Utility of split-VMAT for cardiac protection during left breast cancer radiotherapy

The field-in-field technique of three-dimensional conformal radiotherapy (3D-CRT) is a common treatment technique used in breast radiation therapy. However, when the heart is largely included in the treatment field for the treatment of left breast cancer, it is unavoidable that a large dose will impact the heart. Increases in the mean dose and the maximum dose to the heart increase the hazard ratio for ischemic heart disease. Split-VMAT is a technique that combines tangential field and volumetric modulated arc therapy (VMAT) methods using multiple small partial arcs. Split-VMAT can reduce the maximum dose to the heart and while ensuring target coverage. In this study, we sought to confirm the utility of split-VMAT for patients with left breast cancer whose heart is more than 1 cm inside the treatment field. The split-VMAT plan consisted of 4 fields, set at 20–60° per field, and the treatment time was limited to less than 20 s to enable deep-inspiration breath hold (DIBH). A half-VMAT, consisting of an arc of 180° and a field-in-field approach, using 2 tangential fields symmetrical to each other, were used as comparison groups. In addition, the field-in-field technique was divided into 2 types, 1 that emphasized target coverage and the other that focused on heart protection. For treatment, an Elekta Versa HD linear accelerator was used, and treatment planning was performed using Elekta Monaco software. Both split-VMAT and field-in-field, focused on target coverage achieved better (> 3%) target coverage. However, in field-in-field, focused on target coverage, the maximum dose to the heart was very high. Compared to the both field-in-field-based technique, the average dose to the lungs was about twice as high in the both VMAT-based technique. In addition, the treatment time per field was increased by about 5 s, and the total treatment time was more than doubled. The 3D-CRT field-in-field technique can significantly protect organs at risk (OARs) not included in the radiation field because the gantry is not directed to inside the body. However, if the heart is, the OAR, it may be exposed to high radiation doses. The split-VMAT limits the angle of the arc, constraining the beam directed inside the patient's body, and shortens the treatment time to allow DIBH. In addition, since sufficient target coverage and heart protection are possible, split-VMAT can be a compromise between 3D-CRT, intensity-modulation radiotherapy, and conventional VMAT.