ABSTRACT
<p><b>OBJECTIVE</b>To explore the effect on the iso-center and OAR and PDD of the MV class X-ray coming from the linac when we use the glass mirror to replace an imported one.</p><p><b>METHOD</b>Data collected by using a film and 3D water phantom OAR and PDD measuring methods based on the different conditions. Comparison and analysis of the effect levels for different thickness and material mirrors are made.</p><p><b>RESULT</b>The PDD max error of the 4 mm general glass mirror is 6%.When the 8MV X-ray 720 Gy irradiation is given, the light field appears a dual cross lines and has darker brightness. The PDD max error of the 1.5 mm quartz glass mirror is less than 1%. When it accepts the equal radiation dose, the light field iso-center and OAR and field brightness are agreed well with clinical demands.</p><p><b>CONCLUSION</b>Our practice shows that the 4 mm general glass mirror can't suffer the MV class X-ray irradiation but the thin quartz glass mirror can. So the thin quartz glass mirror can be used for the linac.</p>