Method to prevent the target volume from escaping out of the field in breast irradiation: Forming a "fall-off margin"

We aimed to obtain data that would enable the selection of the appropriate radiotherapy technique for whole breast irradiation (WBI) based on patients' physical characteristics and to evaluate the benefit of the new fall-off (FO) margin technique.

Ten patients with left-sided breast-conserving surgery, treated for breast carcinoma between August 2016 and September 2017, were included. The FO margin was created in five different plans of which two were formed by expanding the target volume out of the skin. The dose evaluation planning was statistically compared by calculating the target volume dosimetric parameters and the doses received by the organs at risk (OARs) for each technique. The volumetric-modulated arc therapy (VMAT) and intensity-modulated radiation therapy plans were considered ideal for WBI homogeneity and conformity indices, while the three-dimensional conformal radiotherapy (3DCRT) plan was considered nonideal.

The increase in the breast x-axis length values and equivalent spherical diameter (ESD) dimension decreased the ideal value, whereas the increase in y-axis length values and ESD dimension correlated significantly with the D98 increase. The techniques were significantly correlated with OARs, such as V5, heart max, left anterior descending artery maximum, ipsilateral lung V5 and V20, and contralateral breast V5. Monitor unit values were significantly low in the 3DCRT and VMAT plans.

The new FO margin structure will have benefits for practical application because the head designs of linear accelerators and collimators and the target-Jaw/MLC distance are adjacent to the breast tissue, which moves during treatment.