Tangent field technique of TomoDirect improves dose distribution for whole-breast irradiation

TomoDirect (TD) is an intensity-modulated radiotherapy system that uses a fixed gantry angle instead of rotational beam delivery. Here, we investigated the effect of the multiple beam technique of TomoDirect on dose distribution compared with commonly-used tangential beams. We included 45 consecutive patients with right breast cancer who underwent postoperative radiotherapy in our institute in the present study. Clinical target volume (CTV) was the whole right breast. The planning target volume (PTV) was created by expanding the CTV by a 0.5 cm margin. Paired TD plans were generated for each patient; a two-beam plan using paired tangential beams and a six-beam plan with four additional beams with modified gantry angles of ± 5° from the original tangential beam set. A prescribed dose of 50 Gy was defined for 50% isodoses of the PTV. The six-beam plan delivered significantly more homogeneous doses to the PTV than the two-beam plan; and the mean dose to the PTV in the six-beam plan more closely reflected the prescribed dose. V20Gy and mean dose to the right lung and mean dose to the whole body were also significantly decreased in the six-beam plan. However, duration of radiation exposure was 1 min longer in the six-beam plan than in the two-beam plan. The dose distribution to the target and organs at risk were improved with the six-beam plan relative to the two-beam plan without increasing the whole-body radiation dose. The six-beam plan using TD is a simple technique that can be routinely applied to whole-breast irradiation in clinical practice.