[Output-factors for squared, rectangular, and elongated photon fields of medical linear accelerators]

For reduction of data scattering in output-factor calculation, a new correction for Stirling's formula is introduced that takes into account the position of the upper and lower collimator jaw settings of linear accelerators. This correction factor requires two simple geometrical data of the treatment head, to describe the collimator exchange effect for open- as well as wedged fields. This model also implies a reduction of output measurements to only three field sizes, the minimum, the maximum, and a reference field size, without significant loss of accuracy in output factors.