In this study, the validity of the effective (virtual) point-detector concept for the CdZnTe (CZT) detector for disc-shaped radioisotope sources has been investigated in the photon energy range of 50-662 keV. It has been found that the virtual point-detector model is valid for conventional CZT detectors. The photon energy dependence of the virtual interaction depth (VID) in the CZT detector does not seem to fit to a good trend. However, in general, the measured VID values are increasing with increasing gamma-ray energy in the range of 50-662 keV, but the VID values always remain within the physical thickness of the CZT detector. For instance, the VID value was measured to be about 4.4 +/- 0.5 mm in the 5 mm thick CZT detector at 662 keV gamma-ray energy. In application, the VID values for photons can be used for efficiency calculation models by scaling the measured detection efficiency as a function of energy and source-to-detector distance.