Megavoltage photon beams are routinely used for external-beam radiotherapy. Recently, new treatment modalities based on dynamic and intensity modulated (IM), beam delivery systems are increasingly used in clinical practice. The purpose of this work is to investigate the energy spectrum and microdosimetric features of these photon beams. A Monte Carlo technique was first used to simulate beam lines of medical accelerators and to compute photon fluence and spectrum per unit dose-to-water inside the irradiated medium. Subsequently, a track structure code was used to compute the lineal energy and its distribution in a 1 micron sphere based on the individual photon spectrum. Results showed that the low energy photon component varied significantly with field size and location within the field due to the presence of the scattered photons. The calculated dose-mean lineal energy <yD> ranged from 2.3 keV.micron-1 at a depth of 1.5 cm along the central axis of a 4 cm x 4 cm field, to 3.7 keV.micron-1 at a depth of 20 cm of the field edge in a 10 cm x 10 cm field. In the tested IM fields at a depth of 10 cm, <yD> ranged from 2.6 to 3.5 keV.micron-1, which was inversely related to the dose intensity in the field. The <yD> values for the clinical photon beams were also significantly greater than that of a reference 60Co beam (1.8 keV.micron-1 from the calculation). The beam quality factor was estimated to vary within 20% due to the change of the energy spectrum for the radiotherapy photon fields.