Purpose: To study the beam characteristics of CyberKnife system, especially the features in
standardization and calibration of small-field dosimetry under non-standard condition, this research utilized
Monte Carlo method to simulate the transporting process of beams in G4 CyberKnife and compared
the results with the outcomes of actual measurement so that the accuracy of the established model
can be verified.
Materials and Methods: BEAMnrc/DOSXYZnrc Monte Carlo system was used for simulating the
physical structure of G4 CyberKnife model (containing 12 collimators of different sizes). With the adjustment
of parameters (the average energy, full width at half maximum of intensity distribution) of
electron beam, data of dose distribution for the fields (percentage depth dose, off-axis ratio, output factor,
beam quality, correction factor, etc.) in the water phantom could be acquired and compared with the
actual measurement so as to determine the parameters of electron beam in the simulated model.
Results: In the comparisons of percentage depth dose, the errors with more than 90% of the points were
smaller than 1% (except 5mm and 7.5mm collimators). And for the off-axis ratios, the errors with more
than 90% of the points were smaller than 1% (except 5mm, 7.5mm and 10mm collimators). The values
of beam output factors in a Monte Carlo system were consistent with the measured values when the collimator
sizes were larger than 20mm, and when the collimator size were smaller than 15mm, a certain
deviation for output factor existed. The value of beam quality in Monte Carlo model was 0.639, which
was close to that of the measured beam quality, 0.635.
Conclusion: The parameters of electron beam were finally determined (the average energy was 6.9MeV
and the full width at half maximum of intensity distribution was 0.4 cm). The Monte Carlo model established
in this research was able to accurately simulate the beam transporting process of CyberKnife and
thus might lay the foundation for the further research.