Spinal osteoid osteoma progressed to osteoblastoma with paraspinal soft tissue mass: a unique presentation.

Osteoid osteoma and osteoblastoma are rare benign bone-forming tumors with very similar histological features. They are nowadays considered as two distinct entities. Progression of an osteoid osteoma to osteoblastoma is considered very rare with only a few cases reported in the literature. Herein we describe a case of an osteoid osteoma of the thoracic spine in a 29-year-old woman that was initially treated conservatively and progressed to osteoblastoma 5 years following the initial diagnosis. Imaging revealed an increase in the size of the spinal lesion that was surrounded by extensive paraspinal abnormal soft tissue that raised suspicion for sarcomatous transformation. The final diagnosis was established by CT-guided biopsy of both the bone lesion and the paraspinal soft tissue, which excluded malignancy and revealed an osteoblastoma surrounded by plasma cell-rich chronic inflammation. The patient then underwent wide surgical excision of the lesion and paraspinal soft tissue component that confirmed the diagnosis. Follow-up with MRI over the next 12 months was unremarkable, with no signs of recurrence or spinal instability. This unique presentation of an osteoblastoma has not been previously described. This case also demonstrates the importance of follow-up of osteoid osteomas that are treated conservatively.