Ct Image Fusion as a Tool for Measuring in 3D the Setup Errors during Conformal Radiotherapy for Prostate Cancer

Abstract

Aims and Background

The importance of optimal daily patient positioning has been stressed in order to ensure treatment reproducibility and gain in accuracy and precision. We report our data on the 3D setup uncertainty during radiation therapy for prostate cancer using the CT image fusion technique.

Methods

Ten consecutive patients scheduled for radiation therapy for prostate cancer underwent 5 prone position CT scans using an individualized immobilization cast. These different setups were analyzed using the image fusion module of the ERGO 3D-Line Medical System (Milan, Italy) treatment planning system. The isocenter and the body marker displacements were measured.

Results

The 3D isocenter dislocations were quantified: systematic error was Σ_3D = 3.9 mm, whereas random error was σ_3D = 1 mm. The mean of the minimum displacements was 0.2 ± 1 mm showing that the immobilization device used allows an accurate setup to be obtained. Single direction errors were also measured showing systematic errors, Σ_AP = 2.6 mm, Σ_LL = 0.6 mm, Σ_SI = 3 mm in the anterior-posterior, latero-lateral, superior-inferior direction, respectively. Related random errors were σ_AP = 1 mm, σ_LL = 0.6 mm, σ_SI = 1.2 mm. In terms of accuracy, our uncertainties are similar to those reported in the literature.

Conclusions

By applying the CT image fusion technique, a 3D study on setup accuracy was performed. We demonstrated that the use of an individualized immobilization system for prostate treatment is adequate to obtain good setup accuracy, as long as a high-quality positioning control method, such as the stereoscopic X-ray-based positioning system, is used.