Monte Carlo calculation of depth doses for small field of CyberKnife

Purpose: A Monte Carlo (MC) model of CyberKnife was developed as a quality assurance tool. The percentage depth dose (%dd) was verified by using this model. Materials and Methods: An MC model was developed with Electron Gamma Shower version 4 (EGS4) in two steps: (1) a model of the CyberKnife treatment head and (2) a model of the collimator and phantom. The bremsstrahlung spectrum was calculated using the first model, and this spectrum was then used to calculate %dds with the second model. The calculated %dds for a large field (60 mmφ) and three small fields (30, 15, and 5 mmφ) were compared with those measured with a diamond detector. Results and Discussion: The MC-calculated and measured %dd-curves for the 60 mmφ field were in excellent agreement (<1.85%), thus confirming the validity of the model. Discrepancies between the calculated and measured %dd-curves increased with decreasing field size, with considerable discrepancy (11.62%) for the 5 mmφ field due to lateral electron disequilibrium. Accurate dose can be determined with MC even in small fields. Conclusion: The MC technique can provide reliable standard data for accurate dose delivery with high-technology radiotherapies using small beams.