Novel Unidirectional Membrane-Like Brachytherapy in the Thoracic Spine: A Cadaveric Study and Model

Introduction: Brachytherapy is an increasingly important treatment modality for a variety of cancers. However, considerable hesitance is necessary when implementing near healthy radiosensitive tissue. In the spine, the posterior portion of the vertebral body is often under dosed leading to adjacent level extension. Traditional multidirectional brachytherapy may deliver radio-ablative doses to the spinal cord. Here, we discuss the feasibility of using a unidirectional, palladium-103, low-dose rate, brachytherapy device that utilizes a bio-absorbable substrate and gold shielding to attenuate directional dose in the thoracic spine.

Methods: Following surgical exposure of the thoracic spinal cord, a unidirectional brachytherapy device was shaped and positioned anterior to the spinal cord. Two 4 mm CMF screws were placed to ensure a planar spatial arrangement on the posterior surface of the vertebral body. An absorbable hemostatic agent (Surgicel Nu-Knit) was placed between the spinal cord and device. The cadaveric specimen was subjected to X-ray, CT and MRI Imaging. Dosimetry and radiation planning models were created using Eclipse and MIM software.

Results: Proper placement of the low-dose rate membrane bound device was achieved in the thoracic spine and confirmed through fluoroscopic imaging. The addition of the hemostatic spacer introduced a 1 mm clearance between the cord and the device creating a notable drop off in dose delivered to the spinal cord. The device demonstrated anisotropic dose distribution, focusing radiation anterior to the vertebral body. Further, 12 sources were implanted with an observed 5.95 mCi per source. There was a total of 47 Gy 5 mm into the vertebral body with a cord max voxel (0.03 cc) = 9.2 Gy. Radiation planning in combination with brachytherapy demonstrated complete coverage of the vertebral body using a two-sided approach.

Conclusion : Surgical implementation of this device is feasible. The device demonstrated the ability to provide appropriate therapeutic doses while sparing healthy, radiosensitive tissue. When combined with adjuvant SRS, these techniques may provide improved local control and warrant further evaluation.