Monte carlo simulation of varian clinac iX 10 MV photon beam for small field dosimetry

The lack of lateral electronic disequilibrium (LED) becomes a main problem in small field. This factor affects the dose in target volume cannot predict correctly. In addi"on, u"liza"on of high-energy linear accelerator (10 MV) can emit some unwanted par"cles (electron contamina"on). Therefore, the aim of this study was to characterize head linear accelerator (linac) Varian Clinac iX 10 MV photon beam for square small field size (1×1, 2×2, 3×3, 4×4 and 5×5 cm2) using Monte Carlo (MC) simula"on.
The commissioning process for this linac, has been conducted for field size 6×6, 10×10 and 20×20 cm2 by comparing the measurement and MC simula"on data. Head linac simula"on was performed with BEAMnrc and dose calcula"on with DOSXYZnrc. The phase space (phsp) data from BEAMnrc was analyzed using BEAMDP to get the par"cles informa"on in scoring plane.
The sca6er angle of par"cles depends on the field size. This factor affects the penumbra width in water phantom. On the other hand, PDD data show that the depth of maximum dose and penumbra width in small field shi:ed correspond with the number of sca6er par"cle. The difference of rela"ve output factor between measurement and MC results were found less than 2%. However, the 2% difference was s"ll acceptable in photon beam dosimetry.
From this simula"on, the electron contamina"on give contribu"on in surface dose of water phantom about 13.0581% and less than 1% for field size 10×10 cm2 and small field size, respec"vely.