SU-E-T-113: Volume Effect Correction Factor KV for Small-Field Photon Dosimetry with Ionization Chambers.

PURPOSE The volume effect of ionization chambers gives rise to a spatial averaging effect that can be expressed mathematically as the convolution of the true dose profile with the detector's response function. The latter has been shown to be best described by Gaussian distribution. Based on this knowledge, the volume effect correction factor kV is derived. METHODS To derive kV, a sixth degree polynomial is fitted to the true dose profile: D(x) = a0 + a2×2 + a4×4 + a6×6. The measured dose profile M(x) is calculated as the convolution product of D(x) with a one-dimensional normalized Gauss function with standard deviation s. Therefore kV at the dose maximum has the value D(0)/M(0), which is a function of the coefficients a0,2,4,6 and the detector specific s. In the case where D(x) is unknown, kV can be derived analogously from M(x) so that M(x) = b0 + b2×2 + b4×4 + b6×6, where kV can now be expressed as a function of the coefficients b0,2,4,6 and s. RESULTS The magnitudes of kV,lat and kV,long were calculated for 1 to 5 cm dose profiles using measured s values, both in the lateral and the longitudinal directions, for a set of common ionization chambers. At field widths above 2 cm, the values of kV,lat fall below 1.01 for all the chambers evaluated, whereas it needs field widths above 4 cm to get all values of kV,long below 1.01. Since the detector's signal is integrated over the sensitive volume, the total kV can be calculated as kV,total = kV,lat . kV,long. CONCLUSIONS In this work, a correction is developed to eliminate the volume effect of ionization chambers when they are positioned in the maxima of dose profiles, particularly for the performance of output factor measurements for the calibration of narrow photon beams.