Three-dimensional conformal setup (3D-CSU) of patients using the coordinate system provided by three internal fiducial markers and two orthogonal diagnostic X-ray systems in the treatment room

Purpose To test the accuracy of a system for correcting for the rotational error of the clinical target volume (CTV) without having to reposition the patient using three fiducial markers and two orthogonal fluoroscopic images. We call this system "three-dimensional conformal setup" (3D-CSU). Methods and materials Three 2.0-mm gold markers are inserted into or adjacent to the CTV. On the treatment couch, the actual positions of the three markers are calculated based on two orthogonal fluoroscopies crossing at the isocenter of the linear accelerator. Discrepancy of the actual coordinates of gravity center of three markers from its planned coordinates is calculated. Translational setup error is corrected by adjustment of the treatment couch. The rotation angles (α, β, γ) of the coordinates of the actual CTV relative to the planned CTV are calculated around the lateral ( x ), craniocaudal ( y ), and anteroposterior ( z ) axes of the planned CTV. The angles of the gantry head, collimator, and treatment couch of the linear accelerator are adjusted according to the rotation of the actual coordinates of the tumor in relation to the planned coordinates. We have measured the accuracy of 3D-CSU using a static cubic phantom. Results The gravity center of the phantom was corrected within 0.9 ± 0.3 mm (mean ± SD), 0.4 ± 0.2 mm, and 0.6 ± 0.2 mm for the rotation of the phantom from 0–30 degrees around the x , y , and z axes, respectively, every 5 degrees. Dose distribution was shown to be consistent with the planned dose distribution every 10 degrees of the rotation from 0–30 degrees. The mean rotational error after 3D-CSU was −0.4 ± 0.4 (mean ± SD), −0.2 ± 0.4, and 0.0 ± 0.5 degrees around the x , y , and z axis, respectively, for the rotation from 0–90 degrees. Conclusions Phantom studies showed that 3D-CSU is useful for performing rotational correction of the target volume without correcting the position of the patient on the treatment couch. The 3D-CSU will be clinically useful for tumors in structures such as paraspinal diseases and prostate cancers not subject to large internal organ motion.