Interpretation of in-air output ratio and its impact on Dose Calculation

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. 2 This quantity is also called inair output factor, collimator-scatter factor, and headscatter factor. 6 The latter two names were somewhat misleading since they emphasized a single component of the output ratio. We retained the symbol Sc because it has been widely used. In the early 90s, the development of conformal radiotherapy based on 3D treatment planning motivated development of models and experimental procedures to quantify different components of the accelerator output. There are multiple factors shown to influence the in-air output ratio, in particular, photons are scattered by structures in the accelerator head (head scatter), photons and electrons are backscattered into the monitor chamber (monitor backscatter), and at very small field sizes, a portion of the x-ray source is obscured by the collimators (source obscuring effect). Various sources of headscatter, the primary collimator, the flattening filter, the secondary collimators, the monitor chamber (and a wedge, if used), have been characterized. Several studies have measured the actual source distributions for the target as well as for the extended headscatter source at the flattening filter. To separate the components of the inair output ratio requires additional measurements. The availability of Monte Carlo simulation has provided a methodology to separate various components of the headscatter to interpret the measurement results, or validate analytical models.