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Report of AAPM Therapy Physics Committee Task Group 74: In‐air output ratio, S c , for megavoltage photon beams
Author(s) -
Zhu Timothy C.,
Ahnesjö Anders,
Lam Kwok Leung,
Li X. Allen,
Ma ChangMing Charlie,
Palta Jatinder R.,
Sharpe Michael B.,
Thomadsen Bruce,
Tailor Ramesh C.
Publication year - 2009
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3227367
Subject(s) - task group , medical physics , physics , photon , dosimetry , nuclear medicine , nuclear physics , medicine , optics , engineering , engineering management
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.