Purpose: To evaluate planning quality and dosimetric differences of clinically deliverable 3D conformal plans generated from Tomotherapy with TomoDirect^TM (TD) and conventional field-in-field approach in prone breast treatment. Materials and methods: Total of twelve randomly selected early stage left breast patients who went through lumpectomy and were previously treated on traditional Linear Accelerator (LINAC) have been re-planned and tested on Tomotherapy TomoDirect module. Baseline prescription dose was chosen at 50.4 Gy (1.8 Gy × 28 fractions) to cover ≥95% of PTV for planning criteria with other critical structure dose constraints in the thoracic region. Planning outcomes such as D₉₅ (95% of volume of PTV receiving the prescribed dose), D₅ and D₁, heart, both lungs as well as the contralateral breast were simultaneously evaluated. Conformity of the prescription isodose/volume to PTV was evaluated as conformity index (CI) and dose uniformity was also evaluated with homogeneity index (HI) in the same study series. All outcome parameters were analyzed and summarized to evaluate dosimetric impact of planning qualities between these two planning platforms. Results: The planning results indicate that CI, HI, D₉₅, D₅ and D₁ of PTV, critical structures such as heart, ipsilateral and contralateral lungs as well as contralateral breast doses were comparable but with better overall statistical end points from TD plans. The D₉₅, D₅ and D₁ of PTV for TD plans were superior in dosimetric analysis and more uniform than those plans generated from Pinnacle^TM field-in-field planning technique. Overall, TD plans have superior planning quality than the conventional method does, with straightforward and automated planning process once the beam delivery parameters were established. Conclusions: From the clinical treatment planning results, plans from TD in general achieved better uniform tumor coverage with fewer hot spots while sparing more critical structures were based upon isodose distribution and Dose Volume Histogram (DVH) analysis. Image guidance of TD delivery automates the setup within the treatment bore without tedious verification process compared to the process with LINAC. Though all plans are deliverable, TD planning possesses dosimetric advantages due to its modulated optimization pattern. However, TD did present a challenge during the simulation if a patient is oversized with long pendulant breast which is hard to fit into the Tomotherapy ring structure. From our analysis, TD plans reserve superior dosimetric outcome with CI, HI, D₉₅, D₅, and D₁ of PTV, and better sparing contralateral lung and breast doses.