[Evaluation of a new table, Exact IGRT(Varian), on the treatment beam attenuation and the image quality].

PURPOSE

A new couch top designed for IGRT procedures, Exact IGRT (EIC), has been developped. It has the advantage of having no removable support bars and is thicker than a standard table (Exact(EC)) in order to create the appropriate rigidity. The aim of this study was to quantify the beam attenuation caused by the EIC and to evaluate the EIC contribution to on-board image quality improvement.

MATERIALS AND METHODS

The treatment beam attenuation and the surface-dose of the patient were measured for simple radiation geometries at different gantry angles and for two clinical cases, head and neck and posterior pelvis. The 2D and 3D image quality was analysed.

RESULTS

The beam attenuation at 215 degrees was 2.0% to 4.1% for 6 MV and 1.2% to 2.2% for 18 MV according to the EIC part. The dose-surface tripled between the measurement without table and with table into the beam (similar results between EIC and EC). The EIC influence was significative for the posterior pelvic tumor, acceptable for the head and neck case. The EIC showed a clear improvement of the low contrast resolution (visualisation of a 7mm and 1% contrast disk [15mm for the EC]).

CONCLUSION

In order to take into account the beam attenuation, the EIC should be modeled in the treatment planning system. However, the good quality of 3D images obtained with the EIC allows us to easily visualize soft tissues.