Radioactive bone cement for the treatment of spinal
metastases: a dosimetric analysis of simulated clinical
The next step in the development of the Bone-Rad brachytherapy bone cement was to use the previously developed and validated Monte Carlo modeling method to evaluate the clinical usefulness of radioactive bone cement. Thus, dose distributions were evaluated for simulated clinical implantations of radioactive bone cement, involving radiation delivery over the life of the radioisotope, varying levels of implanted activity, and more complex bone cement deposition patterns, including simulation of extravasation of the bone cement into the spinal canal.
Voids of varying shapes and sizes were created in cadaveric vertebrae to facilitate the injection of bone cement. The voids were then filled with PMMA cement and the vertebrae were CT scanned. A Monte Carlo model of each vertebra was then created, with the injected volume of bone cement identified on CT scans and simulated as brachytherapy bone cement in the model.
A transverse cross-section of a T2 vertebra, after a void was created and then filled with bone cement, shown in a CT scan image (a) and the corresponding slice of the MCNP model (b). A modified MCNP model for the same specimen is also shown (c), with an enlarged cement volume to simulate cement extravasation penetrating the posterior wall of the vertebral body. The spinal cord is represented by the region inside the black circle.
Isodose curves were generally concentric about the brachytherapy bone cement, despite irregularities in the surface geometry. As a result, clinical dose distributions would be relatively predictable, potentially enabling treatment planning to be based only on the activity concentration and location of the cement, without the need to consider the total cement volume and activity (cement formulation and dosimetry method are patent pending).
Dose-volume histograms calculated for the spinal cord indicated that nearly 100% of the spinal cord volume received a radiation dose of less than 1 Gy in cases without cement extravasation. In cases with cement extravasation into the spinal canal, about 80% of the spinal cord volume received less than 10 Gy, about 90% received less than 50 Gy, and about 5% received more than 200 Gy. Although the effect on the spinal cord due to radiation doses calculated in the extravasation cases is unclear at this time, the need for caution is readily apparent, and the development and use of methods to minimize the risk of extravasation is critical.
Representative isodose contour plots for a T2 (a), T7 (b) and T12 (c and d) vertebra demonstrate that isodose lines are generally concentric about the volume of radioactive cement shown in white within the vertebral body. Note that the lower right portion of the isodose curves in (d) and (f) is influenced by overlapping cement in the adjacent superior slice. Isodose lines represent constant lifetime doses of 100, 50, 25, 12.5 and 1 Gy, from the inner line to the outer line, for an initial activity concentration of 1 mCi/ml. Models for specimens in (c) and (d) were modified to simulate cement extravasation (e and f, respectively).
Spinal cord dose-volume histograms calculated for an initial activity concentration of 1 mCi/ml indicate that, for the cement distribution shown in figure 5(c), nearly the entire spinal cord volume receives less than 1 Gy (a); and, for the cement extravasation case modeled in figure 5(e), about 80% of the spinal cord volume receives less than 10 Gy (b).
Dose distributions for P-32 radioactive bone cement were evaluated for actual bone cement deposition patterns and shown to be clinically useful for treating spinal metastases. Model results indicated that a therapeutic dose could be delivered to tumor and bone within about 4 mm of the cement surface, while maintaining a safe dose to radiosensitive tissue, such as the spinal cord and nerves, beyond this distance. This therapeutic range should be sufficient to treat target volumes within the vertebral body, particularly after tumor ablation techniques are used to create a cavity into which the radioactive cement can be injected. Thus, treating spinal metastases with radioactive bone cement may become an alternative to the conventional two-step approach of a percutaneous strength-restoration procedure followed by radiotherapy.
Source for all figures: Phys Med Biol. 2010 May 7;55(9):2451-63. doi: 10.1088/0031-9155/55/9/002. [LINK]
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