Purpose <p>This study aimed to compare and evaluate plan quality metrics differences between two main methods of radiotherapy: 3D conformal radiation therapy (3DCRT) and volumetric modulated arc therapy (VMAT) for left-sided breast cancer patients after mastectomy in the radiotherapy department of Ain Wazein Medical Village Hospital (AWMV) using the Monaco treatment planning system.</p> Methods <p>The prescription radiation dose was 50&#xa0;Gy delivered in 25 fractions. Plan quality metrics, including target coverage, homogeneity index, conformity number, conformity index, dose spillage index, uniformity index, hotspot (V107%), and dose to critical structures, were assessed. The paired t-test and effect size analysis were conducted to determine if there is a significant difference between the means of the paired observations VMAT and 3DCRT plans by Origin Lab version 2018 software with a significance value of <i>P</i> &lt; 0.05.</p> Results <p>The evaluation of the dose distribution indicates that VMAT outperformed 3DCRT in the target coverage (95.0.27 ± 0.734 vs. 91.063 ± 1.959). The HI values were 0.572 ± 0.187 for 3DCRT and decreased to 0.284 ± 0.141 for VMAT. Better conformity and uniform dose distribution (<i>p</i> = 0.0254) in VMAT than in 3DCRT plans. In terms of the organs at risk, VMAT plans showed higher heart irradiation at lower doses V5Gy and V10Gy than 3DCRT plans, but significantly lower at high doses V25Gy and Dmax (<i>p</i> = 0.0135). The 3DCRT technique showed smaller low-dose parameters in ipsilateral lung V5Gy and V10Gy compared to the VMAT technique, with superior outcomes for high-dose parameter V20.</p> Conclusions <p>VMAT significantly reduces radiation doses to the organs at risk but did not fare well when low-dose radiation was introduced.</p>