Room: Exhibit Hall | Forum 1
Purpose: Investigation of the additional exposure due to CBCT is important to make sure actual irradiated dose for the patient. CBCT dose is normally measured with the CTDI phantom with pencil chamber. However, it is difficult to measure CBCT dose with the CTDI phantom during QA process in many radiotherapy institutions. The purpose of this study is to propose simple technique for evaluating CBCT dose without the CTDI phantom.
Methods: We propose two methods instead of the CTDI phantom: the water-equivalent phantoms (WEP) which built to cubic shape and the IMRT phantom. Radiation dose were obtained by farmer-type chamber in these methods. To make sure the feasibility of these methods, a comparative study between the CTDI phantom method and our methods. Data were obtained in pelvis protocol among two machines. Three types of protocols (Half, Full, and Pelvis protocols) for CBCT acquisition were evaluated by CTDI phantom method, WEP method and IMRT phantom method. TrueBeam (Varian) and Clinac 21EX (Varian) were used in this study.
Results: In CTDI method, radiation dose on Half protocol at center and peripheral points were 12.23 mGy and 20.66 mGy, respectively. On the other hand, radiation dose at center points were 10.17 mGy and 27.65 mGy in WEP and IMRT methods, respectively. In addition, radiation dose at peripheral points were 20.52 mGy and 32.32 mGy in WEP and IMRT methods, respectively. In other protocol, same tendency was obtained. Focusing on both maximum dose and minimum dose points, the trends of WEP method was matched with that of CTDI method.
Conclusion: We proposed simple method to evaluate CBCT dose with farmer-type chamber in this study. Especially, WEP method could obtain similar radiation dose with CTDI phantom method. With WEP method, continuous evaluation of radiation dose due to CBCT could be available in QA process.