[Dose delivered to the patient by megavoltage cone beam computed tomography imaging]

To study the dose delivered by a megavoltage cone beam computed tomography imaging system (MVCBCT) installed on a Oncor Impression linac (Siemens).The acquisition of MVCBCT images was modelled in a treatment planning system by 67 photon beams (6 MV). A study was conducted to: compare the calculated and measured dose at the centre of a cylindrical phantom; compare the calculated and measured dose distributions in the Alderson-Rando phantom (pelvis); study the influence of MVCBCT image acquisition for the repositioning of a prostate cancer patient treated by 3D conformal radiotherapy (prescribed dose of 74 Gy), on the dose-volume histograms (DVH) for the treatment plus seven MVCBCT (protocol D1-3 and weekly), treatment plus 37 MVCBCT (one for each day of treatment).The difference between calculated and measured doses at the centre of the cylindrical phantom was less than 3%. A deviation of 7% maximum was found between the dose distribution calculated in the Rando phantom and the measured doses normalized at the beam isocentre. The dose delivered at the isocentre was equal to 3,7 cGy for a "5 MU" protocol, with a maximum dose of 6 cGy. In the case of the patient considered, the acquisition of 37 MVCBCT corresponded to an additional mean dose to the PTV of 1.2 Gy for a protocol "5MU" with a significant influence on the DVH.In view of this study, it appears that the doses delivered in frequent use of MVCBCT must be taken into account by the radiation oncologist in assessing the therapeutic dose delivered to the target volume and organs at risk.