Benign Notochordal Cell Tumor of the Sacrum with Atypical Imaging\nFeatures: The Value of CT Guided Biopsy for Diagnosis
A 74-year-old man with a long-standing history of hip and back pain presented for evaluation of progressively worsening symptoms. Nonsteroidal anti-inflammatory drugs, narcotics, and physical therapy resulted in pain reduction from a debilitating 9/10 to a manageable 6/10 on a visual analogue pain scale. However, the patient experienced difficulty going up stairs, gait imbalance, progressive weakness, and bowel and bladder incontinence, prompting him to pursue further medical intervention.\nUnenhanced magnetic resonance imaging (MRI) of the lumbar spine with dedicated images of the sacrum (Fig. ) showed a lesion in the S3 segment that extended to the superior aspect of S4. The lesion had a lobulated morphology and demonstrated intermediate signal on T1-weighted and heterogeneous signal on T2-weighted images, with areas of intermediate and high signal. In addition, the lesion contained tiny round foci of increased T1 signal that suppressed on T2-weighted images with fat suppression, indicative of a small component of intralesional fat. Although the lesion was predominantly intraosseous, there was focal permeation through the posterior cortex with a tiny nodular soft tissue mass in the adjacent spinal canal. Intravenous gadolinium was not administered due to patient claustrophobia, which precluded additional image acquisitions. At this time, the differential diagnosis included BNCT, chordoma, hemangioma, metastatic carcinoma, chondrosarcoma, myeloma, or lymphoma.\nUnenhanced CT of the sacrum demonstrated diffuse subtle sclerosis in the lesion without evidence of a destructive osteolytic component (Fig. ). This finding along with the foci of intralesional fat noted on MRI strongly favored a diagnosis of BNCT. However, given the focal permeation of tumor through the dorsal cortex forming a tiny nodular extraosseous soft tissue mass, the lesion was considered atypical for BNCT, and a biopsy was recommended for definitive histologic diagnosis.\nFollowing interdisciplinary discussion, a decision was made to perform a minimally invasive percutaneous CT-guided biopsy rather than pursue a metastatic work-up.\nCT-guided biopsy (Fig. ) was performed with the patient in an oblique prone position on the CT table with the spine and pelvis positioned 15 degrees to the right to align the left S4 pedicle with the gantry. This position facilitated an uneventful biopsy of the intraosseous tumor component at S3 via a left S4 transpedicular approach using coaxial technique with 3 cores obtained using a trephinated 14 gauge needle and one final core obtained with an 11-gauge needle. Although lesions in which chordoma is the leading differential diagnosis are traditionally approached with a (near) midline biopsy tract, this tract was deliberately selected to allow best sampling of a tumor in which the role of the biopsy was to confirm the imaging impression of BNCT. Additionally, in the unlikely event that a chordoma was diagnosed, definitive treatment in this 74 year old gentleman would be proton beam therapy in which the biopsy tract could be sterilized.\nThe submitted needle biopsies showed (Fig. ) typical morphological features of BNCT, with a hypocellular proliferation of bland, eosinophilic, epithelioid cells, often showing striking cytoplasmic vacuolization, and mimicking adipose tissue. Morphological features of chordoma, such as myxoid matrix, cords and chains of epithelioid cells, physaliphorous cells, mitotic activity and necrosis, were absent. By immunohistochemistry the neoplastic cells were strongly positive for cytokeratins using the OSCAR monoclonal antibody and for the notochord-specific transcription factor brachyury.\nAt initial clinical follow-up, the BNCT was thought to be incidental to the pain syndrome. The patient will be followed with periodic MRI of the sacrum to ensure stability of the lesion.