@article{oai:repo.qst.go.jp:00046264,
 author = {Tajiri, Minoru and Watanabe, Kazuhiro and et.al and 田尻 稔 and 渡邊 和洋},
 issue = {2},
 journal = {Medical Physics},
 month = {Feb},
 note = {Purpose: The in-air output ratio (Sc) for a rectangular field is usually obtained using an equivalent square field formula. However, it is well-known that Sc obtained using an equivalent square field formula differs slightly from the measured Sc. Though several correction methods have been suggested for the monitor-backscatter effect, we propose a more simple correction method for a rectangular field. 
Methods: For rectangular fields and equivalent square fields, we assumed that the output variation was the product of six output variations for each backscattering area at the top of the collimator jaws, and the correction factor was the ratio of the output variation for a rectangular field to the output variation for an equivalent square field. The output variation was measured by using a telescope measurement.
Results: The differences between the measured and corrected Sc ranged from -0.20% to 0.28% for symmetric rectangular fields by applying the correction factor to Sc obtained using an equivalent square field formula. This correction method is also available for asymmetric rectangular fields.
Conclusions: We propose a method to correct Sc obtained using an equivalent square field formula, and a method to obtain the output variation for a field defined by collimator jaws.},
 pages = {671--675},
 title = {Correction method for in-air output ratio for output variations occurring with changes in backscattered radiation},
 volume = {39},
 year = {2012}
}