Purpose <p>Monte Carlo simulation-based CT dose calculations incorporating angular and longitudinal tube current modulation (TCM) were performed to examine the trends in dose distribution with and without TCM.</p> Methods <p>We performed dose calculations for a 256-slice CT scanner using the Particle and Heavy Ion Transport code System (PHITS). CT dose index 100 (CTDI₁₀₀) and weighted CTDI (CTDI_w) were computed by arranging a 32-cm diameter CTDI phantom, pencil ionization chamber, and patient table in calculations, to validate the accuracy of the calculations. Additionally, two chest phantoms were employed to obtain absorbed dose distributions with fixed mA and TCM with Noise Index of 13. The modulated tube current data was extracted from the acquisition data files.</p> Results <p>Comparing the calculated CTDI₁₀₀ and CTDI_w with the measured values, the respective differences were 3.13% at the center, 5.78% at the 12 o’clock position, 6.93% at the 3 o’clock position, 10.5% at the 6 o’clock position, and 6.22% at the 9 o’clock position. The difference in CTDI_w was 6.45%. By incorporating TCM into calculations, the axial dose distribution tended to differ depending on the location depending on the presence or absence of TCM.</p> Conclusions <p>The axial dose distribution when TCM was applied showed different trends than when TCM was not applied, depending on the phantom type and location.</p>