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 the M6 CyberKnife system using the equivalent square field method.

Methods The Source-to-Chamber Distance (SCD) was set to 100 cm, with water as the measurement medium. The field size was configured as 83.2 mm×77 mm, equivalent to a 10 cm×10 cm square field. A total of 200 monitor units (MU) was delivered from the CyberKnife, and TPR₂₀ and TPR₁₀ were measured. Their ratio (TPR₂₀/TPR₁₀, \mathrmT\mathrmP\mathrmR_10^20 ) was then calculated to obtain the QI.

Results In the Flattening Filter Free (FFF) mode, there was no significant difference between the three groups of rectangular fields of 80 mm×80 mm and 90 mm×90 mm (P>0.05). Compared with the QI value obtained via the BJR-25 method (QI₂=0.667), the QI determined by the equivalent square field method (QI₁=0.655) was 1.80% lower.

Conclusion Although both approaches yielded negligible differences (<0.2%) in key parameters (PU, Sw/Sa, and K_q), indicating no significant impact on dose calibration or adjustment. However, the equivalent square field method provides a more direct measurement that circumvents uncertainties associated with converting physical parameters from standard linear accelerators in the BJR-25 method. This approach is therefore more straightforward and accurate for routine calibration and quality assurance of the M6 CyberKnife system.