Rapid Growth of a Facet Joint Cyst Mimicking an Aggressive Tumor in the Lumbar Spine.

A 67-year-old patient with a history of fully treated bowel carcinoma presented with a short history of unilateral lumbosacral radiculopathy. No neurological deficit was found on examination. Magnetic resonance imaging (MRI) demonstrated initially L4/5 foraminal narrowing that was believed to be the cause for the patient's symptoms; however, a nerve root block led to no improvement in symptoms. MRI was repeated at 5 months and demonstrated a mass causing compression of the S1 nerve. However, the patient declined surgical decompression. MRI at 14 months subsequently showed rapid growth of the lesion suggestive of an aggressive process such as a metastatic lesion or even a nerve sheath tumor such as a Schwannoma. Open biopsy and decompression revealed the lesion to be a facet joint cyst and the patient recovered well and had satisfactory postoperative imaging at 3 months follow-up. There are no reports in the literature of facet joint cysts growing this quickly and thus mimicking other forms of lesion.