Clinical Implementation and Commissioning of Monte Carlo Dose Calculation Algorithm for CyberKnife Treatment Planning

The purpose of this study is to commission and evaluate the Monte Carlo (MC) dose calculation algorithm for the CyberKnife S7 Robot system (Accuray, Inc.), which utilizes a 6 - MV X - band linear accelerator on a robotic arm with a Multi Leaf Collimator (MLC). We implemented a MC dose calculation algorithm for SRS/SRT treatment planning with the CyberKnife system. This algorithm simulates energy transport and dose deposition in tissue using three independent probability distributions for initial photon characteristics, based on dosimetric data from the Accuray Precision? system. The model was optimized by adjusting maximum energy and source full width at half maximum (FWHM) to minimize discrepancies with measured tissue phantom ratios. MLC plans were calculated and irradiated on various phantoms. Dose and profile measurements were taken with a PTW farmer - type ionization chamber and PTW microdiamond chamber and EBT3 Gafchromic films, followed by comparisons to TPS doses to determine dose differences. Patient - specific quality assurance (QA) was conducted using global gamma index criteria of 2%/2 mm. The optimal parameters for maximum energy and source FWHM were found to be 6.4 MeV and 1.8 mm, respectively. Results of calculations made using these parameters agreed approximately with those actually measured to within ?1% only. Monte Carlo simulation model of MLC covering of CyberKnife is clinically acceptable but tends to underestimate the delivered dose by an average of -1.3%. After 11 cm depths the dosage difference added on further. It is pragmatically advised to rely on the Monte Carlo model with the MLC primarily in heterogeneous regions (7) such as lungs