In vivo measurements of the dose delivered by kilovoltage cone-beam CT during prostate radiation therapy

摘要

The usefulness of CBCT is proven, but this technique can deliver a significant dose to the patient when it is often practiced. The dose is very dependent on the equipment and the parameters used. A study has been undertaken on an anthropomorphic phantom and on patients irradiated for prostate cancer, in order to evaluate the mean dose delivered to the prostate and rectum (organ at risk). Images have been taken using a Varian equipment (model VZW2940FH2-02 -detector Pax-Scan 4030 CB made of aSi) in "half bow-tie" mode, maximum field size, 125 kV, 80 mA, 25 ms. Dosimetry was performed using Gafchromic films and Harshaw - TLD 700H calibrated by comparison with a Keithley ion chamber type 90035.A good agreement has been found between films and TLD results. Films have shown that the dose distribution is very inhomogeneous and that the dose at the center of the phantom depends both on the explored zone thickness and on its position relative to the beam axis. The doses measured at the phantom surface (TLD and films) were maximum at the entrance of the anterior-posterior axis, and their arithmetic mean is equal, or a little greater than the doses measured at mid-thickness of the phantom and at the level of the rectum. After discussion with the medical team, it has been decided that the average between the anterior and posterior skin doses will be systematically evaluated in vivo, and taken as reference for correcting the dose delivered to the patient during radiation therapy. Study performed on 10 patients has shown mean doses per CBCT varying from 5.13 ± 0.23cGy to 7.28cGy ± 0.23cGy. Conclusion is that the doses delivered by CBCT are not negligible and should be taken into account. The RBE value to be applied for kilovoltage x-rays is questionable.