Dose assessment in pediatric Cone Beam CT: an experimental and computational study

Cone beam computed tomography (CBCT) is commonly used for patient setting and pre-treatment verification in image-guided radiation therapy (IGRT). IGRT using CBCT allows visualization of soft tissue targets and critical structures prior to treatment in order visualize and verify the position. Furthermore, since CBCT imaging is used on a daily basis and multiple times per patient, could result potentially high cumulative doses to healthy tissues surrounding exposed target organs. In image-guided radiation therapy, especially for children, managing both imaging dose and positioning accuracy is crucial, due to they have an higher biological susceptibility and they are more likely to develop secondary cancer because of their longer expected lifespan. For this reason, the use of adult CBCT protocols may expose kids to excessive radiation doses, raising their risk of developing cancers brought on by radiation. The aim of this work is to assess the radiation doses in some standard pediatric CBCT protocol and suggest some other protocols that could be used for pediatric patients in order to decrease the absorbed dose. In the first study we investigate the signal-to-noise ratio of scans at various kV of a CBCT (Varian Clinac linear accelerator, equipped with the OBI Varian Medical Systems, Santarem, PT) in order to see where the best image quality parameter is reached while maintaining the same or lower absorbed dose with respect to a standard clinical protocol. ATOR 18 FG Phantom was used to acquire images from 40kV to 150kV in 10kV steps at 25 mA and 50 ms exposure time in the clinical CBCT system The data found were compared at different kilovolts and we obtained that the SNR has peak at 50-60 kV and for higher voltages a decreasing trend. In the second study we experimentallyacquired the spectra with voltages from 40KV to 120KV with an AMPTEK XR-100T-CdTe detector, in order to use real data in Monte Carlo simulations that we performed to assess the radiation dose and decrease the uncertainty on the radiation CBCT spectra.Acquired spectra were quite different from the ones obtained in literature. However, since the filter materials after the anode of the X-ray system were not accurately known, standard literature spectra were used in Monte Carlo (MC) simulations. In the third and last study, three CBCT protocols were analysed at various energies 10-15 year old female antropomorphic phantom, to analyse the radiation dose in the organs for comparison at different kilovoltages and we found that compared to the protocols used today, using an energy of 60kV, the radiation dose could be less than the half.