Evaluation Of Image Quality Of Megavoltage Computed Tomography For Helical Tomotherapy Unit And Aplication For Dose Recalculation

Image-guided radiotherapy (IGRT) is the most advanced treatment technique in radiation oncology. Helical tomotherapy is an innovative means of delivery of IGRT and IMRT, which integrates a helical computed tomotherapy scanner into a linear accelerator. Tomotherapy can generate CT images with the 3.5 MV X-ray fan beam. The megavoltage CT (MVCT) images can be used to verify the patient's position prior to, and potentially during radiation therapy. Daily MVCT images can also be used to assess the shape and size changes in normal anatomy or tumor extent during the radiation treatment.The purpose of this original study is to analyze the image quality of MVCT with Catphan 600, and influencing factors of image quality were also discussed, also to analyze the accuracy of dose recalculation with using MVCT images. The results showed that the MVCT values depended linearly on the physical density of the sample.The MVCT value uniformity was 1.5%; the spatial resolution was 4lp/cm; the low contrast reslotion was low due to the MV Beam.The geometric accuracy was good. MVCT imaging patient dose was 1-3cGy every time, it was far less than treatment dose. As the MVCT values were linear to electron density of material, MVCT images were appropriate for radiotherapy dose recalculations.The use of MVCT images for dose recalculation can be performed with the accuracy similar to that of dose computations in kVCT images. The stability and safety of MVCT system was tvery important for clinical use. This study has also discussed the quality assurance of MVCT on a helical tomotherapy system.The image quality of MVCT was worse than that of kVCT,but was good enough for IGRT. MVCT images of patients are acquired daily on the helical tomotherapy, while these images were primarily used for patient alignment, they can also be used to recalculate the absorbed doses with the the daily images. Firstly, the registration of the MVCT to planning kilovoltage CT (kVCT) image can be used to check, and if necessary, correct the patient's position ensuring proper patient positioning for treatment delivery. Secondly, the daily MVCT images can be used to access anatomical changes and their dosimetric implications. This will provide a good basis for Dose-Guided Radiotherapy (DGRT) and adaptive radiotherapy (ART). We supposed that the protocols of daily, monthly, and annual QA are necessary.