Determination of the Small-Field Output Factor for 6 MV Photon Beam Using EGSnrc Monte Carlo

Accuracy of ionization chamber (IC) to measure the scatter output factor (S(cp)) of a linear accelerator (linac) is crucial, especially in small field (<4 cm × 4 cm). The common IC volume of 0.6 cc is not adequate for small-field measurement and not all radiotherapy centers can afford to purchase additional IC due to the additional cost. This study aimed to determine the efficiency of the EGSnrc Monte Carlo (MC) to calculate the S(cp) for various field sizes including small field in Elekta Synergy (Agility multileaf collimator) linac. The BEAMnrc and DOSXYZnrc user codes were used to simulate a 6 MV linac model for various field sizes and calculate the radiation dose output in water phantom. The modeled linac treatment head was validated by comparing the percentage depth dose (PDD), beam profile, and beam quality (Tissue Phantom Ratio (TPR)(20,10)) with the IC measurement. The validated linac model was simulated to calculate the S(cp) consisting of collimator scatter factor (S(c)) and phantom scatter factor (S(p)). The PDD and beam profile of the simulated field sizes were within a good agreement of ±2% compared with the measured data. The TPR(20,10) value was 0.675 for field size 10 cm × 10 cm. The S(cp), S(c), and S(p) simulated values were close to the IC measurement within ±2% difference. The simulation for S(c) and S(p) in 3 cm × 3 cm field size was calculated to be 0.955 and 0.884, respectively. In conclusion, this study validated the efficiency of the MC simulation as a promising tool for the S(cp) calculation including small-field size for linac.