Calcifying Pseudoneoplasm of the Neuraxis (CAPNON) – Unusual Benign Mass Lesion at the Craniocervical junction of Two Young Patient

Purpose of the Study

Calcifying pseudoneoplasm of the neuraxis (CAPNON) is a rare, benign lesion that can occur anywhere in the neuraxis, including the brain, spinal cord, meninges, and their adjacent tissues. It is characterised by a calcified fibro-osseous structure with a chondromyxoid fibrillary matrix. This case study further investigates its histological characteristics.

Methods

Clinical and histological features of two patients with CAPNON are described.

Summary of Results

Two female patients (39 and 42 years old) with no significant medical history, presented with discomfort and unilateral facial weakness, extending to the cervical and shoulder regions. In both cases, symptoms were present for at least 2 years. MRI revealed a lesion with a homogeneous contrast enhancement and partial calcification at the craniocervical junction in one case, suggestive of a meningioma. In the other case, a lesion at the skull base with heterogeneous contrast enhancement was noted. Histology of both cases showed heavily calcified tissue fragments, consisting of well-demarcated amorphous calcifications, lined by spindle-shaped cells, surrounded by multiple histiocytes and multinucleated giant cells. Both cases showed a chondroid matrix focally. Immunohistochemistry showed vimentin and CD68 positivity, and variable EMA expression in the spindle cells and histiocytes, and a low Ki67 proliferation index (<10%).

Conclusions

CAPNON is a rare, benign lesion easily misdiagnosed on imaging. Histopathological examination is necessary for an accurate diagnosis. Key features include dense calcifications, a chondromyxoid fibrillary matrix, spindle-shaped cells, and multinucleated giant cells. Immunohistochemistry has a limited role due to the lack of specific diagnostic markers.