HOXB13 Is a Sensitive and Specific Diagnostic Marker for Myxopapillary Ependymoma

Myxopapillary ependymoma (MPE) is a rare spinal tumor that remains undercharacterized in systematic investigations. Accurate distinction from histologic mimics in the conus-cauda-filum terminale region is challenging. This study aimed to describe the clinicopathologic features and outcomes of a large single-center MPE cohort, evaluate the diagnostic utility of HOXB13 immunohistochemistry (IHC), and explore the molecular profiles of atypical cases through methylation clustering. HOXB13 IHC was performed on 45 MPEs, 44 spinal ependymomas (SEs), and 35 other nonependymal spinal tumors. Fourteen atypical cases (MPEs with weak HOXB13 or high-level involvement, and HOXB13-positive SEs) were analyzed by DNA methylation microarray with t-SNE and random forest clustering. HOXB13 expression was semiquantitatively graded (0 to 2). Strong positivity was seen in 97.8% (44/45) of MPEs, but only 4.5% (2/44) of SEs and 11.4% (4/35) of nonependymal tumors. Sensitivity and specificity for MPE diagnosis were 97.8% and 92.4%. Methylation clustering showed strong HOXB13 expression aligned with the MPE methylation class (EPN_MPE). Notably, 3 typical MPEs were misclustered due to fresh hemorrhage, stromal sclerosis, or dural components, whereas 2 HOXB13-strong lumbar SEs fell into the EPN_MPE class. MPE patients had 100% 10-year overall survival and 75% progression-free survival (PFS), but PFS was significantly shorter than that of SE patients ( P < 0.01). HOXB13 is a highly specific and sensitive IHC marker for MPE. Histologic artifacts can interfere with methylation-based classification, emphasizing the need for tissue quality assessment and microdissection when indicated. HOXB13 IHC is a simple, reliable diagnostic tool that improves accuracy when added to routine IHC panels.