X-ray spectra emitted by computed tomography (CT) sources differ in shape and mean spectral energy ( E mean ) due to different anode geometries, materials and filtration. Nonetheless, a variety of spectra and monoenergetic photons are employed in Monte Carlo (MC) simulations, despite possible differences in total energy deposition ( E total ) and its spatial distribution ( E spatial ). This study aims at assessing resulting differences and implications. The effect of different measured and generated 120 kV p -equivalent CT X-ray spectra on E total and E spatial in a water phantom (Ø = 32 cm, d = 16 cm) was determined with MC simulations. Spectra were either measured in-house (reference: E mean,ref = 57 . 7 keV @ 0 ° fan angle; and fan-angle dependent spectra (FADS)) or generated (online tool) without and with 3.3 mm aluminum filtration, the latter matching E mean,ref . Additionally, monoenergetic photons matching E mean,ref were applied. Simulations were performed with (FM) and without (NM) beam-shaping. Compared to simulations employing the reference spectrum and using beam-shaping, relative differences in E total are: +2.2% (FADS), −5.4% (online tool, unfiltered), +0.1% (online tool, filtered) and −1.0% (57.7 keV). Maximum relative differences in E spatial are larger: −10% (FADS), −20% (online tool, unfiltered), −3% (online tool, filtered) and −20% (57.7 keV). Ignoring fan-angle dependent fluence modulation causes up to +50% difference in E spatial . For the use of different X-ray spectra, E total and E spatial are comparable for matching peak tube potential and E mean . Additionally, simulations of accurate E spatial require modeling of beam-shaping. Monoenergetic photons matching E mean,ref should not be employed for X-ray source emission modeling due to large inaccuracies in E spatial .