Addressing the dosimetric impact of bone cement and vertebroplasty in stereotactic body radiation therapy.

Abstract

Bone cement used for vertebroplasty can affect the accuracy on the dose calculation of the radiation therapy treatment. In addition the CT values of high density objects themselves can be misrepresented in kVCT images. The aim of our study is then to propose a streamlined approach for estimating the real density of cement implants used in stereotactic body radiation therapy. Several samples of cement were manufactured and irradiated in order to investigate the impact of their composition on the radiation dose. The validity of the CT conversion method for a range of photon energies was investigated, for the studied samples and on six patients. Calculations and measurements were carried out with various overridden densities and dose prediction algorithms (AXB with dose-to-medium reporting or AAA) in order to find the effective density override. Relative dose differences of several percent were found between the dose measured and calculated downstream of the implant using an ion chamber and TPS or EPID dosimetry. If the correct density is assigned to the implant, calculations can provide clinically acceptable accuracy (gamma criteria of 3%/2 mm). The use of MV imaging significantly favors the attribution of a correct equivalent density to the implants compared to the use of kVCT images. The porosity and relative density of the various studied implants vary significantly. Bone cement density estimations can be characterized using MV imaging or planar in vivo dosimetry, which could help determining whether errors in dose calculations are due to incorrect densities.