To limit the entrance dose to normal tissue and achieve the appropriate treatment time (TT) by using three different virtual structures with directional blocks for left-sided post-mastectomy radiation therapy (PMRT) with regional nodal irradiation (RNI).

Ten breast cancer patients who received PMRT by helical tomotherapy were enrolled. Three virtual structures were created for each patient: Organ-based, L-shaped (LB) and C-shaped (CB). The dose to the target and organ at risk (OARs), TT, the volume which received dose 5 Gy (V5Gy), integral dose (ID) and block structure contouring workload (BSCW) of the three virtual block techniques were evaluated. The performance scores were used to explore the suitable technique.

The CB plans showed a significantly better V5Gy, ID and contralateral breast-sparing. However, the CB plans revealed the longest TT and BSCW (p < 0·001). Contrary to the LB, the LB plans showed a significantly reduced TT and BSCW and provided the balance of plan efficiency with the highest score.

The LB technique is considered to be the suitable technique for left-sided PMRT with RNI and provided the advantage of TT, V5Gy, ID and BSCW while maintaining acceptable criteria for the target and OARs.