SU‐FF‐T‐200: Image‐Guided IMRT Dose Verification with a Plastic Photosensitive Dosimeter

Abstract

Purpose: To verify an IMRT treatment, multiple point dose measurements are time‐consuming to acquire. This investigation used a kilovoltage imaging system to localize an optical dosimeter to acquire rapidly three‐dimensional IMRT quality control measurements. Method and Materials: A plastic photosensitive dosimeter was used as a dosimetric verification tool for IMRT. The plastic was formed into a right cylinder 8cm in height and 9cm in diameter that fit precisely into a water equivalent phantom. The system was used to verify an IMRT dose distribution computed by inverse treatment planning and delivered using a medical linear accelerator equipped with a multi‐leaf collimator and a kilovoltage electronic portal imaging device (KV‐EPID). The KV‐EPID was used to determine the location of the dosimeter cylinder within the accelerator coordinate system. Small wires (1mm diameter) inserted into the cylinder at three locations were used as fiducial markers. An auto‐matching algorithm integral to the accelerator control system was used to assure that the position of the dosimeter matched the position in a phantom plan. The Root‐Mean‐Square localization error was measured to be 1.1mm. The dose distribution within the dosimeter was measured by means of an optical cone‐beam imaging system. The measured dose distribution was compared with the calculated dose distribution based on the known locations of the fiducial markers. Results: The KV‐EPID was easily able to visualize the fiducials in the dosimeter and to localize the dosimeter with a RMS error of 1.1mm. Conclusion: The dose distribution delivered by IMRT to a surrogate phantom can be aligned with a mean error of 1mm using the KV‐EPID. This allows optical reconstruction of a photosensitive plastic dosimeter to be aligned with the computed three‐dimensional dose distribution for dosimetric verification of an IMRT treatment plan.