Abstract
According to published data, the absorbed dose used for a CBCT image acquisition with Varian OBI v1.3 can be as high as 100 mGy. In 2008 Varian released a new OBI version (v1.4), which promised to reduce the imaging dose. In this study, absorbed doses used for CBCT image acquisitions with the default irradiation techniques of Varian OBI v1.3 and v1.4 are measured.TLDs are used to derive dose distributions at three planes inside an anthropomorphic phantom. In addition, point doses and dose profiles inside a ‘stack’ of three CTDI body phantoms are measured using a new solid state detector, the CT Dose Profiler. With the CT Dose Profiler, the individual pulses from the X‐ray tube are also studied. To verify the absorbed dose measured with the CT Dose Profiler, it is compared to TLD. The image quality is evaluated using a Catphan phantom.For OBI v1.3, doses measured in transverse planes of the Alderson phantom range between 64 mGy and 144 mGy. The average dose is around 100 mGy. For OBI v1.4, doses measured in transverse planes of the Alderson phantom range between 1 mGy and 51 mGy. Mean doses range between 3‐35 mGy depending on CBCT mode. CT Dose Profiler data agree with TLD measurements in a CTDI phantom within the uncertainty of the TLD measurements (estimated SD ±10%). Instantaneous dose rate at the periphery of the phantom can be higher than 20 mGy/s, which is 10 times the dose rate at the center. The spatial resolution in v1.4 is not as high as in v1.3.In conclusion, measurements show that the imaging doses for default modes in Varian OBI v1.4 CBCT system are significantly lower than in v1.3. The CT Dose Profiler is proven fast and accurate for CBCT applications.PACS number: 87.53.Bn
Highlights
230 Palm et al: Varian CBCT absorbed dose dose cannot be included in the treatment planning process)
CBCT contributes dose to normal tissues that are outside the treated region, but within the imaged region
The highest dose measured by Wen et al[2] was to the hip joint, at 110 mGy. Imaging doses in this range are likely to limit the number of CBCTs that can be performed during the patient’s course of treatment
Summary
230 Palm et al: Varian CBCT absorbed dose dose cannot be included in the treatment planning process). CBCT contributes dose to normal tissues that are outside the treated region, but within the imaged region. In 2008 Varian released a new OBI version (v1.4). Dose-saving improvements include, for example, a change in the convolution filtering which makes better use of the existing data, and the ‘half scan’ option (200° gantry rotation), which allows for a lens saving technique by avoiding imaging at angles ± 80° from the front. Varian declares that v1.4 can reduce the imaging dose in the head and neck region to one-fifth of the dose received with v1.3, for similar image quality, while the dose to the pelvic region can be halved. To our knowledge there is no published dose data on OBI v1.4, other than the values reported by Varian
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