Calcifying pseudoneoplasm of the neural axis (CAPNON)
CAPNON was first described by Rhodes and Davis in 1978 (5), and since then has been found anywhere along the neural axis from L2 to the frontal lobe (1,2). The age range is highly variable as well, ranging from 6 to 83 years (1,3). To date, only 32 cases have been reported in the English literature (1).
Calcifying pseudoneoplasm of the neural axis has multiple histopathologic features that may be present including chondromyxoid matrix, palisading spindled to epithelioid cells, calcifications, fibrous stroma, and a foreign body giant cell reaction (1). Although the components are similar from case to case, the distribution may vary (2), and some features may not be seen at all (1). In our case, the calcification manifested as large areas of chunky and plate-like material. Spindle cells predominated as the cellular component (Fig 4), though occasional areas showed an epithelioid appearance (Fig 5). Multinucleated giant cells were seen scattered throughout the lesion (Fig 6), though they did not have a granulomatous appearance as described in other literature (2). The chondroid matrix, psammoma bodies, and osseous metaplasia, which have been described in other cases (1,2) were not seen in our case.
The differential diagnosis of CAPNON can be fairly broad, depending on the features present. The calcifications seen in this case are very similar to that of phosphaturic mesenchymal tumor and tumoral calcinosis, however the patient did not have a clinical history of hypophosphatemia or hyperphosphatemia. Given the presence of multinucleated giant cells, a granulomatous reaction may be considered. However, there was no necrosis or rim of lymphocytes, and the multinucleated giant cells were scattered rather than localized. The epithelioid component had a whorled pattern similar to a meningioma, but did not stain for EMA or S-100. Chondroid or osteoid are often seen in CAPNON, and if present, a chondrosarcoma or osteosarcoma may be considered. CAPNON is a diagnosis of exclusion, and should not be made until other possibilities have been ruled out.
CAPNON is believed to be a reactive process (1,3) and fibroblasts have been proposed to be the precursors of this lesion (3). It is important to note that the lesion is benign and treated with resection without chemotherapy or radiation (2). There have been fatalities reported due to CAPNON, but from complication related to the anatomic location, and not due to progression of disease (2).
Contributed by Matthew Stemm, MD, Jerome Loew MD, Sukriti Nag, MD, Leonidas Arvanitis, MD