Purpose:To improve the set up accuracy of patient alignment,we developed a new real time patient alignment monitoring system based on laser sensing technology and evaluated its feasibility.Methods:The real time patient alignment monitoring system composed of photo-diode unit,signal conditioning device,control PC,and analysis software was developed for assessing the patient alignment error caused by the movement of the patient during the treatment.A photo-diode unit composed of seven sensors has a spatial resolution of 1 mm and configured to measure the translation error of ±3 mm range from the reference point.Signals obtained from the sensors are sampled by the Arduino(Mega ADK) and transferred to the control PC by the serial communication.To ensure the reliability of the system,reproducibility was assessed in the same light source.In addition,through the phantom test of verifying the displacement of X-axis,Y-axis and Z-axis artificially applied,the usefulness of the system was evaluated.Also,the safety of system operation was evaluated within the treatment irradiation fields.Results:In this study,the reliability of the system was verified by investigating reproducibility of the real time patient alignment monitoring system.Also,the usefulness of the system was verified by phantom study,and the safety of system operation was verified within the treatment irradiation fields.Conclusions:Through this study,economical and efficient system for monitoring the patient alignment in real time was developed using photo sensors.Since photo sensors unit is detachable,it can be positioned optimally to target organ for monitoring and afford to monitor the both sides of the patient unlike the previous camera based system.Through further studies,it is expected to provide meaningful data for real time position calibration utilizing the 6D couch,and the clinical applicability will be evaluated by applying the patient study.