Effect of auto flash margin on superficial dose in breast conserving radiotherapy for breast cancer

To investigate the dose-effect of Auto Flash Margin (AFM) on breast cancer's superficial tissues based on the Treatment Planning System (TPS) in the breast-conserving radiotherapy plan.

A total of 16 breast-conserving patients with early stage breast cancer were selected, using the X-ray Voxel Monte Carlo (XVMC) algorithm. Then, every included case plan was designed using a 2 cm-AFM (the value of AFM is 2 cm) and N-AFM (without AFM). Under the condition of ensuring the same configuration of #MU and collimator, the absorbed dose after a simulated inspiratory motion was calculated again using the new plan center, which moved backward to the linac source. The dose difference between the measurement points between AFM and N-AFM groups was compared.

In the dose results, PTV_V50Gy of the AFM group was superior to that of the N-AFM group, PTV_D2% , PTV_Dmean , Lung_Ipsi_V20Gy , Lung_Ipsi_Dmean , and Body_Dmax . Also, the dose results of the N-AFM group were significantly higher than those of the AFM group. However, there was no significant difference between Lung_Contra_V5Gy , Heart_Dmean , and Breast_Contra_V10Gy in the two groups. In the collimator alignments at the same angle between groups, the AFM group formed an apparent air region outside the collimator compared with the N-AFM group. In the XVMC algorithm feature parameter, the AFM group had less #MU, higher QE, and slightly longer optimization time. The #segments of both groups were close to the 240 control points preset by the plan. The validation results of EBT3 film in both groups were more significant than 95%, meeting the clinical plan's application requirements. The difference in film results between groups was mainly reflected in the dose distribution at the near-source. 4DCT was used to summarize the maximum and minimum inspiratory motion distances of 7.31 ± 0.45 and 3.42 ± 0.91 mm respectively.

These results suggest that the AFM function application could significantly reduce the possibility of insufficient tumor target caused by inspiratory motion and ensure sufficient tumor target exposure.