Purpose: For verification, traditional room laser calibration typically relies on items such as the accuracy of Mylar lines projected by the linac light field, the couch motion axes, and any reference marks on the opposing wall. The objective of this project was to create a laser calibration method for quantifying the position offset, inclination angle, and roll of each of the seven lasers typically used in external beam radiation therapy. The method uses a 3D-printed jig to replicate the room coordinate system registered to the CBCT or MV coordinate systems.
Methods: A foldable laser alignment jig was designed using CAD software and 3D printed with PLA. Gentle application of heat from a soldering iron allowed the PLA to be bent into a box-shape. The completed jig has a 10 cm x 10 cm base and 10.5 cm height. Labels were affixed to the PLA to provide hash marks for quantifying the laser line position on the jig. QR codes associated with each measurement position on the jig will support automated image analysis. The accuracy of laser position, inclination, and roll was assessed by applying known translations or rotations before taking a reading. Further tests were performed to assess the utility of the jig in laser calibration verification.
Results: The position measurement accuracy is 0.15 mm and the rotation measurement accuracy is 0.2 degrees. When registered by CBCT to reference images, the alignment is accurate to 0.2 mm. The jig remains durable after repeated use.
Conclusion: This project complements the existing literature on room laser verification by providing a simple tool that can be easily fabricated in any radiation therapy department equipped with a 3D printer. Future work will include refining the design and developing a camera-phone tool to automate the image analysis.