Purpose: It is known that slight change in machine parameters may cause a large deviation in output or beam profiles of small fields. In this study, sensitivity of beam profiles of small fields to the gantry tilt is determined.
Methods: The cross-line profiles at depths dmax(1.4cm) and 20cm for field-sizes 10cm×10cm, 4cm×4cm, 3cm×3cm, 2cm×2cm and 1cm×1cm were measured using PTW pin-point chamber (volume:0.015cc) in SunNuclear’s 3DRFA scanner with truebeam linac’s gantry angle 359.9deg. for 6MV-FFF beam energy. The same measurement was repeated for gantry 0deg. The levelling of scanning arm was verified with calibrated leveler.
Results: For gantry tilt of 0.1deg. (gantry=359.9deg.), distance between 50% dose points of normalized profiles at depth dmax and 20cm in X1-direction and X2-direction are found as 9mm and 8mm for 10cm×10cm, 6mm and 4mm for 4cm×4cm, 5mm and 3mm for 3cm×3cm, 4mm and 2mm for 2cm×2cm, and 3mm and 1mm for 1cm×1cm field-size. For machine displayed gantry=0deg., no difference noted in distance between 50% dose points of profiles at depth dmax and 20cm in X1-direction and X2-direction for field-sizes 4cm×4cm and 3cm×3cm; however, distance of 4mm and 3mm for 2cm×2cm, 3mm and 2mm for 1cm×1cm are noted in X1-direction and X2-direction respectively.
Conclusion: The difference in distance between 50% dose points of profiles at dmax and 20cm depths noted in X1-direction and X2-direction reflects asymmetry caused by gantry tilt. For gantry tilt=0.1deg., distance in X1-direction and X2-direction are different for all small field-sizes; however, difference is un-noticeable for relatively larger 10cm×10cm field-size. For gantry=0deg., difference noted in distance measured in X1-direction and X2-direction of profile for 2cm×2cm and 1cm×1cm field-sizes only, which mean there may be sub-degree gantry tilt for which the relatively smaller field-sizes are sensitive. The sensitivity of beam profiles to gantry tilt are found to be increasing with reduction in field-size.