Spinal calcifying pseudoneoplasm of the neuraxis (CAPNON) and CAPNON-like lesions: CAPNON overlapping with calcified synovial cysts

Calcifying pseudoneoplasm of the neuraxis (CAPNON) is a rare tumour-like fibro-osseous lesion in the neuraxis including the spine. It is diagnosed by the presence of the following histological features: granular amorphous to chondromyxoid fibrillary cores with calcification/ossification, peripheral palisading of spindle to epithelioid cells, variable fibrous stroma, and foreign body reaction with multinucleated giant cells, as well as positive NF-L immunostaining. Spinal CAPNON is sometimes named as tumoural calcinosis that is tumour-like dystrophic calcification usually in the periarticular tissue and also described in calcified synovial cyst (CSC). We examined clinical, radiological and pathological features of five spinal CAPNONs and 21 spinal CSCs including three recurrent lesions. The results demonstrated some radiological and pathological overlaps between these two entities, as well as distinct features of each entity to be diagnosed. All CAPNONs showed the diagnostic histological features with NF-L positivity mainly in lesion cores and variable CD8+ T-cells. In contrast, CSCs exhibited the synovial lining and variable degenerative/reactive changes with some CAPNON-like features, but mostly no to occasionally limited NF-L positivity and less CD8+ T-cells with statistically significant differences between groups of CAPNONs and CSCs. Four CSCs contained CAPNON-like foci with the CAPNON diagnostic features including prominent NF-L positivity, and some transitional features from CSC to CAPNON. As the pathogenesis of CAPNON is likely reactive/degenerative in association with an inflammatory/immunological process involving NF-L protein deposition, our findings suggest the link between spinal CAPNON and CSC, with possible transition from CSC to CAPNON or CAPNON developing in reaction to CSC.