Monte Carlo study of a Cyberknife stereotactic radiosurgery system

Abstract

This study investigated small-field dosimetry for a Cyberknife stereotactic radiosurgery system using Monte Carlo simulations. The EGSnrc/BEAMnrc Monte Carlo code was used to simulate the Cyberknife treatment head, and the DOSXYZnrc code was implemented to calculate central axis depth-dose curves, off-axis dose profiles, and relative output factors for various circular collimator sizes of 5 to 60 mm. Water-to-air stopping power ratios necessary for clinical reference dosimetry of the Cyberknife system were also evaluated by Monte Carlo simulations. Additionally, a beam quality conversion factor, kQ, for the Cyberknife system was evaluated for cylindrical ion chambers with different wall material. The accuracy of the simulated beam was validated by agreement within 2% between the Monte Carlo calculated and measured central axis depth-dose curves and off-axis dose profiles. The calculated output factors were compared with those measured by a diode detector and an ion chamber in water. The diode output factors agreed within 1% with the calculated values down to a 10 mm collimator. The output factors with the ion chamber decreased rapidly for collimators below 20 mm. These results were confirmed by the comparison to those from Monte Carlo methods with voxel sizes and materials corresponding to both detectors. It was demonstrated that the discrepancy in the 5 and 7.5 mm collimators for the diode detector is due to the water non-equivalence of the silicon material, and the dose fall-off for the ion chamber is due to its large active volume against collimators below 20 mm. The calculated stopping power ratios of the 60 mm collimator from the Cyberknife system (without a flattening filter) agreed within 0.2% with those of a 10 X 10 cm2 field from a conventional linear accelerator with a heavy flattening filter and the incident electron energy, 6 MeV. The difference in the stopping power ratios between 5 and 60 mm collimators was within 0.5% at a 10 cm depth in water. Furthermore, kQ values for the Cyberknife system were in agreement within 0.3% with those of the conventional 6 MV-linear accelerator for the cylindrical ion chambers with different wall material.

Total scatter factors of small beams: A multidetector and Monte Carlo study

“…In particular, for the diode 6 found s c,p = 0.719± 0.015, compared to 0.710 of the present study; for the 7.5 mm collimator they reported s c,p = 0.849± 0.012 and for the 10 mm collimator s c,p = 0.892± 0.011, compared to 0.852 and 0.895 of the present study, respectively. Araki et al 5 reported a graphical representation of average s c,p values from 14 Cyberknife units installed in Japan, giving s c,p = 0.72 for a diode detector and the 5 mm collimator, 0.845 for the 7.5 mm, and 0.885 for the 10 mm collimator. The diode detector employed in those studies was not of the same type reported in this work, however, its characteristics could be considered similar; both detectors were an unshieldedtype diode.…”

“…2 Discrepancies in total scatter factor values have been reported for the smallest collimators of the Cyberknife radiosurgery system. [3][4][5][6][7][8][9] Such discrepancies might be explained as a contribution of several factors: differences in the choice of the dosimeters among centers, possible inaccuracies in the experimental setup ͑in particular for the 5 mm cone͒, and differences in beam parameters between units ͑in particular the electron beam width͒. In fact, it has been observed for other linear accelerators 10 that units with a similar design can have spot sizes that can differ up to 1 mm.…”

Output factor comparison of Monte Carlo and measurement for Varian TrueBeam 6 MV and 10 MV flattening filter‐free stereotactic radiosurgery system

“…One useful tool in dealing with small field effect is Monte Carlo (MC) simulation. Many recent studies have applied Monte Carlo simulations to examine the output factors of small field sizes used for radiosurgery such as CyberKnife, 6 , 7 Gamma Knife, 8 , 9 and Brainlab (10) . In addition, numerous studies have used Monte Carlo simulation to examine the accuracy of diode detectors measurements for small field dosimetry 11 , 12 , 13 , 14 …”

Development of a dose verification system for Vero4DRT using Monte Carlo method

“…Generally, the MC‐based linac head is verified using PDDs and beam profiles for several rectangular fields formed by specifically shaped collimators or variable jaws 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 . On the other hand, the secondary collimator in the Vero4DRT is of a fixed type and the field is formed using the MLC only.…”

“…First, the linac head, exclusive of MLC, is modeled against measurement data as a patient‐independent component. Next, the patient‐dependent MLC model is compared with several measurement data using well‐commissioned phase space data (PSD) from the linac head 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 …”