Using a Toshiba Aquilion 16 CT scanner an adult ATOM dosimetry phantom was exposed to a series of FTC and ATCM CT protocols: FTC - 100,
200,
250,
300 and 400mA; ATCM - low dose+,
low dose,
standard,
quality and high quality,
Three Pitch factors - 0.688,
0.938 & 1.438 and three detector configuration of 0.5×16mm,
1.0×16mm and 2.0×16mm.
For all acquisition protocols,
tube voltage was fixed at 120 kVp and a 0.5 second tube rotation.
Estimations of ER were undertaken using Metal Oxide Semiconductor Field Effect Transistor (MOSFET) dosimeters.
MOSFET detectors were used to measure organ dose (Model TN-RD-70-W,
Best Medical Canada Ltd.,
Ottawa,
Canada).
The detectors were placed inside the adult male CIRS ATOM dosimetry phantom (model 701,
CIRS Inc,
Norfolk,
Virginia,
USA) in locations corresponding to body organs.
The ATOM phantom is 173cm high and weighs 75kg,
it has chest dimensions of 23cm by 32cm,
24 and it comprises 39 cross-sectional slabs,
each of which is 25mm thick.
It has 5mm diameter pre-drilled holes within the slabs to accommodate dosimetry detectors [4].
With only 20 MOSFET dosimeters available the ATOM phantom was loaded and irradiated several times to ensure that all the 273 predrilled holes were covered.
Readings from the MOSFET dosimeters (mV/mGy) were transmitted via a wireless network to a computer where they were saved in an Excel file.
The Phantom was irradiated supine and positioned for adult abdominal CT .
The scanning commenced at the 11th thoracic vertebra and terminated at the superior border of the 5th lumbar vertebra (scan volume; 200mm).
For FTC and ATCM,
the phantom position was the same,
as verified by scanogram.
Mathematical correction of ATCM data
The average tube current for the ATCM protocols was higher than the closest comparable FTC setting in most cases (low dose+=101mA,
low dose=205mA,
standard=366mA,
quality=422mA and high quality=440mA).
Therefore,
to allow a fair comparison of the dose and ER between ATCM and FTC approaches,
the ATCM data was corrected using the methodology described below.
Overall the correction intended to match as closely as possible acquisition conditions for FTC and ATCM so that ER could be compared.
For the purpose of enabling clinically relevant comparisons between FTC and ATCM modalities,
an evaluation of the uncorrected ATCM and FTC are also presented in this poster.
The ER from ATCM data was corrected in accordance with Venkat 3.using the Equation 1 in (Figure 1).
Effective risk calculation
The life-time risk of cancer induction associated with FTC and ATCM adult protocols were calculated using Brenner’s Equation 2 (Figure 2) as described below [5].
To estimate total cancer risk,
the organ-specific radiation doses were converted to organ-specific cancer risks. The cumulative lifetime attributable risk (LAR) was calculated as the sum of risks for all forms of cancer.
The risk estimates are premised on present knowledge of the risk associated with low-levels of ionizing radiation.
This is based on the linear no-threshold model which has been utilised for several CT derived cancer risk estimates in the literature.
Average organ doses were then multiplied by the relevant Lifetime Attributable Risk factors (LAR) obtained from BEIR VII (Table 1) [6].
In this study,
the effective risk was calculated for males and females aged 20-70 years,
in groups of 10 years.