Report of AAPM Therapy Physics Committee Task Group 74: In-air output ratio, S_c, for megavoltage photon beams

The concept of in-air output ratio (S_c) was introduced to characterize how the incident photon fluence per monitor unit (or unit time for a Co-60 unit) varies with collimator settings. However, there has been much confusion regarding the measurement technique to be used that has prevented the accurate and consistent determination of S_c. The main thrust of the report is to devise a theoretical and measurement formalism that ensures interinstitutional consistency of S_c. The in-air output ratio, S _c, is defined as the ratio of primary collision water kerma in free-space, K_p, per monitor unit between an arbitrary collimator setting and the reference collimator setting at the same location. Miniphantoms with sufficient lateral and longitudinal thicknesses to eliminate electron contamination and maintain transient electron equilibrium are recommended for the measurement of S_c. The authors present a correction formalism to extrapolate the correct S_c from the measured values using high- Z miniphantom. Miniphantoms made of high- Z material are used to measure S _c for small fields (e.g., IMRT or stereotactic radiosurgery). This report presents a review of the components of S_c, including headscatter, source-obscuring, and monitor-backscattering effects. A review of calculation methods (Monte Carlo and empirical) used to calculate S_c for arbitrary shaped fields is presented. The authors discussed the use of S_c in photon dose calculation algorithms, in particular, monitor unit calculation. Finally, a summary of S_c data (from RPC and other institutions) is included for QA purposes.