There are a number of questions rega r d ing reliability of stereotactic radiosurgery (SRS), stereotactic radiotherapy (SRT), and small-field dosimetry with radiation detectors used for clinical practice due to the lack of lateral electron equilibrium and uncertainty in beam energy. This paper presents a numerical investigation on the effective point of measurement of thimble ionization chambers in megavoltage photon beams with small fields. A dosimetry study was performed for a Microtron that generated 4 MV,6 MV,10 MV, and 14 MV photon beams. To verify the effective point of measurement, measurements were done with detectors commonly used in SRS, such as small-volume ionization chambers with the chamber axis parallel to the beam axis. The effective point of measurement for relative photon beam dosimetry depends on the details of the chamber design, including the cavity length and cavity radius. Moreover, the effective point of measurement also depends on the beam quality and field size. Once the effective point of measurement with the chamber axis parallel to the beam axis is selected appropriately, measured depth-ionization curves can be equated to measured percent depth dose curves for all depths within ±0.5%.