹⁴ CAPNONs are usually extra-axial lesions, and dural attachments are observed in most cases. Modest enhancement may or may not be visualized, with either intralesional serpentine or peripheral/rim enhancement patterns.¹⁵ Perilesional T2 prolongation, too, is sometimes observed, either representing vasogenic edema or gliosis related to chronic mass effect on the nearby parenchyma.^16,17

The imaging differential of CAPNONs mirrors that of other cerebral calculi (“brain stones”). If extra-axial, osteomas and highly calcified meningiomas are the greatest mimickers of CAPNONs.¹⁸ Osteomas often develop from the calvaria; thus, intracranial osteomas are often contiguous with the inner table or even the diploic space of the adjacent bone. They also typically lack intracranial enhancement.¹⁹ Nevertheless, osteomas can arise from dural attachments, similar to CAPNONs; rarely, some osteomas enhance.²⁰ Large meningiomas are often associated with hyperostosis of the underlying bone, a feature not described in CAPNONs. Meningiomas also usually demonstrate enhancement, though this may be absent in densely calcified lesions. Intra-axial CAPNONs can appear similar to calcified tumors and vascular malformation—particularly oligodendrogliomas and cavernous malformations.^21,22 Dystrophic calcifications, too, can share imaging features of CAPNONs, though these typically arise in post-traumatic or postischemic regions of the brain.²³

Spinal lesions tend to be smaller and less characterizable than their intracranial counterparts. The large majority of spinal CAPNONs (>80%) are in the epidural space.¹⁰ Garcia Duque et al¹⁰ found that the cervical region was the most frequent location of spinal CAPNONs. Other studies, however, have suggested otherwise; a large single-institution study by Ho et al¹⁴ noted that most spinal CAPNONs were in the lumbar spine. Calcified psammomatous meningiomas, calcium pyrophosphate dihydrate deposition disease, exophytic osseous lesions, synovial cysts, calcified hematomas, and herniated discs would be the most likely alternative diagnostic considerations.²⁴

Histologically, CAPNON is typically characterized as a process exhibiting robust calcification in a patchy, nonzonal alternation with chondromyxoid fibrillary matrix. While a chondromyxoid fibrillary matrix has been classically described as a defining feature of this entity, a recent case series has noted that calcification and cartilaginous elements are more consistent features across patients.¹⁴ While the lesion may incite reactive meningothelial cells and macrophages around the periphery of the lesion, these cellular elements should not be an underlying substrate of the lesion itself.

The etiology of CAPNONs remains uncertain. However, both their histologic characteristics and behavior have led to the presumption that CAPNONs are reactive entities.¹⁸ The source of the reactive process can vary, ranging from inflammation to trauma. Ho et al¹⁴ found that nearly one-third of CAPNONs were so-called “collision” lesions, in which CAPNON tissue coexisted with a separate, distinct pathologic entity. The coexistent primary tumors found in such collision lesions included meningiomas, pleomorphic xanthoastrocytomas, dysembryoplastic neuroepithelial tumors, and vascular malformations. Dual lesions have similarly been reported elsewhere.^8,25 Histologic analyses of collision lesions have demonstrated inflammatory tissue outside the CAPNON boundaries, suggesting that the primary lesions initiated a reactive process, ultimately resulting in the formation of a CAPNON.¹⁴ In the spine, too, associated pathologies are also observed, including synovial cysts and degenerated discs.

The management and prognosis of CAPNONs are highly dependent on lesion location. Total resection tends to be curative, though exceptionally rare cases have been reported in which recurrence or progression of lesions or both occur after incomplete resection.¹¹ Still, surgical removal can be particularly challenging when the lesion is located within eloquent regions because piecemeal resection is often required due to the dense intralesional calcifications.²⁶