  We studied the investigation of volume averaging effect for air-filled
cylindrical ionization chambers to determine correction factors in small photon
field for the given chamber. As a method, we measured output factors using
several cylindrical ionization chambers and calculated with mathematical method
similar to deconvolution in which we modeled non-constant and inhomogeneous
exposure function in the cavity of chamber. The parameters in exposure function
and correction factors were determined by solving a system of equations we
developed with measurement data and geometry of the given chamber. Correction
factors (CFs) we had found are very similar to that from Monte Carlo (MC)
simulation. For example, CFs in this study were computed as 1.116 for PTW31010,
and 1.0225 for PTW31016, while CFs from MC were reported as approximately
between 1.17 and 1.20 for PTW31010, and between 1.02 and 1.06 for PTW31016 in
of 6MV photon beam . Furthermore, the result from the method of deconvolution
combined with MC for chamber response function, also showed similar CF for PTW
30013, which was reported as 2.29 and 1.54 in and filed size respectively. The
CFs from our method provided similarly as 2.42 and 1.54. In addition, we
reported CFs for PTW30013, PTW31010, PTW31016, IBA FC23-C, and IBA CC13. As a
consequence, we suggested a newly developed method to measure correct output
factor using the fact that inhomogeneous exposure, force a volume averaging
effect in a cavity of air filled cylindrical ionization chamber. The result
from this method is very similar to that from MC simulation. The method we
developed can easily be applied to clinic.
