[Effects of gantry position on the accuracy of lead block localization and relative quality control methods].

BACKGROUND & OBJECTIVE:To date, the low-melting point lead blocks are commonly used as a beam modifier for the accurate three-dimensional (3D) radiotherapy in China. However, care should be taken when using blocks even though they are verified on the specified simulator or linear accelerator before treatment. This is because that those verifications are only performed under the gantry angle of 0 degree. For the oblique gantry angles, the block may shift in the tray leading to the positioning error. In this study, we explored the influence of gantry position on the accuracy of lead block localization, and the relative quality control methods were also provided. METHODS:After rotating the gantry to 90 degrees and setting the collimator angle to 0 degree on a Siemens linear accelerator, a crosshair was marked on the plastic base of the lead block fitted to the tray according to the crosshair of light field of the linear accelerator. Then the discrepancy between the base crosshair and the light crosshair was investigated for different gantry positions and collimator angles. RESULTS:There was clearance between the plastic base of the lead block and the block tray. As for some collimator angles at oblique gantry positions, the lead block shifted in the tray due to the gravity. This led to the position error of blocked radiation fields from 0 up to 3.6 mm at the isocenter plane. CONCLUSIONS:Accurately marking the crosshair on the plastic base of the lead block is an effective method of correctly fixing the lead block influenced by the plastic base as well as of double checking the accuracy of the localization of the lead block. The positioning error of oblique gantry positions can be minimized by limiting the collimator to some degrees which makes the block tray opening to the ceiling or parallel to the ground while the bisector of the restrictive angle points slanting to the ceiling (that is, the first baseline of the block tray points inclined or right to the ceiling).