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

Overview of attention for article published in Medical Physics, October 2009
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Report of AAPM Therapy Physics Committee Task Group 74: In-air output ratio, Sc, for megavoltage photon beams
Published in
Medical Physics, October 2009
DOI 10.1118/1.3227367
Pubmed ID

Timothy C. Zhu, Anders Ahnesjö, Kwok Leung Lam, X. Allen Li, Chang-Ming Charlie Ma, Jatinder R. Palta, Michael B. Sharpe, Bruce Thomadsen, Ramesh C. Tailor


The concept of in-air output ratio (Sc) 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 Sc. The main thrust of the report is to devise a theoretical and measurement formalism that ensures interinstitutional consistency of Sc. The in-air output ratio, Sc, is defined as the ratio of primary collision water kerma in free-space, Kp, 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 Sc. The authors present a correction formalism to extrapolate the correct Sc from the measured values using high-Z miniphantom. Miniphantoms made of high-Z material are used to measure Sc for small fields (e.g., IMRT or stereotactic radiosurgery). This report presents a review of the components of Sc, including headscatter, source-obscuring, and monitor-backscattering effects. A review of calculation methods (Monte Carlo and empirical) used to calculate Sc for arbitrary shaped fields is presented. The authors discussed the use of Sc in photon dose calculation algorithms, in particular, monitor unit calculation. Finally, a summary of Sc data (from RPC and other institutions) is included for QA purposes.

Mendeley readers

The data shown below were compiled from readership statistics for 163 Mendeley readers of this research output. Click here to see the associated Mendeley record.

Geographical breakdown

Country Count As %
Spain 4 2%
Canada 2 1%
United States 2 1%
Japan 2 1%
United Kingdom 1 <1%
Iran, Islamic Republic of 1 <1%
India 1 <1%
Unknown 150 92%

Demographic breakdown

Readers by professional status Count As %
Other 36 22%
Student > Ph. D. Student 35 21%
Researcher 26 16%
Student > Master 22 13%
Student > Postgraduate 11 7%
Other 33 20%
Readers by discipline Count As %
Physics and Astronomy 95 58%
Medicine and Dentistry 41 25%
Unspecified 13 8%
Engineering 6 4%
Computer Science 2 1%
Other 6 4%