How does to determine equivalent field of irreguler field ?
What is the explicit form about the mathematical equations for calculate the equivalent field of an irreguler field ? Please explain me about clarkson integration or other methods to determine equivalent field for arbitrary field.
Thank you.
There are various formulae for equivalent square. The simplest is E=4A/P where A is the area of the field and P is the perimeter. This works OK for slightly irregular fields, especially ones defined with blocks. For MLC-defined fields this tends to underestimate the equivalent square, since the perimeter of MLC-defined fields is larger than a similar block-defined one.
Clarkson integration works by dividing the field up into a large number of segments (in circular geometry, centered on the point of interest), and for each segment looking up some suitable scatter factor (SAR can be used, or the PSF from BJR25) for a circular field of the same radius as the segment. Averaging round the circle gives an average value of the scatter factor. Looking back into the original data to determine what field size would give the same scatter factor gives you the final answer. (usually as an equivalent diameter, which then needs to be converted back to equivalent square using the methods in BJR25). Various computational tricks are needed for complex shapes if the segment at a agiven angle has gaps in it.
A simpler method for MLC-defined fields is to average the MLC-defined width (for open leaf pairs in the jaw defined field only) to give W, and to add up the number of such leaf pairs times the MLC spacing to give L, and use the formula E=2LW/(L+W). This gives very similar answers to the Clarkson method, for considerably less effort.
There are various formulae for equivalent square. The simplest is E=4A/P where A is the area of the field and P is the perimeter. This works OK for slightly irregular fields, especially ones defined with blocks. For MLC-defined fields this tends to underestimate the equivalent square, since the perimeter of MLC-defined fields is larger than a similar block-defined one.
Clarkson integration works by dividing the field up into a large number of segments (in circular geometry, centered on the point of interest), and for each segment looking up some suitable scatter factor (SAR can be used, or the PSF from BJR25) for a circular field of the same radius as the segment. Averaging round the circle gives an average value of the scatter factor. Looking back into the original data to determine what field size would give the same scatter factor gives you the final answer. (usually as an equivalent diameter, which then needs to be converted back to equivalent square using the methods in BJR25). Various computational tricks are needed for complex shapes if the segment at a agiven angle has gaps in it.
A simpler method for MLC-defined fields is to average the MLC-defined width (for open leaf pairs in the jaw defined field only) to give W, and to add up the number of such leaf pairs times the MLC spacing to give L, and use the formula E=2LW/(L+W). This gives very similar answers to the Clarkson method, for considerably less effort.
What about E = square-root( A * B) used in small fields, how does it compare to E = 4A/P
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