Response to the comments on “The Feasibility Study of a Hybrid Coplanar Arc Technique versus Hybrid Intensity-modulated Radiotherapy in Treatment of Early-stage Left-sided Breast Cancer with Simultaneous-integrated Boost. J Med Phys 2017;42:1-8”

Journal of Medical Physics ¦ Volume 42 ¦ Issue 2 ¦ April-June 2017 100 We highly appreciate your comments. It is well known that there are considerable target position uncertainties during breast radiotherapy. These uncertainties result from respiratory motions, tissue and organ deformation, daily patient setup, and other causes. Respiratory gating and breath control can reduce the target position uncertainties. However, these techniques are not thoroughly implemented at each hospital, including the authors’ institutions. In our manuscript,[1] we proposed hybrid plans (three-dimensional conformal radiotherapy [3DCRT] + intensity-modulated radiation therapy [IMRT] or 3DCRT + volumetric modulated arc therapy [VMAT]) solely for simultaneous integrated boost treatment delivery. As indicated in the manuscript, “The rationale of using two coplanar 90° arcs in the hybrid VMAT technology is as follows: (1) In the VMAT plan, two arcs are needed to optimize dose distribution when dealing with a complex target; (2) the target is an arc that was nearly 90° along the chest wall, and the 90° arc in tangential direction enters the target without irradiating much of the lung.” In the left breast irradiation, the target dose inhomogeneity and doses to the organs at risk (OAR), especially the heart, ipsilateral lung, and contralateral breast, are the major limitations of 3DCRT.[2] IMRT is capable of improving dose homogeneity and conformity and sparing normal tissues.[3-5] Some oncologists illustrate that VMAT has better protection for the adjacent organs than IMRT.[6-8] In our study, the contribution of IMRT/VMAT is only 30% of the prescribed dose, and 3DCRT delivers 70% of dose in the hybrid plan. For this reason, the hybrid plans of 3DCRT + VMAT result in higher doses to OARs than pure VMAT plans. The 3DCRT portals take into account target position uncertainties; meanwhile, target dose uniformity and normal tissue doses are addressed by the IMRT/VMAT fields. As shown in the manuscript, the hybrid VMAT plans have some dosimetric advantages over previously proposed 3DCRT + IMRT plans. Where respiratory gating or breath control is not implemented, the hybrid plans may be considered for patients who underwent breast-conserving surgery.