Parachordomas are rare cutaneous
tumors that show virtually identical histologic findings to
chordomas. Therefore, the major differential diagnosis in a case of parchordoma is metastatic
chordoma. Parachordomas are
benign neoplasms and most often develop on the extremities adjacent to tendons, synovium or osseous structures, as opposed to
chordomas, which are malignant
tumors located along the craniospinal axis. While recurrences may occur in cases of parachordoma,
metastases have not been reported. In this report, two cases of parachordomas are reported and the literature reviewed. By light microscopy, parachordomas show eosinophilic bands of fibrous tissue separating lobules of cells with variably vacuolated cytoplasm (physaliphorous cells) admixed with more epithelioid cells in a myxoid stroma. Parachordomas and
chordomas share immunohistochemical and ultrastructural features. Both
stain with
S-100 protein and
vimentin, and ultrastructurally both demonstrate cytoplasmic vacuoles, intermediate filaments, pinocytotic vesicles, celljunctions, and cytoplasmic membranes with microvillous processes.
Chordomas more frequently express
cytokeratin (98% vs. 66% in parachordomas) and
epithelial membrane antigen (90% vs. 20% in parachordomas) and
chordomas have a larger number of rough endoplasmic reticulum-mitochondrial complexes. Thus, positive staining with
epithelial membrane antigen and the identification of a large number of rough endoplasmic reticulum-mitochondrial complexes are suggestive of metastatic
chordoma. However, the definitive distinction remains a clinical one after appropriate radiologic studies of the skull and spinal chord.