Determination of small-field correction factors for cylindrical ionization chambers using a semiempirical method

Abstract

A semiempirical method based on the averaging effect of the sensitive volumes of different air-filled ionization chambers (ICs) was employed to approximate the correction factors for beam quality produced from the difference in the sizes of the reference field and small fields. We measured the output factors using several cylindrical ICs and calculated the correction factors using a mathematical method similar to deconvolution; in the method, we modeled the variable and inhomogeneous energy fluence function within the chamber cavity. The parameters of the modeled function and the correction factors were determined by solving a developed system of equations as well as on the basis of the measurement data and the geometry of the chambers. Further, Monte Carlo (MC) computations were performed using the Monaco (R) treatment planning system to validate the proposed method. The determined correction factors (k(Qmsr,Q)(fsmf,fref)) were comparable to the values derived from the MC computations performed using Monaco (R). For example, for a 6 MV photon beam and a field size of 1 x 1 cm(2), k(Qmsr,Q)(fsmf,fref) was calculated to be 1.125 for a PTW 31010 chamber and 1.022 for a PTW 31016 chamber. On the other hand, the k(Qmsr,Q)(fsmf,fref) values determined from the MC computations were 1.121 and 1.031, respectively; the difference between the proposed method and the MC computation is less than 2%. In addition, we determined the k(Qmsr,Q)(fsmf,fref) values for PTW 30013, PTW 31010, PTW 31016, IBA FC23-C, and IBA CC13 chambers as well. We devised a method for determining k(Qmsr,Q)(fsmf,fref) from both the measurement of the output factors and model-based mathematical computation. The proposed method can be useful in case the MC simulation would not be applicable for the clinical settings.