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

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• Accurate measurement of scatter output factor (S_cp) is critical for linear accelerator (linac) dosimetry, particularly in small radiation fields (<4 cm × 4 cm).
• Standard ionization chambers (ICs) have limitations in small fields, and acquiring specialized equipment is costly for many radiotherapy centers.

Purpose of the Study:

• To evaluate the efficiency of EGSnrc Monte Carlo (MC) simulations for calculating S_cp across various field sizes, including small fields, for an Elekta Synergy linac.
• To establish MC as a viable and cost-effective tool for S_cp determination, addressing the limitations of conventional IC measurements.

Main Methods:

• Developed and validated a 6 MV linac model using BEAMnrc and DOSXYZnrc MC codes.
• Validated the linac model by comparing simulated percentage depth doses (PDDs), beam profiles, and Tissue Phantom Ratio (TPR_20,10) with IC measurements.
• Calculated S_cp, collimator scatter factor (S_c), and phantom scatter factor (S_p) using the validated MC model.

Main Results:

• Simulated PDDs and beam profiles showed good agreement (±2%) with IC measurements.
• The validated MC model accurately predicted TPR_20,10 (0.675 for 10 cm × 10 cm field).
• MC-calculated S_cp, S_c, and S_p values were within ±2% of IC measurements, with S_c = 0.955 and S_p = 0.884 for a 3 cm × 3 cm field.

Conclusions:

• EGSnrc MC simulation is an efficient and accurate tool for calculating S_cp in radiotherapy.
• MC simulations provide a reliable and potentially more accessible method for dosimetry in small linac fields.
• This study validates MC as a promising alternative to ICs for S_cp calculations, especially in resource-limited settings.