My PDD is lower/higher than expected, what can be the possible causes?
There are several possible causes that lead to a lower or higher PDD, please follow the steps below for troubleshooting:
Go to Common Settings - Equipment Setup - Detectors - Effective Points of Measurement, confirm the effective measurement point setup of the field detector is correct.
Go to the bunker and check the water phantom setup, especially the isocenter position and water surface position. Using hand control to move the sensor back to isocenter position, put on the alignment cap if necessary, check whether the sensor center is in alignment with linac isocenter. Also, verify the water surface position and SSD setting.
Make sure the correct detector holder is used. When using a wrong detector holder, detectors are not in the correct position, and sometimes cannot stay stable during scanning.