Abstract
Purpose: Cone-beam computed tomography (CBCT) is an indispensable procedure for accurate patient positioning during radiation therapy (RT) in many clinical cases. However, the patients get an additional dose using CBCT. This dose is neither therapeutic nor diagnostic. It is very difficult to obtain the reliable information about the dose distribution within the patient using the CBCT. Despite this, there is a need to control the additional dose for the pediatric patients and reduce it. There are different approaches of imaging dose evaluation. Most accurate methods are based on the Monte-Carlo calculation and thermoluminescent dosimeters-based measurements. However, the implementation of these methods is complex and cumbersome, that makes impossible their application in routine clinical practice. The evaluation of dose indexes is an accessible and convenient alternative. The purpose of this study is evaluation of the cone beam computed tomography dose indexes for different imaging protocols and object sizes. Material and methods: The technique based on absolute and relative dose measurements for CBCT was used in this study. Absolute dose measurements were performed at the periphery and center of the FREEPOINT (CIRS) phantom using the Farmer type chamber FC65-P for each CBCT protocols. FREEPOINT (20 cm height, 30 cm width, 30 cm length) was used for imitation big chest and pelvis. Inner insert (16 cm diameter) of the phantom was used for imitation head, small chest and pelvis. The dose profiles were measured using I’mRT MatriXX (IBA) and analyzed by OmniPro-I’mRT software, dose indexes DLP (dose–length product) were calculated. Results: The dose indexes were identified for five protocols corresponding three scanning areas (Head and Neck, Chest and Pelvis). The dose indexes were 51.82 and 90.25 mGy×cm using Head and Neck S20 and Head and Neck M20 protocols respectively. The lowest dose index was obtained 13.28 mGy×cm for Fast Head and Neck S20. It was established that the scanning object size strongly affects on the dose index values and, as result, on the absorbed dose within the patient. The dose indexes were 305.42 and 187.53 mGy×cm using scanning protocol Chest M20 for small and big phantoms respectively. The similar results were obtained for scanning protocol Pelvis M15. The highest dose index was obtained 846.93 mGy×cm for the small phantom, while the dose index was 563.79 mGy×cm for the big phantom. The necessity of several clinical protocols to scan different areas was shown. Using of the Pelvis M15 protocol for head scanning may increase the additional point dose 96 times in comparison with Fast Head and Neck S20 protocol. Conclusion: The dose indexes were evaluated taking into account the size of the scanning object for different imaging protocols. Routine use of CBCT in clinical practice requires a sensible choice of the scanning protocol based on the results of the dose index estimation.
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