[Estimation of exposure dose from CBCT for prostate IMRT and approach to reduce local exposure dose]

Intensity modulated radiation therapy (IMRT) requires a precise set-up for which 3D matching using cone beam computed tomography (CBCT) is useful. Because images are acquired before every irradiation, investigating the exposure dose is essential. Such studies have been reported, but fewer reports have attempted to compare the dose with those in other modalities, as we did in this study. The exposure doses for imaging using 3 devices, including on board imager(OBI) CBCT, were measured using a tough water phantom, a self-made phantom, and a radiophotoluminescence glass dosimeter. The highest measured value was observed on OBI CBCT: the exposure dose was 4.3 times higher at some measurement sites than that from CT. Because this phenomenon is derived from the projection angle, a localized increase in the exposure dose may be prevented by changing the projection angle on each acquisition. Because the exposure dose for OBI CBCT was the highest and acquisition is performed before every irradiation, procedural modification is necessary, such as minimizing acquisition and the use of ultrasonography to confirm urine retention.