WE‐E‐224C‐01: Dose Reconstruction Quality Assurance for Helical Tomotherapy

Abstract

Purpose: To determine if the dose calculation accuracy is suitable for the daily dose verification of patient treatment; and develop quality assurance tests for dose reconstruction with a commercial dose reconstruction system. Methods and Materials: Dose reconstruction accuracy was evaluated using MVCT and kVCT images of three test phantoms: 1.) A Electron Density Phantom phantom (Gammex); 2.) A water‐equivalent cylindrical phantom (TomoPhantom), and 3.) an anthropomorphic RANDO phantom. A parotid‐sparing head & neck plan was created for the phantoms. Using the kVCT images, inverse treatment plans simulating clinical prostate and head & neck helical tomotherapy treatments were created. The phantom MVCT images were fused with the kVCT images using a co‐registration algorithm. Planned‐Adaptive software was then used to re‐calculate the doses on the MVCT images. The kVCT‐based and MVCT‐based doses were then exported to the RIT113 Dosimetry System. Accurate dose reconstruction is dependent on the quality of the MVCT images used. As such, a genetic algorithm was developed to ensure proper image fusion and a principle component analysis was used to determine the most influential factors for image quality. Results: Based on the phantom results, the calculation accuracy of MVCT and kVCT images are typically within ±3% of each other. The greatest discrepancy occurs in the high‐dose gradient regions, which is most likely due to slight alignment errors between the two dose files. PCA indicated a correlation with the person performing the machine warm‐up and image quality. This was confirmed as MVCT dose reconstruction suffered due to an individual's error. Conclusions: Based on comparisons of calculated doses in phantom plans, the accuracy of using MVCT image data in dose reconstruction is typically within ±3%, subject to image quality. A process was developed for assessing the image contrast and resolution on a daily basis.