Effective doses on gender-averaged and age-averaged adults 18–64, from ICRP-103.
Average effective dose (mSv)
Lifetime risk of a cancer-caused death
Parathyroid scan (mibi)
6.4
1 in 3600
CT (neck)
3
1 in 8000
Effective doses from 4DCT, as reported by different authors
4DCT [4]
28
4DCT [5]
10.4
4DCT [6]
5.56
Other radiologic examinations using ionizing radiation, provided for comparison
Chest X-ray (PA + lateral)
0.05
1 in 480000
CT (chest)
7
1 in 3500
Considering Hoang’s data and using modeling from the Biologic Effects of Ionizing Radiation (BEIR) VII report, a 55-year-old woman undergoing 4DCT would incur an increased incidence of cancer, over baseline, of 0.52 %. Using a similar assessment, for a mibi scan, she would incur an increased incidence of cancer, over baseline, of 0.19 % [4].
As background, the average US background radiation exposure is 3 mSv/y. A US radiation worker is allowed to receive a maximum of 50 mSv/y. A representative chest CT often comes at a cost of approximately 7 mSv. When delivered in an acute manner, 400 mSv may induce initial signs of acute radiation sickness.
Summary
While there are a variety of CT acquisition techniques to achieve 4DCT, across the board the effective dose is greater than with mibi. However, the clinician and patient together must take into consideration the risk-benefit ratio of 4DCT versus mibi. 4DCT may confer a larger effective dose, and for elderly patients, this increased effective dose may be justified. Given the relationship of thyroid cancer and age, some suggest caution when considering 4DCT for female patients aged 30 and younger and for male patients aged 20 and younger [5, 7]. Some suggest reserving 4DCT for cases of failed parathyroidectomy or in cases of altered anatomy [8].
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