Session Item

The aim of this work was to investigate the consequences of systematic mechanical errors on selected dosimetric indicators and gamma-pass rate (GPR) results with portal imager for three accelerators.

Ten patients treated with VMAT for H&N cancer were randomely selected. Dose distribution calculations and pre-treatment acquisitions were performed on a TrueBeam (MLC 120), a TrueBeam STx (MLC HD120) and a Halcyon. Eight plans per patient were created to study the impact of changes in 4 parameters that could each be set at three levels: all banks A and B MLC leaves shifted (-1 mm, 0 mm, +1 mm), gantry and collimator (-1°, 0°, +1°). This experimental design included the two most unfavorable presupposed cases in terms of dose impact: plans with all leaves shifted to +1 mm (“All opened plan (AOP)”) and shifted to -1 mm (“All closed plan (ACP)”). Studied indicators were: relative dose differences of PTV (D95%), parotids and thyroid (Dmean), spinal cord (D2%); 2D GPR<1 using three Global/Local Absolute gamma criteria: GA2%/3mm, LA2%/3mm, LA2%/2mm. A multiple linear regression (MLR) was performed to estimate the influence of each mechanical parameter on studied indicators for each patients.

MLR results suggested: gantry and collimator errors as non-significant factors (up to 1° shift); MLC systematic error as a significant influencing factor on dose deviations with a significant interaction between MLC banks. For all accelerators and each organ studied, doses differences are given Figure 1 for both AOP and ACP: median doses deviations were greater than 5% for all organs at risk. For PTV, a median dose deviation of about 5% for a systematic error of ±1 mm was pointed out. Measurements with portal imager were performed on the Halcyon. The AOP gave mean 2D GPR between TPS calculations and portal acquisitions equal to 92.5% (GA2%/3mm, without error: 100%), 89.7% (LA2%/3mm, without error: 99.9%), 73.9% (LA2%/2mm, without error: 99.6%). These first results suggested that LA2%/2mm criteria had the best sensibility to MLC systematic errors as GPR from GA2%/3mm and LA2%/3mm gamma criteria had a majority of GRPs above 90% (Figure 2).

Figure 1: Doses deviations (%) for PTV and OARs between plan without error and both worst cases

Figure 2: Halcyon’s Gamma pass rates (%) for various tolerance criteria

MLC systematic errors influenced significantly dose deviations to PTV and OARs: an increase of about 1% for D95% PTV can be caused by a 0.2 mm systematic shift of MLC leaves bank. 2D GPR results for Halcyon had a good sensibility to MLC systematic positioning errors at the gamma criteria LA2%/2mm while the other criteria gave values above 90%. Works are in progress: to study the dosimetric impacts on conformity/homogeneity indices and the 3D GPR of each organ; to perform portal imager measurements on the TrueBeam and TrueBeam STx; to analyze the correlations between the results of pretreatment controls and all these dosimetric indices combined with a predictive approach.