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CT dose data by CT radiation treatment planning scans
versus CT diagnosis
Poster No.:
C-0561
Congress:
ECR 2015
Type:
Scientific Exhibit
Authors:
S. Johansen, A. Sanderud; Oslo/NO
Keywords:
Breast, CT, Radiation safety, Dosimetric comparison
DOI:
10.1594/ecr2015/C-0561
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Aims and objectives
1) to compare the absorbed dose from Computed Tomography (CT) scans used
for pre radiotherapy planning (RP#CT) against those from diagnostic CT (DG#CT)
examinations. 2) to explore the possible reasons for any dose differences.
Methods and materials
Two groups of patients underwent CT scans of the thorax with either DG-CT or RP-CT.
Each group consisted of 55 patients; patients from each group had similar weight and
body mass index (BMI) (Table 1). Patients were scanned using a GE Light speed PRO CT
machines across two sites.Parameters including CTDIvol, DLP and scan length from the
RP#CT imaging were compared with parameters obtained from DG#CT imaging (Table
2).
A two-tailed T-test with independent samples indicated that the distributions of weight
and BMI were not significantly different between groups (p>0.05). However, there was
a significant difference for the height distribution, mean/SD height for RP-CT group was
170/9 cm whereas for the DG-CT group it was 174/8 cm (p=0.009).
Images for this section:
Table 1
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Table 2
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Results
The mean CTDIvol and DLP values from RP-CT (38.1 mGy, 1472 mGy·cm) are
approximately four times higher than for DG-CT (9.63 mGy, 376.5 mGy·cm). The CT
scan length for both RP-CT (mean 37.8 cm) and DG-CT (mean 37.5cm) were similar
(p=0.549). The CTDIvol in both RP-CT and DG#CT CTDIvol increase with higher BMI,
but the dependence is much stronger in the DG-CT studies than in the RP-CT (DG-CT
r=0.852,p<0.001; RP-CT r=0.279,p=0.044). Variations in CTDIvol with BMI indicates the
ability of the CT system to modulate the mA according to the patient size and that the
DG-CT scans are able to better optimize the mA to the patient size when compared to
the RP-CT (Figure 1). In the DG-CT scans a strong negative linear correlation between
noise index (NI) and mean CTDIvol was also observed (r =-0.954, p=0.004); the higher
NI, the lower CTDIvol. This was not the case in the RP-DG scans.
Images for this section:
Table 1
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Table 2
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Fig. 1: Figure 1: CTDIvol as function of Height, Weight and BMI in CT-thorax and Scan
length as function of BMI from radiation treatment RP CT scans (orange) and DG CT
examinations (blue)
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Conclusion
Our study found that absorbed radiation doses given to patients in RP-CT thorax scans
are four times higher, on average, than the doses given in DG-CT scans. The differences
are largest between patients with a low BMI. It is important to emphasis the use of patient
size modified CT protocols even when high quality images are required. More research
to understand and improve reduce the differences in CT dose between DG-CT and RPCT should be a priority.
Personal information
Corresponding author: [email protected]
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