Study on determining the radiation quality index of M6 CyberKnife based on equivalent square field method

Objective To determine the radiation quality index (QI) of an M6 CyberKnife (referred to as CK-M6) with the BJR-25 method and the equivalent square field size (ESFS) and compares related dosimetric parameters.

Method The QI of the CK-M6 was calculated using the BJR-25 method. The distance from the CK-M6 source to the effective measurement point (SCD) of the ionization chamber was adjusted to 1 000 mm. Water was used as the measurement medium, and the irradiation field was set to dimensions of 83.2 mm×77.0 mm (width×height). When the ESFS was 10 cm×10 cm, the CK-M6 beam output was 200 MU. The tissue phantom ratios (TPR) were measured at water depths of 20 (TPR₂₀) and 10 cm (TPR₁₀), and the TPR₂₀/TPR₁₀ ratio (\mathrmT\mathrmP\mathrmR_10^20 ) was calculated and used as the QI for the ESFS. The QI obtained by the two methods were used in determining the corresponding disturbance factor (P_U), stopping power ratio of water to air (Sw/Sa), and beam conversion factor (K_q) of the ionization chamber. The equivalence of the ESFS method in the flattening filter free (FFF) mode was verified by measuring doses for three groups of 80 mm×80 mm rectangular fields and doses for three groups of 90 mm×90 mm rectangular fields at an SCD=1 000 mm. One-way analysis of variance was used for comparisons of measurement data conforming to normal distribution between groups.

Results The QI calculated using the ESFS method was 0.655, with a deviation of −1.80% when compared with the QI calculated using the conventional BJR-25 method (0.667). The corresponding P_U, Sw/Sa, and K_q were respectively 0.993, 1.122 5, and 0.996 8 for the ESFS method and 0.994, 1.120 5, and 0.995 6 for the BJR-25 method, with deviations of −0.10%, 0.18%, and 0.12%, respectively. The equivalence of the FFF mode with an SCD=1 000 mm was 80 mm×80 mm, and the measured doses of the three groups of rectangular fields were (97.75±0.43), (97.77±0.24), and (97.79±0.31) cGy, respectively (F=6.15, P=0.06). The measured doses of the three groups of 90 mm×90 mm rectangular fields were (99.03±0.04), (99.10±0.02), and (99.04±0.35) cGy (F=1.83, P=0.18). These results suggest that the ESFS method is suitable for application to the FFF mode.

Conclusions The QI obtained by the two methods differed by less than 0.2% in the search for P_U, Sw/Sa, and K_q values but did not affect the calibration and adjustment of the dose of the CyberKnife system. The direct measurement of QI of the ESFS method eliminated uncertainties due to the conversion of some physical parameters of the standard linear accelerator introduced in the BJR-25 method, offering a convenient and accurate method for QI determination.