Radioisotope dosimetry
Radioisotopes and vertebral augmentation: Dosimetric analysis of a novel approach for the treatment of malignant compression fractures

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Abstract

Purpose

Vertebral compression fractures (VCFs), a major cause of morbidity and debilitating pain, often results from secondary tumor metastases to the skeleton. Vertebral augmentation is a palliative technique developed to treat VCFs and involves the injection of polymethyl methacrylate (PMMA) to augment the fractured vertebral body. The authors investigate the feasibility of radionuclide therapy coupled with vertebral augmentation to treat both the tumor metastases and VCFs. Six therapeutic radioisotopes, uniformly mixed in a PMMA bolus, were investigated for their dosimetric properties.

Methods and materials

The MCNP5 Monte Carlo computer code was used to characterize the therapeutic dosimetric distribution within a cortical bone phantom for a 1 mm radial bolus of isotope-infused PMMA. Based on these data, the minimum activity required for a therapeutic treatment was calculated.

Results

The dosimetry from beta emitting Y-90, P-32, and Ho-166 decreased to 10% of its maximum therapeutic dose (R10%) after traveling 1.20 mm, 1.03 mm, and 0.97 mm, respectively, through cortical bone. Low photon energy I-125 had a slightly larger calculated R10% of 1.32 mm. Although F-18 and Tc-99m exhibited a more uniform distribution (R10% = 1.72 mm and 1.94 mm, respectively), the lower dosimetric gradients resulted in significantly greater therapeutic implant activities relative to the other isotopes studied in this report.

Conclusions

Radionuclide therapy coupled with vertebral augmentation is shown to be a feasible technique for the treatment of secondary skeletal metastases and its resulting side effects. Future studies will include a full clinical investigation to determine optimal treatment isotope(s).

Section snippets

Materials and methods

The MCNP5 Monte Carlo computer code was used to characterize the dose delivered to tissue as a function of cortical bone thickness for a 1 mm radial bolus of isotope-infused PMMA. The isotope of interest is assumed to be uniformly distributed within the PMMA. The advantage of studying a bolus of this size is that it may be used as the basis for treatment plans containing complex fracture geometries. From these data, the authors derived the approximate source activity necessary to deliver a

Results

Results from this Monte Carlo analysis are presented in Fig. 3. Here, the absorbed dose deposited over the lifetime of the isotope, based on a 1 μCi (3.7 × 104 Bq) bolus, is presented as a function of cortical bone thickness; cortical bone thickness is defined as the distance from the edge of the PMMA bolus to the critical structure (tally voxel). Perhaps of greater clinical interest, the dosimetry for each isotope was normalized to the absorbed dose at a specific reference depth for that isotope.

Discussion

Targeted therapeutic radionuclide therapy has been used clinically for the treatment of bone metastases. Such therapy uses bone-seeking radiopharmaceuticals to target sites of bone metastases. The most commonly prescribed radiopharmaceuticals clinically are 89Strontium and 153Samarium. These systemic treatments are particularly useful in the setting of multifocal metastatic bony disease when repeated local treatments, particularly repeat external beam radiation therapy over critical normal

Conclusion

Vertebral augmentation is a relatively new and rapidly growing procedure that provides dramatic and often near immediate pain relief for patients with vertebral compression fractures. As the systemic oncology therapies continue to improve, patients with bone metastases are living longer than ever before [31], [32]. While external beam radiation therapy remains the standard of care for the treatment of bone metastases and is often effective in controlling pain, the external beam radiation dose

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    Presented in part at the 49th Annual ASTRO Meeting, October 28–November 1, 2007.

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