Purpose: The purpose of this study is to assess in real-time to the dose delivered by the Cone Beam CT (CBCT) during prostate Volumetric Modulated Arc Therapy (VMAT) and improve the determination of the total dose delivered to the Organ At Risk (OAR) and the target volume.
Methods: The study was carried on a Truebeam (Varian Medical system, Palo Alto, USA) with a Fibermetrix (Strasbourg, France) plastic scintillating dosimeter (PSD) placed under the treatment table. The sensitive part of the PSD is 1mm of diameter and 1m long in order to take account of the complete length of the CBCT acquisition field.The PSD is calibrated following a methodology based on the IAEA human health reports n°5 on the status of computed tomography dosimetry for wide beam scanners in order to measure the delivered dose and CTDI. The measured dose with the PSD was compared to the dose measured by a pencil ionization chamber of 100mm in a CTDI fantom placed at the isocenter on the treatment table. In addition, the CTDI was measured with the PSD for patients benefiting from CBCT repositioning exams during VMAT treatment. Two CBCT protocols were used during this study : Pelvis and Pelvis Large. The delivered dose and CTDI were compared to the OAR and target volume planned dose for prostate VMAT radiotherapy.
Results: The mean difference between the dose measured by the PSD and the ionization chamber were -4,4% and 5,4% for the pelvis and the pelvis large respectively.
Conclusion: Since the dose difference with the ionization chamber is about -5% the PSD system used in this study is an effective way to measure in real-time the dose due to the CBCT during VMAT prostate treatments.
Funding Support, Disclosures, and Conflict of Interest: This work is part of a PhD supported by the FIBERMETRIX company.