Study On Dosimetry Properties Of Cyberknife—Based On Monte Carlo Methold

The paper was based on cyberknife treatment machine with 800 MU/min,and studied its dosimetric properties using the Monte Carlo method.Purpose:To setup the Monte Carlo dose calculation model of cyberknife,and simulate,calculate and analyze dosimetric properties of the cyberknife;To put forward a new calibration method and calculate the correction factor based on Alfonso's new formula.Method:A large number of literature were reviewed and the treatment head model of cyberknife was built with BEAMnrc,and the correctness of the model was adjusted and verified by comparing with measured data;The dosimetry characteristics of cyberknife were analyed with the BEAMDP,DOSXYZnrc and EGS_chamber;A relationship between cyberknife and linear accelerator or ⁶⁰Co was established,and the correction factor(k_QCK,QLINAC or k_QCK,Q60Co) was calculated when cyberknife beam was been calibrated based on Alfonso's new formula.Results:1. The primary source parameters of cyberknife by Monte Carlo simulation was E=6.5MV,FWHM= 2 mm;2. The mass density of epoxy resin of the semiconductor PTW60012 structure was 1.78 g cm -3 with Monte Carlo calculation;3. For the beams larger than 10 mm,PDD errors of each beam were within 2%;And for 5 mm?7.5 mm collimators, PDD errors were relatively larger,errors were reached 4.27% and 2.86% at 20 cm depth in water,respectively;While simulation with semiconductor,the errors between calculated values and measured values were within 2% at different depths;4. Although beam profiles were shown the difference less than 3% in the cones larger than 20 mm,there was the error less than 3.5% in the cones smaller 20 mm;5. The correction factors Sc,p were 0.953,0.956,and 0.973 for the three smallest collimaror of cyberknife beams respectively;6. The peak energy of photon spectra was 0.38 MeV,and the average energy of photon was 1.57 MeV;7. The intensity of Photon was 3 orders of magnitude higher than the intensity of electron;8. The angular distribution of photon was mainly concentrated within the range of 5° off the central axis,while the angular distribution of electron was in a relatively larger range;9. According to the TG-51 agreement and TRS398 report,we measured and converted by equivalent square field size?ESFS?,and got the kQ ?k_Q,Q₀? for farmer ion chamber FC-65G was 0.995 and 0.994, respectively;10. According to the two protocols, the errors between the data measured under cyberknife condition and the data determined by ESFS conversion were 2.77% and 3.6%.The results validated the correction of xiong and Rogers's conclusion;11. When cyberknife beam was been calibrated,the correction factors between cyberknife and standard 6 MV LINAC for three common farmer ion chamber ?NE2571,Exradin A12,PTW30010? were 1.0012,1.0013, and 1.0009,and those between Cyberknife and 60Co?were 0.9942,0.9953,and 0.9909.Conclusion:1.The Monte Carlo model of cyberknife have been established well, and verified the PDD,OAR,and Sc,p;But for the two smallest collimator beam ?5 mm and 7.5 mm?, the measured data from semiconductor can be applied until they have been revised;The cyberknife dosimetry characteristics achieved by Monte Carlo method has certain significance for clinical and radiological protection;2. According to the proposed new method, we can calibrate the cyberknife accurately and conveniently using the simulation results.