Air Kerma Rate Constant V Reference Air Kerma Rate V Apparent Activity

The AIR KERMA RATE in air ($K_{air}$) is given by the following relation:

\begin{align} {(K_{air})}_{air} = \frac{A. \Gamma_{AKR}}{d^2} ) \end{align}

where A is the source activity
and d is the distance between the point of interest and the point source.

The REFERENCE AIR KERMA RATE is the recommended specification for brachytherapy sources:

\begin{equation} K_{air}.({d_{ref})_{air}} \end{equation}

which is defined by the ICRU as the air kerma rate in air at a reference distance of 1 m, corrected for air attenuation and scattering.

The APPARENT ACTIVITY [ $A_{app}$ ] for a given brachytherapy source is defined as the activity of a hypothetical unfiltered point source of the same radionuclide that would give the same air kerma rate in air at a reference distance (typically 1 m) along the perpendicular bisector of the source.

\begin{align} AKR_{reference} = AKR_{constant} . \frac{A_{app}}{d^2} \end{align}

The Air Kerma Rate Constant is an attribute of radionuclide used in a brachytherapy source which relates to the type and energy of the beam produced. The Reference Air KERMA rate is an Air Kerma Rate Constant measured at standard conditions permitting the correlation of dose measurement with unit Activity, while the Apparent activity is expressed as a relative Activity as if the source where 'perfect' (a point source without anisotropy).
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