Pseudomalignant osteoblastoma of the odontoid process.

INTRODUCTION: The anterior elements of the spine, particularly the odontoid processes, are a rare location for osteoblastomas. Pseudomalignant osteoblastomas are themselves rare histologic types and are also extremely rare in this location. Most osteoblastomas are Enneking stage 2 lesions; less frequently, they can be more aggressive with extra-capsular extension (Enneking stage 3). En bloc resection is recommended for aggressive lesions, but the literature is less clear regarding the approach to stage 2 tumors, particularly those with pseudomalignant histologic features.

CASE REPORT: A 6-year-old male child presented with a type III pathologic fracture of the odontoid. The fracture healed but upon 6-month follow-up CT scanning, an expansile lesion was detected. Surgical biopsy revealed an osteoblastoma which was treated with intralesional excision. Meanwhile, the excised specimen showed histological features of a pseudomalignant osteoblastoma. Despite this diagnosis, no further treatment was undertaken. At a 10-year follow-up, the patient was free from pain and had full range of motion of the cervical spine; no recurrence was detected.

CONCLUSION: This unique case of odontoid osteoblastoma illustrates that malignant behavior may not be predicted only by the presence of pseudomalignant features on histology.