MO‐D‐108‐06: Off‐Axis Ratio Based Correction for the Determination of TMR for Flattening‐Filter Free (FFF) Mode Beams

Abstract

Purpose: Due to the nature of the beam fluence of the flattening‐filter free (FFF) mode from Varian TrueBeam linear accelerator, the tissue‐maximum ratio (TMR) conversion formalism from percent‐depth dose (PDD) may not be appropriate at large field sizes and large depths. The purpose of this work was to establish an analytical correction formalism which accounts for the differences in FFF‐mode beam profiles that corrects for small differences in TMR between the calculations and measurements. Methods: Two FFF‐mode beams (6‐MV and 10‐MV) were investigated using Varian TrueBeam linear accelerator. Initially, the analytical correction formalism starts out with the TMR conversion equation which is the product of the PDD, inverse square, and phantom scatter ratio. A ratio of two off‐axis ratios (OARs) at the surface and depth of interest is determined for a given field size. This OAR ratio is multiplied with the phantom scatter term in the TMR conversion equation to account for small differences in TMR values at large field sizes (>= 15 × 15 cm2) and depths (>= 15 cm). Results: For 10‐MV FFF‐mode, the difference between the measured and the standard TMR formalism ranged from ‐ 3.2% to ‐ 0.9% (mean = ‐ 1.8% and 1‐standard deviation = 0.7%) for field sizes >= 15 × 15 cm2 and >= 15 cm depth. When the proposed correction formalism was applied to the TMR conversion formalism, the difference between the measured and the corrected TMRs for field sizes >= 15 × 15 cm2 and >= 15 cm depth ranged from −0.7% to −0.2% (mean = −0.4% and 1‐standard deviation = 0.1%). Similar relationships were observed for 6‐MV FFF‐mode. Conclusion: The analytical correction formalism which accounts for the differences in the FFF‐mode beam profiles has been shown to correct for the small differences in TMR that were observed without the correction.