Assessment of Dose Calculation Accuracy for Bone Cement Implants in Radiation Therapy Planning Systems.

BACKGROUND AND PURPOSE: This study investigated depth-dose alterations caused by bone cement in solid phantoms during vertebroplasty. Dose variations of megavoltage (MV) photon beams after traversing high-density bone cement were measured to estimate the effective density of bone cement.

METHODS

A phantom mimicking human anatomy was fabricated with 2.5-cm acrylic embedded with bone cement. An Elekta VersaHD linear accelerator was used with 6 MV and 6 MV flattening filter-free (FFF) photon beams, field sizes ranging from 3 × 3 cm 2 to 10 × 10 cm 2 . Depth dose, planar dose, and point dose were measured using an Advanced Markus chamber, PTW 1600 two-dimensional array, and Semiflex three-dimensional ion chamber. These were compared with calculated doses from treatment plans with various overridden density values for bone cement.

RESULTS

Minimal differences in percent depth dose were observed after MV photons (6 MV and 6 MV FFF) penetrated the bone cement. At 1 cm beyond cement, measured versus calculated doses differed by 1.2% to 2.7%. Within 1 to 5 cm behind cement, dose discrepancies were within 3% to 5%. At 3-cm depth behind cement, point-dose differences measured by the Semiflex chamber indicated smaller deviations at lower override densities, with an optimal match at 0.6 g/cm 3 . The two-dimensional array showed near-perfect gamma passing rates (∼100%) at an overridden density of 0.8 g/cm 3 , demonstrating the smallest center-point dose discrepancies. A higher override density of 1.4 g/cm 3 yielded results similar to the plan without density override, suggesting minimal advantage at higher override densities.

CONCLUSION

This study revealed that dose perturbations induced by bone cement caused the treatment planning system to underestimate actual delivered doses by approximately 3% to 5%. Optimal density override values for bone cement in treatment planning systems appear to be around 0.6 to 0.8 g/cm 3 . Clinical trials and further data acquisition are necessary to validate precise density values due to variability in cement composition and incomplete cement infusion.