Evaluation of Cone-Beam Computed Tomography imaging protocols in radiation therapy: Dose assessment in pediatric and adult phantoms and a national survey of clinical practice.

INTRODUCTION: Image-Guided Radiotherapy (IGRT) is essential in modern radiotherapy, enabling accurate patient positioning and target localization. Cone-Beam Computed Tomography (CBCT) is widely used; however, repeated CBCT acquisitions contribute additional imaging dose, which is particularly relevant in pediatric radiotherapy due to increased radiosensitivity. This study aimed to characterize pediatric CBCT acquisition practices in Portugal and to quantify CBCT dose using pediatric and adult phantoms. Its routine application results in additional radiation exposure, which is particularly relevant in pediatric radiotherapy due to increased radiosensitivity. This study aimed to characterize pediatric CBCT acquisition practices in Portugal and quantitatively assess CBCT dose using pediatric and adult phantoms.

METHODS: A national survey was conducted to characterize CBCT acquisition parameters used in pediatric radiotherapy across institutions, including tube voltage (kV), tube current-time product (mAs), and number of projections. In parallel, an experimental CBCT dose assessment was performed on a linear accelerator-mounted CBCT system using a 100 mm pencil ionization chamber and Computed Tomography Dose Index (CTDI) phantoms representing pediatric and adult anatomies. Dose was quantified using the weighted Cone Beam Dose Index (CBDIw) for the head, thorax, and pelvis, as well as the Image Gently protocols.

RESULTS: The survey demonstrated marked inter-institutional variability in acquisition parameters, particularly for thorax and pelvis protocols. The largest differences were observed in mAs values, which varied by factors of up to 2.7 for thorax protocols and 10.8 for pelvis protocols. CBDIw values were consistently higher in pediatric phantoms than in adult phantoms for comparable conventional protocols. Image Gently protocols produced the lowest CBDIw values, ranging from 1.23 to 1.65 mGy in pediatric phantoms and from 0.50 to 1.23 mGy in adult phantoms. Inferential analysis showed a statistically significant effect of anatomical region on CBDIw values in both pediatric and adult phantom groups.

CONCLUSION: CBCT imaging dose is influenced by acquisition parameters, anatomical region, and phantom size. The observed national variability in pediatric CBCT protocols, together with higher CBDIw values in smaller phantoms, highlights the need for size-adapted protocol optimization, systematic dose monitoring, and harmonization of clinical practice. These findings provide quantitative evidence to support radiation protection strategies and the development of reference levels for CBCT imaging in pediatric radiotherapy.

PLAIN LANGUAGE SUMMARY: Image-guided radiotherapy uses repeated scans to help position patients accurately, but these scans add extra radiation, which is important to consider in children. This study used a survey and test models to compare how scans are done and how much radiation they give. This study found large differences in scan settings and higher radiation in smaller models, while lower-dose settings reduced exposure. This matters because improving and standardising scan settings can help reduce unnecessary radiation while maintaining accurate treatment for children.