Digital analysis of multi-leaf collimator position accuracy in static intensity modulated radiotherapy / 中华放射医学与防护杂志

ABSTRACT

Objective To investigate the impact of gantry angles on the position accuracy of buildin multi-leaf collimator (MLC) during static (step & shoot) intensity modulated radiotherapy (IMRT).Methods The position accuracy of MillenniumTM 60 pairs multi-leaf collimator (MLC) in Varian 23EX medical linear accelerator during 10 fractions of treatments with static intensity modulated radiotherapy (IMRT) was retrospectively analyzed by resolving dynamic therapy log files and MLC planning files.The single leaf position deviation and gap deviation of each MLC pairs were compared between expected values and actual outcomes.The impact of gantry angle on MLC position accuracy was also investigated with paired t-test between vertical and horizontal beams.Results All the MLC position deviations ranged within ± 1.0 mm,while the gap deviations were at the range of ± 1.5 mm.The statistical standard deviation of MLC position deviations among fractions was less than 0.06 mm.After all the 60 pairs of MLC leaves were taken into account,the M LC position deviation in the field of 103° was smaller than in the field of 0° (t =58.74,P ＜ 0.01),but there was no significant difference between 257° and 0°,while only the moving leaves were put into consideration,the MLC position deviation in the field of 103° was still smaller than in the field of 0°,but the MLC position deviation in the field of 257° was slightly higher than in thefield of 0° (t =41.95、-4.47,P ＜ 0.01).Conclusions The MillenniumTM 60 pairs MLC in Varian 23EX medical linear accelerator meets the mechanical quality assurance requirement during static IMRT treatments.The MLC positioning showed stable repeatability among treatment fractions.Although the MLC position accuracy is slightly affected by gravity from different gantry angles,but still meets the mechanical QA requirements for medical linear accelerator.This dynamic analysis method is expected to be a novel strategy which may improve the existing QA protocols.