Laser markers installed on the walls and ceiling of the bunker are usually used for patient positioning in radiotherapy. Therefore, they directly influence radiotherapy treatment accuracy. The present study aimed to develop a new method without using an external portable laser source or any dedicated laser alignment phantom to align the radiotherapy setup lasers installed in treatment bunkers.

A patient-specific IMRT quality assurance phantom (OCTAVIUS 4D in our study) was used as a phantom having a similar size of patients and surface markers precisely showing the central point of the phantom. After ensuring the precise coincidence between light and radiation isocenters at various gantry angles (0, 90, and 270 degrees), true laser positions were found optically by matching the shadows of all the cross hair lines (CHLs) on the external window of the gantry head and also on the phantom surface at various gantry angles. The distances between the laser lines and light field crosslines on the treatment room walls for our proposed approach were compared with the conventional laser calibration technique (laser calibration with an external laser source positioned in the machine isocenter) and reported as laser alignment accuracy.

The mean displacements performed for laser alignment on the phantom surface were 1.2 ± 0.2, 1.1 ± 0.1, and 1.0 ± 0.1 mm for sagittal, coronal, and axial lasers, respectively. These displacements on the gantry head exit plane were 1.4 ± 0.3, 1.2 ± 0.2, and 1.1 ± 0.1 mm, respectively. Mean value of laser alignment accuracy on the wall of the treatment room was 19.5 ± 0.5 mm for our new method, whilst it was 32.7 ± 0.7 mm for adjusting lasers with the conventional method.

Our new approach for laser alignment was performed and evaluated successfully. This method can provided a more accurate procedure than the conventional method without needing an external laser source or dedicated laser alignment phantom.