This study aimed to compare the post-modified radical mastectomy radiotherapy (PMRMRT) for left-sided breast cancer utilizing 3-dimensional conformal radiotherapy with field-in-field technique (3DCRT-FinF), 5-field intensity-modulated radiation therapy (5F-IMRT) and 2- partial arc volumetric modulated arc therapy (2P-VMAT). We created the 3 different PMRMRT plans for each of the ten consecutive patients. We performed Kruskal-Wallis analysis of variance (ANOVA) followed by the Dunn's-type multiple comparisons to establish a hierarchy in terms of plan quality and dosimetric benefits. P < 0.05 was considered statistically significant. Both 5F-IMRT and 2P-VMAT plans exhibited similar PTV coverage (V95%), hotspot areas (V110%) and conformity (all p > 0.05), and significantly higher PTV coverage compared with 3DCRT-FinF (both p < 0.001). In addition, 5F-IMRT plans provided significantly less heart and left lung radiation exposure than 2P-VMAT (all p < 0.05). The 3DCRT-FinF plans with accurately estimated CTV displacement exhibited enhanced target coverage but worse organs at risk (OARs) sparing compared with those plans with underestimated displacements. Our results indicate that 5F-IMRT has dosimetrical advantages compared with the other two techniques in PMRMRT for left-sided breast cancer given its optimal balance between PTV coverage and OAR sparing (especially heart sparing). Individually quantifying and minimizing CTV displacement can significantly improve dosage distribution.