With the search, 83 institutions were identified. After two reminder e-mails to non-responding institutions, we obtained a response from 50 institutions (60%), and from these institutions we received 37 (45%) questionnaires. Five authors answered that CT colonography was no longer performed, and eight authors responded positively, but finally did not return the questionnaire notwithstanding reminder e-mails. Three authors filled in the questionnaire with insufficient information for calculation of the effective dose, thus 34 institutions remained with complete questionnaires. Of these 34 institutions, 22 performed CTC for both daily practice and screening purposes, 11 only for daily practice and 1 institution only for screening. Indications for patients receiving CTC examinations in daily practice are indicated in Fig. 2.
Data on dose modulation
Seventeen institutions indicated that they used dose modulation for protocols for supine or prone scans, or both. Six of these institutions provided data for patients of 70 kg, and six institutions provided data for patients of another weight. Table 2 shows these weights, the uncorrected mAs values and the estimated mAs values for a patient of 70 kg using the relationship of Fig. 1. Five institutions did not provide unambiguous information on weight, and it was assumed that the weight of the patient was 70 kg.
CT parameters: daily practice and screening protocols
Overall, 37 CT machines were used by 34 institutions; 3 institutions use CT machines from 2 different manufacturers. In Table 3 a summary is given of the protocols for the daily practice patients. No significant differences in effective dose were found between scanners with different detector rows and between protocols with and without dose modulation. The median effective dose in 39 daily practice protocols was 9.1 mSv (range 2.8–22), 5.2 mSv (1.0–14.1) for supine and 3.0 mSv (0.6–9.8) for prone CT acquisition. The median effective dose per institution was also 9.1 mSv (2.8–22).
The median values for the 25 protocols for screening CT colonography in 22 institutions are given in Table 4. No significant difference in effective dose was found between scanners with different detector rows. The median effective dose for the screening protocols was 5.6 mSv (range 2.6–14.7), 2.8 mSv (1.0–6.1) for supine and 2.5 mSv (0.6–9.8) for prone CT acquisition. The median effective dose per institution was 5.7 mSv (2.6–12.2). See Fig. 3 for a histogram of the effective doses of daily practice and screening protocols.
Overviews of CT parameters and effective dose for daily practice protocols and screening protocols per institution are given in Tables 5 and 6. The effective doses for the screening protocols were significantly lower than for the daily practice protocols (p = 0.007).
Sensitivity analysis parameters dose modulation
Recalculations using 50% less or 50% more mAs correction than the nominal correction for the six institutions that provided data for deviant weight (Table 2) produced the following results: Effective doses per institution remained the same except for screening protocols in which the median dose for 50% less correction increased from 5.7 to 5.9 mSv. Recalculations for the five institutions that did not provide unambiguous information on weight resulted in a reduction of the median effective dose for daily practice from 9.1 to 8.9 mSv (for 75 kg) and to 8.2 mSv (80 kg) and for screening from 5.7 to 5.6 mSv and to 5.4 mSv for 75 and 80 kg, respectively.
Overranging planned trajectory of the volume examined
The increase in dose due to overranging of the planned trajectory of the volume examined was calculated for each CT protocol. For 64- and 40-detector-row CT systems the increase in dose was on the average 14%, for 16-detector-row CT systems 10% and for 4-detector-row and single-detector-row CT systems 4%.
Comparison with CTC performed in 2004
We compared effective doses of the 17 institutions that also responded to our questionnaire in the first study. In this study only the effective doses for daily practice were determined. In these institutions the median effective dose for daily practice was at that time 11.0 mSv (range 4.2–21.0). In these figures the effect of overranging has been taken into account . The current median dose in these institutions is 9.7 mSv. This difference is not significant. In the present study, 17 institutions used dose modulation (50%) together with automatic current selection, while in 2004 no institution used this for CTC. Finally we compared the number of detector rows of the CT systems used in the earlier study and now. In 2004, 82% (23/28) of the institutions used a CT system with fewer than 16 detector rows and 18% (5/28) used a 16-detector-row CT system. In 2007 only 18% (6/34) used a CT system with fewer than 16 detector rows and 62% (21/34) used a 64-detector-row CT system.