Selecting noncoplanar beam directions in a patient coordinate system for radiotherapy planning.

To introduce a beam angle selection method based on the patient coordinate system for treatment planning of noncoplanar intensity-modulated radiation therapy (IMRT). Traditionally, in radiotherapy planning, beam directions are selected in the machine coordinate system. A noncoplanar beam direction is established through a treatment table rotation followed by a gantry rotation. However, visualizing the beam direction relative to the patient is difficult. The suggestion here is to describe the beam direction in the patient coordinate system. First, a coplanar beam direction is selected in the transverse plane of the patient coordinate system. The noncoplanar beam direction is then obtained by adjusting the coplanar beam toward the foot or head in the patient coordinate system. Finally, the noncoplanar beam direction is expressed in terms of gantry and table angles in the machine coordinate system via formulae developed in this study. A 3D computer-aided design model of the linear accelerator (linac) is established with Solidworks and used to validate the feasibility of the proposed method. A clinical case was chosen to demonstrate the effectiveness of this method. Treatment plans with the conventional coplanar and noncoplanar beam settings were made. Compared with the conventional coplanar IMRT plan, improved normal tissue sparing of the organs at risk using the noncoplanar IMRT plan is evident. The proposed method for noncoplanar treatment planning makes beam direction selection simpler and straightforward.